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

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 x 10(-4) (2-14 A) and 320 x 10(-4) m(2) (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.