Optimisation and effect of ionomer loading on porous Fe-N-C-based proton exchange membrane fuel cells probed by emerging electrochemical methods

The next generation of proton exchange membrane fuel cells (PEMFCs) require a substantial reduction or elimination of Pt-based electrocatalyst from the cathode, where O-2 reduction takes place. The most promising alternative to Pt is atomic Fe embedded in N-doped C (Fe-N-C). Successful incorporation of Fe-N-C in PEMFCs relies on a thorough understanding of the catalyst layer properties, both ex situ and in situ, with tailored electrode interface engineering. To help resolve this conundrum, we provide a quantitative protocol on the optimisation of I/C for Fe-N-Cs. It is demonstrated that a high pore volume (3.33 cm(3) g(FeNC)(-1)) Fe-N-C catalyst requires a sufficiently high ionomer to catalyst mass ratio (I/C, 2.8 <= I/C <= 4.2) for optimum PEMFC activity under H-2/O-2. Emerging electrochemical techniques (distribution of relaxation times and Fourier transformed alternating current voltammetry) were used to deconvolute for the first time the trade-off between proton and electron resistance and accessible FeNx x active site density with increasing ionomer loading. These findings highlight the significant impact of tuning the I/C ratio based on the catalyst layer properties and feature the power of evolving electrochemical tools for optimising performance in PEMFCs and other electrochemical devices.