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The effects of electrode parameters and several Pt and Pt alloy electrocatalysts on the performance of phosphoric acid doped Poly membrane based high-temperature polymer electrolyte fuel cells is reported. The key parameters on MEA performance are Pt loading; hydrophobicity during heat treatment, catalyst layer thickness and the amount of PTFE in the cathode are investigated. The fuel cell performance is maximized via catalyst utilization by optimizing the phosphoric acid content in the electrodes. Heat treatment of GDE results in increase in hydrophobicity and decrease in phosphoric acid content in the catalyst layer drastically affects the fuel cell performance. Maximum fuel cell performance in the present study was achieved at 160 o C with air of 270 mW cm-2 using a 1 mg Pt cm-2 loading of catalyst (20 % PTFE). The fuel cell performance and the poisoning effect of carbon monoxide in an ABPBI-membrane-based high- temperature PEM fuel cell is investigated with respect to carbon monoxide (CO) concentrations. Fuel cell performance data for cathodes made with alloys of Pt with Ni, Fe and Cu are compared with those with Pt alone as cathode at temperatures between 140, 160 and 180Â Â°C. Pt-Cu/C alloy catalysts show higher performance with lower catalyst loading (0.4Â mgÂ PtÂ cmâˆ’2) than Pt/C catalyst. The performance of alloy catalysts follows the order; Pt-Cu/C > Pt-Fe/C> Pt-Ni/C> Pt/C.
Keywords: Gas diffusion electrode; HT-PEMFCs; ABPBI; Phosphoric acid; membrane electrode assembly (MEA); Pt alloy catalysts; Electrode performance
*Corresponding authors. Tel: +27-21-9599318, Fax: +27-21-9591583
Email addresses: email@example.com (T. Maiyalagan). firstname.lastname@example.org (P.Siva kumar)
The progress of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) in recent years attracted increased attention as an alternative to conventional low temperature PEM fuel cells based on humidified perfluorosulphonic acid (Nafion) membranes . Nafion is the accepted current bench mark perfluoro sulphonic acid polymer membrane used in low temperature fuel cells . The lack of commercialization of low temperature PEM fuel cell is due to limited operation temperature (< 100 o C), Fuel crossover and cost (catalyst and Nafion membrane) . High temperature PEM fuel cells (HT -PEMFCs) can be operated in the temperature range between 120 Â°C and 200 Â°C and offer a numerous potential advantage compared to low temperature PEM fuel cells (LT-PEMFCs), e.g., high CO tolerance, elimination for humidification of reactant gases, and simplified heat management of the fuel-cell system. The efficiency of the single fuel cell of HT-PEMFCs shows a slightly poorer performance than the LT-PEMFCs. HT-PEMFCs offer the reduction of significant system components such as gas purification and humidification and in parallel an increase of operational stability of the fuel cell stacks and the overall system. The waste heat from HTPEM fuel cell (~160 Â°C) is very usable and ideally suited for combined heat and power (CHP) applications than LTPEM fuel cell (~70Â°C) [4,5]. Fuel cells powered micro-CHP systems are viable for commercialization. Japan has already installed nearly 3,000 micro-CHP fuel cell systems under the trade name ENEA-FARM.
The performance of HT-PEMFCs MEA strongly depend on phosphoric acid content in PBI (= poly (2, 2'-(m-phenylene)-5, 5'- bibenzimidazole) electrolyte . Alternatively chemically related poly (2, 5-benzimidazole) (ABPBI) membranes have emerged as a promising alternative for high-temperature PEM fuel cells and have similar fuel cell performance to PBI membranes [6-10]. ABPBI membrane has low cost, good mechanical strength, high phosphoric acid uptakes, as compared to PBI membranes [10, 11]. HT-PEMFC MEAs based on an ABPBI membrane saturated with phosphoric acid became commercially available under the trade name as celtec (BASF) and Advent [12, 13].
