Inducing porosity in xylose-derived FeNC electrocatalysts for alkaline oxygen reduction

Abstract

Iron–nitrogen–carbon (FeNC) electrocatalysts are emerging as a low-cost alternative to Pt-based materials for electrochemical oxygen reduction at the cathode of alkaline exchange membrane hydrogen fuel cells. The valorisation of waste biomass is a sustainable pathway that could allow the large-scale production of such catalysts. By means of hydrothermal carbonization (HTC), a biomass-derived carbohydrate can be converted into a carbonaceous framework, however, the electrocatalytic performance of the metal–nitrogen–carbon electrocatalysts prepared through HTC is suboptimal owing to the lack of microporosity in the highly crosslinked carbon frameworks. In this work, we address this issue by adding polystyrene sulfonate (kayexalate) in the HTC of xylose. Kayexalate's negative charges mitigate particle aggregation, resulting in smaller carbon-based particles, with the O₂ activation leading to a four-fold increase in specific surface area (127 vs. 478 m² g⁻¹). Subsequent high-temperature pyrolysis in the presence of an N and Fe source leads to an active FeNC. This produces a corresponding increase in the electrocatalytic activity for the oxygen reduction in alkaline media in a rotating disk electrode (1.45 vs. 14.3 A g⁻¹ at 0.8 V vs. RHE) and in a gas diffusion electrode at high current densities (≥2 A cm⁻²). The sustainable character of the reported catalyst as well as the high electrocatalytic activity at industrially relevant current densities provides a pathway to catalyst design for low-cost cathodes in alkaline exchange membrane fuel cells.