Much effort has been devoted in the development of efficient catalysts for electrochemical reduction of CO. Molecular level understanding of electrode-mediated process, particularly the role of bicarbonate in increasing CO reduction rates, is still lacking due to the difficulty of directly probing the electrochemical interface. We developed a protocol to observe normally invisible reaction intermediates with a surface enhanced spectroscopy by applying square-wave potential profiles. Further, we demonstrate that bicarbonate, through equilibrium exchange with dissolved CO, rather than the supplied CO, is the primary source of carbon in the CO formed at the Au electrode by a combination of in situ spectroscopic, isotopic labeling, and mass spectroscopic investigations. We propose that bicarbonate enhances the rate of CO production on Au by increasing the effective concentration of dissolved CO near the electrode surface through rapid equilibrium between bicarbonate and dissolved CO.

Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, a selective electrocatalyst is lacking. Here we show that vanadium nitride (VN) nanoparticles are active, selective, and stable ENRR catalysts with an ENRR rate and a Faradaic efficiency (FE) of 3.3 × 10 −10 mol s −1 cm −2 and 6.0% at −0.1 V within 1 h, respectively. ENRR with 15 N 2 as the feed produces both 14 NH 3 and 15 NH 3 , which indicates that the reaction follows a Mars− van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified VN 0.7 O 0.45 as the active phase in the ENRR. Operando X-ray absorption spectroscopy and catalyst durability test results corroborate this hypothesis and indicate that the conversion of VN 0.7 O 0.45 to the VN phase leads to catalyst deactivation. We hypothesize that only the surface N sites adjacent to a surface O are active in the ENRR. An ammonia production rate of 1.1 × 10 −10 mol s −1 cm −2 can be maintained for 116 h, with a steady-state turnover number of 431.

Localized concentration gradients within the electrochemical double layer during various electrochemical processes can have wide-ranging impacts; however, experimental investigation to quantitatively correlate the rate of surface-mediated electrochemical reaction with the interfacial species concentrations has historically been lacking. In this work, we demonstrate a spectroscopic method for the in situ determination of the surface pH using the CO2 reduction reaction as a model system. Attenuated total reflectance surface-enhanced infrared absorption spectroscopy is employed to monitor the ratio of vibrational bands of carbonate and bicarbonate as a function of electrode potential. Integrated areas of vibrational bands are then compared with those obtained from calibration spectra collected in electrolytes with known pH values to determine near-electrode proton concentrations. Experimentally determined interfacial proton concentrations are then related to the resultant concentration overpotentials to examine their impact on electrokinetics. We show that, in CO2-saturated sodium bicarbonate solutions, a concentration overpotential of over 150 mV can be induced during electrolysis at −1.0 V vs RHE, leading to substantial losses in energy efficiency. We also show that increases in both convection and buffering capacity of the electrolyte can mitigate interfacial concentration gradients. On the basis of these results, we further discuss how increases in concentration overpotential affect the mechanistic interpretations of the CO2 reduction electrocatalysis, particularly in terms of Tafel slopes and reaction orders.

Understanding reaction pathways and mechanisms for electrocatalytic transformation of small molecules (e.g., H2O, CO2, and N2) to value-added chemicals is critical to enabling the rational design of high-performing catalytic systems. Tafel analysis is widely used to gain mechanistic insights, and in some cases, has been used to determine the reaction mechanism. In this Perspective, we discuss the mechanistic insights that can be gained from Tafel analysis and its limitations using the simplest two-electron CO2 reduction reaction to CO on Au and Ag surfaces as an example. By comparing and analyzing existing as well as additional kinetic data, we show that the Tafel slopes obtained on Au and Ag surfaces in the kinetically controlled region (low overpotential) are consistently ∼59 mV dec–1, regardless of whether catalysts are polycrystalline or nanostructured in nature, suggesting that the initial electron transfer (CO2 + e– → CO2 –) is unlikely to be the rate-limiting step. In addition, we demonstrate how initial mechanistic assumptions can dictate experimental design, the result of which could in turn bias mechanistic interpretations. Therefore, as informative as Tafel analysis is, independent experimental and computational techniques are necessary to support a proposed mechanism of multielectron electrocatalytic reactions, such as CO2 reduction.

In situ surface-enhanced spectroscopic and reactivity investigations of the electrochemical reduction of CO2 at low overpotentials (<0.7 V) was conducted on Cu surfaces. Vibrational bands corresponding to adsorbed hydrogen (Had) and carbon monoxide (COad) on Cu have been identified at 2090 and 2060 cm–1, respectively. Spectroscopic investigations show that Had is capable of partially displacing COad; however, COad is unable to displace Had to any detectable level. The preferential adsorption of H over CO on Cu is consistent with the high selectivity toward the hydrogen evolution reaction at potentials >−0.8 V versus reversible hydrogen electrode.