CA2778688A1

CA2778688A1 - Osmium anode for direct borohydride fuel cells and batteries

Info

Publication number: CA2778688A1
Application number: CA2778688A
Authority: CA
Inventors: Elod Gyenge; Vincent Lam
Original assignee: University of British Columbia
Current assignee: University of British Columbia
Priority date: 2012-05-31
Filing date: 2012-05-31
Publication date: 2013-11-30

Abstract

A method of manufacturing an anode for a direct borohydride fuel cell or battery comprises: providing a porous and electronically conductive monolithic substrate;
roughening deposition surfaces of the substrate; applying a surfactant to the substrate;
electrodepositing osmium catalyst material onto the roughened deposition surfaces such that agglomerates of osmium (Os) nanoparticles are formed on the substrate; and removing the surfactant from the substrate. The substrate can be a fibrous graphitic substrate such as AvCarb.TM. P75 or GF-S3.

Description

Osmium Anode for Direct Borohydride Fuel Cells and Batteries Field This invention relates generally to anodes, and in particular to anodes for direct borohydride fuel cells and batteries.
Background Direct fuel cells and batteries have found emerging applications in portable electronics, where there is a growing demand for high energy density power supplies.
Organic fuels such as methanol and ethanol suffer metal catalyst deactivation via COad poisoning, and require typically high precious metal catalyst loadings. An alternative to organic fuels are inorganic alkaline solutions of hydrazine and borohydrides. These are attractive fuels due to high theoretical energy density and the lack of carbonaceous species. The absence of carbon in the fuel eliminates the catalyst poisoning by CO
formation and adsorption. As a result, the durability of the fuel cell anode can be extended.
Similar to other fuel cell systems, many direct borohydride fuel cell ("DBFC") studies employed expensive noble metal catalysts such as platinum (Pt). The price of Pt has been climbing for over a decade, and as of December 2011 was about USD $49 per gram. The reliance on fuel cell systems on Pt or Pt based catalysts is a hindrance to their commercialization.
As an alternative to Pt or Pt based catalysts, the ability of Osmium (Os) based catalyst to oxidize BH-4was demonstrated by V.W.S. Lam and E.L. Gyenge in "High-Performance Osmium Nanoparticle Electrocatalyst for Direct Borohydride PEM
Fuel Cell Anodes" J. Electrochem. Soc 155 (September 22, 2008) B1155. This article studied osmium nanoparticles that were deposited onto larger Vulcan XC 72 carbon particles using a method known as the Bonneman Organosol Method. The article found that Os nanoparticles are kinetically superior and stable catalysts for borohydride electro-oxidation compared to Pt and PtRu, and found that Os favours the direct oxidization of BH-4 by a total of seven electrons as opposed to in site hydrogen V84911CANAN_LAW\ 990785\ 1 generation. This article provides early stage experimental data that suggests that Os-based anodes as a promising alternative to Pt based anodes for direct borohydride hydride fuel cells.
Summary According to one aspect of the invention, there is provided a method of manufacturing an anode for a direct borohydride fuel cell or battery, comprising: providing a porous and electronically conductive monolithic substrate; roughening deposition surfaces of the substrate; applying a surfactant to the substrate; electrodepositing osmium catalyst material onto the roughened deposition surfaces such that agglomerates of osmium (Os) nanoparticles are formed on the substrate; and removing the surfactant from the substrate.
The surfactant can be selected from the group consisting of: non-ionic, cationic, anionic, and zwitter ionic. In particular, the surfactant can be one of Triton-X 102, Triton-X 100, and Triton-X 114.
The step of roughening the substrate can comprise electrochemically treating the substrate by potential cycling of the substrate in a concentrated basic electrolyte bath.
Alternatively, the step of roughening the substrate can comprise chemically treating the substrate in an acidic solution. The steps of applying the surfactant and electrodepositing can comprise immersing the substrate and a pair of counter electrodes in an electrodeposition media comprising an Os salt such as Ammonium Hexachloroosmate ((NH4)20sCI6) and the surfactant. The step of removing the surfactant can comprise heating and rinsing the anode in a solvent, then refluxing the anode.
According to another aspect of the invention, there is provided an anode for a direct borohydride fuel cell or battery comprising: a porous and electronically conductive monolithic substrate; and agglomerates of Os particles deposited on deposition surfaces of the substrate. The agglomerates of Os particles can be electrodeposited on V84911CA\VAN_LAW\ 990785\ 1 the deposition surfaces of the substrate. The substrate can be selected from the group consisting of graphite felt, carbon paper, carbon fiber-carbon particle composite, a reticulated carbon structure, metal mesh, metal fibers, and metal foam. In particular, the substrate can be a fibrous graphitic substrate such as AVCarbTM 75 or GF-S3. The Os agglomerates can have a characteristic diameter of between 1 and 100 nm.
The Os nanoparticles can have a characteristic diameter of between 1 and 50 nm. The substrate can have a thickness between 30 microns and 3000 microns, a porosity between 0.6 and 0.98 and a conductivity between 10 S m-1 and 105 S m-1.
Brief Description Of Drawings Figure 1 is a flow chart showing a method of manufacturing an anode for a direct borohydride fuel cell or battery according to an embodiment of the invention.
Figures 2(a) and (b) are SEM images of a first anode manufactured by the method shown in Figure 1, wherein Figure 2(a) shows a substrate composed of AVCarbTM

