Nickel-Based Anode Catalysts for Efficient and Affordable Anion-Exchange Membrane Fuel Cells

ConspectusLow-temperature ion-exchange membrane hydrogen fuel cells, as zero-emission power sources, can largely preserve the merits of gasoline engines, including rapid fueling, extended cruising range, and low maintenance cost. To enable the widespread prevalence of fuel-cell automobiles, the U.S. Department of Energy (DOE) has set a long-term fuel-cell system cost target of US$30 kW^-1. Over past decades, proton-exchange membrane fuel cell (PEMFC) technology has developed rapidly, resulting in the first commercial sales of fuel-cell-powered vehicles. Although there has been great success, the mass market penetration of PEMFCs is currently hindered by the excessive reliance on expensive platinum group metal (PGM) catalysts. Anion-exchange membrane fuel cells (AEMFCs), because of the alkaline environment that permits the use of PGM-free catalysts, have become an alternative technology with inherent long-term cost advantages. Thus far, significant progress has been made in the exploration of PGM-free catalysts for the oxygen reduction reaction at the AEMFC cathode, some of which have shown intrinsic catalytic properties comparable to PGM catalysts. However, the development of PGM-free catalysts for the anodic hydrogen oxidation reaction (HOR) has lagged behind, presumably owing to its sluggish kinetics in alkali. In alkaline media, the HOR kinetics is about 2 orders of magnitude slower than that in acid, which demands higher PGM loadings to reach similar fuel-cell performance in PEMFCs. Since Raney nickel (Ni) was explored for alkaline HOR catalysis in 1960s, research on Ni-based HOR catalysts has begun and now is flourishing, primarily thanks to their favorable adsorption energies of key HOR intermediates (e.g., Ni-H_ad and Ni-OH_ad). At present, a number of strategies have been developed to improve HOR performances of Ni-based materials, such as alloying, Ni nitridation, and alloy amorphization, which yield cost-effective HOR catalysts that rival or even exceed the activity and stability of PGM counterparts.In this Account, we describe our recent research endeavors toward the development of efficient Ni-based HOR catalysts for practical AEMFC anodes. First, we briefly highlight the important merits of AEMFC technology and why Ni-based materials are appealing for alkaline HOR catalysis. Critical innovations in the design of Ni-based nanostructured and bulky catalysts were then discussed, showing their great promise to catalyze alkaline HOR that traditionally relied on PGMs. To demonstrate utility, performances of the elaborately designed Ni-based catalysts under realistic fuel-cell conditions were examined, along with an initial effort to develop a CO-tolerant AEMFC anode. We conclude by outlining future research directions that allow access to next-generation PGM-free HOR catalysts for advanced AEMFCs.