Enabling technologies for the continuous electrically driven conversion of CO2 and water to multi-carbon products at high current densities

Abstract

Herein, we demonstrate greatly improved conversion of CO₂ using a gas diffusion electrode (GDE) with flowing electrolyte configuration for CO₂ gas delivery in combination with a high surface area nickel phosphide electrocatalyst. This configuration achieves 40–50% selectivity for total carbon products over H₂ formation (HER) at total current densities ranging from 50 to 300 mA cm⁻². We developed a soft-templating method using CTAB detergent micelles for synthesis of phase-pure Ni₂P, achieving a 260-fold larger surface area (BET) and porous sponge-like morphology that produces stable currents. This catalyst produces mainly one C-product, methylglyoxal (MG, C₃H₄O₂) with 38–47% overall selectivity, the highest reported selectivity for a 12-electron reduction product. The versatile soft-templating method for electrocatalyst synthesis uses low-temperature (185 °C) that is permissive for incorporation of co-catalysts that are otherwise destroyed by the high temperatures used in traditional solid-state synthesis (SSS). The non-porous Ni₂P-SSS catalyst produces mainly H₂ at these current densities. Achieving these high currents and C/H selectivity benefits from use of hydrophobic polymers as co-catalyst binders (cationic = Nafion, anionic = PFAEM and neutral = PTFE) to improve CO₂ conversion. PFAEM is the better ionomer for the CO₂RR at high current density, postulated as due to suppressing CO₂ conversion to inactive bicarbonate. Precipitation of the carbon products as a polycarbonate polymer occurs at high currents.