material and Figure 2(b) shows Os particles electrodeposited on this substrate.
Figures 3(a) and (b) are SEM images of a second anode manufactured by the method shown in Figure 1, wherein Figure 3(a) shows a substrate composed of GF-S3 material and Figure 3(b) shows Os particles electrodeposited on this substrate.
Figure 4 is a schematic perspective view of an apparatus for small scale manufacturing of the anode according to the method shown in Figure 1.
Figure 5 is a schematic illustration of an apparatus for large scale manufacturing of the anode according to the method shown in Figure 1.
Figures 6 to 15 relate to an exemplary experimental embodiment, wherein Figure 6 is graph of cathode potential profile during Os electrodeposition on AvCarbTM P75 substrate at 4 mA cm-2 and 341 K. Inset shows the cathode potential profile during the first 120 s of deposition. The bath was composed of 10 mM (NH4)20sCI6 with 12.5 vol% of Triton-X 102 OCP = open-circuit potential.
V84911CANAN_LAW\ 990785\ 1 Figure 7 are TEM images of Os deposits on (A) AVCarbTM P75 and (B) GF-S3 substrates.
Figure 8 is a XRD wide scan of Os electrodeposited onto AVCarbTM P75 (Os loading:0.3 mg cm-2) and GF-S3 (Os loading 0.2 mg cm-2).
Figure 9 is a cyclic voltammogram of (A) Os/AVCarbTM P75 in 10 mM NaBF14 -2 M
NaOH ( __ ), Os/AvCarb P75 in 2 M NaOH (- - - ), (B) Os/GF-S3 in 10 mM NaBF14 -NaOH ( ___ ), Os/ GFS3 in 2 M NaOH (- - - ).
Figure 10 shows (A) an XPS wide scan of Os/ AVCarbTM P75 and (B) an XPS narrow scan of the same sample.
Figure 11 is a comparison graph of DBFC performance with Os/GF-S3 and Os/
AVCarbTM P75 anodes.
Figure 12 is a graph of the effect of temperature on DBFC operation with Os/AvCarb P75 anode with 0.2 mg cm-2 Os loading.
Figure 13 is a graph of the effect of Os loading on the DBFC performance with Os/
AVCaIbTM P75 anode.
Figure 14 is a graph of the reproducibility of the DBFC performance with Os/AVCarbTM
P75 anode prepared by the quadruple (4X) deposition procedure.
Figure 15 is a graph of a longer-term performance evaluation of the DBFC with Os/AVCarbTM P75 anode prepared by the quadruple (4X) deposition procedure.
Detailed Description of Embodiments of the Invention According to one embodiment of the invention and referring to Figures 1 to 5, a method of manufacturing an Os catalyst anode for a direct borohydride fuel cell or battery is disclosed. Instead of using the Bonneman or polyol type methods to produce a slurry of colloidal nanoparticles in solution followed by physical adsorption on smooth surfaces or V84911CANAN_LAW\ 990785\ 1 carbon particle supports in a manner as known in the art, the method of the present embodiment employs surfactant-mediated electrodeposition to deposit agglomerates of Os nanoparticles onto deposition surfaces of a porous and electronically conductive monolithic substrate that has a relatively thick "three-dimensional" structure with a rough electrodeposition surface. The three-dimensional nature of such a structure is expected to provide benefits for the electro-oxidation of borohydride such as high electronic contact area per unit electrode volume, high residence time for electroactive species, and promotion of turbulence to increase mass transport. The rough nature of the electrodeposition surfaces is expected to provide favourable nucleation and deposition sites for the Os catalyst, and the morphology of the Os catalyst particles prepared by the surfactant-mediated electrodeposition on this rough surface is expected to favour the direct oxidization borohydride.
A suitable substrate can be: a graphite felt, a carbon paper, a carbon fiber-carbon particle composite, a reticulated glassy carbon structure, a metal mesh, metal fibers, and a metal foam. A fibrous graphitic substrate is particularly suitable wherein the deposition surfaces are fibres located throughout the substrate (external and internal);
two commercially available fibrous graphitic substrates that are suitable as use for the substrate are AVCarbTM P75 and GF-S3. A suitable thickness of the substrate is between 30 microns and 3000 microns and typically around 300 microns, a suitable porosity is between 0.6 and 0.98 and typically around 0.95, and a suitable conductivity is between 10 S m-1 and 105 S m-1.
A process of manufacturing an Os catalyst anode using a fibrous graphitic substrate will now be described in detail, with reference to the method steps shown in Figure 1. For preparing the anode by Os electrodeposition, the substrate is first immersed in a pre-treatment bath (step 10) where surfaces of the fibres constituting the substrate ("deposition surfaces") will be electrochemically roughened in order to produce favourable sites for Os electrodeposition. A suitable electrochemical pre-treatment oxidative bath is a concentrated basic electrolyte bath, such as 2M NaOH.
Alternatively, the substrate can be chemically pre-treated in a chemical pre-treatment bath such as HNO3 or H202. For metallic and carbon based substrates, the pre-V84911CANAN_LAW\ 990785\ 1 treatment can consist of soaking the substrate in an acid bath and/or using a "strike" in which the metal substrate is soaked in an acid bath and metal salt bath. For a strike, a pulsing DC voltage is applied to roughen the substrate surface and deposit metal precursors on the metal surface. For example, for a nickel strike, the nickel substrate is subjected to an oxidizing potential for a period of time, then a reducing potential for a period of time while immersed in a nickel salt ¨ HCI bath. This process can be repeated more than once.
Once immersed in the electrochemical pre-treatment bath, together with a counter electrode, the substrate is subjected to potential cycling to roughen the deposition surfaces, until the surface morphology of graphite fibres in the substrate are changed (step 12). This requires a number of potential cycles typically between 10 and 500, in a potential range selected by those skilled in the art depending on the type of substrate used (e.g. graphitic carbon or metallic) For graphitic carbon, a typical potential range is between 0 and 2 V vs. SHE. This roughening process is expected to create carbon functional groups, which may result in favourable deposition sites on the fibres that facilitate the nucleation of Os ions on the substrate. The roughened deposition surfaces may also provide better mechanical adhesion, create greater surface area to allow for increased deposition sites, and for certain types of substrates, may decrease the hydrophobicity of the substrate and allow increased wetting of the substrate.
After the potential cycling has been completed, the substrate is rinsed in deionized water and dried (step 14). The substrate is then immersed in an electrodeposition media bath, which is a solution containing a specified Os salt and a specified surfactant for mediating the electrodeposition process, i.e. controlling the deposition morphology and structure. A suitable Os salt is Ammonium Hexachloroosmate ((NH4)20sC16).
The surfactant can be non-ionic, cationic, anionic, or zwitter ionic and in particular can any one of Triton-X 102, Triton-X 100, and Triton-X 114 by Sigma-AldrichTM.
Electrodeposition is then carried out at a specified elevated temperature, for a specified deposition time, and at a specified current density (step 16) until Os particles are deposited onto the deposition substrate thereby forming an anode. At the end of the Os V8491 ICANAN_LAW\ 990785\ 1 deposition step, the substrate is removed and subjected to a post treatment step (step 18) to remove surfactant from the electrode surface. The post treatment step comprises heating and rinsing the electrode immersed in a solvent such as acetone, followed by refluxing the electrode in the same solvent or another solvent or mixture of solvents.
Removal of surfactant can also be accomplished by using commercial cleaners and/or a heat treatment process. In this embodiment, the anode is removed and soaked in acetone. After soaking, the substrate is then refluxed until the surfactant is removed from the deposition surface. The solvent reflux cleaning can be carried out at a temperature close to the solvent boiling point. Then, the substrate is air dried to complete the process.
=
The specified deposition time is selected depending on the desired morphology of the deposited Os. A suitable deposition time is between 30 seconds and 30 minutes.
The selection is also limited by type and concentration of Os salt in the electrodeposition bath. The Os salt should be selected such that to possess adequate solubility in the aqueous-surfactant electrodeposition bath. A goal during the electrodeposition process is to minimize the parallel, parasitic, hydrogen evolution reaction in order to increase the current efficiency of the electrodeposition and to control the morphology of the osmium deposit. The surfactant present in the electrodeposition bath also plays a role in lowering the hydrogen evolution rate during electrodeposition in addition to controlling the osmium deposit morphology. Further, a high concentration of Os salt, near to the solubility limit will also decrease the applied potential required to deposit the Os at a constant current density and minimize the effect of hydrogen evolution which affects the current efficiency of the system.
The specified electrodeposition temperature is selected to satisfy the conditions required for the surfactant to form micelles which act as a template to control the Os deposit morphology and size. The temperature will also affect the rate in which Os deposits are formed on the deposition substrate where higher temperatures will increase the Os deposition rate. The temperature of the electrodeposition media bath is chosen to be below the boiling point of the aqueous bath, and to ensure that the surfactant would not reach its "cloud point", where unwanted phase separation would V84911CA\VAN_LAW\ 990785\ 1 occur. For kinetic reasons, the bath temperature should be kept high enough to facilitate an efficient deposition process, but below the boiling point of the bath or below the cloud point of the surfactant, whichever is smaller. For morphology reasons the temperature should be maintained within a temperature range in which the surfactant forms the micelle structure. The specified temperature range will vary depending on what type of surfactant is used and the concentration used; a suitable electrodeposition temperature for a Triton-X surfactant for example, can be determined by referring to the phase diagram of this surfactant and selecting the temperature where this surfactant is in a phase which forms micelles.
The specified concentration of Os is selected depending on the desired loading of Os deposits in the substrate as noted above, and the specified concentration of surfactant is selected dependent on the desired phase which forms the desired template. A