To achieve maximum performance of the membrane electrode assemblies operating in the temperature range of 120 -200 Â°C is highly dependent upon optimization of catalyst layer and electrode structure. Additionally, the manufacture of membrane electrode assemblies using the new membrane materials and optimized electrode structure of high-performance fuel cells for commercialization still requires research and development efforts. The aim of the present work is to improve the performance of ABPBI based MEAs by optimizing the electrode parameters, and the fabricated MEAs were tested in laboratory-scale 25 cm2 cells at 160 Â°C to evaluate the most promising MEA formulation. The catalyst and electrode structure in the MEAs play a major role in determining fuel cell performance. This paper explored the effects of electrode parameters such as Pt loading, catalyst thickness, PTFE content, and carbon supports on the performance of the cathode for ABPBI based HT-PEMFC. Furthermore, the performance of an ABPBI-membrane-based high- temperature PEM fuel cell in the presence of carbon monoxide (CO) at various concentrations was reported.
2.1 Physiochemical characterization of the catalysts
The commercial catalysts used in the MEAs were analyzed by the powder X-ray diffraction (PXRD) technique, which was performed at room temperature in the angular range of 2Î¸Â =Â 10-80Â° with a scan step width of 0.02Â and a fixed counting time of 1Â s/step using an automated Bruker D2Â X-ray diffractometer with Cu KÎ± radiation (Î»Â =Â 1.5418Â Å). Nitrogen adsorption-desorption isotherms were recorded at 196 Â°C using a Micromeretics ASAP 2020. Total surface area and pore volume were determined using the BET equation and the muti point method.
2.2 Phosphoric acid doped ABPBI Membranes
The ABPBI (=poly (2, 5-benzimidazole)) membranes (supplied by FuMA-Tech GmbH in an acid-free form, dÂ =Â 35 (Â±5) Î¼m, mass per unit area 4.5Â mgÂ cmâˆ’2). The dry ABPBI membranes (previously cut into 7 cm x 7 cm) were doped by immersing them in Phosphoric acid solutions (85 %) and the membrane was heated at 120 oC for 12 h for effective doping of phosphoric acid across the membrane. The doped ABPBI membrane was removed from phosphoric acid solution, and the excess phosphoric acid has been wiped out. Thus, the obtained phosphoric acid doped ABPBI membrane was fabricated on gas diffusion electrodes (GDE) to form MEAs.
2.3 Preparation of gas diffusion electrodes
GDEs (carbon paper) incorporated with a wet-proofed microporous layer (H2315 CX196) obtained from Freudenberg (FFCCT, Germany) were used as substrates to deposit the catalyst layer for both anode and cathode. For the fabrication of gas diffusion electrodes (GDEs), slurries containing appropriate amounts of platinum catalysts (supported on carbon) with precious metal weight fractions from 20 to 60 wt. % (HiSPEC 4000, from Johnson Matthey) and aqueous dispersions of PTFE (Dyneon) were prepared by ultrasonic agitation for 20-40 min in a mixture of water and isopropanol. The catalyst ink was highly dispersed by an ultrasonic homogenizer several times for 30Â s each. This dispersion was agitated, until it turned into slurry. The viscosity of the inks was controlled by addition of isopropyl alcohol (IPA). These inks were uniformly sprayed on the microporous layer of a commercially available GDL (Freudenberg.) by a spraying technique followed by an air drying step. The GDE were produced with a platinum loading of 1.0Â mgÂ cm-2.
2.4 Phosphoric acid impregnation on GDE
The catalyst layers were impregnated with predefined amounts of phosphoric acid was carried out by pipetting a H3PO4 to the surface of the GDEs and allowed to evaporate overnight. The Phosphoric acid impregnated gas diffusion electrode is then combined with Phosphoric acid doped ABPBI membranes to form an MEA.
