Invited: Non-Precious Metal Electrocatalysts: Accomplishments and Challenges

With increasing demand for energy, the development of the energy storage and conversion technologies has become the focus of an intensive research effort in recent years. Among various technologies, fuel cells, batteries, supercapacitors, and water electrolyzers have been recognized as potentially feasible and efficient devices for portable, stationary, and transportation applications. For both the fuel cells and metal-air batteries, the oxygen reduction reaction (ORR) cathode catalysts play a major role in the device performance characteristics, as determined based on the power output, open circuit voltage, charge-discharge rate, energy efficiency, cycling life (for batteries), etc. Currently, Pt-nanoparticle catalysts, supported on high surface-area carbons, represent the state-of-the-art electrocatalysts for hydrogen oxidation reaction (HOR) and ORR at the polymer electrolyte fuel cell (PEFC) anode and cathode, respectively. However, the prohibitive price and scarcity of Pt have limited its widespread implementation in the PEFC, especially at the cathode, which accounts for approximately 80% of the Pt loading in fuel cells. As a result, non-precious metal catalysts (NPMCs) for the ORR have received more attention in recent years as a possible replacement of precious-metal catalysts.

A successful ORR catalyst must combine high activity with good long-term stability – a major challenge in the strongly acidic environment of the PEFC cathode. Since the early work of Jasinski in the 1960s (Nature 201, 1212, 1964), recent breakthroughs in the synthesis of high-performance non-precious metal catalysts (NPMCs) (e.g., Lefèvre et al., Science 324, 71, 2009; Wu et al., Science 332, 443, 2011) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility, though the activity and especially durability of the resulting catalysts need to be improved before the technology can become viable.

The NPMC performance depends on the selection of precursors, the synthesis chemistry; and especially the catalyst nanostructure. In addition to those, the CN_x structures likely play a major role in the performance of ORR active site(s). Apart from possible direct participation in the active site, the transition metal is crucial to in-situ formation of carbon nanostructures (nanotubes, onion-like structures, graphene) by catalyzing the decomposition of the nitrogen/carbon precursor(s) at a high temperatures (800-1000°C). The formation of different carbon and nitrogen-doped carbon nanostructures can be controlled during the synthesis of such NPMCs. The highly-graphitized carbon nanostructures likely serve as a matrix for the formation of the ORR-active groups with improved catalytic activity and durability, containing nitrogen and possibly also metal atoms.

Future NPMC synthesis approaches are certain to focus on the precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment, with the main purpose being the maximizations of the population of active sites, optimization of nitrogen doping levels, and generation of carbon morphologies capable of hosting active and stable ORR sites. This is evident not only in catalysts developed for PEFCs but also in materials specifically designed for alkaline fuel cells (Chung et al., Nat. Commun. 4:1922 doi:10.1038/ncomms2944, 2013). In the end, however, the much needed progress in ORR electrocatalysis at NPMCs, especially in acid media, will be decided by better understanding of the origin of the NPMC activity and the nature of the active site. That key part of NPMC development will be addressed in this presentation along with the summary of the progress achieved to date and challenges still awaiting non-precious metal electrocatalysis in polymer electrolyte fuel cells.

Acknowledgment

Financial support from DOE-EERE Fuel Cell Technologies Office and Los Alamos National Laboratory Laboratory-Directed Research and Development (LDRD) Program and is gratefully acknowledged.