suitable concentration of surfactant is dependent on the type of surfactant and the desired surfactant phase but for Triton-X type of non-ionic surfactants typically it can be between 1 wt% and 50 wt%.
The specified deposition current density is selected based on a desired rate at which the Os is deposited and also the morphology and size of the Os deposits.
Higher deposition currents will also result in more negative cathode potentials, which can affect the current efficiency due to hydrogen evolution. A suitable current density is between 0.4 mA cm-2 and 40 mA cm-2.
Optionally, the deposition process (step 16) can be repeated multiple times on the substrate, in which case the substrate is washed in acetone or other suitable material between depositions and is immersed in a freshly prepared electrodeposition bath for each deposition step. Multiple depositions can be advantageous to increase the Os loading on the substrate with high current efficiency, as opposed to running a single step deposition for a longer time.
As an alternative to surfactant mediation, other approaches to controlling the morphology of Os deposits can be used, such as current or potential pulsing V84911CA\VAN_LAW\ 990785\ 1 techniques. These techniques can be deployed with our without surfactant present in the electrodeposition bath.
Referring now to Figures 2, 3 and 16, an Os electrodeposited anode 20 produced by the above method has a unique microstructure and is suitable for use in a direct borohydride fuel cell or battery. Figures 2(a) and 3(a) are SEMs showing two different types of fibrous graphitic substrates 22, namely AVCarbTM P75 in Figure 2(a) and GF-S3 in Figure 3(a). In both cases, agglomerates of Os particles 24 are formed on the deposition surface of the substrates 22, as shown in Figures 2(b) and 3(b) respectively.
The Os deposits 24 are fairly homogeneously distributed through the thickness of both substrates 22. The agglomerates 24 have a characteristic diameter of approximately 50 nm, and in turn comprise Os catalyst nanoparticles having a characteristic diameter of approximately 5 nm. However, the agglomerates can vary in characteristic diameter between 1 and 100 nm , and the Os catalyst nanoparticles can have a characteristic diameter of between 1 and 50 nm. It is theorized that these relatively large agglomerates 24 further favour the direct oxidation of BH-4which is desirable, over the heterogeneous decomposition to produce H2 which is undesirable.
A notable structural feature found in the substrate 22 shown in Figure 2(a) is the presence of interstitial carbon deposits in the form of rough plates amongst the fibres.
These interstitial carbon plates appear to be particularly favourable sites for the electrodeposition of Os particles to form agglomerates. In contrast, dense and fairly uniform deposition of agglomerates occurred on the substrate shown in Figure 3(a).
Referring now to Figure 4, an electrodeposition apparatus 30 is provided for manufacturing the Os anode according to the above disclosed method, at a relatively small scale. The apparatus 30 comprises a container 32 for containing the electrodeposition media bath. The apparatus also comprises a pair of perforated counter electrodes 34 sandwiching a chemical resistant cartridge 36; these electrodes are made from conductive plate material such as titanium, platinized titanium or graphite. The cartridge 36 contains a template 38 for holding the deposition substrate 22 in place within the cartridge 36. The cartridge 36 has an opposed openings aligned V8491ICANAN_LAW\ 990785\ 1 with the major surfaces of the deposition substrate 22, and the perforations of the counter electrodes 34 are aligned with the cartridge openings. These perforations and openings allow fluid communication between the electrochemical media in the container 32 and the substrate 22 in the cartridge 36. The apparatus 30 can also be used to perform the pre-treatment.
Referring now to Figure 5, an electrodeposition apparatus 40 is provided for manufacturing the Os anode according to the above disclosed method, at a larger scale than the apparatus shown in Figure 4. This electrodeposition apparatus 40 includes a roll conveyor 42 for conveying a sheet of deposition substrate 22; the deposition substrate can be provided in discrete sheets, or fed continuously from a roll 44. The roll conveyor 42 communicates with a electrodeposition module 46 containing a viscous electrodeposition media bath. A pair of static perforated counter electrodes 48 are positioned in the bath and on either side of the roll conveyor, such that a segment of the deposition substrate is sandwiched between the counter electrodes 48. Upstream of the electrodeposition module 46 is a continuous surface pretreatment module 50 that contains a pretreatment bath that is in communication with the roll conveyor 42 such that the deposition substrate 22 is immersed in the pre-treatment bath when fed into the pre-treatment module 50 by the roll conveyor 42. Downstream of the deposition module 46 is a post-treatment (cleaning) module 52 that is in communication with the roll conveyor 42 and which contains an organic solvent washing means, an aqueous solution washing means, means for evaporating the washing solution, and heating means (not shown). The application of washing solutions can be performed by spray jets. The evaporation of the washing solutions from the surfaces can be performed by blowing hot inert gas such as He or N2 Example Experimental Method AVCarbTM P75 (uncompressed thickness of about 210 mm and porosity of 0.85, supplied by FuelCellStore.com) and GF-S3 (uncompressed thickness of 350 mm and porosity of 0.95, supplied by Electro-Synthesis Company Inc.) were used as deposition V84911CA\VAN_LAW\ 990785\ 1 substrates. Prior to electrodeposition of the Os catalyst, the substrates were electrochemically pretreated in 2 M NaOH by potential cycling between 0 and 2 VSHE
for 50 cycles at 50 mVs-1 at 295 K using Ti counter electrodes and Hg/Hg0/0.1 MKOH
reference electrode. The substrates were then thoroughly rinsed with 18 MU
deionized water and air-dried at 333 K in an oven. The Os electrodeposition media consisted of a mM (NH4)20sCI6 (Alfa Aesar) solution prepared with 18 MU deionized water.
Triton-X 102 (12.5 vol%) (Sigma Aldrich) was added to control the deposit morphology and structure. The electrodeposition media composed of Triton-X 102 and (NH4)20sCI6 solution was heated and stirred on a hot plate at 341 K for at least 30 min prior to deposition. Any evaporation of solution during the pre-heating procedure was replaced with 18 MU deionized water.
Electrodeposition was carried out at 341 K at a constant specified current density of 4 mA cm-2 for 30 min. At the end of the Os deposition, the substrate was removed and soaked in acetone for 10 min at 330 K. After soaking in acetone, the substrate was refluxed in 50:50 v/v hexane and acetone for 1 h at 363 K to remove the surfactant Triton-X 102 from the surface of the three dimensional electrode. Afterwards, the substrate was dried in air at 295 K.
When more than one electrodeposition step was performed on the same substrate, the substrate was washed in acetone at 330 K for 10 min between depositions.
Reflux cleaning was only applied at the very end of the complete electrodeposition procedure.
The morphology and structure of the Os electrodeposit were characterized by XPS
(Leybold Max 200 and Kratos AXIS Ultra), TEM (FEI Tecnai G2 200 kV
Transmission Electron Microscope), XRD (D8 Advance Brukerdiffractometer with Cu Ka1 source), SEM (Hitachi S-4700 and Hitachi S-4500 Field Emission Scanning Electron Microscopes), and ICP-MS for Os loading determination.
For half-cell electrochemical evaluation of the electrocatalytic activity toward BI-14-oxidation of the Os electrodeposited GF-S3 and AvCarb P75, solutions of 10 mM
NaBH4 were prepared using a powdered form of 98%1D NaBH4 (Acros Inc.) in 2 MNa0H. A three electrode setup composed of the Os three-dimensional working V8491 I CANAN_LAW\ 990785\ 1 electrode (6.5 cm2 geometric area) with Ag/AgCl/3 M KCI reference electrode (Cypress Systems Inc.) and a platinized titanium counter electrode (6.45 cm2 geometric area) was connected to a PARSTAT 2263 potentiostat (Princeton Applied Research). For the voltammetry data, in each electrolyte composition (i.e., 2 M NaOH or 10 mMNaBH4 -2M
NaOH), 50 scans were run at 50 mVs-1 prior to 10 slow scans at 10 mVs-1. The 60th scan in each test is presented here.
Fuel cell experiments were conducted using the Fideris MTK test station. The membrane electrode assembly comprised of the Os electrodeposited AVCarbTM P75 or GF-S3 anode and a Nafion 117 membrane with 4 mg cm-2 Pt black on the cathode side (Lynntech Inc.). The end plate material was stainless steel with a serpentine flow field.
Flow rates for the anolyte (0.5 MNaBH4e2 MNa0H) and oxidant (02) were 10 mL
min-1 and 0.25-1.25 SLPM (i.e., I min-1 at 273 K, 1 atm), respectively. The 02 absolute pressure was 4.4 atm, while the anode side pressure was about 1 atm.
Temperatures used during fuel cell performance testing were 298 K or 333 K.
Pretreatment of the fibrous carbon substrates by potential cycling in 2 M NaOH
The AVCarbTM P75 and GF-S3 substrates before and after pretreatment were analyzed using XPS. Elemental analysis of the untreated AvCarbTM P75 substrate shows much lower content of heteroatoms (particularly oxygen and nitrogen) as compared to untreated GF-S3, which contains approximately 72 wt% carbon and 23 wt% oxygen (table 1). After pretreatment, the surface oxygen content of AVCarbTM P75 increased by about 6 wt%, indicating oxidation of the substrate forming functional groups such as C-OH, C=0, C00-, which were also separately identified by deconvolution of the carbon spectra. The pretreatment of the GF-S3 substrate on the other hand resulted in a decrease of the oxygen content with about 4 wt% (table 1), which suggest possible electro-oxidation of functional groups on graphite felt (such as C-OH and COON) to CO2,(g).
After pretreatment, the oxygen content of GF-S3 is approximately three times that of AvCarb P75 and the former also contains nitrogen in higher proportion (table 1). The V84911CA\VAN_LAW\ 990785\ 1 surface oxygen and nitrogen functional groups could act as active sites for Os nucleation and deposition.
Table 1: Elemental Composition (in wt%) of the three dimensional fibrous carbon deposition substrates before and after pre-treatment Sample C Ca Cl N Na 0 S Si AvCarb P75 98.4-99.4 - 0.6-1.6 Pretreated 90.7-91.4 - 0.1 0.2-0.5 1.6-2.3 6-7.3 AvCarb P 75 GF-S3 70.7-72.2 0.5-0.8 3.1-3.2 1.0-1.2 21.7-23.4 .6 0.2-0.6 Pretreated GF-S3 75.1-76.1 0.4-0.5 4-4.4 0.7-1.1 18.3-18.5 0.1 0.3-0.4 OS electrodeposition and structural characterization Fig. 6 shows the cathode potential profile during the electrodeposition of Os on the AVCarbTM P75 substrate. There appears to be two deposition stages. The first stage, shown in the first 20-30 s in the inset of Fig. 1, involves the nucleation of Os deposits on energetically favorable sites. From about 30 s onward, the second stage involves the bulk deposition of Os. At deposition times much greater than 30 min (not shown here), severe mass transport limitation is imposed by the depletion of Os salt concentration within the deposition substrate. It should be noted that the electrodeposition media was not stirred. A similar profile to Fig. 6 was observed when electrodepositing Os onto the GF-S3 substrate.
SEM images of AvCarbTm P75 and GF-S3 are shown in Figures 2(a) and 3(a), respectively. Figures 2(b) and 3(b) shows the respective substrates with electrodeposited Os. The unique structural feature of the AVCarbTM P75 is the presence of interstitial carbon deposit in the form of rough plates among the fibers.
These interstitial carbon plates appear to be favorable sites for electrodeposition on AVCarbTM
P75 as shown by Figure 2(b). For GF-S3, dense and fairly uniform deposition of aggregates occurred on the fiber (Fig. 3(b)). Low magnification SEM images (not shown V84911CA\VAN_LAW\ 990785\ 1 here) revealed a fairly homogeneous distribution of the Os deposits throughout the thickness of both substrates.
Upon closer inspection of the electrodes with TEM (Fig. 7 A and B), it was found that the larger deposits are actually agglomerates composed of nano-sized Os particles, with particle sizes of 5 nm or lower. A similar morphology was observed in our previous study of electrodeposited PtRu on GF-S3 using a similar electrodeposition procedure.
The Os nanoparticle distribution within the aggregate appeared to be more uniform in case of GF-S3, which could be explained by the higher proportion of surface heteroatoms (particularly oxygen and nitrogen).
Fig. 8 shows the XRD spectra from 2E1= 34-90 for an Os/ AvCarbTm P75 electrode with a metal loading of 0.3 mg cm-2 and an Os/GF-S3 electrode with a metal loading of 0.2 mg cm-2. The spectra for the Os/GF-S3 electrode indicates the presence mainly of the (101) plane with a low intensity compared to the strongly polycrystalline Os/
AvCarbTm P75 where the (100), (101), (103), (110) and (111) planes could be identified.
Among the latter planes, the (101) plane generated the highest intensity. The crystallite sizes for the Os/ AVCarbTM P75 and Os/ GF-S3 electrodes were calculated to be approximately 2 nm. Another interesting distinguishing feature between the two electrodes is the strong graphite XRD signal from AVCarbTM P75 and its absence in case of GF-S3 (Fig. 8). This shows significant structural differences between the two graphitic substrates, with possibly a high content of amorphous-like (less ordered) graphite in case of GF-S3.
Cyclic voltammetty investigation of borohydride oxidation on the electrodeposited three-dimensional Os electrodes Fig. 9A and B shows the cyclic voltammogram of Os/ AVCarbTM P75, and Os/ GF-53 respectively, in 2 M NaOH in the absence and presence of borohydride. The broad anodic peaks in the absence of borohydride at _0.56 V in Fig. 9A (dashed line) and -0.59 V in Fig. 96 (dashed line) represent the electrooxidation of H2 generated at the cathodic starting potential of -0.9 V. Fig. 9A (dashed line), also reveals that on Os/AvCarb P75 weak underpotential formation of hydrogen occurred on the reverse V849I ICANAN_LAW\ 990785\ 1 cathodic scan between approximately -0.57 V and -0.77 V, which was absent for the Os/ GF-S3 electrode (Fig. 9B (dashed line)). The strongly polycrystalline Os/AvCarb P75 surface (Fig. 8) is more likely to generate underpotentially deposited hydrogen atoms. Furthermore, there were no Os oxidation peaks within the potential range of the cyclic voltammograms, although the formation of 0s02 is thermodynamically favorable at high pH and at potentials greater than -0.3 V.
When 10 mM NaBH4 was added to the 2 M NaOH solution, on the Os/ AvCarbTm P75 electrode two characteristic oxidation peaks were observed, one at -0.48 V in the anodic direction and the other at -0.43 V on the return cathodic scan (Fig. 9A (solid line)). For the Os/GF-S3 electrode, there was a slight positive shift for the forward oxidation peak to -0.44 V, whereas the cathodic return scan oxidation peak remained at -0.43 V (Fig.
9B (solid line)). The anodic peak currents (expressed as Os mass specific activities) on both electrodes were between about 3-4 times larger with BH4- in 2 M NaOH.
This shows clearly the activity of the electrodeposited Os electrodes toward BH4-electrooxidation.
The observed peaks on Os/ AvCarbil" P75 in the presence of BH4" (Fig. 9A
(solid line)) are similar in appearance to the characteristic dominant peaks "al" (on the anodic scan) and "cl" (cathodic return scan) found on Pt. Recent literature has suggested that the electrooxidation reactions on Pt occurring at the "al" and "cl" peaks are complex from a mechanistic point of view. The "al" peak has been attributed to the composite oxidation of BH4-ad and Had, the latter formed also through partial hydrolysis of BH4-. For Os nanoparticles prepared by a modified Bonneman method and supported on Vulcan XC72, Lam and Gyenge reported a seven electron oxidation of 61-14-corresponding to peak "al" through analysis of the peak current and potential of their voltammetry data.
In the present work, the number of electrons associated with the voltammetric peaks from Fig. 9A and B was not determined because application of the standard voltammetry theory to responses from three-dimensional electrodes was deemed unreliable. A detailed electrode kinetic model for the BH4- oxidation on three-dimensional electrodes is very much required in future work corroborated by V84911CA\VAN_LAW\ 990785\ I

