Review ArticleDurability challenges and perspective in the development of PGM-free electrocatalysts for the oxygen reduction reaction
Introduction
High energy density, high efficiency, fast oxidation kinetics and low environmental impact of hydrogen fuel make the polymer electrolyte fuel cell (PEFC) one of the most promising energy conversion technologies. However, successful integration of PEFC technology into automobile and stationary power applications requires significant reduction of the overall fuel cell stack cost, which is mainly attributed to the expensive platinum-based catalysts needed for the pertinent electrochemical reactions, that is, the oxygen reduction reaction (ORR) occurring at the cathode and the hydrogen oxidation reaction (HOR) at the anode. The ORR is six orders of magnitude slower than the HOR, requiring significant amounts of platinum catalyst to carry out the sluggish reaction [1]. Therefore, to reduce the PEFC stack cost and further accelerate the commercialization of PEFC technology, the development of durable and highly active, low-cost platinum group metal-free (PGM-free) ORR electrocatalysts is crucial.
An important effort attracting significant attention is the development of ORR electrocatalysts from earth-abundant elements such as carbon, nitrogen, and transition-metals, namely, Me–N–C 2.•, 3., 4.. From the initial studies reporting ORR activity from metal chelates in alkaline electrolytes [5] to the discovery of pyrolyzed Me–N–C precursors with ORR activity in acid electrolytes [6], substantial advancements have been made to improve the activity of PGM-free electrocatalysts 7.••, 8., 9.. Nevertheless, the durability of these novel materials, that is, retention of initial high activity over the required lifetime, remains the biggest challenge. According to the U.S. Department of Energy Fuel Cell Technology Office (DOE-FCTO), current targets for PEFCs are 5000 hours for transportation and 60 000 hours for stationary applications [10]. While the state-of-the-art PGM-based PEFC can operate over thousands of hours, PGM-free-based PEFCs have only been demonstrated over hundreds of hours of operation 11., 12., 13.••. This manuscript highlights current durability status and challenges faced by PGM-free electrocatalysts providing a perspective for the study and development of next-generation electrocatalysts with increased activity and long-term stability [14].
Section snippets
Current durability status
Improvement of the PGM-free electrocatalyst fuel cell performance has been achieved by control of the catalyst pore structure. Macro/mesopores facilitate transport of reactants and products to/from PGM-free active sites, while micropores act as active site hosts 15., 16.. This hierarchical pore structure has been obtained through a number of different processes, for example, mesoporous templates 12., 17., 18., metal organic frameworks (MOFs) 8., 19., 20., 21., 22., 23., 24., N-precursors acting
Proposed degradation mechanisms
Based on a similar rationale to PGM electrocatalysts, the proposed degradation mechanisms encompass two main categories: (1) atomic-scale degradation of active sites: demetalation, carbon/nitrogen corrosion of active site, and active site poisoning; and (2) macro- and meso-scale degradation affecting the catalyst layer structure: carbon corrosion, hydroperoxyl radical attack, and polymer electrolyte degradation (Figure 1).
Development of techniques to investigate degradation mechanisms
In an effort to solve the active site conundrum, a number of poisons have been tested as molecular probes. For instance, following the idea of a heme-like, or metal-centered, active site structure, researchers have both shown successful [49] and unsuccessful [50] attempts to poison PGM-free electrocatalysts with CO as a molecular probe. Correlating the number of probe molecules with a loss in the measured activity after poisoning can be used to calculate the number of active sites in the
Modeling of possible degradation mechanisms
Quantum chemical modeling can aid experimental efforts to better understand and mitigate activity loss in PGM-free materials. Such approaches allow for the generation of atomic-scale structure-to-function relations which are exceedingly difficult to generate experimentally for highly heterogeneous materials such as PGM-free Me–N–C electrocatalysts [61•]. Knowledge about the binding properties of probe molecules to different atomic-scale structures is important to understand the specificity of
Future outlook
The discussed recent durability studies highlighting proposed degradation mechanisms and techniques to gain further understanding of the source of activity loss in PGM-free ORR electrocatalysts highlight key knowledge gaps that must be overcome for targeted durability improvement. It should be noted that degradation mechanisms in aqueous electrochemical cell (“half-cell”) versus MEA studies are intrinsically different. Due to the complexity of PGM-free electrocatalysts and the mystery of the
References and recommended reading
• Paper of special interest
•• Paper of outstanding interest.
Acknowledgments
The authors’ work on the development of PGM-free electrocatalysts was supported by the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy (DOE) through the Fuel Cell Technologies Office Electrocatalysis Consortium (ElectroCat).
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