WO2012007931A2 - Catalyseur à électrode d'argent - Google Patents

Catalyseur à électrode d'argent Download PDF

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Publication number
WO2012007931A2
WO2012007931A2 PCT/IE2011/000038 IE2011000038W WO2012007931A2 WO 2012007931 A2 WO2012007931 A2 WO 2012007931A2 IE 2011000038 W IE2011000038 W IE 2011000038W WO 2012007931 A2 WO2012007931 A2 WO 2012007931A2
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Prior art keywords
electrode
catalyst
electrodes
dbsa
modified
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PCT/IE2011/000038
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WO2012007931A3 (fr
Inventor
Anthony Joseph Killard
Aoife Morrin
Laura Gonzales-Macia
James G. Carton
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Dublin City University
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Publication of WO2012007931A3 publication Critical patent/WO2012007931A3/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8663—Selection of inactive substances as ingredients for catalytic active masses, e.g. binders, fillers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H01M4/8803—Supports for the deposition of the catalytic active composition
    • H01M4/8807—Gas diffusion layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/88—Processes of manufacture
    • H01M4/8878—Treatment steps after deposition of the catalytic active composition or after shaping of the electrode being free-standing body
    • H01M4/8892—Impregnation or coating of the catalyst layer, e.g. by an ionomer
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the invention relates to a silver electrode catalyst and uses thereof.
  • the invention relates to catalysing the decomposition and electrochemical reduction of hydrogen peroxide.
  • the electrocatalytic reduction of hydrogen peroxide is important for a wide range of applications including chemical sensors, biosensors, fuel cells and batteries, chemical synthesis, and decomposition processes.
  • Hydrogen peroxide is an extremely widely used raw material due to its oxygen content and its electron-donating properties.
  • the catalytic reduction of hydrogen peroxide can be used in a wide variety of applications. Often, this process is coupled electrochemically either for analytical purposes or to use hydrogen peroxide as an oxidant source in fuel cells and batteries.
  • Many catalysts are available for the decomposition and reduction of hydrogen peroxide, most notably metallic silver and platinum.
  • precious metal catalysts such as these are extremely expensive, can easily become poisoned and are relatively inefficient compared to biological catalysts such as enzymes (e.g., peroxidase and catalase).
  • enzymes e.g., peroxidase and catalase
  • a solution to the creation of suitable catalytic systems would be a material that had good environmental stability such as precious metal catalysts, but with minimal cost implications.
  • Silver is a precious metal that is approximately 100 times cheaper than its platinum counterpart. It is a well-known catalyst used for example, in the production of formaldehyde from methanol and air. Silver is the only catalyst available today to convert ethylene to ethylene oxide.
  • a type of Fuel Cell that does not employ noble-metal catalysts is the alkaline Fuel Cell. Because of facile oxygen reduction kinetics at a high pH, non-precious metals can be utilised. However, it suffers drawbacks due to problems with liquid electrolyte management and electrolyte degradation. In addition, this approach cannot be used for hydrogen peroxide reduction as the process requires an acidic medium.
  • H 2 0 2 analysis Another market that demands H 2 0 2 analysis is the clinical field, where exhaled breath is often monitored for H 2 0 2 . Hydrogen peroxide in the breath is indicative of lung diseases such as asthma and chronic obstructive pulmonary diseases.
  • a common method to collect the exhaled breath condensate is to use a special cooling collector including a freezing cooling tube (usually cooled using ice or liquid nitrogen) and cooling machine, which has a refrigerator's circuit. Once collected, the condensate is analyzed off-line by techniques, such as spectroscopy, with assistance of peroxidase.
  • H 2 0 2 is also the most valuable marker for oxidative stress, recognized as one of the major risk factors in progression of disease-related pathophysiological complications in diabetes, atherosclerosis, renal disease, cancer, aging and other condition.
  • Hydrogen peroxide sensors are also needed for the development of biosensors based on enzyme oxidases.
  • Universal Sensors market an amperometric peroxide electrode based on a platinum electrode for these types of applications.
  • a catalyst comprising a silver electrode wherein the electrode has been modified with surfactant-salt solution.
  • the electrode comprises silver colloids, silver particles, or silver nanoparticles.
  • the electrode is a silver paste electrode.
  • the electrode may be a screen printed electrode.
  • the surfactant is present at a concentration of from 0.01 to 0.1 M. In one embodiment the salt is present at a concentration of at least 0.001 M.
  • the surfactant may, for example, be selected from: dodecyl benzene sulphonic acid (DBSA)
  • CTAB cetyl trimethylammonium bromide
  • the catalyst is selected from DBSA and SDS.
  • the salt solution may, for example be a solution selected from :
  • the molar ratio of surfactant and salt to achieve maximum catalysis may be approximately 1 :3. This concentration ratio may optimise the formation of complex structures which are particularly suitable for deposition onto the silver electrode substrate.
  • the surfactant-salt solution contains approximately 0.033M DBSA and approximately 0.1 M KCI.
  • the surfactant-salt solution contains approximately 0.033M SDS and approximately 0.1 M NaCl.
  • the surfactant-salt solution contains approximately 0.0033 M CTAB and approximately 0.1 M NaBr. In another case the surfactant-salt solution contains approximately 0.033M Triton X- 100 and approximately 0.0001 M KCI.
  • the electrode may be modified with the surfactant-salt solution for about 3 hours.
  • the catalyst is coated with a coating such as nafion and/or chitosan and/or cellulose acetate.
  • the catalyst may be a non-enzymatic catalyst.
  • the catalyst may be a peroxide catalyst such as a hydrogen peroxide catalyst.
  • catalysts operate by reducing the activation energy required for a chemical transformation, increasing its kinetics. This is typically achieved by the formation of an energetically favourable transition state structure between the catalyst and the species being transformed. This is true of either homogeneous catalysts or heterogeneous (surface) catalyst such as that proposed here.
  • heterogeneous catalysts has, in the past, been limited to a number of materials which exhibit good catalytic properties such as metals (e.g., platinum, gold, silver) as well as metallic oxides.
  • metals e.g., platinum, gold, silver
  • Important species in this regard would include, but not be limited to a range of gasses such as hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogenous gasses, hydrogen sulphide, chlorine and other halogens, ammonia, phosgene, methane and other higher alkanes. It would also include, but not be limited to low molecular weight species including hydrocarbons, carbohydrates, organic and inorganic salts, transition metal complexes, fluorocarbons and related halogenated carbons, organophosphorous compounds and nitrogenous chemicals.
  • gasses such as hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogenous gasses, hydrogen sulphide, chlorine and other halogens, ammonia, phosgene, methane and other higher alkanes. It would also include, but not be limited to low molecular weight species including hydrocarbons, carbohydrates, organic and inorganic salts, transition metal complexes, fluorocarbons and related halogenated carbons, organophosphorous compounds and nitrogenous chemicals.
  • macromolecules including, but not limited to proteins, complex carbohydrates, lipids, phospholipids and derivatives thereof, supramolecular assemblies including, but not limited to species such as carbon nanotubes, fullerenes, graphenes and derivatives thereof.
  • the invention also provides a sensor system comprising a catalyst of the invention.
  • the sensor may be a biosensor, or a chemical sensor.
  • the invention further provides a cholesterol sensor comprising a catalyst of the invention.
  • the invention also provides a glucose sensor comprising a catalyst of the invention.
  • the invention also provides a fuel cell comprising a catalyst of the invention.
  • the invention further provides a cathode (such as a fuel cell cathode) comprising a catalyst of the invention.
  • a cathode such as a fuel cell cathode
  • the catalyst is provided in a gas diffusion layer.
  • the silver may be provided in the gas diffusion layer which is then modified with a surfactant - salt solution.
  • the invention also provides a method for manufacturing a fuel cell cathode comprising providing silver in a gas diffusion layer, and subsequently modifying the silver with a surfactant - salt solution.
  • a porous substrate such as a carbon cloth material
  • the silver may be in the form of an ink for application to the substrate.
  • the invention provides the use of a catalyst of the invention in fuel cells
  • the invention provides a method of detecting hydrogen peroxide comprising the steps of: providing a silver electrode;
  • the method may comprise rinsing the electrode with water before exposing the electrode to the sample.
  • the rate at which the hydrogen peroxide molecule can be catalytically cleaved to hydroxy! radicals and hydroxide ions which can then be reduced electrochemical ly at an electrode at low applied reduction potentials is increased.
  • catalytic structures composed of dodecyl benzene sulphonic acid (or other appropriate amphiphilic molecules) and metal halides (potassium bromide, sodium chloride or others) form at the surface of a silver electrode surface. It is believed that these structures catalyse the initial decomposition of the hydrogen peroxide which in turn oxidises the surface of the silver electrode. The oxidised silver electrode surface is then returned to its neutral state via electrochemical reduction resulting from an applied cathodic potential to the electrode.
  • One application of the invention is demonstrated as a suitable platform for cholesterol detection.
  • a modified screen printed electrode SPE
  • HyPer modified screen printed electrode
  • the HyPer electrode has been used to quantitatively detect 1 - 5 mM cholesterol, which is the relevant clinical range for this analyte.
  • Another application of the invention is demonstrated as a suitable platform for glucose sensing.
  • Inset magnified diagram of voltammograms (a) and (b);
  • Fig. 2 are amperometric i-t curves of Ag SPEs measured at - 0.1 V (vs Ag/AgCI) in PBS pH 6.8: (a) unmodified and (b) modified with 3.3 ⁇ 10 "2 M DBSA/ 0.1 M C1 at H 2 0 2 concentration from 1 to 5- 10 "3 M;
  • Fig. 3 are amperometric i-t curves of Ag SPEs measured at - 0.1 V (vs Ag/AgCI) in PBS pH 6.8: (a) unmodified; (b) 3.3 ⁇ 10 "2 M DBSA (c) 0.1 M C1 and (d) 3.3 ⁇ 10 "2 M DBSA/ 0.1 M KG modified electrodes, at H 2 0 2 concentration ranges from 1 to 5- 10 "3 M;
  • Fig. 4 are plots of current vs log [DBSA] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCI) at 5 10 "3 M H 2 0 2 .