experiments using electrodeposited Os RDE to establish the total number of electrons involved and to assess the feasible mechanistic pathways.
A notable difference between Os on AVCarbTM P75 and GF-S3 is the shape of oxidation peak "c1" on the return cathodic scan (Fig. 9A and B (solid lines)). For Os/GF-S3, this peak is broader and its peak current is over two times smaller than on Os/
AVCaIbTM
P75. The "c1" peak has been attributed to oxidation of strongly adsorbed intermediates such as BOHad and BH20Had. The broader peak on Os/GF-S3 could be indicative of a wider range of adsorbed species present on the surface compared to the Os/
AVCarbTM
P75, where a sharp "c1" peak was observed. Furthermore, the large "c1"
oxidation peak current density for Os/ AVCarbTM P75 suggests more effective oxidation of the adsorbed intermediates. For Os/GF-S3 the BH4" cyclic voltammogram was similar to that on Os nanoparticles prepared by a modified BOnneman method and supported on Vulcan_ XC72.
To better understand the Os/ AVCarbTM P75 surface, XPS was performed after the electrode had been subjected to electrochemical half-cell tests (Fig. 10). A
narrow scan was performed on the Os 4f double peak. Deconvolution revealed four peaks with binding energies at 50.8 eV, 51.7 eV, 53.5 eV, and 54.4 eV. It was found that the binding energies at 50.8 eV and 53.5 eV corresponded to the literature values indicating the presence of Os(0) (4f712 and 4f512, respectively). The binding energies at 51.7 eV and 54.4 eV likely indicate the presence of 0s02, which is also supported by the literature.
The partial oxidation of the Os surface cannot be explained by anodic oxidation since the corresponding anodic peak expected at potentials greater than -0.3 V, was absent in the cyclic voltammograms (Fig. 9A and B). Thus, it is surmised the partial surface oxidation (0s02 formation) occurred during the electrode preparation procedure by exposure to ambient air.
DBFC experiments: comparison between the three-dimensional anodes at low Os loading (0.2 mg cm-2) In Fig. 11, the fuel cell performance at 333 K of the two Os three dimensional anodes with AVCarbTM P75 and GF-S3 substrates, respectively, are compared at an Os loading V84911CA\ VAN_LAW\ 990785\ 1 of 0.2 mg cm-2. The AVCarbTM P75 electrode generated a superior peak power density compared to the GF-S3, namely 25.2mWcm-2 versus 18.3mWcm-2. Both electrodes exhibited similar characteristics between cell voltages of 1.1 V (open circuit) and 0.9 V.
The polarization curves differ at cell voltages below 0.9 V (Fig. 11). An explanation of this difference can be sought by calculating and comparing the effective ionic and electronic conductivities of the two three-dimensional electrodes and evaluating possible electrodekinetic differences.
The Bruggeman equation can be used to describe the effective ionic conductivity of a three-dimensional porous electrode:

