Development of a continuous reactor for the electro-reduction of carbon dioxide to formate – Part 2: Scale-up

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This paper reports experimental and modeling work for the laboratory scale-up of continuous “trickle-bed” reactors for the electro-reduction of CO2 to potassium formate. Two reactors (A and B) were employed, with particulate tin 3D cathodes of superficial areas, respectively, 45 × 10−4 (2–14 A) and 320 × 10−4 m2 (20–100 A). Experiments in Reactor A using granulated tin cathodes (99.9 wt% Sn) and a feed gas of 100% CO2 showed slightly better performance than that of the tinned-copper mesh cathodes of our previous communications, while giving substantially improved temporal stability (200 vs. 20 min). The seven-fold scaled-up Reactor B used a feed gas of 100% CO2 with the aqueous catholyte and anolyte, respectively [0.5 M KHCO3 + 2 M KCl] and 2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet temperature 295 to 325 K. For a superficial current density of 0.6–3.1 kA m−2 Reactor B achieved corresponding formate current efficiencies of 91–63%, with the same range of reactor voltage as that in Reactor A (2.7–4.3 V), which reflects the success of the scale-up in this work. Up to 1 M formate was obtained in the catholyte product from a single pass in Reactor B, but when the catholyte feed was spiked with 2–3 M potassium formate there was a large drop in current efficiency due to formate cross-over through the Nafion 117 membrane. An extended reactor (cathode) model that used four fitted kinetic parameters and assumed zero formate cross-over was able to mirror the reactor performance with reasonable fidelity over a wide range of conditions (maximum error in formate CE = ±20%), including formate product concentrations up to 1 M.