  • the electrodes were modified only with DBSA (a) or 0.1 M KC1 and DBSA (b);
  • Fig. 5 are plots of current vs log [KCl] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCl) at 5- 10 "3 M H 2 0 2 .
  • Fig. 6 is a plot of current vs modification time obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCl) at 5- 10 "3 M H 2 0 2 . Data at 0 s corresponds to unmodified electrode;
  • Fig. 7 are plots of i cat vs electrolyte pH for Ag SPEs (a) with and (b) without DBSA/KC1 modification in 5- 10 "3 M H 2 0 2 at - 0.1 V vs Ag/AgCl.;
  • Fig. 8 are amperometric responses of the DBSA/KC1 modified electrodes in 20 ⁇ 10 "6 M aliquots of H 2 0 2 (20 - 160 10 "6 M) at -0.1 V (vs Ag/AgCl) in PBS pH 6.8.
  • Input Plot of cathodic current vs H 2 0 2 concentration at -0.1 V (vs Ag/AgCl) in PBS pH 6.8;
  • Fig. 9 are illustration of reproducibility study using three different electrodes: (a) electrode 1 ; (b) electrode 2; (c) electrode 3. Amperometric responses of the DBSA/KCI modified electrodes in 20- 10 "6 M aliquots of H 2 0 2 (20 - 160 - 10 "6 M) at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8. Each electrode was measured three times: ( ⁇ ) repeat 1 ; ( A) repeat 2; ( ⁇ ) repeat 3;
  • Fig. 10 are SEM images using secondary electron (SE) detection of Ag SPEs (a) unmodified; (b) DBSA/KCl-modified; (c) DBSA/KCI-modified after electrochemical reduction of 5- 10 "3 M H 2 0 2 ; (d) AgCl electrochemical ly deposited; (e) KCl modified.
  • SE secondary electron
  • Fig. 1 1 are XPS images of Ag SPEs (a) unmodified and (b) DBSA/KCI modified;
  • Fig. 12 are amperometric i-t curves of Ag SPEs measured at - 0. 1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified; (b) AgCl electrodeposited and (c) DBSA/KCI modified electrodes, at H 2 0 2 concentrations from 1 to 5- 10 "3 M;
  • Fig. 14 are plots of change in mass per unit area versus time of (a) unmodified and (b) modified Ag SPE in 2 ml 1 M H 2 0 2 ;
  • Fig. 15 are plots of the change in mass per unit area versus time of (a) unmodified and (b) modified Ag SPE in 2ml 1 M H 2 0 2 solution for 1 hour. Each electrode was measured three times and the number of the repetition is indicated besides each graph;
  • Fig. 16 are SEM images of unmodified Ag SPEs (A) before and (B) after the measurements in 1 M H 2 0 2 solution. SEM images of Ag SPEs modified with DBSA/ C1 (C) before and (D) after exposure to 1 M H 2 0 2 solution. Accelerating voltage of 20 kV. (5.0 k x magnification);
  • Fig. 17 are SEM images of Ag SPEs modified with DBSA/ C1 (A) before exposure to 1 M H 2 0 2 ; and after (B) 1 h, (C) 2 h and (D) 3 h exposure to 1 M H 2 0 2 solution. Accelerating voltage of 20 kV. (5.0 k x magnification);
  • Fig. 18 are plots of current vs log [SDS] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCl) at 5- 10 "3 M H 2 0 2 .
  • the electrodes were modified (a) only with SDS or (b) SDS and 0.1 M NaCl;
  • Fig. 19 are plots of current vs log [salt] obtained during amperometric measurements of
  • Fig. 20 are illustrations of (i). Simplified surfactant structure, (ii). Typical surfactant aggregates: (a) cylindrical structures, (b) lamellar structures and (c) spherical micelles; Fig. 21 are plots of current vs log [XCl] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCl) at 5 10 "3 M H 2 0 2 .
  • Fig. 22 are amperometric i-t curves of metallic silver electrodes measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified; (b) 3h; (c) 1 day DBSA/KCI modified, at H 2 0 2 concentrations from 1 to 5- 10 "3 M;
  • Fig. 23 are SEM images using SE detection of metallic Ag (99.9 %) electrodes (a) unmodified and (b) DBSA/KCI modified. Accelerating voltage of 20 kV. (5.0k x magnification);
  • Fig. 24 are amperometric i-t curves of Ag wire electrodes measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified; (b) DBSA/KCI modified; (c) AgCI electrochemical formation after 2 s at + 1 V (vs Ag/AgCl) in 0.1 M HCl (d) AgCI electrochemical formation after 5 s at + 1 V (vs Ag/AgCl) in 0.1 M HCl three times, at H 2 0 2 concentrations from 1 to 5 10 3 M.
  • Fig. 25 are plots of the change of mass per unit area versus time for (a) unmodified and (b) modified 92.5 % Ag electrodes upon repeated exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposures is indicated besides each graph;
  • Fig. 26 are plots of the change of mass per unit area versus time for (a) unmodified and (b) modified 99.9 % metallic Ag electrodes upon exposure to 1 M H 2 0 2 solution for 1 hour;
  • Fig. 27 are plots of the change in mass per unit area versus time for (a) unmodified and (b) modified (circle) 92.5 %, (triangle) SPE and (square) 99.9 % Ag electrodes when exposed to 1 M H 2 0 2 solution for 1 hour. These data correspond to the first repetition of each electrode;
  • Fig. 28 are SEM images of unmodified sterling 92.5 % Ag electrodes (A) before and (B) after being exposed to 1 M H 2 0 2 solution and 99.9 % Ag metallic electrodes (C) before and (D) after H 2 0 2 decomposition measurements. Accelerating voltage of 20 kV. (5.0 k x magnification);
  • Fig. 29 are SEM images of modified sterling 92.5 % Ag electrodes (A) before and (B) after exposure to 1 M H 0 2 solution and 99.9 % Ag (C) before and (D) after H 2 0 2 decomposition measurements. Accelerating voltage of 20 kV. (5.0 k x magnification);
  • Fig. 30 are amperometric i-t curves of metallic Au (99.9 %) electrodes measured at - 0.1 V (vs Ag/AgCI) in PBS pH 6.8: (a) unmodified and (b) DBSA/KCI modified, at H 2 0 2 concentrations from 1 to 5 ⁇ 10 "3 M;
  • Fig. 3 1 are plots of change in mass per unit area versus time for (a) unmodified and (b) modified metallic Au (99.9 %) electrodes upon repeated exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposures is indicated besides each graph;
  • Fig. 32 are plots of change in mass per unit area versus time for (a) unmodified and (b) modified Au SPEs after exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposure is indicated besides each graph;
  • Fig. 33 are plots of change in mass per unit area versus time for (a) unmodified and (b) modified (square) Au (99.9 %) metallic electrodes and (circle) Au SPEs upon exposure to 1 M H 2 0 2 solution for 1 hour. These data correspond to the first repetition of each electrode;
  • Fig. 34 are plots of change in mass per unit area versus time for (a) unmodified and (b) modified Pt SPEs upon repeated exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposures is indicated besides each graph;
  • Fig. 35 is a plot -In [H 2 O 2 ]/[H 2 O 2 ] 0 versus time for (a) unmodified and (b) modified Ag SPEs after exposure to 1 M H 2 0 2 solution for 1 hour;
  • Fig. 36 is a plot of the kinetics of H 2 0 2 decomposition on (a) unmodified and (b) modified Ag SPEs;
  • Fig. 37 is a calibration curve of cathodic current increase against cholesterol concentration.
  • Working Electrode HyPer.
  • Buffer solution Made up according to Audit Diagnostics protocol with 200-fold increase in cholesterol oxidase concentration.
  • Inset Signal based on increase in amperometric current from trough to peak;
  • Fig. 38 is a plot of the effect of addition of various solutions to BSA to monitor the current drop Working Electrode: HyPer. (a) 3.2 mM Cholesterol in Deionised Water (500 ⁇ ); (b) Buffer, pH 7 (500 ⁇ ,); (c) Deionised Water (500 ⁇ ). A current decrease is observed when cholesterol was added to the system. Negligible, if any oxidation current was observed for buffer or H 2 0 additions to the system;
  • Fig. 39 is a calibration graph of maximum amperometric cathodic response of HyPer electrode to additions of lyophilised cholesterol over the concentration range 0.5 - 3.2 mM;
  • Fig. 40 is a calibration graph of the slope of the amperometric cathodic response of HyPer electrode to additions of lyophilised cholesterol over the concentration range 0.5 - 3.2 mM. (Slopes measured by approximating response as straight line and by dividing net response from net time taken to reach that response);
  • Fig. 41 is a calibration graph of the time taken to reach the maximum amperometric cathodic response of HyPer electrode to additions of leophilised cholesterol over the concentration range 0.5 - 3.2 mM;
  • Fig. 42 is a graph of the response of HyPer electrode to 10 mL H 2 0 2 injections (each addition was equivalent to 0.5 mM H 2 0 2 ).
  • Bulk solutions (a) PBS with Choi Ox and Lipo-lipase, (b) Pipes/MgCl 2 with no enzyme, (c) PiPes/MgCl 2 with Choi Ox and Lipo- lipase;
  • Fig. 43 are amperometric responses of the Nafion-coated HyPer electrodes (A) and bare HyPer electrodes (B) to H 2 0 2 (0.5 mM) in the presence of varying amounts of BSA in bulk solution: (a) 0.5% BSA, (b) 0.25% and (c) 0.125%.