K Kõ (02 (1) where k is the effective ionic conductivity of the three-dimensional electrode [Sm_1], k0 is the conductivity of the electrolyte (2 M NaOH) [S m-1], and 3 is the porosity of the substrate.
The AvCarb P75 is a denser material, with an average porosity of 0.85 and a thickness of 210 mm whereas the GF-S3 has a porosity of 0.95 and a thickness of 350 mm.
It was assumed that both electrodes were compressed in the fuel cell to the same final thickness (approximately 100 mm), which was determined by the thickness of the gasket (Table 2). The ratio of the effective ionic conductivities was calculated with Eq.
(2) and the results are shown in Table 2.
Table 2: Calculation of the effective ionic (k') and electronic (s') conductivity ratios fo the GF-S3 to AvCarb P75 Three dimensional electrodes under the employed experimental conditions Uncompressed Compressed k' s' electrode electrode thickness (mm) thickness (mm) GF-53 350 100 1.25 0.45 V8491ICA\VAN_LAW\ 990785\ 1 AvCarb P75 210 100 1.25 0.45 =\ 3/2 = K
GF-S3 eGF-S3 C AvCarh175 \S AvCarbP75 (2) The compressed porosity of the three-dimensional electrodes can be evaluated with a linear relationship:
, c T00¨ eo) =1 (3) where T, To are the compressed and uncompressed thickness of the three-dimensional electrode, respectively, and co is the uncompressed (original) porosity of the substrates (AvCarbTM P75 and GFS3, respectively).
From Table 2, it is clear that the GF-S3 three-dimensional electrode has a higher effective ionic conductivity than AvCarb P75. Therefore, the advantage of the Os/
AVCarbTM P75 over the Os/GF-S3 anode at fuel cell voltages lower than 0.9 V
(Fig. 11) cannot be explained by the effective ionic conductivity in the anode compartment.
The effective transverse electronic conductivity of a graphite fiber matrix such as GF-S3 and AVCarbTM P75 can be approximated by the following relationship:
0- =10 + 2800 1--o (4) where a is the effective electronic conductivity of the graphite fiber matrix, 3 is the compressed porosity, and co is the uncompressed porosity.
It is likely that the parameters in Eq. (4) determined by fitting of experimentally measured electronic conductivity data, will differ based on the compositional and structural features of the employed carbon-based fiber matrix. In the absence of specific correlations for either AvCarbTM P75 or GF-S3, Eq. (4) can provide a preliminary V84911CA\VAN_LAW\ 990785\ 1 estimate of the respective electronic conductivities as a function of porosity. The ratio of the electronic conductivities is:
, _s3 =
AvearbP75 (5) The calculations reveal that compressed AVCarbTM P75 had about 2.2 times higher electronic conductivity compared to GF-S3, counteracting the approximately 25%
lower effective ionic conductivity (Table 2). The higher effective electronic conductivity and possibly lower contact resistance with the end plate could have some benefits for the fuel cell polarization performance of Os/AvCarb P75 in the ohmic potential loss controlled region.
An additional effect that probably is the most significant for cell voltages lower than 0.9 V in Fig. 11 is the accumulation and electrooxidation of surface adsorbed intermediate species such as BH20Had and/or BOHad. The fuel cell polarization data points are recorded in a sequential fashion hence, there is an in-built time factor in the polarization curves. The accumulation in time of strongly adsorbed species coupled with their sluggish oxidation will increase the anodic overpotential due to a higher effective (or apparent) Tafel slope, lowering the power output. As discussed in relation to the cyclic voltammetry study and peak "c1" (Fig. 9), Os/ AvCarbm" P75 was more effective in oxidizing the adsorbed intermediates. This beneficial electrode kinetic effect could have contributed to the superior fuel cell performance of the Os/ AVCarbTM P75 anode compared to Os/GF-S3 (Fig. 11).
DBFC experiments using Os/AVCarbTM P75 anode: effect of temperature and Os loading Based on Fig. 11, Os/ AVCarbTM P75 was retained as anode for further studies.
A
significant performance improvement was observed when the fuel cell temperature increased from 298 K to 333 K at an Os loading of 0.2 mg crn-2 (Fig. 12).
Improvements in electrode kinetics, ionic conductivity and the open circuit voltage were all evident.
V84911CA\VAN_LAW\ 990785\ I

