GB2485028A - Zinc-Oxygen cell and its application - Google Patents

Abstract

The subject matter of the invention is a zinc-oxygen cell, comprising a zinc anode, an electrolyte and an enzyme-modified biocathode, characterised in that the said biocathode is a ceramic carbon electrode, CCE, modified with laccase and a redox probe. The invention comprises also the application of such cell as an implantable power source.

Description

Zinc-oxygen cell and its application.

The subject matter of the invention is a zinc-oxygen cell, comprising a zinc anode, an electrolyte and an enzyme-modified biocathode, as well as the application of said cell as an implantable power source.

In recent years it has been proposed to construct an implantable biofuel cell, where the fuel and the oxidising agent are glucose and dioxygen dissolved in body fluids (Heller 2004, Phys. Chem. Chem. Phys. 6: 209-216). Making use of the difference between the oxidation potential of glucose and the reduction potential of dioxygen such a device produces electric voltage between two electrodes.

Miniaturisation of such a cell requires that it is caseless and has no membrane separating the near-electrode areas. To meet the requirement the catalysts of electrode reactions must be permanently immobilised at the surface of the electrodes (anode and cathode). Proposed catalysts are enzymes allowing to achieve a significant difference between the oxidation potential of glucose and the reduction potential of dioxygen that cannot be achieved with inorganic catalysts, and mediators acting as an electron relay between the enzyme and the electrode.

The models of the glucose-oxygen biofuel cells constructed so far allowed for powering with power density of the order of miliWatt per square centimeter (mW cm2) at operating voltage and current density for the maximum power being equal 0.3 V and 5 mA cm2, respectively (Sakai 2009, Energy & Environmental Science 2: 133-138). Because of the instability of enzymes the values of these parameters rapidly decrease and the enzymes' operating time is several days. Therefore it has been proposed to replace the bioanode with a zinc electrode. In this cell zinc is used as a fuel. The operating time of the electrode is, however, significantly longer than the operating time of enzyme-modified electrodes constructed so far. Moreover, because of a low standard oxidation potential of zinc, the voltage of this cell exceeds 1 V, which is particularly important because of difficulties with serial connection of cells in vivo.

The problem of penetration of Zn2 ions that are formed as a result of cell operation has been eliminated. In physiological solutions they react with P043 ions forming a durable insoluble coating Zn3(PO4)24H2O on the surface of the anode (Shin 2005, J. Am. Chem. Soc. 127: 14590-14591). The coating protects also the zinc electrode against oxidation by oxygen dissolved in a body fluid.

The cells consisting of a zinc anode and an enzyme-modified oxygen biocathode as presented to date display a moderate power density below 18 1iW cm2 (Szot 2009, Electrochim. Acta 54: 4620-4625). Unknown so far are also any studies on cells composed of a redox probe and laccase-modified ceramic carbon electrode and a zinc anode.

According to the invention, the zinc-oxygen cell, comprising a zinc anode, an electrolyte and an enzyme-modified biocathode is characterised in that the said biocathode is a ceramic carbon electrode, CCE, modified with laccase and a redox probe.

The redox probe can be any electroactive substance with a redox potential lower than the redox potential of the laccase prosthetic group, displaying reversible electroreduction reaction and having a reduced form capable of reducing the prosthetic group of the enzyme.

In a preferable embodiment, the said redox probe is a substance selected form the group comprising ammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate), ABTS, syringaldazine and mixtures thereof.

Preferably, the said electrolyte comprises phosphate buffer. In this case, its concentration is preferably from 0.02 M to 1 M, more preferably about 0.1 M. In a preferred embodiment, the said phosphate buffer has pH from 4 to 8, more preferably about 7.4.

In another preferable embodiment, the said electrolyte comprises additionally NaCI solution. In this case, concentration of the NaCI solution is preferably from 0.01 M do 1 M, more preferably about 0.15 M. Preferably, the cell according to the invention is capable of generating power with power density greater than 130 kiW cm2 at a voltage higher than 1.27 V. Preferably, it is characterised by an open circuit voltage higher than 1.2 V. The invention comprises also application of such a cell as an implantable power source.