  • the effect of Nafion or of increasing the amount of BSA in the solution does not affect the sensor response to hydrogen peroxide;
  • Fig. 44 is a graph of the response of HyPer electrode to H 2 0 2 injections (each addition was equivalent to 0.5 mM H 2 0 2 ) in the presence of: (a) PBS (0.13813 g/10 ml), (b) MgCl 2 (0.005 g/10 ml), (c) PIPES (0.13813 g/10 ml) and (d) PIPES/MgCl 2 (as per AD Buffer);
  • Fig. 45 are amperometric responses to hydrogen peroxide (0.5 mM additions) of: (a) Nafion-coated and (b) unmodified HyPer electrode in AD PIPES buffer. It can be observed that there is no significant effect on the initial response of the electrode to hydrogen peroxide when a Nafion membrane is employed;
  • Fig. 46 are amperograms showing electrode responses to injections of H 2 0 2 in buffers: (a) PBS (HyPer electrode) and Pipes/MgCl 2 (b) Chitosan-coated and (c) unmodified HyPer electrode. It can be seen that the HyPer electrodes response could be improved in Pipes buffer by employing the chitosan membrane;
  • Response time from the chitosan- HyPer is 700 s, while the response time of the bare HyPer electrode was approx. 1500s;
  • Pipes MgCl 2 / Cholesterol Oxidase AD Buffer with various electrode barriers (a) Amperometric response from a chitosan HyPer electrode, (b) Amperometric response from a HyPer electrode, (c) Amperometric response from a cellulose acetate—coated HyPer electrode Response time from the chitosan-modified electrode is 750 s, while the response time of the bare electrode was approx. 1500s;
  • Fig. 51 are amperometric i-t curves of Ag_DBSA/KCl SPEs measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) H 2 0 2 sensing (b) glucose sensing, with l mg/ml GOx in solution and (c) H 2 0 2 sensing after glucose sensing, at H 2 0 2 and glucose concentration from 1 to 5 10 ⁇ 3 M;
  • Fig. 52 are amperometric i-t curves of Ag_DBSA/KCI SPEs measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) without and (b) with a cellulose acetate film, at H 2 0 2 concentration from 1 to 5- 10 "3 M;
  • Fig. 53 are amperometric responses of the DBSA/KCl CA GOx modified Ag SPEs in
  • Fig. 54 is an illustration of a hydrogen peroxide fuel cell
  • Fig. 55 is an illustration of the gas diffusion layer and its modification with the catalyst
  • Fig. 56 is a typical current voltage diagram for a fuel cell indicating the points on the I-V curves that correspond to different losses;
  • Fig. 57 is a graph of test results of fuel cells
  • Fig. 58 is a photomicrograph of a gas diffusion layer material before modification
  • Fig. 69 are photomicrographs of gas diffusion layer material after modification with the hydrogen peroxide catalyst
  • Fig. 60 are profilometry images after modification with the hydrogen peroxide catalyst.
  • Dodecylbenzenesulfonic acid (DBSA-D0989) was purchased from TCI Europe. Poly(vinylsulfonic acid) (PVS), sodium dodecyl sulphate (SDS), cetyl trimethylammonium bromide (CTAB), Triton X-100, lithium, sodium, potassium and caesium chloride (LiCI, NaCI, CI, CsCl), sodium bromide (NaBr) and potassium dihydrogen phosphate ( H2PO4) were purchased from Sigma-Aldrich. Di-sodium hydrogen phosphate (Na 2 HP0 ) was purchased from Riedel-de Haen. 30% (v/v) hydrogen peroxide solution was purchased from Merck.
  • Gold and platinum screen-printed electrodes (4 mm diameter) were purchased from Dropsens (Asturias, Spain).
  • Silver conductive ink (Electrodag ® PF-410) was purchased from Henkel (Scheemda, The Netherlands).
  • Poly(ethylene) terephthalate substrates were Melinex ® (pre-shrunk) films obtained from HiFi Industrial Film Ltd. (Dublin, Ireland). All the solutions were prepared using 1 8 ⁇ Milli-Q water.
  • phosphate buffered saline solution PBS
  • the buffer solution is 0.1 M phosphate, 0.1 37 M NaCI and 0.0027 M KCI. This was prepared by mixing solution 1 (0.1 M Na 2 HP0 4 , 0.137 M NaCI and 0.0027 M KCI) and solution 2 (0.1 M KH 2 P0 4 , 0.137 M NaCI and 0.0027 M KCI) to a pH of 6.8.
  • Cholesterol oxidase/Pipes buffer was prepared as follows, according to an AD protocol unless otherwise stated: 0.13813g Pipes Di-sodium Salt and 0.00526 g of magnesium chloride 6- hydrate were diluted with water and the pH adjusted to between 6.9 - 7.0, the enzymes cholesterol oxidase (-20 mg) and lipoprotein lipase ( ⁇ 1 1 mg) and the entire solution total volume was 10 ml, unless otherwise stated.
  • Phosphate buffered saline PBS
  • PBS Phosphate buffered saline
  • Bis-Tris was also investigated at the same concentration (-0.13813 g/10 ml).
  • Hydrogen peroxide was prepared to a concentration of 50 mM by diluting 0.5 ml to 5 ml with deionised water. 10 ⁇ was added to 1 ml of the appropriate enzyme/buffer or buffer solution to give an overall concentration of 0.5 mM in order to examine the electrode response. Lyophilised cholesterol samples were diluted with 2.5 ml deionised water and added to the buffer to a final concentration of 1.6 mM, unless otherwise stated.
  • 1 % (w/v) Nafion solution was prepared by diluting 100 ⁇ into 10 ml deionised water.
  • 1 % (w/v) Chitosan solution was prepared by dissolving of 0.1 mg into 10 ml of 10% Acetic Acid.
  • Cellulose Acetate 1 % (w/v) suspension was prepared by diluting 0.1 mg into 10 ml deionised water.
  • Silver screen-printed electrodes were fabricated using an automated DE 248 machine (Weymouth, UK). Briefly, a layer of silver paste was deposited onto PET substrate and cured in a convectional oven at 120°C for 5 minutes. Then, an isolating tape layer was deposited to define the working electrode area (0.126 cm 2 ).
  • SEM Scanning Electron Microscopy
  • SE Secondary Electron
  • EDX Energy Dispersive X- ray
  • H 2 0 2 decomposition was assessed by the decrease in mass registered when the electrodes were placed in 2 ml of 1 M H 2 0 2 .
  • the electrodes were dipped into the H 2 0 2 solution and the vials were covered with parafilm to avoid losses due to water evaporation.
  • a small hole was made in the parafilm.
  • the mass data were registered every minute for 1 hour and each electrode was measured three times, with a resting time of 10-15 minutes between measurements.
  • Silver electrodes were prepared by a screen printing method using the DEK 248 printing system. The electrodes were conditioned by placing them in a solution of 0.1 M KG and 0.033 M DBSA for at least 2 hours followed by a washing step with deionised water before using them as sensors. These electrodes are referred to throughout the text as HyPer electrodes. RESULTS AND DISCUSSION
  • Cyclic voltammetry and time-based amperometry were carried out in order to assess the catalytic effect observed when Ag SPEs had been modified with surfactant-based solutions. Cyclic voltammetry and amperometry were performed on the unmodified home-made electrodes in PBS pH 6.8 as controls. Subsequently, Ag SPEs were dipped into a solution of 3.3 ⁇ 10 "2 DBSA/ 0.1 M KC1 for 3 h and rinsed with distilled water to remove any excess of modifying solution on the surface and they were also subjected to cyclic voltammetry and amperometry in PBS pH 6.8.
  • volume ( ⁇ ) aliquots of 1 M H2O2 were sequentially added to the bulk so H 2 0 2 concentration in the solution increased from H O "3 to 5 ⁇ 10 "3 M.
  • One of the main requirements of a substrate to be used as a platform in an electrochemical sensing process is that it be electrochemical ly inert in the selected range of potential.
  • a narrow window of potential was chosen for the voltammetric measurements (-0.2 to + 0.025 V vs. Ag/AgCl), as a wider potential range would lead to the oxidation of Ag (at more positive potentials) or 0 2 interferences (at more negative potentials), distorting H 0 2 responses.
  • the cathodic current in the presence of 5- 10 "3 M H 2 0 2 was more than 100 times higher for electrodes after being modified with 3.3- 10 "2 M DBSA/ 0.1 M KC1 when compared to unmodified electrodes.
  • the cathodic current at -0.1 V from the cyclic voltammograms in the presence of 5- 10 "3 M H 2 0 2 was approx. 4.4 ⁇ 10 "5 A for the modified electrode, after subtracting the background current, whereas the modified one exhibited only a 3.8 ⁇ 10 "7 A current at the same potential.
  • Inset magnified diagram of voltammograms (a) and (b).
  • -0.1 V seemed to be a suitable potential to study the catalytic effect of the surfactant- based modification on Ag SPEs towards H 2 0 2 reduction.
  • Typical applied potentials used for the amperometric determination of H 2 0 2 reduction in the literature range from - 0.7 V (GC/PABS- modified electrodes), ' - 0.5 V (Ag nanoparticles in a polyvinyl alcohol film on a Pt electrode) 3 , - 0.1 V commonly applied in enzymatic biosensors (such as a HRP-modified electrodes) 4 " 5 and up to 0 V with Prussian Blue.
  • Modified Ag SPEs showed cathodic currents of approx. 3.60- 10 5 A at - 0.1 V in the presence of 5- 10 "3 M H 2 0 2 whereas the current obtained with the unmodified ones reached approx. 4.2- 10 "7 A. This difference of almost two orders of magnitude was previously observed in the cyclic voltammograms in Fig. 1. Moreover, the greater steady state background current exhibited by the modified amperometric curve (approx. 1 .6- 10 "6 A) compared to the unmodified one (approx. 9.0- 10 "8 A) makes it evident that the Ag SPE surface undergoes a modification following the exposure to DBSA/KCI solution. Further controls employing DBSA or KCI alone were performed.
  • FIG. 3 shows the amperometric curves of an unmodified Ag SPE and with DBSA (0.033 M) and KCI (0.1 M) together and separately.