The positive effect of the increased temperature on the open circuit voltage is due to reduced mixed potential polarization losses on the cathode and/or anode. A
mixed potential between BH4- and 02 is established on the cathode due to BH4-crossover across Nafion 117, whereas on the anode a mixed potential between BH4" and 02 can form due to 02 crossover. The 02 cathode pressure was kept at 4.4 atm (abs) while the anode operated at approximately atmospheric pressure. The pressure differential minimized the BH4- crossover but it cannot be completely discounted. The mixed potential loss on the Pt cathode is lessened at higher temperature because of enhanced hydrolysis on Pt of the BH4- that crossed over to the cathode. As a result, the mixed potential polarization on the cathode will involve H2-02 as opposed to the BH4-couple. Furthermore, the structure of the three-dimensional electrode could also play a role in improving the open circuit voltage by reducing the BH4" crossover flux. The ability for three-dimensional electrodes to decrease fuel crossover has been proven by Lam et al. in a direct methanol fuel cell (DMFC) system. They found that the open circuit voltage and methanol crossover of the DMFC improved with increased anode layer thicknesses.
In addition, another potential complicating factor on the anode could be the generated either by thermocatalytic hydrolysis of NaBH4 (Eq. (6)) or incomplete electrooxidation of BH4".
NaBH4 +2H20 NaB02+4H2 (6) If the rate of reaction (6) would be significant, it is expected that the presence of H2 at the anode lowers the open circuit cell voltage by partially replacing the anodic reaction (7) with reaction (10), creating an in-situ alkaline hydrogeneoxygen fuel cell, instead of the direct fuel cell reaction (9). At higher temperatures the rate of the thermocatalytic hydrolysis (reaction (6)) is exponentially accelerated thereby enhancing the evolution. The fact that increasing the temperature from 298 K to 333 K
increased the open circuit cell voltage as opposed to reducing it (Fig. 12), implies that the competing H2 evolution/oxidation was probably not the predominant factor. At present, the relative rates as a function of temperature and concentration of borohydride hydrolysis (Eq. (6)) V84911CANAN_LAW\ 990785\ 1 and direct BH4- and H2 electrooxidations, respectively, are unknown on the electrodeposited Os catalysts and need to be separately investigated in future work.
This would yield the effective borohydride utilization efficiency, which is an important figure of merit for the present system.
NaBH 4 +80H- --> NaB02+6H20 + 8e- E a,298K = -1.24 VSHE (7) 202 + 4H20 + 8e- ¨> 80H- E c,298K = 0.4 VSHE (8) NaBH4 + 202 ¨> NaB02+2H20 E ce11,298K = 1.64 V (9) 4H2 + 80H- ¨>8H20 +8e- E a,298K ¨ -0.83 VSHE (10) The effect of Os loading was tested in the case of Os/AvCarb P75 (Fig. 13).
Higher Os loadings were achieved by simply repeating the electrodeposition process on the same substrate a number of times, using a fresh bath for each deposition step (see Section 2). Thus, Os loadings of 0.3, 0.9, and 1.7 mg cm-2 were achieved with single (1X), double (2X) and quadruple (4X) depositions, respectively. The corresponding peak power densities were 65.2 mWcrn-2, 75.9 mWcm-2, and 102.8 mWcm-2, respectively (Fig. 13). It is important to note that the specific peak power densities per Os loading, decreased with increasing Os loading: 203.8 mW mg-1, 89.3 mW mg-1, and 61.2 mW

