The pH at the electrocatalyst surface plays a key role in defining the activity and selectivity of the CO 2 reduction reaction (CO 2 RR). We report here operando Raman measurements of the catalyst surface in a customized CO 2 RR flow cell that enable the measure of pH. Using this flow cell, we were able to measure surface pH as a function of time, current density, and proximity to the catalyst surface during the electrolysis of bicarbonate solutions. We observed that increasing the current density from 0 to 200 mA cm −2 increased the surface pH from 8.5 to 10.3. We also show here that operation at elevated temperatures (70 °C) results in an increased surface pH and serves to suppress the competing and undesirable hydrogen evolution reaction.

Gas-fed CO2 electrochemical flow reactors are appealing platforms for the electrolytic conversion of CO2 into fuels and chemical feedstocks at commercially relevant current densities (≥100 mA/cm2). An inherent challenge in the development of these reactors is delivering sufficient water to the cathode to sustain the CO2 reduction reaction, while also preventing accumulation of excess water at the porous cathode (i.e., flooding). We present herein experimental evidence showing cathode flooding in a zero-gap electrolyzer at 200 mA/cm2. This flooding causes a 37% decrease in partial current density for CO production (j CO) along with a 450 mV increase in cell voltage (E cell). We show that the detrimental effects associated with this flooding can be mitigated by pairing thin membranes (i.e., ≤40 μm) with hydrophobic cathodes to enable CO2 electrolysis at commercially relevant conditions (j CO ≥ 100 mA/cm2 and E cell < 3 V).

Lattice strain can enhance the activity and selectivity of electrochemical reactions by breaking the linear scaling relationship. Notwithstanding, the explicit use of strain to affect the CO2 reduction reaction (CO2RR) is rarely reported. In this Perspective, we highlight the opportunity to use strain to affect the activity and selectivity of CO2RR electrocatalysts. We summarize the existing challenges in isolating the influence of strain from convoluting factors (e.g., size, shape, electronic, and surfactant effects) that result from typical methods of inducing strain. We also propose ways to isolate strain effects using the application of mechanical strain to thin-film CO2RR catalysts. We designed this Perspective to motivate  the use of joint empirical and computational studies to investigate CO2RR strain–activity–selectivity relationships.

Electrocatalytic palladium membrane reactors (ePMRs) use electricity and water to drive hydrogenation reactions without forming H2 gas.

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