  • DBSA 0.033 M
  • KCI 0.1 M
  • the highest cathodic current was obtained when the Ag SPE was modified with a mixture of both DBSA and KCI.
  • Such pre- treated electrodes provided cathodic responses of 3.6- 10 "5 A, which represented a more than eighty-fold greater response to H 2 0 2 than those without any modification. Therefore, the relative order of the modified Ag SPEs regarding the catalytic response to 5- 10 "3 M H 2 0 2 is the following: Unmodified ⁇ 3.3- 10 "2 M DBSA modified ⁇ 0.1 M KCI modified ⁇ 3.3- 10 "2 M DBS A/ 0.1 M KCI modified
  • DBSA and KCI concentrations were optimised. Cyclic voltammetry and amperometry were performed using first 0.1 M KCI over a range of DBSA concentrations followed by 3.3 I 0 "2 M DBSA over a range of KCI concentrations. The cathodic responses to 5- 10 "3 M H 2 0 2 were correlated with DBSA or KCI concentrations.
  • Fig. 4 shows the responses of Ag SPEs after dipping into several DBSA concentration solutions for 3h, in the absence and the presence of 0.1 M KCI in the modifying solutions. At low DBSA concentrations, the presence of KCI provided approx.
  • FIG. 5 shows the results of a similar study, using three DBSA concentrations and a range of KCI concentrations.
  • Fig. 4 highlighted that the optimum DBSA concentration in the modification solution was between 0.1 and 0.01 M. Therefore, DBSA concentrations of 0.1 , 3.3- 10 "2 and 1 0 "2 M were studied.
  • Fig. 5 presents the results for these experiments and compares them to those obtained using KCI alone.
  • the catalytic effect on H 2 0 2 reduction was again observed for those electrodes modified with both DBSA and KCI reagents whereas those electrodes pre-treated only with KCI displayed a small catalytic increase at 0.1 M, which was also seen with amperometry in Fig. 3.
  • the enhancement of the H 2 0 2 reduction current was shown to be dependent on KCI
  • the catalytic ratio shown by the modified electrodes went along the following relative order: unmodified ⁇ 10 " 2 M DBSA/KC1 ⁇ 3.3 ⁇ 10 "2 M DBSA/KC1 ⁇ 0.1 M DBSA/KC1. Electrodes exhibited enhanced catalysis at lower KCI concentrations when DBSA concentrations were increased. Electrodes modified with 0.1 M DBSA/KC1 solutions showed catalytic activity from approx. 10 "4 M KCI concentrations whereas the onset of the catalysis occurred from 10 "3 M KCI for the electrodes modified in 3.3- 10 "2 M DBSA/KC1 or 10 "2 M DBSA/KCI solutions.
  • surfactants are well-known to form organized structures when they are in aqueous solution. 7 ' 8 At low concentrations but above the critical micelle concentration (CMC), typical surfactant aggregations are in the form of spheroidal micelles. As the concentration of surfactant in solution increases, greater aggregations such as hexagonal or lamellar structures are observed. The addition of a salt or cosurfactant to the solution allows the formation of these high-aggregation structures at lower surfactant concentrations. This was observed by Sein et al. 9 with sodium dodecylbenzenesulfonate (NaDoBS) in the presence of several chloride salts. The CMC for DBSA is approx.
  • modification time i.e. the time Ag SPEs were deposited in the DBSA/KCl solution before their application in H 2 0 2 detection.
  • Several Ag SPEs were submerged into 3.3 ⁇ 10 "2 M DBSA/ 0. 1 M KCI solutions for different periods of time, from a few seconds to one day. Then, the electrodes were rinsed thoroughly with distilled water to remove any non-adherent species which might remain on the surface and these were measured amperometrically in 5- 10 "3 M H 2 0 2 in PBS pH 6.8.
  • Fig. 6 shows the plot of these data vs modification time.
  • the cathodic currents displayed by Ag SPEs after 60, 300 and 600 s modification time were all approx. 2.9 ⁇ 10 "5 A.
  • the highest cathodic current value of 3.4- 10 "5 A was obtained with electrodes following exposure to DBSA/KCI modification solution for 2 hours. Further exposure times such as 1 day or 1 week did not improved the response; on the contrary, the catalytic current started to decrease. To ensure that the highest catalytic currents were obtained, 3 hours was chosen as the optimum modification time for the experiments. This was used for all subsequent experiments.
  • FIG. 6 Plot of current vs modification time obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCl) at 5- 10 "3 M H 2 0 2 .
  • Data at 0 s corresponds to unmodified electrode.
  • Influence of pH on the catalytic process The influence of the pH of the bulk electrolyte solution during the reduction of 5- 10 "3 M H 2 0 2 was investigated for both unmodified and DBSA/KCI modified Ag SPEs. Amperometric measurements at - 0.1 V were performed (Fig. 7) using the same electrode while changing the pH. H 2 0 2 reduction at the unmodified Ag SPE showed little catalytic response in the pH range from 2 to 10, unlike a DBSA/KCI modified one.
  • the cathodic currents of the unmodified electrodes rang ed from 3.4 ⁇ 10 "7 A at pH 2 to 3.6 ⁇ 10 "6 A at pH 10, with a maximum value of 4.4- 10 "6 A at pH 6.8.
  • H 2 0 2 reduction occurs at more negative potentials than when the concentration is lower. This behaviour is typical of a mechanism of reaction in which H + appears as a reactant or OH " as a product of the reaction, which agrees with many of the proposed mechanisms for H 2 0 2 reduction," "15 as for example the following:
  • X-Ray Photoelectron Spectroscopy (XPS) measurements were performed using a Kratos AXIS 165 spectrometer (University of Limerick). The samples analysed were unmodified and DBSA/KCl modified Ag SPEs. Spectra and concentration percentages are presented in Fig. 1 1 and Table 1 , respectively. As might be expected, Ag SPE surfaces showed the presence of CI, K and S after the modification in DBSA/KCl solutions. However, a higher proportion of CI (4.3 %) over K (0.6%) was detected on the electrode surface, which is approx. 7.5-fold greater than that which would be expected on an equimolar basis. This fact could be explained by the formation of AgCl on the substrates during the modification which, unlike K would remain on the surface after the electrodes were rinsed.
  • Figure 1 XPS images of Ag SPEs (a) unmodified and (b) DBSA/KCl modified.
  • the process began with the stripping of Ag to form Ag-Cl complex and the subsequent cathodic electrodeposition of Ag + to form Ag nanoparticles on the Ag substrate.
  • the Ag-Cl complex decreased the diffusion rate of Ag + and made the reduction of Ag + more difficult, which was helpful in the formation of small Ag nanoparticles.
  • the activity of the roughed electrodes towards ⁇ 2 was assessed as a function of the number of cycles, KC1 concentration and scan rate during the cyclic voltammetry.
  • the catalytic current due to H2O2 reduction increased with cycle number and, therefore, Ag surface area of the electrodes. 1 , 20- 21
  • DBSA/KC1 modified Ag SPEs were shown by contact angle measurements in water to be more hydrophilic compared to the unmodified electrodes, as is shown in Fig. 13. This increase in hydrophilicity and surface energy would be consistent with modification with an amphiphile in a "head-up" orientation.
  • Dominguez et al. 27 have already reported the difference in contact angle for unmodified and surfactant modified surfaces. They performed contact angle measurements on graphite surfaces modified with SDS in the presence and the absence of different NaCI concentrations to show the different characteristics of the aggregates on the electrode surface. They found that the contact angle decreased as the salt concentration was increased. This indicated that the SDS aggregates wet the surface more thoroughly as the salt concentration increased.
  • Figure 14 Plot of change in mass per unit area versus time of (a) unmodified and (b) modified Ag SPE in 2 ml 1 M H 2 0 2 .
  • Figure 15 Change in mass per unit area versus time of (a) unmodified and (b) modified Ag SPE in 2ml 1 M H 2 0 2 solution for 1 hour. Each electrode was measured three times and the number of the repetition is indicated besides each graph.
  • Ag SPEs were dipped into solutions with different concentrations of SDS (0.5 - 10 "7 M), with and without 0.1 M NaCl for 3 h. Once rinsed, the electrodes were placed in the working cell (containing 10 ml PBS pH 6.8) and amperometry was performed. Cathodic current data from the amperometric responses to 5- 10 "3 M H 2 0 2 were correlated with SDS concentration. Fig. 18 shows the comparison between modification with and without NaCl.
  • FIG. 18 Plot of current vs log [SDS] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCI) at 5- 10 "3 M H 2 0 2 .
  • the electrodes were modified (a) only with SDS or (b) SDS and 0.1 M NaCl.
  • Ag SPEs modified solely with SDS solutions showed little catalytic effect towards H 2 0 2 reduction, even though SDS concentration was increased to 1 .5 .
  • the presence of NaCl in the modification solutions caused a noticeable enhancement in the cathodic current when Ag SPEs were measured in the presence of 5 10 "3 M H 2 0 2 .
  • Table 2 shows the structures and main characteristics of the surfactants under study.
  • the Group I halogen salt used with the surfactants in the modification solutions are also presented in Table 2.
  • Cathodic current data from amperometric measurements using 5 ⁇ 10 "3 M H2O2 are shown in Fig. 1 .
  • Ag SPEs exposed to the different surfactant solutions showed enhanced catalysis in a manner similar to that seen with DBSA/ C1 and SDS/NaCl. No remarkable enhancement effect was noticed when the concentration of the respective salt in the modification solutions remained below 10 "5 M. However, above this concentration, a significant catalytic effect towards H 2 0 2 reduction was observed, providing the highest reduction current values at salt concentrations in the modifying solutions of 10 "2 to 1 .5 M.
  • CMC critical micelle concentration
  • Triton X-100 (0.2-0.24) ⁇ CTAB (0.9- 1 ) ⁇ DBSA (2) ⁇ SDS (7- 10)
  • surfactants can form several types of organized structures in aqueous solutions as a function of concentration and/or experimental conditions. 7"9- 27 Typical surfactant aggregate structures are shown in Fig. 20. Figure 20. (i). Simplified surfactant structure, (ii). Typical surfactant aggregates: (a) cylindrical structures, (b) lamellar structures and (c) spherical micelles.
