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

Abstract

This paper reports experimental and modeling work for the laboratory scale-up of continuous ''trickle-bed'' reactors for the electro-reduction of CO 2 to potassium formate. Two reactors (A and B) were employed, with particulate tin 3D cathodes of superficial areas, respectively, 45 · 10 À4 (2-14 A) and 320 · 10 À4 m 2 (20-100 A). Experiments in Reactor A using granulated tin cathodes (99.9 wt% Sn) and a feed gas of 100% CO 2 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% CO 2 with the aqueous catholyte and anolyte, respectively [0.5 M KHCO 3 + 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 À2 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.Keywords Carbon dioxide Á Continuous reactor Á Electro-reduction Á Formate Á 3D electrode Á Scale-up Á Tin granule cathode Nomenclature a 1 ,a 2 Tafel constant for reaction 1, 2 (V) b 1 ,b 2 Tafel slope for reaction 1, 2 (V decade À1 ) C Concentration of KCl (M) CECurrent efficiency (-) d p, average Average particle diameter (m) E a,1 , E a,2 Activation energies for reactions 1 and 2 (kJ kmol À1 ) E cell Full-cell operating voltage (absolute value) (V) E 1 , E 2 Electrode potential for reaction 1 and 2 (V(SHE)) E r,1 , E r,2 Reversible electrode potential for reaction 1 and 2 (V(SHE)) G Gas flow rate (mL STP min À1 ) H Height of 3D cathode (m)Exchange current densities for reactions 1 and 2 (kA m À2 ) j 1 ,j 2 Partial real current density for reaction 1, 2 (kA m À2 ) j 1L CO 2 mass transfer limited current density for reaction 1 (kA m À2 ) k 1 , k 2 Electrochemical rate constants for reactions 1 and 2 (m s À1 ) L Catholyte liquid flow rate (mL min À1 ) P cathode Cathode side pressure (kPa(abs))

Microstructural surface changes of electrodeposited Pb on gas diffusion electrode during electroreduction of gas‐phase CO2

“…Azuma et al [19] reported current efficiency as low as 16.7% for CO 2 reduction into formic acid in 0.05 M KHCO 3 on Pb electrode at −1.8 V, where the current decreased with increasing potential. Li and Oloman [3] reported a current efficiency of 50% at 67 mA/cm 2 with a concentration of formate of 45 mM with a larger electrode area. They further reported increasing current efficiencies with a decrease in current density, the reduction of which took place in 0.45 M KHCO 3 .…”

“…However, a variety of metals have different reactivities and stabilities and hence their catalytic activities may be affected by reduction media, size and shape of catalyst or prevailing conditions that can lead to their inactivation or changes in form or morphologies. [3,10] Changes may include structural and crytalline forms, depending on the nature of agents the metals are exposed to. It has been reported that some metals may undergo structural changes when exposed to atmospheric agents [11] and different temperature conditions.…”

Electrochemical Conversion of Carbon Dioxide to Formate in Alkaline Polymer Electrolyte Membrane Cells

“…The use of gas diffusion electrodes for improving current density was explored by Cook et al 12 Li and Oloman have investigated the engineering variables that determine the rate and efficiency in the continuous electrochemical processing of carbon dioxide. [13][14][15][16] Previous research on carbon dioxide reduction has focused on using alkaline solution electrolytes, porous separators, and solid metallic electrode structures. There are numerous benefits to using a cell design based on a solid polymeric ion-conducting membraneelectrode assembly ͑MEA͒ with porous catalytic electrodes.…”

Transformation of Carbon Dioxide to Useable Products through Free Radical‐Induced Reactions

“…Recently, several reactors with different designs have been used for CO 2 reduction to HCOOH [119][120][121][122][123][124][125][126]. Many of these designs have been based on a fuel cell in which mostly polymer electrolyte membranes were used to separate the anode and cathode.…”