Electrocatalytic Reduction of Carbon Dioxide

In the October issue of Chem, Han et al. reported that the recycling of carbon dioxide (CO2) for use as fuels and chemicals can be achieved by a combination of nanotechnology and molecular engineering to give robust, active, and selective organic-inorganic hybrid catalysts in the electrochemical reduction of CO2. In the October issue of Chem, Han et al. reported that the recycling of carbon dioxide (CO2) for use as fuels and chemicals can be achieved by a combination of nanotechnology and molecular engineering to give robust, active, and selective organic-inorganic hybrid catalysts in the electrochemical reduction of CO2. The removal and sequestration of carbon dioxide (CO2) by its conversion to value-added chemicals—such as carbon monoxide (CO), formic acid, methanol, or hydrocarbon—is an ideal way to tackle both environmental concerns of ever-increasing global CO2 levels and energy challenges. From the energy aspect, CO2 can serve as an economical and renewable C1 feedstock for the production of various energy-dense carbon-based fuels. Successful achievements in this highly topical theme would drastically affect our society, especially by decreasing current energy dependence on fossil fuels. The number of publications and patents on CO2 reduction over the past decade has remarkably increased (Figure 1), which clearly reflects the worldwide interest in and importance of this research. Electrochemical reduction of CO2 is appealing because of its multiple merits. It operates under ambient temperature and pressure conditions, and when it's interfaced with a solar input (a photo-electrochemical cell), the energy for CO2 reduction can come from sunlight. It can also offer a way to store excess renewable electricity as a chemical off the grid. CO2, however, is quite an inert molecule. Because of the large reorganization energy imposed by its extremely stable linear structure, the first single-electron reduction step to form a bent CO2⋅− radical is highly unfavorable and has a highly negative formal reduction potential (−1.90 V versus a normal hydrogen electrode in aqueous solution at pH 7). Concerted or sequential proton-coupled electron transfer is required for facilitating the activation of CO2, especially for the production of further reduced products via processes involving multiple electrons and protons. Another concern is the suppression of the hydrogen evolution reaction, which is usually the prevailing side reaction in many catalytic systems for CO2 reduction in aqueous media. Currently, most existing electrocatalysts suffer from one or more of the following problems: a competitive side reaction of hydrogen evolution, poor product selectivity, the high cost and poisoning of noble metals, and low long-term stability. Scaling up to meet the industrial need is also a challenge for their successful commercialization. Scientists have long desired to develop a highly efficient, selective, and durable electrocatalytic system with economic viability for practical CO2 reduction. In the October issue of Chem, Han et al. reported a polymeric cobalt phthalocyanine supported on a carbon nanotube (CNT) as a highly efficient electrocatalyst for CO2 reduction.1Han N. Wang Y. Ma L. Wen J. Li J. Zheng H. Nie K. Wang X. Zhao F. Li Y. et al.Chem. 2017; 3: 652-664Abstract Full Text Full Text PDF Scopus (298) Google Scholar The potential of transition-metal phthalocyanines as electrocatalysts for CO2 reduction was recognized as early as the 1980s. Despite several decades of exploration, transition-metal phthalocyanines still suffer from a rather high overpotential and poor stability. Han et al. have demonstrated that these challenges can be overcome through an effective catalyst design: organic-inorganic hybridization. They engaged a template-directed polymerization of cobalt phthalocyanine to form a uniform polymeric coating around a CNT. This hybridization approach yielded an enlarged electrochemically active surface area on the conductive scaffold, which improved physical and chemical robustness. With unique physicochemical properties, CNTs have been widely used in many applications. In electrochemical reactions, CNTs used as electrode materials can provide high surface area, hollow geometry, high electronic conductivity, and mechanical strength, all of which contribute to the promotion of electron transfer. Reaping the benefits of porous and conductive structure, a number of electrocatalysts—including cobalt phthalocyanine supported on a CNT—have been previously reported. Most of this work has anchored the electrocatalysts on a CNT surface by covalent grafting (via pending pyrene groups2Kang P. Zhang S. Meyer T.J. Brookhart M. Angew. Chem. Int. Ed. 2014; 53: 8709-8713Crossref PubMed Scopus (200) Google Scholar or pyrene substituents in a porphyrin complex3Maurin A. Robert M. J. Am. Chem. Soc. 2016; 138: 2492-2495Crossref PubMed Scopus (210) Google Scholar) or non-covalent interaction.4Zhang X. Wu Z. Zhang X. Li L. Li Y. Xu H. Li X. Yu X. Zhang Z. Liang Y. Wang H. Nat. Commun. 2017; 8: 14675Crossref PubMed Scopus (516) Google Scholar In contrast to these usual approaches, the authors used in situ selective polymerization and cross-linking of 1,2,4,5,-tetracyanobenzene together with cobalt ions and CNTs to afford poly-phthalocyanines as a thin conformal polymer coating layer around CNTs, probably mediated through the strong π-π interaction between the benzene rings and CNT sidewalls. Such a formation of an inorganic-organic hybrid structure with a one-dimensional CNT core and a uniform polymeric electrocatalyst sheath brings manifold advantages: suppressed aggregation of electrocatalysts, enlarged active surface area, and improved chemical and physical robustness. Also, the surface concentration of electrochemically active cobalt sites has shown to be significantly higher (∼14.5% of the total cobalt sites) than that reported for exquisitely designed cobalt porphyrin containing covalent organic frameworks (∼4%).5Lin S. Diercks C.S. Zhang Y.-B. Kornienko N. Nichols E.M. Zhao Y. Paris A.R. Kim D. Yang P. Yaghi O.M. Chang C.J. Science. 2015; 349: 1208-1213Crossref PubMed Scopus (1649) Google Scholar As a result, Han et al. achieved a highly selective conversion of CO2 to CO with a faradic efficiency of 90% and a turnover frequency of 4,900 hr−1 at η = 0.5 V. Moreover, the hybrid catalytic system showed excellent long-term stability for more than 24 hr, which the authors believed arose from the polymerization of cobalt phthalocyanine on CNTs with improved chemical and physical robustness of the electrochemically active species. With scientists' and engineers' increasing efforts to understand and design electrochemically driven processes for the reduction of CO2, we are optimistic that this challenging problem will eventually be solved and that one day we will all enjoy clean energy and a clean environment because of the inexpensive, efficient, and robust catalysts for CO2 reduction. Supported Cobalt Polyphthalocyanine for High-Performance Electrocatalytic CO2 ReductionHan et al.ChemOctober 5, 2017In BriefHan et al. have prepared a type of organic-inorganic hybrid material by template-directed polymerization of cobalt phthalocyanine on carbon nanotubes for a selective CO2 reduction reaction to CO with a large faradic efficiency, exceptional turnover frequency, and excellent long-term durability. Full-Text PDF Open Archive