Unexpectedly, the cell according to the invention displays a more favourable power density as compared with the existing technology of cells consisting of a zinc anode and an enzyme-modified oxygen cathode due to use of mediator-modified oxygen cathodes.

The cell according to the invention can be applied as an implantable power source.

Electric power generated by this cell is sufficient to power implantable devices such as microvalves, drug dispensers, temperature sensors and other low power electronic devices.

The invention is now described in preferable embodiments, with reference to the accompanying figures where: Fig. 1 shows a scheme of a zinc-oxygen cell according to the invention (PBS -phosphate buffer); Fig. 2 shows (A) current-voltage characteristics of a cell consisting of a zinc anode coated with a thin Nafion film and CCE (*) or ABTS-CCE/Lc (.) immersed in an oxygen-saturated 0.1 M phosphate buffer pH 5. (B) Current-voltage characteristics of a cell consisting of a zinc anode coated with a thin Nafion film and ABTS-CCE/Lc immersed in a 0.1 M phosphate buffer pH 5 (.), pH 7.4, or electrolyte with a blood-serum-like composition (A); Fig. 3 shows current-voltage (A) and current-power (B) characteristics of a cell consisting of a zinc anode coated with a thin Nafion film and ABTS-CCE/Lc immersed in an oxygen-saturated 0.1 M phosphate buffer pH 5 at temperatures: 24°C (.,n), 37°C (.,o) and 50°C (A,L\); Fig. 4 shows current-voltage (A) and current-power (B) characteristics of a cell consisting of a zinc anode coated with a thin Nafion film and Syr-CCE/Lc immersed in an oxygen-saturated 0.1 M phosphate buffer pH 5 (.), pH 7.4 (.) and electrolyte with a blood-serum-like composition (A), and Fig. 5 shows current-voltage (A) and current-power (B) characteristics of a cell consisting of a zinc anode coated with a thin Nafion film and Syr-CCE/Lc immersed in an electrolyte with a blood-serum-like composition at temperatures: 24°C (.), 37°C(.)and 50°C(A).

The constructed zinc-oxygen cell (Fig. 1) consists of a zinc anode and a ceramic carbon electrode that are immersed in a solution with composition similar to human physiological fluids. The anode is a zinc wire coated with a thin film of Nafion (ionic conducting sulfonate derivative of polytetrafluorethylene). The cathode is a ceramic carbon electrode modified with laccase -an enzyme catalysing dioxygen reduction and mediator -ammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) (Nogala 2006, Electrochem. Commun. 8: 1850-1854) or syringaldazine (Nogala 2007, J. Electroanal. Chem. 608: 31-36). Both electrodes have similar surface areas: 0.02 cm2.

Reagents The cathodes were prepared using 98% methyltrimethoxysilane (MTMOS), tetramethoxysilane (TMOS) purchased at Sigma-Aldrich, 35% hydrochloric acid (HCI) purchased at Chempur, methyl alcohol (CH3OH) purchased at Chempur, graphite powder MP-300 purchased at Carbon GmbH, syringaldazine purchased at Sigma-Aldrich, ammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) purchased at Sigma-Aldrich, sodium glass capillary tubing (opening diameter 1.5 mm) and laccase obtained according to a previously published procedure (Rogalski 1999, Journal of Molecular Catalysis B-Enzymatic 6: 29-39).

The anode was prepared using a 0.25 mm diameter zinc wire purchased at GoodFellow, Nafion (5 % solution in isopropanol) purchased at Sigma-Aldrich, isopropyl alcohol (C3H7OH) purchased at Merck.