2.5 Single Fuel cell Tests
The fuel-cell tests for demonstrating the HT-PEMFC are carried out in 25Â cm2 single cell setups using graphite flow field plates. Freudenberg gaskets made of fluorinated polymer with thickness of 50 Âµm were used as sealants. The cell was assembled in cell compression unit from pragma industries for uniform pressure on the MEA. The MEAs were assembled by contacting GDEs impregnated with phosphoric acid with doped ABPBI membrane in to the test cells without a preceding hot-pressing step. The prepared MEAs was assembled in Cell Compression Unit (CCU) which operates at constant force with high accuracy and be able to precisely measure the influence of compression over MEA and GDL. A compression force of 1.5 N/m2 was applied throughout all the experiments. The performance of the MEAs was evaluated after conditioning for 24 h at 0.6 V where the cell is operated at 160 oC, with the stoichiometric amount of air and hydrogen using an in house built up fuel cell test system. A fuel cell test system consisted of control of the electronic load Hocherl & Hackl GmBH load and labview controlled program is employed to record polarization curves. Unless otherwise noted, the cells are operated at 160Â Â°C in ambient pressure using dry hydrogen and air with stoichiometries of 0.6 and 1.8, respectively.
3. Results and discussion
3.1. Effect of membrane doping
Pristine ABPBI membrane has a negligible conductivity and requires a substantial amount of Phosphoric acid to facilitate proton conduction. The higher the doping level of phosphoric acid used, the higher the conductivity would be. So in the present study optimized high amount of PA acid doping of ABPBI membranes was adopted. However, the mechanical properties and tensile stress of ABPBI deteriorate dramatically on increasing the doping level of phosphoric acid. Since, ABPBI membranes disintegrate in presence of concentrated H3PO4 at higher temperatures, one has to be careful in treating the membrane with H3PO4. In our study, the ABPBI membrane was treated in 85 % (or 14.8 M) phosphoric acid at 120 oC for 12 hours. The treated membrane contained 3.1 molecules of phosphoric acid per repeat unit .The specific conductivity for ABPBI x 3.1 H3PO4 at 140 oC corresponds to 79 mS cm-1 is reported [14, 15].
3.2 Physicochemical characterization of electro-catalysts
The electro-catalysts surface area was studied by nitrogen physisorption method are shown in Fig 1. The surface area results are summarized in Table 1. The BET surface area of the catalysts tends to decrease on increase in metal loading on the carbon support . The catalyst 60 % Pt/C exhibited a lower surface area and total pore volume than the 40 % Pt/C and 20 % Pt/C. 60 % Pt/HSAC has higher surface area compared to 20 %, 40 % and 60 % Pt/C due to replacement of conventional Vulcan carbon by high surface area carbon catalyst support. The Pt particle size of 60 % Pt/HSAC catalyst is 4.5Â nm which is smaller than those of commercial 60 % Pt/C catalysts. This small Pt particle size increases the active surface area of Pt on the support.
The BET surface area, total pore volume, micropore volume and pore size distribution of Pt/C catalysts vary with Pt loading shown in Table 1. Pore size distribution data were calculated from the adsorption branches of nitrogen isotherms by the Barrett-Joyner-Halenda (BJH) method. Typical pore size distribution for the Pt/C electrocatalysts is shown in Fig.Â 2. The catalysts with low Pt loading have larger total pore volumes and a higher proportion of micropores than the high-loaded catalysts. In particular, the decrease of the micropore volume and micropore area was more significant in increasing the catalyst loading on the carbon supports. However, a more significant decrease of microporosity was observed from the case of 60 % Pt/Vulcan, strongly indicates that Pt was mainly distributed throughout the micropore structure. 60 % Pt/HSAC has higher content of microporosity than 20, 40 and 60 % Pt/C catalytsts. Large amount of Pt nanoparticles distributed in the micropores has the difficulty of reactant accessibility will probably have little electrochemical activity.
Table 1. The results of N2 physisorption on the various Pt/C catalysts used in this work.
Particle sizea (nm)
BET surface area (m2Â gâˆ’1)
Total pore volume
Micropore area (m2Â gâˆ’1)
Micropore volume (cm3Â gâˆ’1)
at cell voltage of 0.3 V
20 % Pt/C
40 % Pt/C
60 % Pt/C
60 % Pt/HSAC
a Estimated from XRD.
bCurrent density of HTPEMFC single cells measured at 0.3 V and 160 0C.
Fig. 1. Nitrogen Physisorption isotherms of (a) 20 % Pt/C (b) 40 % Pt/C (c) 60 % Pt/C and (d)
60 % Pt/HSAC catalysts.
Fig. 2. Pore size distribution of (a) 20 % Pt/C (b) 40 % Pt/C (c) 60 % Pt/C and (d) 60 % Pt/HSAC catalysts.
