Abstract
PtNi alloy and hybrid structures have shown impressive catalytic activities toward the cathodic oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, such promise does not often translate into improved electrode performances in PEMFC devices. In this contribution, a Ni impregnation and subsequent annealing method, translatable to vertically aligned nanowire gas diffusion electrodes (GDEs), is shown in thin-film rotating disk electrode measurements (TFRDE) to enhance the ORR mass activity of Pt nanowires (NWs) supported on carbon (Pt NWs/C) by around 1.78 times. Physical characterisation results indicate that this improvement can be attributed to a combination of Ni alloying of the nanowires with retention of the morphology, while demonstrating that Ni can also help improve the thermal stability of Pt NWs. These catalysts are then tested in single PEMFCs. Lower power performances are achieved for PtNi NWs/C than Pt NWs/C. A further investigation confirms the different surface behaviour between Pt NWs and PtNi NWs when in contact with electrolyte ionomer in the electrodes in PEMFC operation. Indications are that this interaction exacerbates reactant mass transport limitations not seen with TFRDE measurements.
Highlights
In order for proton exchange membrane fuel cells (PEMFC) to become viable for full scale commercialisation, the catalyst activities and utilisation ratio of the precious metal catalysts, i.e., Pt/C in catalyst electrodes needs to increase to improve the fuel cell power performance and reduce the system cost
Based on the similarities of these agglomerates with previous investigations of Pt NWs grown using formic acid, it seems that the size and shape of the Pt NW agglomerates depends primarily on the underlying seed formed on the support [21,26], and the nanowire growth rate [27]
Of more importance is the fact that all samples are derived from the exact same initial batch of Pt NWs/C, thereby all differences found from the thin-film rotating disk electrode measurements (TFRDE) and Membrane Electrode Assemble (MEA) tests are solely due to the inclusion of Ni and annealing temperature used, and not differences in initial Pt NW structure and distribution
Summary
In order for proton exchange membrane fuel cells (PEMFC) to become viable for full scale commercialisation, the catalyst activities and utilisation ratio of the precious metal catalysts, i.e., Pt/C in catalyst electrodes needs to increase to improve the fuel cell power performance and reduce the system cost. The as-synthesised PtNi NWs showed 3-fold improvements in mass activity for the ORR over Pt NWs, aided by the retention of high electrochemically active surface areas (ECSA) Post treatment, such as thermal annealing [15] and acid treatment [16], have been investigated to tailor these materials for practical use in PEMFCs. a large disparity is still observed between the intrinsic catalytic activity of the PtNi NWs and their performance in electrodes in fuel cells [17]. The relation between the intrinsic activities of the 1D NWs determined by thin-film rotating disk electrode (TFRDE) measurement and their power performance in operating PEMFCs is explored
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