mg-1 for 1X, 2X and 4X depositions, respectively. The most probable explanation for this behavior is the increased agglomeration and reduced specific electrocatalytic surface area of Os as the loadings increased on the substrate.
Furthermore, Fig. 13 shows that the slope of the polarization curves gradually decreased with increase of the anode catalyst loading. This could be attributed to a decrease of the effective anodic Tafel slope at higher Os loadings, which is beneficial for the anode kinetic performance. Future studies should attempt the modeling of the three-dimensional Os anode DBFC polarization.
DBFC experiments using Os/ AvCarbTM P75 anode: experimental reproducibility and longer-term durability V849I ICANAN_LAW\ 990785\ 1 The reproducibility of the anode performance was investigated by preparing three Os/
AVCarbTM P75 electrodes by quadruple electrodeposition (loading of 1.7 mg cm-2), and testing them under the same conditions in DBFC with the same cathode half-MEA
each time (Fig. 14). The cathode half-MEA was washed with deionized water and purged with air between the repeated experiments. The error bars in Fig.14 represent the standard error of the mean at 90% confidence level. At open circuit and current densities up to 100 mA cm-2 the error was small (virtually negligible), indicating that the electrodeposition procedure was generating reproducible Os catalytic surfaces with very similar electrode kinetic performance. The error in the measured cell voltage increased progressively with increasing current density, as ohmic and possibly mass transfer limitations gained more important roles (Fig. 14). Inherent structural differences among the AVCarbTM P75 samples used could have caused some morphological variations for the electrodeposited Os affecting the borohydride mass transport to the active sites.
Furthermore, the reuse of the membrane and the cathode (i.e., half-MEA) could have generated somewhat deteriorating performance over time. The average peak power density was 109 mWcm-2 with a standard error of the mean +/-12 mWcm-2 at 90%
confidence level (Fig. 14). This error can be considered acceptable and shows good reproducibility for the electrodeposited Os three-dimensional anodes.
The longer-term performance of the DBFC equipped with Os/ AvCarbTM P75 anode prepared by the quadruple electrodeposition procedure (see Figs. 13 and 14), was investigated at a constant current density of 290 mAcm-2 over a total operational time of 540 min, which was interrupted by two shutdowns of 12 h each, after 180 and 360 min, respectively (Fig. 15). After the first shutdown the fuel cell was not reconditioned in any form (such as by washing with distilled water or purging an inert gas at the cathode).
During the first 180 minutes the DBFC cell voltage decreased by about 30 mV, which can be attributed to a slight deactivation of the Os catalytic surface over the first 180 min, amounting to approximately 10 mVh-1 voltage loss. This low Os degradation rate, due probably to the incomplete removal of some of the strongly adsorbed intermediates (as discussed before in relation to the cyclic voltammograms), was also confirmed in our previous study of DBFC with Os nanoparticle anode catalyst prepared by the modified Bonneman method and supported on Vulcan XC72.
V84911CANAN_LAW\ 990785\ 1 , When the cell was restarted after 12 h shutdown, the voltage dropped by an additional 50 mV. It is hypothesized that the voltage loss at the first restart was due to crystallization and carbonation of the NaOH electrolyte particularly in the gas diffusion cathode which has narrower pores than the three-dimensional anode. This is a common occurrence in DBFC systems and in alkaline fuel cells and batteries in general. To alleviate the cathode performance loss, after the second shutdown (i.e., after 360 min) the cathode compartment was washed with copious amounts of deionized water followed by drying with high flow rate of air.
Therefore, following the second restart the cell voltage was the same as in the last portion of the previous operation sequence and it remained virtually constant for the last sequence of the testing period (between 360 and 540 min) (Fig. 15). Clearly, reconditioning the cathode was effective for recovering some of the performance.
V84911CA\VAN_LAW\ 990785\ 1