Fig. 3. XRD pattern of various percentage of Platinum supported carbon catalysts.
The XRD patterns for all commercial catalysts are shown in Fig. 3. All samples showed the characteristic peaks of the FCC structure of platinum. However, the intensity and width of the peaks, which are related with the Pt crystallite size, depended upon the carbon material used as support. The average Pt crystallite size was determined from the XRD diffractograms using the Scherrer's equation, and the values obtained are summarized in Table 1. The particle size of the catalysts determined from XRD and Platinum surface area are shown in Table 1. It can be clearly seen higher the platinum percentage in the catalysts, larger the platinum particle size resulting in lower Platinum surface area. The value of increase in platinum utilization with Platinum percentage in the catalysts has been well reported .
3.3 Electrode performance varies with PTFE content
The performance of ABPBI MEAs with various loading of PTFE contents in the cathode catalyst layer are shown in Fig. 4. The influence of PTFE content used on fuel cell performances has been studied [17, 18]. The anode and cathode are consisted of 40 wt % Pt/C electrocatalysts and the electrode parameters of the anode are fixed. Polytetrafluoroethylene (PTFE) content in the fuel cell catalyst layer plays an important role in the performance of High temperature polymer electrolyte membrane fuel cell (HT-PEMFC).The amount of phosphoric acid in the GDE was kept constant (20 mg/cm2) and most notably nearly identical for the anode and cathode of each individual MEA. Typically, 20-60 wt % PTFE is added to the cathode catalyst layer in phosphoric acid doped ABPBI membrane fuel cells. PTFE can offer several advantages, such as enhancing mass transport through a more porous structure [17, 18]. However, excess PTFE will lead to increased resistance, due to a thicker layer (more porous) and less ionic and electronic conductivity. The fuel-cell performance at 160 oC with varying PTFE content in cathode catalyst layer is shown in Fig. 3. The PTFE content in the cathode catalyst layer was varied up to 60 wt %. A minimum appears for a PTFE loading between 20 and 40 wt%. The clear trend that increasing the PTFE content decreases the fuel cell performance can be observed in Fig.4. The increase of PTFE content in the catalyst layer decreases the phosphoric acid content in the catalyst layer which tends to decrease the proton conductivity of the electrode. Electrode with PTFE loading of 20 wt. % in GDEs shows higher fuel cell performance. The MEA with 60 % PTFE content shows relatively lower fuel cell performance.
Fig. 4. Fuel cell performance of electrodes with different amount of PTFE contents in 40 % Pt/C cathode catalyst (1.0 mg Pt cm-2).
3.3. Electrode performance with cathode catalyst loading
High-temperature ABPBI PEM fuel cells with phosphoric acid electrolyte require relatively higher catalyst loadings compared to low temperature PEM fuel cell. This has been attributed to anion adsorption on platinum as well as to low solubility of oxygen in and its slow diffusion through phosphoric acid within the cathodic catalyst layer. The effect of catalyst loading (0.5, 1 and 2 mg Pt cm-2) on the fuel cell performance, at 160 oC using air as an oxidant, is shown in Fig. 5. The anodes used 40 % Pt/C with a loading of 1 mg Pt cm-2 and 20 mg cm-2 phosphoric acid as an electrolyte. PTFE binder was kept constant for all the electrodes. Open circuit potentials were more than 0.9 V and increased slightly with the increase in catalyst loading.
Fig. 5.Â Fuel cell performance of ABPBI based MEAs with GDEs of the same composition (40 % Pt/C, 40 % PTFE, 20Â mgÂ H3PO4Â cm-2 per electrode) but different cathode catalyst loadings.
3.4 Electrode performance on the Heat treatment of GDE
Hydrophobicity of the catalyst layer (PTFE content), membrane acid content and cathode catalyst content played major roles in determining the fuel cell performance. These factors dictate the acid (electrolyte) volume fraction in the catalyst layer and therefore the three phase boundaries. The GDE was dried in air in two steps at 120 oC for l h and at 230 oC for 30 min, and then finally heat treated at 350 oC for 15 min under an inert atmosphere. 40 % Pt/C (anode: 1mg, cathode: 1mg) heat treated GDE and GDE boiled in conc H3PO4 at 180 oC for 12 hrs.The cell performance measured at 160 o C, before and after the heat treatment of the GDE with the same catalyst loading, and PTFE contents is shown in Fig. 6.
