Electrochemical Reduction of Carbon Dioxide Using a Copper Rubeanate Metal Organic Framework

Abstract

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...

Facile Synthesis of M-MOF-74 (M=Co, Ni, Zn) and its Application as an ElectroCatalyst for Electrochemical CO2 Conversion and H2 Production

“…Meanwhile, Zn-MOF-74 led to 21.72% CO 2 conversion and 78.28% H 2 production. The CO selectivity obtained from Zn-MOF-74 (15.40%) was greater than that reported for a Cu rubeanate-based MOF (2%) [28] but smaller than that reported for an Fe porphyrin-based MOF (50%) [25]. However, the CO productivity (-0.69 mA cm ) of CO partial current was higher than that of Cu rubeanate-based (-0.36 mA cm -2 ) and Fe-porphyrin-based MOFs (-0.65 mA cm -2 ) at the same reduction potential.…”

“…Recently, there have been attempts to apply metal organic frameworks (MOFs) as catalysts for the electrochemical conversion of CO 2 [25][26][27][28] and H 2 production [29,30]. MOFs are made of metal ions coordinated to organic ligands, giving rise to characteristic three-dimensionally ordered crystalline structures.…”

“…These extraordinary properties of MOFs are attractive for use in the electrochemical conversion of CO 2 and production of H 2 . Previous studies focused on different types of MOF structures such as porphyrin-based [25,26], benzenetricarboxylate (BTC)-based [27], and rubeanate-based MOFs [28] for CO 2 reduction. Even though some samples showed promising activity, the previously suggested MOF structures required long and multiple steps for synthesis.…”

“…Hence, M-MOF-74 with various central metals can be obtained via an inexpensive and high-yield process. Thus far, only Fe [25], Co [26], and Cu [27,28] have been investigated as the central metal in the MOF structure for the electrochemical conversion of CO 2 , regardless of the type of MOF. From this experiment, we expect to demonstrate the effect of the type of central metal in MOF-74 on the electrochemical conversion of CO 2 , in terms of the reduced product and conversion efficiency.…”

Metal‐Organic Framework‐Based Nanomaterials for Electrocatalysis

“…[121] The onset potential was about 0.2 V more positive than that of a Cu electrode in an aqueous KHCO 3 medium, with HCOOH to be almost the only product. The product on the CR-MOF electrode was 13-fold greater than that of Cu metal electrode and the stability was confirmed, proving that highly selective reduction of CO 2 was promising through rational design of MOF structures.…”