Conversion of Reactive Carbon Solutions into CO at Low Voltage and High Carbon Efficiency

Abstract

Electrolyzers are now capable of reducing carbon dioxide (CO 2 ) into products at high reaction rates but are often characterized by low energy efficiencies and low CO 2 utilization efficiencies. We report here an electrolyzer that reduces 3.0 M KHCO 3 (aq) into CO(g) at a high rate (partial current density for CO of 220 mA cm −2 ) and a CO 2 utilization efficiency of 40%, at a voltage of merely 2.3 V. These results were made possible by using: (i) a reactive carbon solution enriched in KHCO 3 as the feedstock instead of gaseous CO 2 ; (ii) a cation exchange membrane instead of an anion exchange membrane, which is common to the field; and (iii) the hydrogen oxidation reaction (HOR) at the anode instead of the oxygen evolution reaction (OER). The voltage reported here is the lowest reported for any CO 2 to CO electrolyzer that operates at high current densities (i.e., a partial current density for CO greater than 200 mA cm −2 ) with a CO 2 utilization efficiency of greater than 20%. This study highlights how the choice of feedstock, membrane, and anode chemistries affects the rate and efficiency at which CO 2 is converted into products.

Permeability Matters When Reducing CO2 in an Electrochemical Flow Cell

“…CO 2 electrolyzers can be fed with a gaseous CO 2 feedstock or with “reactive carbon solutions” enriched with (bi)­carbonate ions. , In this study, we focus on the electrolysis of 3.0 M KHCO 3(aq) into CO (Figure ). ,− The distinguishing feature of “bicarbonate electrolyzers” is that H + supplied by the anode to the cathode through a membrane reacts with the bicarbonate feedstock to form CO 2 in situ (“ i -CO 2 ”) (eq ). , It is this i -CO 2 that is reduced into CO at the cathode surface (eq ).…”

“… ,− The distinguishing feature of “bicarbonate electrolyzers” is that H + supplied by the anode to the cathode through a membrane reacts with the bicarbonate feedstock to form CO 2 in situ (“ i -CO 2 ”) (eq ). , It is this i -CO 2 that is reduced into CO at the cathode surface (eq ). …”

MOF-Derived Ni Single-Atom Catalyst with Abundant Mesopores for Efficient Mass Transport in Electrolytic Bicarbonate Conversion

“…13 Zhang et al further designed a 500 μm buffer layer between porous Ag foam and Nafion membrane to inhibit the HER activity, which obtained a 71% FE CO at 100 mA/cm 2 . 16 The use of Ag-based catalysts during bicarbonate electrolysis led to a huge burden on the commercial application. As a result, the designed micro- structure of the catalyst (NaCl@Ni/Zn-6) can promote reactant mass transport, which enhances the local reactant concentration around the active sites and increases the electrochemical performance.…”

“…Typically, the energy requirements during CO 2 capture and separation account for >76% in a traditional postcombustion CO 2 capture system . Electrolytic bicarbonate conversion to fuels or value-added chemicals (e.g., CO, formate, and C 2 H 4 ) can bypass these energy-intensive processes (i.e., CO 2 desorption and compression), providing an alternative approach for the future commercial application. − Previous life cycle assessments confirmed that electrolytic bicarbonate conversion to CO had a 1.93 net energy ratio, much lower than that of electrolytic gaseous CO 2 conversion to CO (6.19), highlighting the energy-efficient characteristics of the electrolytic bicarbonate conversion …”

“…Most studies reported so far mainly used precious Ag nanoparticles (NPs) , or porous Ag foam , as the catalyst to support the electroreduction of bicarbonate to CO, delivering a low selectivity and a poor economy, which prevented the current system from commercial applications. For example, Li et al.…”