Catholyte‐Free Electrocatalytic CO₂ Reduction to Formate

Abstract

Electrochemical reduction of carbon dioxide (CO 2 ) into value-added chemicals is ap romising strategy to reduce CO 2 emission and mitigate climate change.O ne of the most serious problems in electrocatalytic CO 2 reduction (CO 2 R) is the lows olubility of CO 2 in an aqueous electrolyte,w hich significantly limits the cathodic reaction rate.T his paper proposes af acile method of catholyte-free electrocatalytic CO 2 reduction to avoid the solubility limitation using commercial tin nanoparticles as acathode catalyst. Interestingly,as the reaction temperature rises from 303 Kto363 K, the partial current density (PCD) of formate improves more than two times with 52.9 mA cm À2 ,despite the decrease in CO 2 solubility. Furthermore,asignificantly high formate concentration of 41.5 gL À1 is obtained as aone-path product at 343 Kwith high PCD (51.7 mA cm À2 )and high Faradaic efficiency (93.3 %) via continuous operation in afull flowcell at alow cell voltage of 2.2 V.The electrocatalytic CO 2 reduction (CO 2 R) into useful chemicals or fuels has attracted significant interest for renewable energy storage and environmental sustainability. [1][2][3][4] There are several electrochemical pathways to convert CO 2 into valuable products, [1,5,6] among which formic acid or formate has received particular attention over the past few years because the CO 2 Ri nto formic acid is regarded as as uitable reaction for industrial scale production. [7][8][9] However,there still remain some problems to be solved, including alarge overpotential, low Faradaic efficiency(FE), and poor stability of electrocatalysts.Inparticular, the low solubility of CO 2 in the catholyte significantly limits the reaction rate,thus greatly hindering the large-scale application of the CO 2 R. [1] Furthermore,t he liquid electrolytes dilute the reduction products,w hich increases the separation costs for product recovery.Some efforts have been reported to overcome the solubility limitation and enhance the current density of formate production, including development of ah ighly porous structure of SnO 2 ,t unable alloys,a nd ionic liquids (ILs). [2,[10][11][12][13] Despite these significant results,t heir kinetics of CO 2 Rconversion into formate is still limited by the low CO 2 concentration in electrolytes.I nr ecent years,s ome research groups reported remarkable improvement in the current density of CO 2 Rt of ormate using gas diffusion electrodes (GDEs) with metal nanoparticles. [5,[14][15][16][17][18][19] This type of electrode provides at hree-phase interface of gas/ electrolyte/ catalyst, in which the CO 2 molecules only have to diffuse through av ery thin liquid layer of electrolyte to reach the catalyst surface. [19] Castillo et al. [5] achieved ah igh formate concentration of 2.5 gL À1 via continuous operation of a10cm 2 filterpress cell using Sn-GDEs under galvanostatic condition of 150 mA cm À2 .A lthough they obtained an enhanced performance over previous results,their method still suffered from ahigh cell voltage of 3.7 V, alow FE of 70 %, and t...

Towards Higher Rate Electrochemical CO2 Conversion: From Liquid-Phase to Gas-Phase Systems

“…Mainly, various types of PEMs, such as an anion, cation exchange membrane, and bipolar membrane, have been adopted, and the PEM is the key component to determine the overall performance of the CO 2 electrolysis cell [24,59,92,93]. Also, in the MEA system, the suppression of side products causing the deterioration of the membrane and the phase of the delivered CO 2 are significant issues to improve the CO 2 RR activity [26,33]. On the other hand, a microfluidic reactor type was first proposed by Cook et al, with the configuration of a gas diffusion region/Cu catalysts/aqueous electrolyte [60].…”

“…In the following sub-sections, we derive the perspectives by reviewing the previous reports for three different final products in MEA and microfluidic gas-phase CO2RR reactor types. Formate production has been intensively studied with MEA reactors, in particular with Sn electrocatalysts [24,26,28,58,64,67,86,[88][89][90][91]. Nevertheless, the current density achieved in most studies is not adequate for scalable technology.…”

“…Thus, this liquid-phase-based cell configuration is not appropriate for commercially lity. In contrast to the liquid-phase reactor, gas-phase cell systems have recently shown an anding performance, in particular, recording a high current density for target chemical products out regard for the mass transport limit of the reactant [17,18,[23][24][25][26][27][28][29]. The general trend of the reported current densities for CO2RR (jCO2 Reduction) is plotted in Figure 4.…”

“…Through this work, we finally outline the significant issues and perspectives of the gas-phase reactor systems toward a practical application level. The trend of partial current density for CO2 reduction (jCO2 Reduction) in the case of liquid- [3][4][5][6][7][8]10,11,13,16,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] and gas-phase reactors [15][16][17][18][23][24][25][26][27][28][29][30][31][32]. The gas-phase reactors are divided into membrane assembly and microfluidic types.…”

Looking Back and Looking Ahead in Electrochemical Reduction of CO2

“…A similar approach was further developed by Lee et al, namely, catholyte-free electrocatalytic system for ECRP. [56] Gaseous CO 2 with water vapor was supplied to cathode channel of a 25 cm 2 flow cell and then reacted to form HCOO À as depicted in Figure 9b. Interestingly, the partial current density assigned to HCOO À significantly increased with increase in the reaction temperature despite the decrease in CO 2 solubility, which is disagreement with several previous reports.…”