Density functional theory (DFT) calculations are performed to investigate the energetics of the CO2 electrochemical reduction on metal (M) porphyrin-like motifs incorporated into graphene layers. The objective is to develop strategies that enhance CO2 reduction while suppressing the competitive hydrogen evolution reaction (HER). We find that there exists a scaling relation between the binding energy of the catalyst to hydrogen and that to COOH, a key intermediate in the reduction of CO2 to CO; however, the M-H bond is stronger than the M-COOH bond, driving the reaction toward the HER rather than the reduction of CO2 to CO. This scaling relation holds even with axial ligation to the metal cation coordinated to the porphyrin ring. When 4f lanthanide or 5f actinide elements are used as the reactive center, the scaling relation still holds but the M-COOH bond is stronger than the M-H bond, and the reaction favors the reduction of CO2 to CO. By contrast, there is no scaling relation between the binding energy of the catalyst to H and that to OCHO, the key intermediate in CO2 reduction to formic acid. Interestingly, we find that coordination of a ligand to an unoccupied axial site can make the M-OCHO bond stronger than the M-H bond, resulting in preferential formic acid formation. This means that the axial ligand effectively enhances CO2 reduction to formic acid and suppresses the HER. Our DFT calculations have also identified several promising electrocatalysts for CO2 reduction to HCOOH with almost zero overpotentials.

The discovery of electrocatalysts that can efficiently reduce CO2 to fuels with high selectivity is a subject of contemporary interest. Currently, the available analytical methods for characterizing the products of CO2 reduction require tens of hours to obtain the dependence of product distribution on applied potential. As a consequence, there is a need to develop novel analytical approaches that can reduce this analysis time down to about an hour. We report here the design, construction, and operation of a novel differential electrochemical mass spectrometer (DEMS) cell geometry that enables the partial current densities of volatile electrochemical reaction products to be quantified in real time. The capabilities of the novel DEMS cell design are demonstrated by carrying out the electrochemical reduction of CO2 over polycrystalline copper. The reaction products are quantified in real time as a function of the applied potential during linear sweep voltammetry, enabling the product spectrum produced by a given electrocatalyst to be determined as a function of applied potential on a time scale of roughly 1 h.

There are a number of recent reports on the use of oxidation/reduction cycling of Cu surfaces to improve their selectivity for ethylene formation in the aqueous CO2 reduction reaction. Here, the oxidation/reduction process is examined in detail. It is found that the faradaic efficiencies for both ethylene and ethanol are enhanced after oxidation/reduction cycling in the presence of halide anions. Specifically, cycling of the electrode in the presence of chloride, bromide, or fluoride anions allows for an ethylene faradaic efficiency of approximately 15.2 %, a factor of 1.5 higher than that for polycrystalline copper (at −1.0 V vs. RHE). The faradaic efficiency for ethanol is also enhanced from 2.65 to approximately 7.6 %. The effects of electrochemical oxidation/reduction with the chloride anion were investigated by using in situ Raman spectroscopy, and the changes in the surface morphology of copper were monitored by using SEM. Consistent with prior reports, it is observed that during the oxidation part of the cycle, anodic corrosion forms a Cu2O layer, which consists of cubical crystals of about 150 nm. During the reduction sweep, it is converted to metallic copper, which forms irregular Cu nanoparticles of around 20 nm in diameter. The enhancement in ethylene formation is presumably attributed to the formation of grain boundaries, which may serve as active sites.

Electrolyte cation size is known to influence the electrochemical reduction of CO₂ over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pK_a for cation hydrolysis decreases and is sufficiently low for hydrated K⁺, Rb⁺, and Cs⁺ to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO₂. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H₂ and CH₄, and an increase in Faradaic efficiencies for CO, C₂H₄, and C₂H₅OH, in full agreement with experimental observations for CO₂ reduction over Ag and Cu.

In the last few years, there has been increased interest in electrochemical CO₂ reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk electrolyte CO₂ concentration. Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm^-2, the temperature increase was less than 4 °C and a decrease of <10% in the dissolved CO₂ concentration is predicted. In contrast, limits on the CO₂ gas-liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO₂ concentration, significant undersaturation of CO₂ is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm^-2. Undersaturation of CO₂ produces large changes in the faradaic efficiency observed on Cu electrodes, with H₂ production becoming increasingly favored. We show that the size of the CO₂ bubbles being introduced into the cell is critical for maintaining the equilibrium CO₂ concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equilibrium CO₂ concentration at current densities up to 15 mA cm^-2.

Carbon materials are frequently used as supports for electrocatalysts because they are conductive and have high surface area. However, recent studies have shown that these materials can contain significant levels of metallic impurities that can dramatically alter their electrochemical properties. Here, the electrocatalytic activity of pure graphite (PG), graphene oxide (GO), and carbon nanotubes (CNT) dispersed on glassy carbon (GC) are investigated for the electrochemical CO2 reduction reaction (CO2RR) in aqueous solution. It was observed that GO and CNT dispersed on GC all exhibit significant electrochemical activity that can be ascribed to impurities of Ni, Fe, Mn, and Cu. The level of Cu in GO can be particularly high and is the cause for the appearance of methane in the products produced over this material when it is used for the CO2RR. Washing these supports in ultrapure nitric acid is effective in removing the metal impurities and results in a reduction in the electrochemical activity of these forms of carbon. In particular, for GO, nearly all of the catalytically relevant metals can be removed. Electrochemical deposition of Cu on GO and PG supported on GC, and on GC itself, increased both the electrochemical activity of these materials and the production of methane via the CO2RR. Particularly high rates of methane formation per unit of Cu mass were obtained for Cu electrodeposited on GO and PG supported on GC. We suggest that this high activity may be due to the preferential deposition of Cu onto defects present in the graphene sheets comprising these materials.