  • Spheroidal micelles are the simplest aggregates formed by surfactants at low concentrations. When the surfactant concentration increases, cylindrical structures are observed, and above 30- 40 % (w/v) of surfactant, liquid crystalline phases are formed. These structures result from the aggregation of surfactant molecules into large domains, often of hexagonal or lamellar structures. However, surfactant concentration is not the only cause of changes in shape and structure. There are also other methods of inducing aggregate growth, which includes the addition of a salt such as NaCI, the addition of cosurfactants, changes in the counterions and the use of surfactant mixtures.
  • a salt such as NaCI
  • Such structures would be formed in the solution and might become subsequently deposited or assembled at the electrode surfaces.
  • Baryla et al. 24 recently studied the adsorption mechanisms and aggregation properties of CTAB and used atomic force microscopy (AFM) to image and determine the aggregate morphology of the surfactant on coated surfaces. They also reported the surfactant concentration dependence of the micellar coating on fused silica substrates as well as the effect of ionic strength on the surface assembly of CTAB. At low phosphate buffer ionic strength, CTAB formed spherical aggregates on the substrates whereas at higher ionic strength (0.1 M) a combination of short rods and spherical aggregates was observed.
  • surfactant monomers form aggregates in aqueous solution because of the hydrophobic interactions between the long hydrocarbon chains which seek to minimise their interaction with water, and the hydrophilic head groups which favour interactions with water.
  • the favourable interactions of adjacent amphiphiles are limited by the unfavourable electrostatic repulsion between the polar headgroups.
  • Increasing the ionic strength of the buffer minimizes this repulsion by ionic screening, which leads to the formation of different morphologies such as lamellae. Therefore, surfactants in such a conformation are likely to be deposited onto a surface in different morphologies depending on the salt concentration in the solution.
  • FIG. 21 Plot of current vs log [XC1] obtained during amperometric measurements of Ag SPEs at - 0.1 V (vs Ag/AgCI) at 5- 10 "3 M H2O2.
  • Dominguez et al. have also recently reported the formation of hemicylindrical aggregates of SDS molecules on graphite surfaces at different salt (NaCl)/water solutions. They performed a molecular dynamics simulation study and the results were compared to those obtained experimentally by atomic force microscopy (AFM). Again, the aggregates exhibited different structures as the salt concentration was increased. Without salt, the hemicylindrical aggregates showed only two well-defined layers due to the adsorbtion of the SDS tails on the graphite surface. At low NaCl concentration, a third layer was observed, which vanished at high salt concentration. Any change in solution properties which causes a reduction in the effective size of hydrophilic head groups, i.e.
  • Electrochemical characterization Planar Ag (99.9 %) metallic electrodes were used to study the catalytic effect on H2O2 previously observed with Ag SPEs.
  • the electrodes were polished using 0.3 ⁇ first and 0.05 ⁇ then, and sonicated in distilled water for 5 min to remove any possible impurity on the surface. Next, they were dipped into 3.3 ⁇ 10 "2 M DBSA/ 0. 1 M KC1 solutions for 3 h, rinsed copiously with distilled water and placed in a working cell containing 10 ml PBS pH 6.8. Cyclic voltammograms and amperometric time-based measurements were performed in 0-5 ⁇ 10 "3 M H2O2. The measurements obtained with the modified electrodes were compared to those from the unmodified ones (Fig. 22).
  • FIG. 22 Amperometric i-t curves of metallic silver electrodes measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified; (b) 3h; (c) 1 day DBSA/KC1 modified, at H 2 0 2 concentrations from 1 to 5- 10 "3 M. As no enhancement was observed following the modification, the experiment was repeated leaving the metallic Ag (99.9 %) electrode exposed to the surfactant-based solution for 1 day (Fig. 22c). The longer pre-treatment time seemed to provide greater catalysis of H 2 0 2 reduction.
  • FIG. 23 SEM images using SE detection of metallic Ag (99.9 %) electrodes (a) unmodified and (b) DBSA/KCI modified. Accelerating voltage of 20 kV. (5.0k x magnification).
  • AgCI was also electrochemically formed onto the Ag wire electrodes by applying the same conditions as those above mentioned in section 3.1.2. 18 and the electrodes were subsequently tested in the presence of H 2 0 2 (Fig. 24).
  • the increased deposition of AgCI on the Ag wire electrode did lead to an increase in the catalytic current.
  • the cathodic current at 5- 10 3 M H 2 0 2 was 1.0 10 6 A for a Ag wire electrode after 2 s in 0.1 M HCl and reached 2.3 ⁇ 10 "6 A after 3 x 5 s exposures in 0.1 M HCl.
  • the formation of AgCl on the surface also led to an increase in the background noise and current.
  • 4.6- 10 "8 A was observed for the unmodified electrode whereas 2.7 ⁇ 10 "6 A was exhibited by the electrode modified 5 s in 0.1 M HCl three times, which reflected the increasing level of surface modification.
  • FIG. 24 Amperometric i-t curves of Ag wire electrodes measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified; (b) DBSA/KCl modified; (c) AgCl electrochemical formation after 2 s at + 1 V (vs Ag/AgCl) in 0.1 M HCl (d) AgCl electrochemical formation after 5 s at + 1 V (vs Ag/AgCl) in 0.1 M HCl three times, at H 2 0 2 concentrations from 1 to 5- 10 "3 M.
  • H 2 0 2 decomposition H 2 0 2 decomposition was then analysed on Ag metallic electrodes (92.5 % and 99.9 %) using the same conditions as for the Ag SPEs. The initial aim was to check if the catalytic effect was also observed on these substrates and to assess the influence of Ag surface on the H 2 0 2 decomposition process. The mass differences were expressed per unit area (g/cm 2 ). The area values used in the experiments were the geometric areas because the narrow potential range in which Ag is not electroactive has so far prevented the accurate calculations of their electroactive areas.
  • Figure 27 shows the compiled responses of SPE and metallic 92.5 % and 99.9 % Ag electrodes, before and after the DBSA/KC1 modification.
  • the data correspond only to the first of the measurements for each electrode.
  • the catalytic effect observed after surfactant-base modification of Ag SPEs was partially shown by the 92.5 % Ag electrodes.
  • the catalytic responses after modification were only three times higher than before for the 92.5 % Ag electrodes, whereas this was about eight times better in the case of SPEs.
  • No enhancement in H 2 0 2 decomposition was observed on the 99.9 % Ag electrode after surfactant-based modification.
  • DBSA/KC1 modification seemed to enhance H 2 0 2 decomposition only on Ag SPE and metallic 92.5 % Ag electrodes whereas no improved catalytic process was observed on metallic 99.9 % Ag electrodes.
  • FIG. 29 SEM images of modified sterling 92.5 % Ag electrodes (A) before and (B) after exposure to 1 M H 2 0 2 solution and 99.9 % Ag (C) before and (D) after H 2 0 2 decomposition measurements. Accelerating voltage of 20 kV. (5.0 k x magnification)
  • Electrochemical characterization Other noble metallic substrates were assessed in order to check if it was an isolated effect from Ag surfaces or a general behaviour from this group of metallic elements. Several gold-based electrodes were evaluated and their responses compared to those obtained with silver-based electrodes. In addition, the relationship between metallic Au electrodes and Au paste electrodes was also assessed.
  • Au SPEs Gold screen-printed electrodes
  • Au SPEs showed some enhancement of the H 2 0 2 reduction current after the modification with DBSA/ C1, although this effect was not so marked as that shown by Ag SPEs.
  • the highest ratio of cathodic density current obtained for a modified Au SPE compared to an unmodified one was approx. 12, whereas almost 90 was the ratio obtained with Ag SPEs.
  • Au SPE AT (cured at high temperature) showed a more noticeable catalytic effect after DBSA/KC1 pre-treatment, with approx. 8.03 ⁇ 10 "6 A/cm 2 , than Au SPE BT (cured at low temperatures), on which the cathodic current was only 0.5 H O "6 A/cm 2 .
  • Au SPE AT cured at high temperature
  • Au SPE AT showed a more noticeable catalytic effect after DBSA/KC1 pre-treatment, with approx. 8.03 ⁇ 10 "6 A/cm 2
  • Au SPE BT (cured at low temperatures), on which the cathodic current was only 0.5 H O "6
  • electrodes cured at higher temperature show rougher surfaces because the organic compounds presented in the printable ink are evaporated from the substrates when the temperature increases. 28
  • Au SPEs AT may be more prone to DBSA/ CI modification because they would present a higher surface area than Au SPEs BT and this would increase the amount of H 2 0 2 able to be reduced on them.
  • differences in organic binder composition cannot be excluded. In any case, the catalytic effect obtained by the gold-based SPEs was very low in comparison to that of Ag SPEs.
  • Figure 30 Amperometric i-t curves of metallic Au (99.9 %) electrodes measured at - 0.1 V (vs Ag/AgCl) in PBS pH 6.8: (a) unmodified and (b) DBSA/KCl modified, at H 2 0 2 concentrations from 1 to 5 0 "3 M.
  • DBSA/KCl modification did not seem to improve the catalytic features of metallic Au (99.9 %) electrodes, as can be observed by comparing the amperometric responses from the unmodified and DBSA/KCl modified electrodes. While 41.1 1 uA/cm 2 was shown by the unmodified Au metallic electrode in the presence of 5- 10 "3 M H 2 0 2 , only 3 1 .72 ⁇ /cm 2 was obtained with the DBSA/KCl modified one. Furthermore, comparing these data to those presented for Ag SPEs, metallic Au (99.9 %) showed higher reduction current values, which is probably due to the available metallic surface area of conductivity. However, the Ag SPEs showed greater increases in catalytic currents when treated with DBSA/KCl than did metallic Au. This may relate to availability of surface defects or the deposition/modification sites not available on planar metallic Au (99.9 %).
