Hydrogen produced from water and renewable energy could fuel a large fleet of proton-exchange-fuel-cell vehicles in the future. However, the dependence on expensive Pt-based electrocatalysts in such fuel cells remains a major obstacle for a widespread deployment of this technology. One solution to overcome this predicament is to reduce the Pt content by a factor of ten by replacing the Pt-based catalysts with non-precious metal catalysts at the oxygen-reducing cathode. Fe- and Co-based electrocatalysts for this reaction have been studied for over 50 years, but they were insufficiently active for the high efficiency and power density needed for transportation fuel cells. Recently, several breakthroughs occurred that have increased the activity and durability of non-precious metal catalysts (NPMCs), which can now be regarded as potential competitors to Pt-based catalysts. This review focuses on the new synthesis methods that have led to these breakthroughs. A modeling analysis is also conducted to analyze the improvements required from NPMC-based cathodes to match the performance of Pt-based cathodes, even at high current density. While no further breakthrough in volume-specific activity of NPMCs is required, incremental improvements of the volume-specific activity and effective protonic conductivity within the fuel-cell cathode are necessary. Regarding durability, NPMCs with the best combination of durability and activity result in ca. 3 times lower fuel cell performance than the most active NPMCs at 0.80 V. Thus, major tasks will be to combine durability with higher activity, and also improve durability at cell voltages greater than 0.60 V.

In this study, we explored the behavior of NC_Ar + NH₃, an initially highly active catalyst for oxygen electroreduction, in H₂/air fuel cells from 0.8 to 0.2 V at 80 °C and 25 °C, in order to find the causes of its instability. We discovered that the decay of the current density always involves the superposition of fast and slow first order kinetics, for which half-lives were obtained. The half-life of the fast decay was practically the same at all potentials and temperatures with a value of around 138 ± 55 min, while the half-life of the slow decay greatly varied from a minimum of ≈2400 min (40 h) to infinity. From the adsorption–desorption isotherm of NC_Ar + NH₃, it was deduced that the Fe/N/C carbonaceous catalyst is characterized by interconnected open-end slit-shaped micropores, in which water (with dissolved H⁺ and O₂) quickly flows in the fuel cells if their width is ≥0.7 nm as it has no interaction with the hydrophobic walls of the micropores. The driving force of this quick water flow is the humidified air streaming through the working cathode. Fe–N₄-like active sites are thermodynamically stable in stagnant acidic conditions, but according to the Le Chatelier principle, they demetalate in the flux of water running into the micropores. This specific demetalation is the cause of the initial loss of ORR activity of NC_Ar + NH₃ catalysts assigned to the fast current decay in fuel cells.