Oxygen Reduction Reaction on Metal and Nitrogen–Doped Carbon Electrocatalysts in the Presence of Sodium Borohydride

Investigation of non-precious metal cathode catalysts for direct borohydride fuel cells

Borohydride crossover in anion exchange membrane (AEM) based direct borohydride fuel cells (DBFCs) impairs their performance and induces cathode catalyst poisoning. This study evaluates three non-precious metal catalysts, namely LaMn0.5Co0.5O3 (LMCO) perovskite, MnCo2O4 (MCS) spinel, and Fe-N-C, for their application as cathode catalysts in DBFCs. The rotating disk electrode (RDE) testing shows significant borohydride tolerance of MCS. Moreover, MCS has exhibited exceptional stability in accelerated durability tests (ADTs), with a minimal reduction of 10 mV in half-wave potential. DFT calculations further reveal that these catalysts predominantly adsorb over , unlike commercial Pt/C which preferentially adsorbs . In DBFCs, MCS can deliver a peak power density of 1.5 W cm-2, and a 3% voltage loss after a 5 hours durability test. In contrast, LMCO and Fe-N-C have exhibited significantly lower peak power density and stability. The analysis of the TEM, XRD, and XPS results before and after the single-cell stability tests suggests that the diminished stability of LMCO and Fe-N-C catalysts is due to catalyst detachment from carbon supports, resulting from the nanoparticle aggregation during the high-temperature preparation process. Such findings suggest that MCS can effectively mitigate the fuel crossover challenge inherent in DBFCs, thus enhancing its viability for practical application.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.