The electrolytes were prepared using sodium dihydrogen phosphate (NaH2PO42H2O) purchased at POCh, sodium hydroxide (NaOH) purchased at Chempur, sodium chloride (NaCI) purchased at Chempur, demineralised water (p> 15 MO cm) obtained from Millipore Elix purification system and oxygen (99.99999 %) -Multax.

Apparatus The current-voltage characteristics and power characteristics of the cells were determined using a galvanostat from the EcoCemie Autolab PGSTAT3O electrochemical system.

Preparation of electrodes and electrolyte Carbon ceramic electrode used as cathodes in presented examples of cells were prepared in accordance with a procedure reported earlier (Nogala 2010, Bioelectrochemistry 79: 101-107). The hydrophobic polysilicate matrix was obtained in a sol-gel process from a sol obtained by mixing 1 mL MTMOS, 1.5 mL methanol, 50 RL HCI and 12 mg ABTS, or 78 mg syringaldazine. The mixture was sonicated for 2 minutes. Next, 1.25 g graphite microparticles was added and the whole was sonicated for another minute. The suspension so obtained was placed into a 1.55 mm inner diameter glass tubing with a copper wire, at a depth 2 mm.

The electrode was allowed to dry at room temperature for 48 hours, and subsequently polished with printing paper, washed with demineralised water and dried. The next stage of modification was to coat the surface of so prepared electrodes with a thin film of hydrophilic polysilicate with immobilised laccase. For that purpose, TMOS, H20 and 0.04 M HCI were mixed in a volume ratio 18:4.5:1. The mixture was sonicated for 20 mm, diluted with water in a volume ratio 1:1, and subsequently sonicated for 3 mm, diluted with water in a volume ratio 1:100 and sonicated for another 3 mm. Finally, 145 ig laccase was added to 250 RL of sol so prepared and 10 kL was sprayed on the electrode surface. The electrode was then allowed to dry for 20 hours at room temperature. The biocathodes obtained with this method are referred to hereinafter as ABTS-CCE/Lc and Syr-CCE/Lc.

The zinc anode was prepared with a method similar to that reported earlier (Shin 2005, J. Am. Chem. Soc. 127: 14590-14591). A zinc wire (0 = 0.25 mm) was coated with a thin Nafion film by immersing in a 0.5% Nafion solution in isopropanol and drying. The surface areas of both the cathode and the anode were equal: 0.019 cm2.

The electrolytes were oxygen-saturated aqueous 0.1 M phosphate buffers prepared by pH-metric titration of NaH2PO4 solution with a concentration slightly above 0.1 M with a concentrated NaOH solution, followed by bringing the titrated solution to such volume that the concentration of phosphate ions was 0.1 M. Using similar method the electrolyte with physiological fluid-like composition was prepared, said electrolyte consisting of 0.15 M NaCI and 20 mM phosphate buffer pH 7.4.

Results of measurements To illustrate the performance of typical cells their current-voltage characteristics were recorded. For that purpose, a current preset using the galvanostat was allowed to flow through the cells. After 100 s since the preset current had been applied, the cell voltage was recorded and subsequently plotted as a function of current density. The power densities (product of voltage and current density) were also plotted as a function of current density.

Example 1. A cell with ABTS and laccase modified cathode (ABTS-CCE/Lc) To illustrate the effect of biocatalysis, Fig. 2a compares the voltage-current characteristics of the cells with unmodified CCE electrode as cathode with those with an ABTS-CCE/Lc biocathode. One can notice that the reduction of oxygen takes place also on the unmodified CCE electrode at significantly lower potential, resulting in a higher activation voltage drop for the cell with a CCE cathode than that with an ABTS-CCE/Lc biocathode. This is probably a result of a lower overpotential of bioelectroreduction of oxygen on the ABTS-CCE/Lc.

In view of potential application of such biocell as an implantable power source, its characteristics were also studied with an electrolyte with pH 7.4 and an electrolyte with blood-serum-like composition (20 mM phosphate buffer pH 7.4 and 0.15 M NaCI). Current-voltage characteristics of the cells with the electrolytes mentioned above are shown in Fig. 2B. The highest efficiency is displayed by the cell with 0.1 M phosphate buffer pH 5, lower efficiency is displayed by the cell with the same electrolyte with pH 7.4, and the lowest voltages were recorded for the cell with the electrolyte with blood-serum-like composition.

