Agglomerate Size Effect on the PEMFC Performance of a Non-Noble Metal Oxygen Reduction Catalyst

Abstract

Proton exchange membrane fuel cells (PEMFCs) are expected to be a key contributor to environmentally friendly transportation, but the high cathode Pt-loadings required to catalyze the oxygen reduction reaction (ORR) inhibit their widespread commercialization. In order to address this issue, tremendous efforts are being devoted to develop inexpensive, non-noble metal catalysts (NNMCs), and novel syntheses established in recent years have led to NNMCs with initial ORR-activities comparable to those of Pt-based materials [1]. However, NNMC cathode loadings need to be much higher (≈ 4 mgNNMC·cm−2) than those of Pt-based catalyst layers (CLs, with loadings ≤ 0.3 mgPt·cm−2) to compensate for their intrinsically lower ORR-activity compared to Pt. Therefore, the resulting, thick NNMC CLs (up to ≈ 100 μm) suffer from mass transport issues [2] which prevent them from reaching the high current densities at high potentials required for automotive applications. Moreover, NNMC CLs display a remarkably poor durability for which the reasons remain under debate [3].In this context, our group recently proposed a novel synthesis [4] that yields NNMCs with a high initial ORR-activity in PEMFC tests (≈ 10 A·g−1 at 0.8 V in H2:O2 at 80 °C, 1.5 barabs, and 100% relative humidity) but poor mass transport properties. The latter was caused by the NNMC’s large particle size, which we have efficiently reduced by substituting the dry ball-milling (BM) originally used to mix the catalyst precursors [4] with wet BM (i.e., in the presence of a solvent). This new approach resulted in a reduction of the NNMC aggregate size from > 2 μm to ≈ 100 nm (see Fig. 1). The electrochemical behavior of both kinds of NNMCs was subsequently studied through rotating ring disk electrode (RRDE) voltammetry measurements at various catalysts loadings and temperatures (50 – 500 μgNNMC·cm−2 and 20 – 50 °C, respectively), which revealed the negligible effect of the former parameter on the catalysts’ ORR-activity and H2O2-yields, along with a significant enhancement in the limiting current of the NNMC with a smaller particle size. Subsequently, PEMFC tests at temperatures between 40 and 80 °C were conducted for both kinds of NNMCs with different ionomer-to-catalyst ratios, including electrochemical impedance spectroscopy measurements from which we estimated the CLs’ proton-transfer resistance [5]. These in terms allowed us to decouple the different overpotential contributions to the overall PEMFC performance, which confirmed the enhanced mass transport properties of the wet-milled NNMC, and allowed us to compare the kinetic parameters of both materials in RRDE vs. fuel cell configurations. Acknowledgments: The authors acknowledge the financial assistance of the Swiss National Science Foundation through Sinergia grant CRSII5_18033.