We synthesized a copper rubeanate metal organic framework (CR-MOF) which has the potential to improve the catalytic activity of electrochemical reduction of CO 2 due to its characteristics of electronic conductivity, proton conductivity, dispersed reaction sites, and nanopores. Synthesized CR-MOF particles were dropped on carbon paper (CP) to form a working electrode. The onset potential for CO 2 reduction of a CR-MOF electrode was about 0.2 V more positive than that observed on a Cu metal electrode in an aqueous electrolyte solution. Our analysis of the reduction products during potentiostatic electrolysis showed formic acid (HCOOH) to be virtually the only CO 2 reduction product on a CR-MOF electrode, whereas a Cu metal electrode generates a range of products. The quantity of products from the CR-MOF electrode was markedly greater (13-fold at −1.2 V vs. SHE) than that of a Cu metal electrode. Its stability was also confirmed. The electrochemical reduction of carbon dioxide (CO 2 ) into useful products at ambient pressures and temperatures has been a focus of research interest. A huge number of studies have been carried out in the past several decades especially for metal electrodes in aqueous media.1-4 Most metals have little catalytic activity for CO 2 reduction and mainly evolve hydrogen due to electrolysis of water, a competing reaction. Some metals produce carbon monoxide (CO) and formic acid (HCOOH), and, especially for HCOOH formation, a high applied potential of below −1.5 V vs. SHE is generally needed to obtain sufficient current for the process. Copper (Cu) has the unique property of producing hydrocarbons 3,5,6 and would appear to be the most hopeful candidate as a catalyst for CO 2 electrochemical reduction; however, Cu generates a range of reaction products and the selectivity for each product tends to be low. To improve its selectivity, there have been several explorations with metal alloys. 7 The basic idea is to disperse and specify the reaction site and to change the concentration of protons that are needed for electrochemical reduction of CO 2 . These protons are adsorbed by the atoms adjacent to each Cu site, where they induce a change in local reaction conditions and alter the reduction products of CO 2 . 8 Metal Organic Frameworks (MOFs) which have backbones constructed from metals and organic ligands have also been studied intensively. MOFs are structured and porous materials with nano-scale pores, and have been tested for applications including gas storage, 9,10 gas separation, 9,10 and heterogeneous catalysis. 9,11,12 Interestingly, some MOFs, such as copper rubeanate and its derivatives, show proton conductivity 13 that results from the adsorption of a proton to a nitrogen atom in the ligand, accompanied by electron transfer from Cu(II) ions.14 Those MOFs have been applied as electrochemical catalysts in ethanol fuel cell systems and have demonstrated selective formation of acetaldehyde as an oxidant of ethanol. 15 In spite of the interest shown in this area, there have until now been n...

In the aqueous electrochemical reduction of CO 2 , the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both experimental and simulation approaches, we report how the buffer capacity of the electrolytes affects the kinetics and equilibrium of surface reactant species and the resulting reaction rate of CO 2 with varying partial CO 2 pressure. Electrolytes investigated include KCl (nonbuffered), KHCO 3 (buffered by bicarbonate), and phosphate-buffered electrolytes. Assuming 100% methane production, the simulation successfully explains the experimental trends in maximum CO 2 flux in KCl and KHCO 3 and also highlights the difference between KHCO 3 and phosphate in terms of pK a as well as the impact of the buffer capacity. To examine the electrolyte impact on selectivity, the model is run with a constant total current density. Using this model, several factors are elucidated, including the importance of local pH, which is not in acid/ base equilibrium, the impact of buffer identity and kinetics, and the mass-transport boundary-layer thickness. The gained understanding can help to optimize CO 2 reduction in aqueous environments.

We achieved highly selective electrochemical reduction of CO 2 to C 2 H 4 (faradaic efficiency of 25%) by crystalline copper phthalocyanine (CuPc) supported on carbon black. Remarkably, noncrystalline CuPc generated by treatment of crystalline CuPc with sulfuric acid did not give C 2 H 4 selectively, suggesting that catalyst crystallinity is crucial for the selective conversion of CO 2 to C 2 H 4 . The stability of crystalline CuPc under electrochemical reduction conditions was also evaluated, showing that crystalline CuPc can selectively convert CO to C 2 H 4 in the initial stage (<10 000 s), as long as the crystallinity of the catalyst is maintained.

CO2 reduction with water and light illumination is realized using a gallium nitride (GaN) photoelectrode in which excited electrons induce CO2 conversion at the counter electrode. For the counter electrode, a copper (Cu) plate was chosen. The low affinity and wide gap of the nitride semiconductor enable us to create an electron–hole pair which has a sufficient energy for both CO2 reduction and water oxidation, in spite of the fact that a high energy for CO2 reduction is required. Within this system, the generation of formic acid (HCOOH) with 3% Faradic efficiency was confirmed by light illumination alone.

Applying combinatorial technology to electrochemical CO2 reduction offers a broad range of possibilities for optimizing the reaction conditions. In this work, the CO2 pressure, stirring speed, and reaction temperature were varied to investigate the effect on the rate of CO2 supply to copper electrode and the associated effects on reaction products, including CH4. Experiments were performed in a 0.5 M KCl solution using a combinatorial screening reactor system consisting of eight identical, automatically controlled reactors. Increasing the CO2 pressure and stirring speed, or decreasing the temperature, steadily suppressed H2 production and increased the production of other reaction products including CH4 across a broad range of current densities. Our analysis shows that the CO2 pressure, stirring speed, and reaction temperature independently contributed to the limiting rate of CO2 supply to the electrode (Jlim). At a constant temperature, the limiting current density of CH4 increased proportionally with Jlim, illustrating that the production rate of CH4 was proportional to CO2 supply. Varying the CO2 pressure and stirring speed hardly affected the maximum Faradaic efficiency of CH4 production. However, changes to the reaction temperature showed a significant contribution to CH4 selectivity. This study highlights the importance of quantitative analysis of CO2 supply in clarifying the role of various reaction parameters and understanding more comprehensively the selectivity and reaction rate of electrochemical CO2 reduction.