Effect of Catalyst Aggregate Size on the Mass Transport Properties of Non-Noble Metal Catalyst Layers in PEMFC Cathodes

Abstract

Non-noble metal catalysts (NNMCs) are regarded as ultimate promising alternative to the high-cost Pt-based catalysts required to catalyze the oxygen reduction reaction (ORR) in the cathode of proton exchange membrane fuel cells (PEMFCs). Indeed, novel syntheses established in recent years have led to NNMCs with initial fuel cell performances comparable to those of Pt-based materials [1]. However, due to their intrinsically lower ORR-activity compared to Pt, NNMC cathodes catalyst layers (CLs) require much higher loadings than those of Pt-based (≈ 4 mgNNMC·cm−2 vs. ≤ 0.3 mgPt·cm−2, respectively) to achieve similar performances. This makes NNMC CLs much thicker (up to ≈ 100 μm) than conventional Pt-based ones, which results in additional mass transport issues [2] that prevent them from reaching the high current densities at high potentials required for automotive applications.In this context, our group recently proposed a novel synthesis [3] 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 due to the NNMC’s large particle size. In this study, we have efficiently reduced the aggregate size of this NNMC type from the > 2 μm in Ref. [3] to ≈ 100 nm by substituting the dry ball-milling (BM) originally used to mix the catalyst precursors with wet BM (i.e., in the presence of a solvent). The electrochemical behavior of both kinds of NNMCs was studied through rotating ring disk electrode (RRDE) voltammetry measurements, which revealed significant enhancement in the ORR limiting current of the NNMC with a smaller particle size. Moreover, 3 times higher H2O2 yield (≈ 5% to ≈ 15%) was observed for both catalyst types when lowering their RRDE-loading from 0.5 to 0.1 mgNNMC·cm-2 while keeping the homogenous film quality for both catalyst at low loadings. Subsequently, PEMFC tests were conducted for both kinds of NNMCs with different loadings (1 or 4 mgNNMC·cm-2) using both O2 and air as the cathode reactant feeds. The mass transport overpotential (ηtx) was then assessed by subtracting the polarization curves recorded in O2 vs. air (and correcting for the shift in reversible potential caused by the concomitant change in the O2 partial pressure) [4]. As displayed in Figure 1, increasing the loading of the dry-BM sample resulted in a larger mass transport over-potential, while in the case of wet-BM sample ηtx barely changed as a consequence of the larger loading. Overall, this reduction of the catalyst’s aggregate size led to a ≈ 75 mV improvement in ηtx at low loading, which became more than 2 times larger (≈ 175 mV) in the case of the CLs with a high loading, thus confirming the enhanced mass transport properties of wet-BM NNM-catalyst layer. Acknowledgments: The authors acknowledge the financial assistance of the Swiss National Science Foundation through Sinergia grant CRSII5_18033.