Enhanced Activity of Extended Surface Platinum Nickel Nanowires for the Oxygen Reduction Reaction

Abstract

Increasing energy demands combined with dwindling fossil fuel resources require renewable energy solutions. Polymer electrolyte fuel cells (PEFCs) are a promising option but the slow kinetics of the oxygen reduction reaction (ORR) inhibit widespread implementation. Additionally, state-of-the-art platinum on high surface area carbon (Pt/HSC) is expensive. To address these concerns, various approaches have been explored, including creating highly faceted surfaces, alloying Pt with a non-noble metal, or creating core-shell structures, but achieving simultaneously high specific activity, mass activity, and surface area remains a critical challenge. Extended surface catalysts offer improved activity and durability compared to Pt/HSC,1 and extended surface platinum nickel (PtNi) nanowires (NWs) improve surface area limitations typically faced by this type of catalyst. These NWs, synthesized via spontaneous galvanic displacement (SGD), demonstrated promising surface areas, initial ORR activities over 10 times greater than Pt/HSC, and considerably improved durability in preliminary rotating disc electrode (RDE) testing,2 but viability for scale-up in membrane electrode assemblies (MEAs) was hindered due to low batch sizes and limited reproducibility from batch to batch observed in SGD-derived materials.3,4 A new generation of PtNi NW-based catalysts was synthesized using atomic layer deposition (ALD) to scale production while preserving morphological control over NW synthesis. An advanced suite of characterization techniques was used to gain a comprehensive understanding of structure-property-performance relationships. The catalyst was first investigated using a combination of x-ray absorption near-edge structure (XANES) spectroscopy, extended x-ray absorption fine structure (EXAFS) spectroscopy, x-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) coupled with energy dispersive x-ray spectroscopy (EDS) hypermapping in order to obtain detailed complementary information about distribution and speciation of platinum and nickel, discerning differences between surface and bulk. RDE testing was conducted to assess activity initially and post-durability cycling. Differences between ALD- and SGD-synthesized samples were discovered, including varying effects of similar post-processing steps. The robustness of ALD-derived NWs allowed introduction of an additional post-processing step to improve performance that was not possible with SGD. Catalysts were also studied after integration into MEAs and performance was correlated to information about electrode structure. STEM/EDS was used to characterize interfaces between catalyst, carbon, and ionomer, while transmission x-ray microscopy (TXM) was used to track catalyst distribution across the electrode. These results offer a promising approach to harness the robustness of ALD-produced wires and create an active, durable, and scalable ORR catalyst.