Short communicationPerformance losses at H2/O2 alkaline membrane fuel cell
Introduction
Much research has been focused on alkaline membrane fuel cell (AMFC) which allows the use of non-precious metal catalyst for oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). Presently, the performance of H2–O2 AMFC employing commercial anion exchange membrane (AEM) and anion exchange ionomer (AEI) is low in relation to proton exchange membrane fuel cell (PEMFC) even with benchmark catalyst like Pt/C [1], [2], [3]. Major reasons for low performance suggested in the literature are insufficient water supply at cathode even with humidified Air/O2 at practical stoichiometries [4], large anodic over potential due to water flooding at anode [5], and low ionic conductivity of membrane and ionomer [6]. However, systematic quantification of all these losses at the fuel cell level is scarce in literature. In this work, AMFC performance is systematically evaluated at various catalyst loadings to measure anodic and cathodic overpotential. Furthermore, performance losses due to the ionic resistance of membrane and ionomer are quantified by determining ionic resistance of catalyst layer (CL) and membrane.
Section snippets
Experimental
Two types of fuel cell membrane electrode assemblies (MEAs) were prepared, Type-1 with various low anode and cathode catalyst loadings and Type-2 with high anode and cathode catalyst loadings. Type-1 MEAs were prepared by spray coating the required amount of catalyst ink on A201 membrane (Tokuyama Corp Japan) or Nafion-115 membrane (QuinTech Germany) maintained at 65 °C. Typical catalyst ink was prepared by sonicating the mixture comprised of 50 mg Pt/C (60 wt.% Pt, Alfa Aesar, United Kingdom), 250
Performance evaluation
Performance evaluations of AMFC were carried out with Type-1 MEAs with low loadings between 0.07 and 0.16 mgPt·cm− 2. Fig. 1(a) shows the I–V curve for two MEAs with varying catalyst loading at anode. The obtained power density of 90 mW·cm− 2 at 0.6 V is reasonable compared to literature values [1], [2], [3] considering the low loading of catalyst used here. Performance is mostly not sensitive to change in anode loading. This indicates that mass transport of H2 is facile and kinetic limitation from
Discussion
The performance of AMFC is significantly lower below 0.85 V in relation to PEMFC even after correcting for ionic resistance of membrane and CL (Fig. 3(b)) and the mass transfer effect in AMFC starts to become dominant only below 0.5 V as shown above in Fig. 2(a). Hence, a major performance loss between 0.85 V and 0.5 V is attributed to low water concentration at the catalyst surface. In PEMFC, the ORR is dependent on O2 and H+ concentration where H+ ions are abundantly available via proton exchange
Conclusions
Measurement of performance curves with various anode and cathode loading shows that the HOR kinetics in AMFC is fast and can be neglected for potential losses at least at high anode flow rates. Significant improvement in AMFC performance can be obtained by increasing the H2O concentration in oxidant for example by operating at higher temperatures. Analysis of the AMFC performance curves corrected for catalyst layer and membrane resistance show that in relation to PEMFC, at a given current the
Conflict of interest
No conflict of interest.
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