  • H 2 0 2 decomposition Following the study of the catalytic effect of H 2 0 2 decomposition on silver-based electrodes, the same effect was studied on gold-based electrodes.
  • Metallic Au (99.9 %) electrodes were polished using 0.3 and 0.05 ⁇ alumina, and subsequently rinsed with distilled water and sonicated for 5 min in order to clean the surfaces before the modification. Then, they were immersed in fresh 3.3 ⁇ 10 "2 M DBSA/ 0.1 M KC1 solution for 3 hours. Au SPEs were directly dipped into the surfactant-based solution without any pre-treatment. Subsequently, H 2 0 2 decomposition was analyzed by submerging the electrodes in 1 M H 2 0 2 solution for 1 h.
  • Figure 32 Change in mass per unit area versus time for (a) unmodified and (b) modified Au SPEs after exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposure is indicated besides each graph.
  • Figure 33 Change in mass per unit area versus time for (a) unmodified and (b) modified (square) Au (99.9 %) metallic electrodes and (circle) Au SPEs upon exposure to 1 M H 2 0 2 solution for 1 hour. These data correspond to the first repetition of each electrode.
  • Pt SPEs Platinum screen-printed electrodes from two different sources were modified following the same procedure as that used for Ag and Au SPE pre-treatment. After the modification in 3.3 ⁇ 10 "2 M DBSA/ 0.1 M KC1 solution, amperometric measurements were performed and the cathodic currents obtained for 5- 10 "3 M H 2 0 2 are shown in Table 4. Pt is the best known catalyst for H 2 0 2 reduction, and this fact is supported by the cathodic current data obtained even with unmodified Pt SPEs.
  • Pt SPEs were modified with DBSA/KCI in order to analyze if the catalytic effect observed with the silver-based electrodes was also exhibited by this material.
  • the electrodes were modified following the same procedure previously reported for Ag and Au SPEs and were subsequently submerged in 1 M H 2 0 2 (2 ml) for 1 h.
  • the results for the unmodified and modified substrates are presented in Fig. 34.
  • Pt SPEs modified with the DBSA/KCI solution showed at least ten-fold greater rates of catalytic decomposition when they were exposed to 1 M H 2 0 2 than those registered for the unmodified substrates.
  • Figure 34 Change in mass per unit area versus time for (a) unmodified and (b) modified Pt SPEs upon repeated exposure to 1 M H 2 0 2 solution for 1 hour. The number of exposures is indicated besides each graph. Comparing the data obtained for different metal SPEs, both Ag and Pt showed an enhancement in the H 2 0 2 decomposition process after being treated with DBSA/KCl solutions. The losses of mass per area after 1 hour reaction were approx. 8 and 10 times higher than those obtained by the same unmodified Ag and Pt electrodes, respectively. However, Au SPEs did not show any catalytic effect after the above mentioned modification, presenting the same behaviour before and after dipping into DBSA/KCl solution.
  • a catalyst is a substance that participates in a chemical reaction by increasing the rate of reaction.
  • rate constants of the decomposition reactions were calculated.
  • the reaction rate is defined as the change in the advancement of the reaction with time, i.e. the change in the number of moles of a given species (reactant or product) with time:
  • R is the intensive reaction rate, v, is related to the stoichiometric coefficient of species i and [i] is the molarity of species i.
  • the rate of a reaction will generally depend on the temperature, pressure and concentrations of species involved in the reaction as well as the phase or phases in which the reaction occurs.
  • the constants a and ⁇ are the reaction orders with respect to A and B, respectively, and k is referred to as the rate constant for the reaction.
  • the rate constant is independent of concentration, but dependent on pressure and temperature. 34
  • the rate constant of H 2 0 2 decomposition was determined by measuring the mass of 0 2 liberated as a function of time at room temperature. The mass data were rearranged to be expressed as the H 2 0 2 concentration remaining in the vial versus time to study the kinetics of the reaction. All obtained results revealed that such decomposition followed pseudo first-order kinetics:
  • Figure 35 Plot -In [H 2 O 2 ]/[H O 2 ] 0 versus time for (a) unmodified and (b) modified Ag SPEs after exposure to 1 M H 2 0 2 solution for 1 hour.
  • the data appears to be linear during the first ten minutes, after which it deviates from linearity. This observation could be as a result of the formation of a diffusion layer at the surface, which could become a limitation of the reaction, or as a result of bulk substrate limitation (mass transport).
  • mass transport mass transport
  • the decrease in the catalytic responses observed for the modified electrodes as a function of the repeat could be explained as a loss of the surface modification or surface inactivation with the time, even though, H 2 0 2 decomposition was up to four times faster than the same process carried out on the unmodified electrodes. More remarkable is the catalytic effect observed on the sterling 92.5 % Ag electrodes after the modification.
  • the unmodified electrode showed an increase of decomposition rate as the substrate was re-measured. Thus, this electrode exhibited an initial rate constant of 0.68 s " ' for the first measurement and 2.94 and 5.66 s " ' for the subsequent exposures to 1 M H 2 0 2 .
  • the apparent rate constants must be normalized to the surface area of the catalysts.
  • Falcon and Carbon io showed that the heterogeneous reaction for peroxide decomposition is a slow process and the transport to the surface would therefore be diffusion controlled.
  • Unmodified metallic 99.9 % Ag showed the highest heterogeneous rate constant for H2O2 5 decomposition and it did not change after its treatment with DBSA/KCl. Unlike metallic 99.9 % Ag and SPE, sterling 92.5 % Ag electrodes improved their H 2 0 2 decomposition responses up to 10 times after exposure to the surfactant-based solution. Similar enhancement was presented by Pt SPEs after being modified with DBSA/KCl. The responses obtained with unmodified gold- based electrodes were close to those shown by the other metallic electrodes under study, except0 for metallic 99.9 % Ag. However, no improvements were observed after the surfactant-based modification, as is shown by no change in the heterogeneous rate constants before and after the modification. The overall data from the unmodified electrodes were of the same order of magnitude than those previously reported. 37 5 SUMMARY
  • the surfactant-based modification on several metallic-based electrodes also seemed to induce an enhancement on H2O2 decomposition.
  • the greater differences of mass due to the release of O2 during H2O2 decomposition after the electrodes were treated with DBSA/KCl showed the surfactant-based solution produced an improvement in the catalytic process.
  • Such an enhancement was proven by the increase of the heterogeneous rate constants obtained after the kinetics study carried out on both unmodified and modified electrodes. It showed that DBSA/KCl modification on the metallic surfaces provided an easier mechanism for H 2 0 2 decomposition.
  • Au and Pt substrates did not show as high a catalytic effect on both electrochemical H 2 0 2 reduction and H2O2 decomposition after DBSA/KCl modification as Ag electrodes.
  • HyPer sensors modified Ag SPEs were used as described in Materials and Methods for cholesterol sensing.
  • the electrochemical cell was filled with 1 ml enzyme buffer solution (buffer according to Audit Diagnostics protocol where cholesterol oxidase concentration was increased 200-fold).
  • cholesterol solution lyophilized sample
  • Cholesterol concentrations used were 4.8 mM, 3.2 mM and 0.86 mM. (Note: these cholesterol concentrations were achieved by adding different volumes of lyophilised cholesterol solution, as opposed to prior preparation of a dilution series of varying concentrations.
  • Figure 37 Calibration curve of cathodic current increase against cholesterol concentration.
  • Working Electrode HyPer.
  • Buffer solution Made up according to Audit Diagnostics protocol with 200-fold increase in cholesterol oxidase concentration.
  • Inset Signal based on increase in amperometric current from trough to peak.
  • Figure 38 Effect of addition of various solutions to BSA to monitor the current drop.
  • Working Electrode HyPer. A current decrease is observed when cholesterol was added to the system. Negligible, if any oxidation current was observed for buffer or H 2 0 additions to the system.
  • the calibration experiments were repeated using cholesterol standard solutions in the range of 0.2 to 3.2 mM by preparing a dilution series of a 3.2 mM cholesterol stock solution. For each of experiment, 500 of the appropriate cholesterol solution was added to 1000 ⁇ of enzyme buffer.
  • Figure 41 Calibration graph of the time taken to reach the maximum amperometric cathodic response of HyPer electrode to additions of leophilised cholesterol over the concentration range 0.5 - 3.2 mM.
  • the effect of the enzyme buffer on the electrochemistry of H 2 0 2 at the electrode surface was determined by analysing the response of a HyPer electrode to the addition of a 10 L aliquot of 50 mM H 2 0 2 . Buffers examined were PBS (pH 7.0) and Pipes (with and without enzyme) (Fig. 42).
  • Figure 42 Response of HyPer electrode to 10 mL H 2 0 2 injections (each addition was equivalent to 0.5 mM H 2 0 2 ).
  • the buffer of PBS containing cholesterol oxidase and lipo-lipase showed much more significant response to H 2 0 2 than compared to the Pipes/MgCl 2 .
  • the Pipes/MgCI 2 buffer with and without enzyme showed comparable responses again illustrating that the presence of the enzyme was not the source of the deleterious effect on the sensor's response to H 2 0 2 .
  • BSA cholesterol oxidase enzyme/protein in the sample, e.g., by non-specific adsorption
  • BSA was used in place of cholesterol oxidase enzyme in varying concentrations and the sensor's response to H 2 0 2 investigated. Modification of the electrode with a Nafion membrane was also investigated to see would this have any beneficial effect. The amperograms are shown below in Fig. 43. The performance of the electrode was monitored by adding 10 ⁇ of a 50mM H 2 0 2 solution at 100 s intervals.
  • Figure 43 Amperometric responses of the Nafion-coated HyPer electrodes (A) and bare HyPer electrodes (B) to H 2 0 2 (0.5 mM) in the presence of varying amounts of BSA in bulk solution. The effect of Nafion or of increasing the amount of BSA in the solution does not affect the sensor response to hydrogen peroxide.