Heat treatment produced a dramatic reduction in the cathode performance, where the heat-treated cathode exhibited a high degree of Hydrophobicity repelling any mobile acid coming from the membrane. Therefore, the cathode acid (electrolyte) content was low, resulting in a small active three-phase zone close to the membrane boundary, whereas the remainder of the cathode layer remained relatively inactive. This is clearly seen in the large fall in potential at low current densities, due to kinetic losses and low active electrochemical surface area. A similar behavior was obtained using a 60 % Pt/C catalyst and thus a thinner catalyst layer.
Fig. 6.Â Fuel cell performance of ABPBI based MEAs with GDEs of the same composition (40 % Pt/C, 40 % PTFE, 20Â mgÂ H3PO4Â cm-2 per electrode) with and without heat treatment.
3.5 Influence of the Platinum percentage on carbon in the cathode catalysts on fuel cell performance
The Pt to carbon weight ratio and the catalyst layer thickness of the electrode determines the fuel -cell performance. At 160 oC, with air operation the 60 % Pt/C catalyst gave the best fuel cell performance and the MEA with 20 % Pt/C shows the lowest fuel cell performance (Fig. 7). 20 % Pt/C had smaller XRD crystalline sizes and larger surface areas than the 40 % Pt/C and 60 % Pt/C catalysts (Table 1.). At a loading higher than 40 % on Vulcan XC-72, the XRD particle size of the 60 % Pt/C catalysts increases but that of the 40 % Pt/C catalyst increases very slightly with the loading. Thus, although the surface area of the 60 % Pt/C catalyst was lower than that of the 40 % catalyst, the former probably benefited from better catalyst area utilization, better mass transport and higher conductivity as the cathode was thinner. The MEA with 20 % Pt/C shows lowest fuel cell performance. 60 % Pt/C catalysts are good cathode catalysts for ABPBI based HT-PEMFCs. This could be due to reduction in the catalytic layer thickness, facilitating the access of reactant gases towards the catalyst active sites . Mostly, catalysts layer thickness is assumed proportional to carbon loading. The carbon loading for each electrode was 4, 1.5 and 0.66 mg/cm2 for 20, 40 and 60 % Pt/C respectively. The BET surface areas and pore volumes of the 20 %, 40 % and 60 % Pt/C catalysts are summarized in Table 1. As expected, the BET surface area and the pore volume values for the catalyst with 60 % metal are lower than those for the catalyst containing 20 % metal and those for the support. These differences are due to the incorporation of the Pt particles into the carbon support and mainly block the micropores of the support. Catalyt with high Pt content gives the better fuel cell performance. At 1 mg Pt/cm-2 catalyst loading 140, 160 and 180 oC, with air operation the 60 % Pt/C catalyst gave the best fuel cell performance and the 20 % Pt/C exhibits the lowest fuel cell performance.
Fig. 7.Â Fuel cell performance of ABPBI based MEAs with GDEs of the same PTFE content but different cathode catalyst (a) 60 % Pt/C (b) 40 % Pt/C and (c) 20 % Pt/C catalysts ( Pt loading : 1.0 mg/cm-2).
3.6. Effect of electro-catalyst support on fuel cell performance
Replacement of conventional electro-catalysts supports (Vulcan XC-72 supported Pt catalysts) by high surface area carbon supported Pt catalysts on fuel cell performance was investigated. High surface area carbon black (HSAC) supported catalysts shows smaller particle size which increases the active Pt surface area. The influence on the carbon support with the different surface area on the fuel cell performance at 160 oC was examined. Two kinds of carbon were used as cathode electro-catalysts support, vulcan carbon (Surface Area (SA) = c.a. 250 m2 g-1), and high surface area advanced carbon support (HiSPECÂ® 9100) (SA= c.a. 800 m2 g-1). The weight ratio of platinum to the catalyst was around same 60 % in both catalysts. The fuel cell performance influences on the morphology, hydrophobic properties, density and surface area of the carbon support , Safety data sheet of Vulcan XC-72 R, Cabot Corporation, France (2007) www.cabot-corp.com..
