Catalyst Activity Targets for Performance Parity in Hydroxide Exchange Membrane Fuel Cells
Authors: Setzler Brian P. , Wittkopf Jarrid A. , Zhuang Zhongbin , Yan Yushan 
Published: February 2020

HEMFCs offer several potential cost advantages over proton exchange membrane fuel cells (PEMFCs), most notably improved kinetics and stability for platinum group metal (PGM)-free catalysts. However, these materials cost advantages can be offset by the lower power density of HEMFCs. To support the ultimate goal of performance parity between HEMFCs and PEMFCs, we have developed catalyst activity targets based on a model of HEMFC performance and compared existing literature against these targets1.

Despite the similarities with PEMFCs, it is important to recognize the unique features and challenges of HEMFCs and set priorities accordingly. In PEMFCs, the hydrogen oxidation reaction (HOR) is so fast that near zero anode overpotential can be achieved with ultra-low Pt loadings. In base, HOR is 200 times slower than in acid, yet few studies have explored PGM-free HOR catalysts. Additionally, PGM-free catalysts are benchmarked against Pt/C without considering whether Pt/C is an appropriate target. In fact, the increased anode and membrane overpotentials in HEMFCs require a substantial improvement in cathode performance to reach the same overall cell performance and therefore a higher target than Pt/C. These differences highlight the importance of setting model-based activity targets to support an overall performance goal rather than using ad-hoc targets based on PEMFC catalysts.

Using a standard porous electrode model, required catalyst activity was determined as a function of desired performance, transport properties, and electrode thickness. To represent PEMFC performance parity, a cell potential of 0.636 V at 1.5 A/cm2 was targeted. This point was taken from the reference polarization curve provided by Kongkanand and Mathias for a state-of-the-art PEMFC with a 0.1 mgPt/cm2 PtCo/C cathode2. The required HOR and ORR activities are shown in Figure 1 as a function of electrode thickness. Even for a costless catalyst, loading is constrained by the opposing processes of ionic conduction and reactant mass transport as illustrated in Figure 1. For any combination of transport properties and current density, an optimal electrode thickness exists. In the case simulated here, the activity targets estimated by assuming an electrode thickness of 100 µm and ignoring transport losses are an order of magnitude lower than those determined by the model for optimal electrode thickness.

When extrapolated to room temperature RDE tests, the activity targets are 50 A/cm3 for HOR exchange current and 370 A/cm3 for ORR at 0.9 VRHE. The volumetric activity of a representative sample of PGM-free catalysts from the literature will be compared against these targets, and recommendations for closing the one to two order of magnitude gap will be presented. While catalyst activity alone cannot guarantee a high-performance fuel cell, it does set the upper bound, and it is essential to set goals for the long-term competitiveness of HEMFC technology.

References

1. B. P. Setzler, Z. Zhuang, J. A. Wittkopf, and Y. Yan, Nat. Nanotechnol., 11, 1020–1025 (2016).

2. A. Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7, 1127–1137 (2016).

Figure Caption:

Figure 1: Influence of transport losses and electrode thickness on the catalyst volumetric activity required to match PEMFC performance. a) Required HOR volumetric exchange current density at 80 °C to achieve anode half-cell potential of 0.080 VRHE at 1.5 A/cm2. b) Required ORR volumetric activity at 80 °C and 0.9 VRHE to achieve cathode half-cell potential of 0.806 VRHE at 1.5 A/cm2. Conditions: 80 °C, 100% relative humidity, 150 kPaabs pressure, H2 and CO2-free air at differential flow conditions. Parameters: ionomer effective conductivity: 2 S/m, HOR transfer coefficient: 0.5 (anodic and cathodic), ORR transfer coefficient: 0.75, O2 reaction order: 0.79, H2 reaction order: 0.5, O2 effective diffusivity: 7.34 × 10-7 m2/s, O2 GDL transport resistance: 24.3 s/m. H2:O2 diffusivity ratio: 4.

Figure 1