Elucidating the Role of Ionomer in the Performance of Platinum Group Metal-free Catalyst Layer via in situ Electrochemical Diagnostics

Abstract

The ionomer content in platinum group metal (PGM)-free polymer electrolyte fuel cell (PEFC) cathode catalyst layer (CCL) plays an important role in the electrode gas transport properties, proton conductivity, and hence, membrane electrode assembly (MEA) performance. In this work, the ionomer content in the CCL is varied, influencing electrode microstructure by altering porosity, tortuosity, as well as ionomer distribution and coverage of the catalyst particles. A novel technique consisting of a H2 pump, combined with a Pt black sensor layer, is used to measure the bulk mass transport resistance of a series of PGM-free CCL prepared with different ionomer contents. The values for bulk electrode mass transport resistance are contrasted with electrode proton transport resistance in the cathode catalyst layer, establishing a clearly defined trade-off between two key performance limiting phenomena and identifying a need for novel PGM-free electrode fabrication strategies.

Cathode Design for Proton Exchange Membrane Fuel Cells in Automotive Applications

“…PGM-free catalysts, including transition metal-based and non-metallic catalysts, are potential candidates to further reduce the cost of PEMFCs in the future. The most promising PGM-free catalysts toward ORR for PEMFC applications are carbonaceous materials doped with nitrogen and one (or more) active 3D-period transition metals (e.g., Fe, Co, Mn, Ni) [48]. The breakthroughs of Fe-N-C catalysts have been very recently reviewed by Wang et al [49] with reference to DOE (Department of Energy) standards and targets.…”

The influence of the anion exchange membrane on mass-transport limiting phenomena in bipolar interface fuel cells with Fe–N/C based cathode catalyst layers

“…This is usually related to the large thickness or insufficient porosity and thus high uidic resistance of the oxygen transport in the PGM-free electrode. 17,29 For PEMFCs using the same class of Fe-N/C catalysts, it has been shown that an excessive increase in the ionomer content in the CCL can reduce the overall electrode porosity and therefore cause severe gas-phase transport limitations at the electrode due to ionomer swelling during operation. 29 Since the AEM dispersion nearly completely inltrates into the upper part of the CCL during the manufacturing process, it is reasonable to suspect changes in the electrode morphology, which could lead to severe mass transport limitations in the gas phase, which is schematically illustrated in Fig.…”

“…From literature, it is well known that gas-phase transport limitations are enhanced when air is used as an oxidant instead of pure oxygen. 2,29 For that reason, for the very same samples, the oxidant was switched to synthetic air aer ten polarizations in oxygen and additional ve polarizations were recorded and the last polarization (Fig. 8) was compared to the data collected under pure oxygen conditions.…”

“…The AEI in the CCL is well known to affect the overall electrode morphology, like the pore size distribution. 29 On the other hand, it provides the ionic conduction pathway towards the bipolar junction for generated OH − at the catalytic active sites. The main task of the AEM is to form the bipolar interface while shielding the Fe–N/C catalyst from contact with the PEM.…”

Ionomer Films Impact on The Structure, Flow Regime, and The Wettability of The Catalyst Layer of PEMFC

“…Thus, the pore size distribution of a catalyst layer may be shifted towards a lower average effective radius as the ionomer swells [15,16]. This will have an impact on reactant transport resistances [17,18,19]. Cycling the relative humidity of the catalyst layer results in the absorption/release of water to/from ionomer films, which in turn leads to structural stresses and potential defect (crack) growth [20].…”