Effects of electrolyte, catalyst, and membrane composition and operating conditions on the performance of solar-driven electrochemical reduction of carbon dioxide

Abstract

Solar-driven electrochemical cells can be used to convert carbon dioxide, water, and sunlight into transportation fuels or into precursors to such fuels. The voltage efficiency of such devices depends on the (i) physical properties of its components (catalysts, electrolyte, and membrane); (ii) operating conditions (carbon dioxide flowrate and pressure, current density); and (iii) physical dimensions of the cell. The sources of energy loss in a carbon dioxide reduction (CO2R) cell are the anode and cathode overpotentials, the difference in pH between the anode and cathode, the difference in the partial pressure of carbon dioxide between the bulk electrolyte and the cathode, the ohmic loss across the electrolyte and the diffusional resistances across the boundary layers near the electrodes. In this study, we analyze the effects of these losses and propose optimal device configurations for the efficient operation of a CO2R electrochemical cell operating at a current density of 10 mA cm(-2). Cell operation at near-neutral bulk pH offers not only lower polarization losses but also better selectivity to CO2R versus hydrogen evolution. Addition of supporting electrolyte to increase its conductivity has a negative impact on cell performance because it reduces the electric field and the solubility of CO2. Addition of a pH buffer reduces the polarization losses but may affect catalyst selectivity. The carbon dioxide flowrate and partial pressure can have severe effects on the cell efficiency if the carbon dioxide supply rate falls below the consumption rate. The overall potential losses can be reduced by use of an anion, rather than a cation, exchange membrane. We also show that the maximum polarization losses occur for the electrochemical synthesis of CO and that such losses are lower for the synthesis of products requiring a larger number of electrons per molecule, assuming a fixed current density. We also find that the reported electrocatalytic activity of copper below -1 V vs. RHE is strongly influenced by excessive polarization of the cathode and, hence, does not represent its true activity at bulk conditions. This article provides useful guidelines for minimizing polarization losses in solar-driven CO2R electrochemical cells and a method for predicting polarization losses and obtaining kinetic overpotentials from measured partial current densities.

Direct Observation of the Local Reaction Environment during the Electrochemical Reduction of CO2

“…Conversely, these observations suggest that there is a second mode of CO 2 depletion that becomes relevant at potentials cathodic of −1 V vs RHE at scan rates below 20 mV/ s. This additional mode of CO 2 depletion is hypothesized to be the reaction of CO 2 with hydroxyl anions evolved at the cathode surface, which produces bicarbonate anions and depletes the local CO 2 concentration. 6,7 The extent to which this reaction depletes the local CO 2 concentration scales with the concentration of hydroxyl anions produced at the cathode surface during the linear potential sweep, which decreases as the scan rate increases. It is important to realize that the intrinsic electrocatalytic activity can only be measured if the extent of concentration polarization is minimized.…”

“…As the hydrodynamic boundary layer thickness increases, the local pH should increase at a fixed current density, leading to CO 2 depletion by acid−base reaction at more anodic potentials. 7 To validate this hypothesis, the mass-ion current signal associated with CO 2 was plotted as a function of the deconvoluted massion current signal associated with CO for each linear potential sweep performed. Figure 5B shows that as the electrolyte flow rate is increased, the point at which the CO 2 signal rapidly declines shifts to higher CO evolution rates, which correspond to increasingly cathodic potentials.…”

“…As a result, the pH and CO 2 concentration near the cathode surface deviate significantly from those in the bulk of the electrolyte. 6,7 Determining the extent to which the observed electrocatalytic activity is influenced by concentration polarization is difficult because conventional analytical techniques sample species from the gasphase effluent and the bulk electrolyte. Additionally, there is currently little experimental evidence to support any of the proposed mechanisms by which CO 2 is reduced to multicarbon products over Cu.…”

Standards and Protocols for Data Acquisition and Reporting for Studies of the Electrochemical Reduction of Carbon Dioxide

“…28,29 Even modest current densities cause the pH and CO 2 concentration near the cathode surface to vary significantly from that in the bulk electrolyte. 30,31 The magnitude of the concentration gradients depends largely on the hydrodynamics of the electrochemical cell. As a result, the electrolyte needs to be mixed vigorously to ensure sufficient mass transport to and from the cathode.…”

Effects of temperature and gas–liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO2reduction electrocatalysts

“…[19][20][21][22] In order to drive progress in the CO2R electrocatalysis community, commonly accepted, easily reproducible testing conditions need to be established, 23 similar to those suggested for photoelectrochemical water splitting. 24 Hori and co-workers have already identified many good practices for experimentation in this area 10,13,25,26 and some further insight into polarization losses was provided by Singh et al 27 However, examining the recent literature, one can observe that several different methods of analyzing CO2R electrocatalysts have been used which employ different cell designs and testing conditions.…”