The BET surface areas and pore volumes of the 60 % Pt/C and 60 % Pt/HSAC catalysts are summarized in Table 1. As expected, the BET surface area and the pore volume values for the catalyst with 60 % Pt on High surface area carbon are higher than those for the catalyst containing 60 % Pt on Vulcan carbon support. The comparison of fuel cell performances of 60 % Pt/C and 60 % Pt/HSAC (HiSPECÂ® 9100) catalyst are shown in Fig. 8. HSAC catalyst support outperforms at lower current density than that of the Vulcan carbon support. The overall fuel cell performance of 60 % Pt/HSAC is less than 60 % Pt/C, even though Pt particle size is smaller in 60 % Pt/HSAC catalyst compared with commercial 60 % Pt/C catalysts. High surface area carbon support used in the present study failed to improve the fuel cell performance. It is reported and well known that the performance of carbon support with high surface area is not good in mass transfer limitation region . High surface area advanced carbon has lot of micropores than mesopores. The lower fuel cell performance of High surface area advanced carbon support. The lower fuel cell performance of High surface area advanced carbon support is due to Pt particles mainly distributed in the micropores has limited acess to the reactants.
The other possible explanation is given by mamlouk et al  is lower interaction with the phosphoric acid electrolyte (more hydrophobic than Vulcan) leading to a lower acid content in the catalyst layer and therefore, lower conductivity and accessible electrochemical surface area. Another explanation is that High surface area advanced carbon support had a lower density than that of Vulcan carbon leading to thicker catalyst layer thickness (higher IR losses) and, since the acid electrolyte can diffuse from the membrane to a limited catalyst layer thickness, a large portion of the catalyst surface would become inaccessible, resulting in slower kinetics. It is also reasonable to consider the combination of both factors. Similar behavior has been observed for 60 % Pt/HSAC shows lesser performance than 60 % Pt/C for PBI based high- temperature PEM fuel cell has been reported .
Fig. 8.Â Fuel cell performance of ABPBI based MEAs with different carbon support (a) 60 % Pt/Vulcan and (b) 60 % Pt/HSAC catalysts (Pt loading: 1.0 mg/cm-2).
3.7 Effect of CO tolerance on anode performance of ABPBI based MEAs at various temperatures
In real fuel cell conditions, the fuel -cell system will often run on a hydrogen- rich gas made from reformed fuels. The reformate gas will contain some reasonable percentage of CO. Carbon monoxide would slow the kinetics of hydrogen oxidation because of its adsorption (poisoning) on platinum active catalytic sites, the performance of the fuel cell is reduced. As the temperature rises, Pt tolerance to CO poisoning increases. HT-PEM fuel cells are highly tolerant of CO. HT-PEM fuel cells shows 100 times CO tolerance in comparison to low temperature PEM fuel cells [22-25]. We investigated the influence of CO on performance using different CO concentrations at 160 oC. The anode exhibited a high tolerance to impurities in the gas feed at temperature 160 oC. An advantage of fuel cell operating at elevated temperatures is improved platinum CO tolerance. CO tolerance increases with temperature. At 180 oC CO tolerance is higher than 120 oC.
There is no much significant loss in fuel cell performance when a mixture of H2 and CO (0.05 %) were used as a fuel feed at 160 Â°C. The cell showed capable tolerance to carbon monoxide. Above 1 % vol CO a dramatic fall in fuel cell performance was observed due to poisoning: the anode performance was limited and current densities above 700Â mAÂ cmâˆ’2 were not achievable. However similar less performance loss has been reported at all current densities even with 1.0 % CO at 150 â-¦C for the PBI and ABPBI membrane based fuel cells [26, 27]. In agreement with other authors, the CO tolerance can be further improved by humidification in CO and H2 mixture. It was reported the Presence of water vapour in the H2 + CO mixture decreases the CO partial pressure and favours OH adsorption on Pt sites. Enhanced CO tolerance of Pt catalysts by gas humidification suggests that the removal of adsorbed CO by well-known bifunctional mechanism [25, 28].
