Water Management in PEM Fuel Cells with Non-Precious Metal Catalyst Electrodes

The cost of polymer electrolyte membrane (PEM) fuel cells is a key challenge to developing commercially viable systems for transportation and stationary power applications. Non-precious group metal (non-PGM) catalysts offer a promising solution to lowering the system cost by eliminating the use of costly Pt-based catalysts. Recent studies have reported non-PGM catalysts with promising performance in fuel cells with Nafion^® membranes and Nafion^® as ionomer in electrodes [1]. Optimizing the electrode structure to improve transport and mitigate flooding is an ongoing task.

The three-dimensional structure of the membrane-electrode assemblies (MEAs) was imaged using X-ray computed tomography (XCT), Figure 1. MEA 1 had conventional Pt/C cathode, while MEAs 2 and 3 had PANI-derived cathodes (with Fe salt also involved in the synthesis [1]). Cathode thickness was ~60 mm for all MEAs shown in Figure 1. All anodes were Pt/C based (bright regions in Figure 1 correspond to Pt in the electrodes). MEAs 1 and 2 had carbon-cloth gas diffusion layers (GDL), with microporous layer (MPL). MEA 3 had carbon-fiber-mat based GDLs with MPLs.

Water distribution was recorded in situ, i.e. in H₂/air operating fuel cells. We employed high-resolution neutron imaging at NIST [2], and small single-serpentine 2.5 cm² cells [3], designed for the imaging of water profiles across the cell thickness (Figure 2). Cathode catalyst layer of non-PGM MEA2 had significantly higher (roughly double) water content compared to the Pt/C cathode in MEA 1. Pt and non-PGM electrodes were deliberately made with the similar thickness for straightforward comparison of the water content. The severe flooding in non-PGM cathode was correlated with the lower performance and higher mass-transport resistance measured from electrochemical impedance spectroscopy (EIS). Thus water management is critical to performance in fuel cells with non-PGM electrodes.

Figure 1 also depicts an improvement in the fabrication of non-PGM MEAs, going from gas-diffusion electrodes (GDEs), MEAs 1 and 2, to the more recent MEAs with catalyst-coated membrane (CCM), MEA 3. The latter enables the use of modern GDL materials, with the goal to improve water management in non-PGM MEAs. To alleviate the cathode flooding levels, we employed novel GDL materials, known to help reduce water content in the cathode [4, 5] or to enhance water removal from cathode through the anode side [6]. Finally, we compare water management and fuel cell performance with varying non-PGM cathode thickness.

The authors acknowledge funding from LANL Laboratory Directed Research and Development (LDRD) program, and US DOE EERE FCTO program. NIST authors were supported by the U.S. Department of Commerce, the NIST Radiation and Physics Division, the Director's office of NIST, the NIST Center for Neutron Research, and the Department of Energy interagency agreement No. DE_AI01-01EE50660.

References:

1. G. Wu et al., Science 332 (6028), 443 (2011).
2. D. Hussey et al., J. Appl. Phys. 112 (10), 104906 (2012).
3. J. D. Fairweather et al., J. Electrochem. Soc. 160 (9) F980 (2013)
4. R.  Mukundan et al., ECS Trans. 33 (1), 1109 (2010)
5. R. Schweiss et al., J. Power Sources 220, 79 (2012)
6. A. J. Steinbach et al., ECS Trans. 33 (1) 1179 (2010)