In situ deposited copper nanodendrites are herein proven to be a highly selective electrocatalyst which is capable of reducing CO2 to ethylene by reaching a Faradaic efficiency of 57% at a current density of 170 mA cm−2. It is found that the desired structures are formed in situ under acidic pH conditions at high electrode potentials more negative than −2 V versus Ag/AgCl. Detailed investigations on the preparation, characterization, and advancement of electrode materials and of the electrolyte have been performed. Catalyst degradation effects are intensively followed by scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (HR‐TEM) characterization methods and found to be a major root course for selectivity losses.

The mixtures of room temperature ionic liquid 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate ([EMIM]TFO) and water as electrolytes for reduction of CO2 to CO are reported. Linear sweep voltammetry shows overpotentials for CO2 reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM]TFO concentration in the range from 4 × 10−3m (0.006 mol%) to 4869 × 10−3m (50 mol%). A steady lowering of overpotentials up to an optimum for 334 × 10−3m is identified. At 20 mol% and more of [EMIM]TFO, a significant CO2 reduction plateau and inhibition of HER, which is limited by H2O diffusion, is noted. Such a plateau in CO2 reduction correlates to high CO Faraday efficiencies. In case of 50 mol% [EMIM]TFO, a broad plateau spanning over a potential range of 0.58 V evolves. At the same time, the overpotential for HER is increased by 1.20 V when compared to 334 × 10−3m and, in turn, HER is largely inhibited. The Faraday efficiencies for CO and H2 formation feature 95.6% ± 6.8% and 0.5% ± 0.3%, respectively, over a period of 3 h in a separator divided cell. Cathodic as well as anodic electrochemical stability of the electrolyte throughout this time period is corroborated in 1H NMR spectroscopic measurements.

Abstract:The direct electro-reduction of CO 2 to functional molecules like ethene is a highly desirable variant of CO 2 utilization. The formation of, for example, ethene from CO 2 is a multistep electrochemical process going through various intermediates. As these intermediates are organic species, the CO 2 reducing electro-catalyst has to be competent for a variety of organic functional group transformations to yield the final product. In this work, the activity of an in situ-grown nano-structured copper catalyst towards a variety of organic functional group conversions was studied. The model reagents were selected from the product spectrum of actual CO 2 reduction reaction (CO 2 RR) experiments and from proposals in the literature. The CO 2 bulk electrolysis benchmark was conducted at 170 mAcm −2 current density with up to 43% Faradaic Efficiency (FE) for ethene and 23% FE for ethanol simultaneously. To assure relevance for application-oriented conditions, the reactivity screening was conducted at elevated current densities and, thus, overpotentials. The found reactivity pattern was then also transferred to the CO reduction reaction (CORR) under benchmark conditions yielding additional insights. The results suggest that at high current density/high overpotential conditions, also other ethene formation pathways apart from acetaldehyde reduction such as CH 2 dimerization are present. A new suggestion for a high current density mechanism will be presented, which is in agreement with the experimental observations and the found activity pattern of copper cathodes toward organic functional group conversion.

Aqueous ionic liquid electrolytes featuring the promising 1,3‐dialkyl‐ and 1,2,3‐trialkyl‐imidazolium cations are reported in CO2 electrolysis up to 200 mA cm−2. A close‐to‐application flow cell setup equipped with silver gas diffusion electrode to overcome CO2 mass transport limitations was used. Faraday efficiencies for CO as high as 68 % were achieved, while inhibiting the hydrogen evolution reaction. Particular attention is paid to the stability of the ionic liquids, which is determined by 1H NMR spectroscopic measurements. The root cause of decomposition is the formation of hydroxide ions through reduction reactions. A high local pH arises that depends on the current density. The water content is also found to play a key role. An overall mechanism is proposed from analysis of the decomposition products including methylamine, ethylamine, formate, acetate and N1‐ethyl‐N2‐methylethane‐1,2‐diamine. Methylation of the susceptible C2 position of the imidazolium ring fails to stabilize the system under high conversion rate electrolysis conditions.