Fig. 9. Fuel cell performance of ABPBI based MEAs at 160 oC, using different CO concentrations with Hydrogen (Pt loading: 1.0 mg/cm-2).
3.7 Effect of Pt alloys catalyst on cathode performance ABPBI -MEAs
Platinum alloyed with other transition metals are generally found to have enhanced ORR activity over pure Pt/C . Mukerjee et al. investigated various Pt bimetallic alloys supported on carbon and found a two-three fold increase in the ORR activity for the alloy catalysts . The formation of Pt-OH intermediates, which are considered to block the active Pt site for ORR, is generally found to be delayed in Pt alloys, thus enhancing the ORR activity under PEMFC operating conditions . Pt alloys cathode catalysts were studied for this work are 20 % Pt/C. 20 % Pt-Fe/C, 20 % Pt-Ni/C, and 20 % Pt-Cu/C (E-TEK, USA). All the studied alloys catalysts had atomic ratios of (1:1 a/o). The platinum loading for the alloy catalyst was kept as 0.4Â mgÂ PtÂ cmâˆ’2 and maintain a desired catalyst layer thickness to minimize mass transport effects on the fuel cell polarisation. Fig. 10. shows the Pt alloys cathode catalyst at all temperatures 140, 160 and 180 oC exhibited good electrochemical performance than Pt/C catalysts. At low temperature 140 Â°C the fuel cell performance of Pt-Fe catalysts performs better at higher current density than Pt-Ni/C catalysts and at low current density 0.4 A/cm2 both Pt-Fe and Pt-Ni gives same performance. Interestingly, Pt-Cu/C shows the superior performance than all Pt-alloy catalysts at all temperature s. At low temperature 140 Â°C, Pt and Pt-Fe/C shows similar performance at higher current density, however at higher temperature (180 Â°C) Pt-Fe/C performs better than Pt-Ni/C. The observed increased fuel cell performance of Pt-Fe/C catalyst was not due to the catalyst layer structure, as both electrodes had similar Pt: C ratios (20Â wt %) and loading of 0.4Â mgÂ PtÂ cmâˆ’2. The data confirm the reported advantage of Pt alloying (with iron) increases ORR kinetics at higher temperature . The fuel cell performance of the catalysts decreased in the order of Pt-Cu/C > Pt-Fe/C > Pt-Cu/C > Pt/C. Pt-Cu/C alloy turned out to be best catalysts shows 4 times higher activity compared to pure Pt towards oxygen reduction [33,34].Under identical conditions, at 160 Â°C Pt-Cu/C exhibited a good electro catalytic performance than the Pt/C, with a current density (1420 mA cm-2 mg-1Pt) 1.42 times higher than that of the Pt/C (1000 mA cm-2 mg-1 Pt) which proved that the Pt-Cu/C catalyst as potential catalysts than other Pt alloys in a practical environment .
Fig. 10. Fuel cell performance of ABPBI based MEAs using different Pt alloy cathode catalysts
at (a) 140 (b) 160 and (c) 180 oC (Pt loading: 0.4 mg cm-2).
The phosphoric acid doped ABPBI membrane fuel cell performance has significant effects of various electrode parameters such as cathode catalysts loading, heat treatment effects, PTFE content and electro-catalyst support. The electrodes with different platinum percentages on the carbon support (20, 40 and 60 %) with constant Pt loading of 1 mg cm-2 were physiochemically characterized and their fuel cell performances was evaluated. Among the Pt alloy catalysts studied, Pt-Cu/C catalysts shows superior performance with same metal loading. The results of ABPBI based MEAs with optimized electrode parameters show higher fuel cell performance and CO tolerance has a great potential for commercialization, especially for stationary CHP applications. Therefore, further extensive research on this material and electrode is required, with particular emphasis on stability over a long lifetime.
The author T.M would also like to thank Prof V. Linkov for the post-doctoral fellowship. The work was performed in research facilities provided through a South African Department of Science and Technology's (DST) National Hydrogen and Fuel Cell Technologies Research, Development and Innovation Strategy