  • Fig. 44 shows the amperograms with H 2 0 2 additions where the effect of the presence of PBS, Pipes MgCI, MgCI 2 and Pipes was monitored.
  • the Pipes/MgCli containing buffer showed much lower initial responses to H 2 0 2 .
  • Pipes buffer or MgCl 2 alone, gave higher responses to that of the Pipes/MgCl 2 buffer indicating that they appear to have a combined effect.
  • FIG 44 Response of HyPer electrode to H 2 0 2 injections (each addition was equivalent to 0.5 mM H 2 0 2 ) in the presence of PBS (0.13813 g/10 ml), PIPES/MgCI 2 (as per AD Buffer), MgC (0.005 g/10 ml) and PIPES (0.13813 g/10 ml). Investigation of the addition of membrane layers
  • Nafion and Chitosan were both examined as membrane layers to reduce the impact of buffer effects.
  • Nafion solution (3 ⁇ , 1 % w/v) was drop-coated onto the HyPer electrode and allowed to dry. The performance of the electrode was monitored by adding 10 ⁇ of a 50mM H 2 0 2 solution (overall concentration: 0.5 mM) to a BSA protein buffer solution at 100 s intervals (BSA was employed as a non-specific control protein).
  • BSA BSA protein buffer solution at 100 s intervals
  • Chitosan-coated HyPer electrodes were prepared similarly (according to Section 2.1 ) and similar responses were observed. Neither chitosan nor Nafion were shown to have any negative effects on the electrode, and so were proved as potentially a valid approach for protecting the electrode surface.
  • Figure 45 Amperometric responses to hydrogen peroxide (0.5 mM additions) of the Nafion- coated HyPer electrode in AD PIPES buffer. It can be observed that there is no significant effect on the initial response of the electrode to hydrogen peroxide when a Nafion membrane is employed.
  • the Nafion-coated electrode showed increased amperometric responses to H 2 0 2 as compared to an equivalent HyPer electrode (Fig. 45).
  • Chitosan a linear polysaccharide, which is present in the shells of shellfish, was also looked at as a potential membrane modification for the electrode. With injections of 10 50 mM H 2 0 2 . the response was monitored (Fig. 46).
  • a HyPer electrode it was again evident that the Pipes/MgC electrolyte buffer was the cause of the reduction of the electrochemical response, as indicated by the purple line in Fig. 46.
  • the red line shows a HyPer electrode in PBS buffer, which has an initial response of more than double that of the electrode in Pipes.
  • HyPer electrode coated with 3 ⁇ 1 % solution of chitosan was also examined for its response to H 2 0 2 .
  • This electrode was placed in the Pipes/MgCl 2 solution and the initial response for the first injection of H 2 0 2 was greater than that of the bare HyPer electrode in PBS. It was evident that the chitosan barrier has a beneficial effect on the detection of H 2 0 2 using the HyPer electrode in Pipes/MgCl 2 .
  • Figure 46 Amperograms showing electrode responses to injections of H2O2 in buffers PBS and Pipes. It can be seen that the HyPer electrodes response could be improved in Pipes buffer by employing the chitosan membrane.
  • the HyPer sensors were fabricated with a range of volumes of chitosan ( 1 % w/v) dropped onto the surface of the HyPer electrode and tested with cholesterol oxidase and cholesterol at 1 .6 mM.
  • a range of volumes of chitosan 1 % w/v
  • cholesterol oxidase and cholesterol 1 .6 mM.
  • Fig. 47 Further optimisation of this layer will need to be investigated to further reduce its thickness and impact on diffusion, while also maintaining an effective barrier to the buffer constituents.
  • Fig. 48 shows the amperometric response of the electrodes to cholesterol (500 mL of Cholesterol - overall concentration 1.6 mM) in the presence of AD buffer containing cholesterol oxidase comparing the HyPer electrode with a 1 % chitosan-coated HyPer electrode. This does appear to show that the slope of the response for the chitosan-modified electrode is greater than the unmodified electrode (response time of 750 s as compared to 1480 s).
  • Red line is the amperometric response from a chitosan-coated HyPer electrode.
  • Black line is the amperometric response from a bare HyPer electrode.
  • Response time from the chitosan- HyPer is 700 s, while the response time of the bare HyPer electrode was approx. 1500s.
  • Table 2 Pipes/MgCl 2 Cholesterol oxidase buffer for cholesterol response with various electrode barriers.
  • Red line is the amperometric response from a HyPer electrode.
  • Purple line is the amperometric response from a chitosan-coated HyPer electrode.
  • Blue line is the amperometric response from a cellulose acetate— coated HyPer electrode Response time from the chitosan-modified electrode is 700 s, while the response time of the bare electrode was approx. 1 500s.
  • HyPer technology developed to date This research was commenced on the basis that there was preliminary data showing that the HyPer electrode responded to enzymatically generated hydrogen peroxide.
  • the main issue with the system was that the response times were prohibitively long (1300 s approx.) and so was not suitable in that form for the AD assay.
  • This feasibility study involved looking into the reasons behind the long response times as well as strategies to reduce the response time of the sensors to less than 600 s. It was found that the co-factor MgCI 2 as well as the PIPES buffer appeared to have negative effects on the electrode response. The actual interaction is not fully understood, however several strategies were looked at over the study to overcome these effects.
  • Membranes were examined as potential materials that could isolate the electrode from the negative effects of the buffer solution - Nafion, chitosan and cellulose acetate were examined. Both Nafion and Chitosan were shown to be promising materials as membranes. It was shown that using chitosan, the response time was decreased approximately 2-fold, reducing the peak response time to 750 s approximately in the PIPES buffer.
  • the platform in place can detect cholesterol ( 1 .6 mM) within 700 s over the concentration range - 0 - 5 mM.
  • further work is required to generate solid analytical calibration data to show the improvements that the chitosan layer has on the system when compared to the bare electrode.
  • Dodecylbenzenesulfonic acid (DBSA-D0989) was purchased from TCI Europe.
  • Sodium and potassium chloride NaCl, KC1
  • potassium dihydrogen phosphate KH 2 P0 4
  • alpha-D-(+)- glucose cellulose acetate (CA)
  • HMDA hexamethylenediamine
  • Type II- S glucose oxidase
  • Type II- S from Aspergillus niger, 20% protein
  • Di-sodium hydrogen phosphate Na 2 HP0 4
  • 30% (v/v) hydrogen peroxide solution was purchased from Merck.
  • Glutaraldehyde (GA) was purchased from Fluka Chemika.
  • Acetic acid glacial was purchased from Fisher Scientific.
  • Silver conductive ink (Electrodag® PF-410) was purchased from Henkel (Scheemda, The Netherlands).
  • Poly(ethylene) terephthalate substrates were Melinex® (pre-shrunk) films obtained from HiFi Industrial Film Ltd. (Dublin, Ireland). All the solutions were prepared using 18 ⁇ Milli-Q water.
  • phosphate buffered saline solution PBS
  • the buffer solution is 0.1 M phosphate, 0.137 M NaCl and 0.0027 M C1. This was prepared by mixing solution 1 (0.1 M Na 2 HP0 4 , 0.137 M NaCl and 0.0027 M C1) and solution 2 (0.1 M H 2 P0 4 , 0.137 M NaCl and 0.0027 M KC1) to a pH of 6.8.
  • Hydrogen peroxide was prepared to a concentration of 1 M by diluting 0.5 ml to 5 ml with deionised water. 10 ⁇ was added to 1 ml of the appropriate enzyme/buffer or buffer solution to give an overall concentration of 1 mM in order to examine the electrode response. A 0.2 M glucose solution was prepared and aliquots of 50 ⁇ were added to the bulk to obtain a final concentration of 1 mM, unless otherwise stated.
  • % (w/v) cellulose acetate solution was prepared by dissolving I mg into 5 ml of glacial acetic acid.
  • glucose oxidase solution 25 mg/ml glucose oxidase solution was prepared by dissolving 50 mg into 0.4 ml PBS pH 5.
  • Silver screen-printed electrodes were dipped into a freshly-prepared solution of DBSA/KC1 for 3 hours. The electrodes were then rinsed with water to remove the excess of modification solution on it and measured directly.
  • Silver electrodes were prepared by a screen printing method using the DEK 248 printing system.
  • the electrodes were conditioned by placing them in a solution of 0.1 M KC1 and 0.033 M DBSA for 3 hours followed by a washing step with deionised water before using them as sensors. These electrodes were used as hydrogen peroxide sensors.
  • Glucose biosensors were prepared by immersing the DBSA/KC1 modified electrodes in a cellulose acetate solution (20% cellulose acetate in glacial acetic acid) for 3 s to create a thin and uniform layer of the polymer on the electrode. After the immersion, the electrode was placed for ten minutes in cold deionised water to accelerate the polymer solidification phase.
  • Activation of the cellulose acetate layer was carried out by immersing the electrode into a 5% HMDA aqueous solution for 20 min. After washing in deionised water, the electrode was immersed for 20 min in 2.5% GA aqueous solution. After further washing with deionised water, the electrode was kept overnight at 4 °C in a 25 mg/ml of GOx in PBS pH 5 for enzyme immobilization.
  • the whole process of covalent binding of glucose oxidase to the cellulose acetate layer is shown below: 38
  • DBSA/ Cl modified Ag SPEs were used as H 2 0 2 sensors, those devices were used as a platform in the manufacture of a glucose biosensor.
  • Ag DBSA/KCl electrodes were checked as glucose sensors when the glucose oxidase enzyme was in solution.
  • the modified electrode was first used to detect H 2 0 2 by amperometric i-t curve at -O. l V (vs Ag/AgCl) in order to assess if it worked properly (Fig. 5 l a).
  • the electrode was placed in a cell containing 1 mg/ml GOx in PBS pH 6.8 with compressed air passing through the solution and an amperometric i-t curve was performed at -0.1 V (vs Ag/AgCl).