This dependence results from a strong dependence of laccase activity on pH (Zawisza 2006, J. Electroanal. Chem. 588: 244-252). At pH 5 laccase shows an activity close to the maximum one, whereas at pH 7.4 it is significantly lower. In addition, the reduction of oxygen can also take place on graphite, which is a component of the ABTS-CCE/Lc, but at significantly lower potentials, which causes a drop in voltage of such cell. Chloride anions, known as laccase inhibitor (Xu 1996, Biochemistry 35: 55-55), present in the electrolyte simulating the blood serum are responsible for additional voltage drop of the biocathode (Fig. 2B).

Because the implantable power sources are required to operate at temperature 37°C, the current-voltage characteristics of the cell were also determined at various temperatures (Fig. 3A). Increasing temperature results in a slight increase of the cell voltage, which virtually has no effect on the power density (Fig. 3B). One can notice that the current-voltage characteristics do not display a maximum in this range of current densities. This results also from the fact that at higher current densities oxygen is reduced on graphite microparticles that are a component of the ABTS-CCE/Lc. Then oxygen reduction can lead to H202 formation. To avoid oxygen reduction on graphite, the cell was operated at such currents as not to exceed current density of 110 RA cm2. The highest power density achieved from the cell was obtained with a voltage over 1.28 V and was more than 130 iW cm2.

Example 2. A cell with syringaldazine and laccase modified cathode (Syr-CCE/Lc) Current-voltage characteristics and current-power characteristics of the cell with biocathode Syr-CCE/Lc are shown in Fig. 4. According to the pH dependence of laccase, the cell voltage decreases with increasing pH. The inhibition of laccase by chloride anions results also in additional losses of the cell voltage.

Similarly as for the cell from example 1, both the voltage and the density of the generated power increase with increasing temperature (Fig. 5). In the present example the dependence is stronger, which complies with a lower activation energy of oxygen reduction on the Syr-CCE/Lc than for biocatalysis with ABTS (Nogala 2007, J. Electroanal. Chem. 608: 31-36). Unexpectedly, one can notice an increasing cell voltage with growing current density > 60 lA cm2 at temperature 37 °C. It can be related to a decreasing anode potential due to its activation.

Claims (10)

1. Patent claims 1. A zinc-oxygen cell, comprising a zinc anode, an electrolyte and an enzyme-modified biocathode, characterised in that the said biocathode is a ceramic carbon electrode, CCE, modified with laccase and a redox probe.
2. 2. A cell according to claim 1, characterised in that the said redox probe is a substance selected form the group comprising ammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate), ABTS, syringaldazine and mixtures thereof.
3. 3. A cell according to any of the foregoing claims, characterised in that the said electrolyte comprises phosphate buffer.
4. 4. A cell according to claim 3, characterised in that the said phosphate buffer's concentration is preferably from 0.02 M to 1 M, more preferably about 0.1 M.
5. 5. A cell according to claim 3 or 4, characterised in that the said phosphate buffer's pH is from 4 to 8, preferably about 7.4.
6. 6. A cell according to claim 3, 4 or 5, characterised in that the said electrolyte comprises additionally NaCI solution.
7. 7. A cell according to claim 6, characterised in that the said NaCI solution's concentration is from 0.01 M to 1 M, preferably about 0.15 M.
8. 8. A cell according to any of the foregoing claims, characterised in that it is capable of generating power with power density greater than 130 iW cm2 at a voltage higher than 1.27 V.
9. 9. A cell according to any of the foregoing claims, characterised in that it is characterised by an open circuit voltage higher than 1.2 V.
10. 10. Application of the cell according to any of the foregoing claims as an implantable power source.