Our presentation will address recent progress in the development, characterization, and modeling of next generation NSTF ORR electrocatalysts, and performance and durability characteristics of NSTF-based fuel cell and water electrolyzer membrane electrode assemblies (MEAs).
Recent developmental NSTF ORR electrocatalysts are based on two distinct thin film morphologies: nanoporous thin film (NPTF) and ultra-thin film (UTF) . The formation of high activity and durable NPTF and UTF ORR electrocatalysts depends strongly on alloy composition (alloying elements, alloy mole fractions) and process-induced structure control. Tailoring of these compositional and structural properties has resulted in several electrocatalysts yielding specific activities up to 7x higher than Pt nanoparticles in MEA testing (Fig. 1A).
Electrocatalyst and electrocatalyst support durability remain key barriers to wide-spread commercialization of economically-competitive PEM fuel cells and water electrolyzers. In fuel cell applications, electrocatalysts must be tolerant of many 10s of thousands of load cycles and numerous off-nominal operations including stop/starts and fuel starvation. NSTF electrocatalysts are substantially robust towards support corrosion losses. In fuel cells, NSTF catalysts have exceeded U.S. Department of Energy (DOE) Fuel Cell Support Accelerated Stress Test (AST) 2020 targets , and NSTF OER electrocatalysts have yielded stable electrolyzer performance for 5000 hours (Fig. 1B).
NSTF ORR electrocatalyst durability depends strongly upon composition and morphology. Previous generation “whiskerette” PtCoMn/NSTF electrocatalysts achieve 30% mass activity loss after the U.S. DOE Electrocatalyst AST, while nanoporous PtNi/NSTF mass activity losses exceed 60% , well above the 40% DOE target. NPTF and UTF PtNi ORR electrocatalyst durability has been improved by integration of Ir, resulting in mass activity losses approaching the DOE target and substantially stable H2/Air performance at ultra-low PGM loadings (Fig. 1C).
The ultra-thin, ionomer-free NSTF electrode minimizes reactant, ionic, and electronic transport distances and can enable substantially high power densities at low absolute electrode loadings and surface areas. An experimental UTF fuel cell MEA has demonstrated a specific power density of 8.1kW/gPGM with only 0.077 mgPGM/cm2 total MEA loading , exceeding the DOE 2020 targets of 8.0kW/gPGM and 0.125mgPGM/cm2. With water electrolysis MEAs, current densities exceeding 15A/cm2 have been demonstrated with only 0.50mgPGM/cm2 total MEA loading.
While enabling very high specific power densities, the ultra-thin traditional NSTF electrode structure also brings unique challenges. In fuel cells, traditional NSTF electrodes are susceptible to water flooding, causing larger-than-desired performance sensitivities to operating conditions. The operational robustness has been substantially addressed by both electrode-extrinsic and electrode-intrinsic approaches [7, 8]. In water electrolysis applications, H2 crossover from cathode to anode can yield higher than acceptable H2 concentrations in the O2 effluent stream. Effective H2 crossover mitigation has been developed, and H2crossover has been reduced two orders of magnitude (Fig. 1D) with little apparent impact on performance or durability.
We acknowledge 3M Company and the US Department of Energy, which provided funding for this work under grants DE-EE0007270, DE-SC0004192, DE-SC0007471, and NASA for funding under grant NNX12CE73P.
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