CO 2 reduction: the quest for electrocatalytic materials Bahareh Khezri, ab Adrian C. Fisher bc and Martin Pumera * ab Rising levels of carbon dioxide (CO 2 ) are of significant concern in modern society, as they lead to global warming and consequential environmental and societal changes. It is of importance to develop industries with a zero or negative CO 2 footprint. Electrochemistry, where one of the reagents is electrons, is an environmentally clean technology that is capable of addressing the conversion of CO 2 to value-added products. The key factor in the process is the use of catalytic electrode materials that lead to the desired reaction and product. Significant progress in this field has been achieved in the past two years. This review discusses the progress in the development of electrocatalysts for CO 2 reduction achieved during this time period. 1. Introduction Carbon dioxide (CO 2 ) emitted from fossil fuels is responsible for the majority of anthropogenic CO 2 emissions. Concerns of continuous increases in atmospheric CO 2 concentration causing global warming are growing. As a result, environmen- talists are seeking help to limit fossil fuel consumption to avoid and reduce CO 2 emissions. 1 However, technological advance- ments in renewable energy and nuclear power have been incorporated to reduce the consumption of fossil fuels. None- theless, renewable energies are not accessible in many regions, which continue heavy use of fossil fuels. Although new tech- nologies are contributing signicantly to lower CO 2 emissions, the replacement of all current technologies is not yet possible. The capture and utilization of CO 2 has attracted attention worldwide since this process can convert carbon dioxide to a wide range of value-added chemicals 2,3 such as carbon monoxide, formic acid, methanol, ethanol, and ethylene through radiochemical, 4 thermochemical, 5,6 biochemical, 5 photochemical, 5–8 and electrochemical 2,5,9–15 reactions using homogenous/heterogeneous catalysts where required. However, the use of electrochemical methods for CO 2 reduction on a real scale remains a challenge; 16,17 a great deal of research is going on in this eld. Electrochemical conversion can be performed under ambient temperature and pressure conditions. In most cases, the co-reactant is water and the required electricity can be obtained by using renewable energy resources. 2,10,18,19 The literature indicates that high overpotential, catalyst lifetime, and low selectivity are key challenges that this eld still faces. The development of efficient electrocatalysts plays an important role in the electrochemical reduction of CO 2 in terms of activity and selectivity/faradaic efficiency (FE). 9,20 In addition, the mechanism of reduction is not exactly known in most cases, it is only hypothetical. A fundamental understanding of the reaction kinetics and thermodynamics is necessary to overcome these challenges. Despite good progress and numerous studies in this eld, large-scale electrochemical CO 2 reduction is chal- lenging. It requires new strategies, materials, and advance- ments in the fundamental understanding of the reactions involved to make it economical and practical. 21 This review will discuss the most recent advancements in electrochemical CO 2 conversion, focusing on years 2015–2016. CO 2 conversion electrocatalysts are usually categorized into two main classes: homogenous and heterogeneous. Homogenous electrocatalysts have attracted increased attention; several reviews cover their advancement. 10,22 In contrast, our review focuses on heterogeneous catalysts. Heterogeneous catalysts will be discussed in three main categories: (i) metallic catalysts including noble metals, copper, hydride catalysts, molecular metal catalysts, and carbon supported metal catalysts; (ii) metal- free catalysts; and (iii) transition metal dichalcogenide catalysts. 2. The chemistry behind CO 2 reduction The nal products of CO 2 reduction are mostly dened based on the electrocatalyst (cathode) employed in the electro- chemical reaction. This multi-step reaction generally involves adsorption on the surface of the catalyst (cathode), electron/ proton transfer and desorption from the electrocatalyst surface. Hori et al. 23 suggested that the formation of CO 2  c is a Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore. E-mail: pumera@ ntu.edu.sg; pumera.research@gmail.com b Cambridge Centre for Advanced Research and Education in Singapore, CREATE Tower, 1 Create Way, 138602, Singapore c Department of Chemical Engineering and Biotechnology, New Museum Site, Cambridge University, Pembroke Street, Cambridge CB2 3RA, UK Cite this: J. Mater. Chem. A, 2017, 5, 8230 Received 15th November 2016 Accepted 2nd April 2017 DOI: 10.1039/c6ta09875d rsc.li/materials-a 8230 | J. Mater. Chem. A, 2017, 5, 8230–8246 This journal is © The Royal Society of Chemistry 2017 Journal of Materials Chemistry A REVIEW Published on 21 April 2017. Downloaded by University of California - San Diego on 11/05/2017 09:04:58. View Article Online View Journal | View Issue