Claims

1. A method of manufacturing an anode for a direct borohydride fuel cell or battery, comprising:
(a) providing a porous and electronically conductive monolithic substrate;
(b) roughening deposition surfaces of the substrate;
(c) applying a surfactant to the substrate;
(d) electrodepositing osmium catalyst material onto the roughened deposition surfaces such that agglomerates of osmium (Os) nanoparticles are formed on the substrate; and (e) removing the surfactant from the substrate.

2. A method as claimed in claim 1 wherein the surfactant is selected from the group consisting of: non-ionic, cationic, anionic, and zwitter ionic.

3. A method as claimed in claim 2 wherein the surfactant is selected from the group consisting of Triton-X 102, Triton-X 100, and Triton-X 114.

4. A method as claimed in claim 1 wherein the substrate is selected from the group consisting of graphite felt, glassy carbon, carbon paper, carbon fiber-carbon particle composite, a reticulated carbon structure, metal mesh, metal fibers, and metal foam.

5. A method as claimed in claim 1 wherein the substrate is a fibrous graphitic substrate and the step of roughening is carried out until the morphology of graphite fibers of the substrate is changed.

6. A method as claimed in claim 5 wherein the substrate is selected from the group consisting of AvCarb.TM. 75 and GF-S3.

7. A method as claimed in claim 1 wherein the step of roughening the substrate comprises electrochemically treating the substrate by potential cycling of the substrate in a concentrated basic electrolyte bath.

8. A method as claimed in claim 1 wherein the step of roughening the substrate comprises chemically treating the substrate in an acidic solution.

9. A method as claimed in claim 1 wherein the steps of applying the surfactant and electrodepositing comprise immersing the substrate and a pair of counter electrodes in an electrodeposition media comprising an Os salt and the surfactant.

10. A method as claimed in claim 9 wherein the Os salt is Ammonium Hexachloroosmate ((NH4)2OsCl6).

11. A method as claimed in claim 1 wherein the step of removing the surfactant comprises heating and rinsing the anode in a solvent, then refluxing the anode.

12. A method as claimed in claim 1 wherein the Os agglomerates have a characteristic diameter of equal to or less than 100 nm.

13. A method as claimed in claim 1 wherein the Os nanoparticles have a characteristic diameter of between 1 and 50 nm.

14. An anode for a direct borohydride fuel cell or battery comprising:
(a) a porous and electronically conductive monolithic substrate; and (b) agglomerates of Os nanoparticles deposited on deposition surfaces of the substrate.

15. An anode as claimed in claim 14 wherein the agglomerates of Os nanoparticles are electrodeposited on the deposition surfaces of the substrate.

16. An anode as claimed in claim 14 wherein the substrate is selected from the group consisting of graphite felt, carbon paper, carbon fiber-carbon particle composite, a reticulated carbon structure, metal mesh, metal fibers, and metal foam.

17. An anode as claimed in claim 14 wherein the substrate is a fibrous graphitic substrate selected from the group consisting of AvCarb.TM.75 and GF-S3.

18. An anode as claimed in claim 14 wherein the agglomerates have a characteristic diameter of less than 100 nm.

19. An anode as claimed in claim 14 wherein the Os nanoparticles have a characteristic diameter of between 1 and 50 nm.

20. An anode as claimed in claim 14 wherein the substrate has a thickness between 30 microns and 3000 microns, a porosity between 0.6 and 0.98 and a conductivity between 10 S m-1 and 10 5 S m-1.