Hierarchically porous carbon supports enable efficient syngas production in electrified reactive capture

Abstract

Direct-air capture (DAC) of CO₂ often uses alkali hydroxides (e.g. KOH) as sorbent, and relies on an energy-intensive thermal CaCO₃/Ca(OH)₂ step to release CO₂ and regenerate the alkali hydroxide. Reactive capture instead uses alkali carbonate post-capture liquid as feedstock, seeking to convert the captured CO₂ to value-added products while regenerating the capture liquid. Here we investigate the origins of low prior performance in electrochemical reactive capture systems, finding that the catalyst becomes starved of CO₂ even at moderate current densities leading to a rapid decline in faradaic efficiency (FE). We then study how the catalyst support can be redesigned to tackle this problem, and construct hierarchical carbon supports featuring interconnected mesopores and micropores, our purpose to increase the interaction between in situ generated CO₂, i-CO₂ – the limiting reagent – and the catalyst. We find that the attachment chemistry of the catalyst to the support is critical: only when we disperse and tether the molecular catalyst do we prevent catalyst aggregation and deactivation under bias. We report as a result carbonate electrolysis at 200 mA cm⁻² at 2.9 V with FE of 47 ± 1% for CO, this corresponding to an energy efficiency (EE) to 2 : 1 syngas of 50% at 200 mA cm⁻² when H₂ is added using a water electrolyzer. This represents a 1.5× improvement in EE at this current density relative to the most efficient prior carbonate electrolysis reports. The CO FE remains above 40% at current densities as high as 500 mA cm⁻², and all systems studied herein achieve < 1% CO₂ in the outlet stream. The cradle-to-gate carbon intensity is lowered to −1.49 tonCO₂ per tonsyngas as a result of the increase in EE, and a CO₂-free tailgas stream is provided that minimizes separation costs.