  • the glucose oxidase enzyme was then deposited onto the surface after the activation of the cellulose acetate film by hexamethyldiamine and glutaraldehyde.
  • the electrode was rinsed, its catalytic activity towards glucose was assessed by amperometry at -0.1 V (vs Ag/AgCl).
  • the electrode was placed in a batch cell containing 10 ml PBS pH6.8 and aliquots of 0.2 M glucose solution were added so glucose concentration ranged from 1 to 5 itiM.
  • the response of the glucose sensor is shown in Fig. 53.
  • a calibration curve with the catalytic response of the sensor versus glucose concentration was built and the limit of detection and sensitivity of the device were calculated.
  • the sensitivity was found to be 0.77 pAcm "2 mM " ' .
  • the LOD was calculated from the regression line obtained from the plot of the cathodic currents versus glucose concentration. These data are presented in the inset of Fig. 53. Although the lowest concentration measured was 1 T 0 "3 M, the lowest theoretical LOD of the sensor devices was found to be 3.6- 10 " 4 M. The range of LOD and sensitivity calculated for glucose sensor and found in the literature is very wide and depending on the modification and enzyme immobilization method.
  • This section describes the use of the hydrogen peroxide catalyst material in a direct hydrogen peroxide fuel cell for stable and efficient production of electric current by the electro-catalytic reduction of the hydrogen peroxide, via a cathode comprising of the hydrogen peroxide catalyst, coupled with the oxidation of a hydrogen containing fuel by means of proton transfer across a proton conducting electrolyte.
  • the hydrogen peroxide fuel cell consists of a proton conducting electrolyte, a porous electrical conducting substrate that supports the hydrogen peroxide catalyst and two porous or channelled flow manifolds that allow flow of the hydrogen peroxide (H202) to the cathode and a hydrogen containing fuel to the anode (Fig. 54).
  • the proton conducting electrolyte 1 such as an ion-conducting polymer comprising of a perfluorinated polymer containing sulfonic or carboxylic ionic functional groups of suitable type, durability and thickness as to allow efficient operation of the hydrogen peroxide fuel cell.
  • the cathode porous electrical conducting substrate or Gas Diffusion Layer (GDL) contains a hydrophobic agent 2.
  • the silver-based ink composite consisting of metallic component, binder components and solvent components is deposited by any suitable painting, deposition, coating process, on at least one side of the substrate. The application of the inks should not block the porous electrical conducting substrate.
  • the porous electrical conducting substrate is then modified with the catalytic modification layer, again using any suitable means such as dip- coating or printing methods.
  • the porous electrical conducting substrate, with the hydrogen peroxide catalyst layer is allowed to dry.
  • the anode porous electrical conducting substrate contains a hydrophobic agent 3.
  • a suitable anode electro-catalyst, for a hydrogen containing fuel, is deposited by any suitable painting, deposition, coating process, on at least one side of the substrate. The application of the anode electro-catalyst should not block the porous electrical conducting substrate.
  • the porous or channelled flow manifold or flow plate 4, 5, that allows flow of the hydrogen peroxide to the cathode and a hydrogen containing fuel to the anode, has at least one input orifice and at least one output orifice.
  • the design of the flow manifold is such that it allows effective flow of the fluids to the electro-catalysts of the anode or cathode, supports the cell and enables a conductive path to the external circuit.
  • the anode and cathode porous electrical conducting substrate or Gas Diffusion Layer (GDL) 2, 3 can be incorporated onto the flow manifold if the flow manifold is constructed of suitable metallic or conductive porous matrix. This would negate the use of a porous electrical conducting substrate or GDL and the anode and cathode catalysts would then be coated directly to the conductive porous matrix of the anode and cathode flow manifolds 4, 5.
  • the cathode porous electrical conducting substrate or GDL with the hydrogen peroxide catalyst 1 is laid onto the proton conducting electrolyte 2, so that the catalyst-coated side faces the proton conducting electrolyte.
  • the cathode porous or channelled flow manifold 4 is placed onto the cathode porous electrical conducting substrate and secured into position.
  • the same procedure is completed for the anode side of the hydrogen peroxide fuel cell.
  • the anode porous electrical conducting substrate or GDL with a suitable anode electro-catalyst, for a hydrogen containing fuel 3, is laid onto the other side of the proton conducting electrolyte 2, so that the coated anode electro-catalyst side faces the proton conducting electrolyte.
  • the anode porous or channelled flow manifold is placed 5 onto the anode porous electrical conducting substrate and secured into position.
  • the ideal cell voltage (thermodynamic reversible cell potential) of the hydrogen fuel cell would be 1 .23 volts, at 25°C and one atmosphere [43].
  • the formation of water from hydrogen and oxygen gases is an exothermic reaction, which has an enthalpy of -286 kilojoules of energy per mole of water formed.
  • the free energy available to perform work decreases as a function of temperature. At 25°C and one atmosphere the free energy available to perform work is about -237 kilojoules per mole. This energy is observed as electricity and heat.
  • the fuel cell's performance is summarised with the graph of its current and voltage characteristics, polarisation curve (I-V curve) shown in Fig. 56.
  • overvoltage refers to the difference between the ideal cell voltage and the operating cell voltage. This, in due course, represents the losses in the cell.
  • Activation losses are a result of the energy required to initiate the reaction and losses due to the slowness of the electrochemical reactions taking place on the surface of the electrodes. This is a result of the catalyst. The better the catalyst the less activation energy required. Platinum forms an excellent catalyst however there is much research underway for cheaper materials.
  • a limiting factor to power density available from a fuel cell is the speed at which the reactions can take place.
  • the cathode reaction (e.g., the reduction of oxygen or hydrogen peroxide) is about 100 times slower than that of the reaction at the anode, thus it is the cathode reaction that limits power density.
  • the losses depend on factors such as electrode material properties, ion intersection and electrolyte characteristics. These losses can be reduced by increasing the operating temperatures however increasing the temperature may cause other losses. Electrochemical reaction kinetics cause the voltage to decrease and these losses can mainly be seen in an I-V curve as the cell begins to produce current, as seen on the left-hand side of 2.
  • the Tafel equation is used to model the losses, it is
  • Ohmic losses are a result of the combined electrical resistances of the various fuel cell components that produce heat. These include the electrode material resistance, the electrolyte membrane resistance and the various interconnection resistances. The major proportion of the losses within a fuel cell occur as more current is drawn from the cell, as seen in Fig. 56 (middle 67
  • V (vs Ag/AgCl) providing a sensitivity of 0.56 ⁇ "1 and a linear range from 0.01 to 6.4 mM.
  • 41 A higher sensitivity (30.64 uAcm ⁇ 2 mM ⁇ ') and LOD (2.5 ⁇ 10 "6 M) was obtained with an amperometric glucose biosensor based on layer-by-layer (LbL) electrostatic adsorption of GOx and dendrimer-encapsulated Pt nanoparticles (Pt-DENs) on multiwalled carbon nanotubes (CNTs). Glucose was detected by amperometric measurements at 0 V (vs Ag/AgCl) in PBS pH 6.8.
  • the device from the present work Ag DBSA/ Cl CA GOx, has shown to be feasible as a glucose sensor, although further studies should be performed to improve its sensitivity and LOD.
  • a high through-put fabrication approach of printing could be adopted to fabricate the cathodic electrodes, aligning it with potential in the printed electronics area.
  • the invention provides:
  • Tlohmic i Eqn. (4) where, i is the current density given in Acm "2 and R is the specific resistance given by:
  • Mass transport (concentration) losses result from the reduction of the concentration of hydrogen and oxygen gases at the electrodes. For example, following the reaction new gases must be made immediately available at the catalyst sites. With the build up of water at the cathode, particularly at high currents, catalyst sites can become clogged, restricting oxygen access. This is responsible for the sharp decline in the potential at high current densities and can be seen at the tail end of the I-V curve in Fig. 57.
  • the loss can be modelled by the following equation [44].:
  • Tlconc m exp(ni) Eqn. (7) where m is 3e “5 V and n is 8e “3 cm W [46].
  • V E rev - Aln(i/b) - iR - m exp(ni) Eqn. (9)
  • E rev is the reversible OCV given by ⁇ 1.23 V ⁇ A is the slope of the Tafel line ⁇ 0.03V ⁇ R is the area-specific resistance ⁇ 2.45 e-4 ⁇ m ⁇ 3e-5V ⁇ and n ⁇ 8e-3 cm 2 mA * ' ⁇ are the constants in the mass-transfer overvoltage. These values are taken from Ballard Inc. [46].
  • Fig. 58 The results presented in Fig. 58 are a batch of tests on 1 new hydrogen GDL run after each other in one day.
  • the fuel cell showed open circuit potentials in excess of 1 V, which shows excellent performance in this preliminary fuel cell application example.
  • Many improved configurations could be envisaged such as further optimisations of the design and fabrication of the cathode material, use of alternative fuel sources, alternative membrane types, the potential for use and application in biofuel cell systems and microbial fuel cell systems, use of the hydrogen peroxide catalyst as a support for standard platinum cathodes employing the oxygen reduction reaction.
  • Figs 58, 59 and 60 show microscopic analysis of the gas diffusion layer material before and after modification with the hydrogen peroxide catalyst showing deposition of high contrast ratio materials, most likely silver, onto the gas diffusion layer.

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Abstract

Le catalyseur ci-décrit comprend une électrode d'argent, ladite électrode ayant été modifiée avec une solution de tensioactif-sel. L'électrode peut comprendre des colloïdes d'argent, des particules d'argent, ou des nanoparticules d'argent, notamment, une électrode à base de pâte d'argent. L'électrode peut être une électrode formée par sérigraphie. Le catalyseur peut être un catalyseur de peroxyde d'hydrogène non enzymatique. Il peut être utilisé dans un système de capteur tel qu'un capteur de glucose ou de cholestérol.
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CN114807984A (zh) * 2022-04-26 2022-07-29 大连理工大学 一种模拟自然光下以光电极利用溴化钠生产水溶性环氧化物的方法
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