|Fig. 1: Schematic Diagram of Electrochemical Reduction of Carbon Dioxide into Fuels. (Source: Z. Liang)|
Carbon dioxide is the most dominating greenhouse gas. In order to overcome excessive, environmentally harmful carbon dioxide emissions and control the carbon balance, conversion of carbon dioxide into useful chemicals and fuels gained a position of paramount research interest recently. Compared with other conversion methods including thermal and photochemical reactions, electrochemical reduction of carbon dioxide is a simple and cost-effective approach under mild conditions, which provides high energy-density reduction products such as formic acid, methanol and methane.  The very fundamental challenge for this electrochemical reduction process is the hydrogen evolution. During the reduction of carbon dioxide, protons in the electrolyte and carbon dioxide molecules compete for the incoming electrons. Most materials have a strong preference for proton reduction over carbon dioxide reduction in aqueous electrolytes unless extreme overpotentials are applied, which compromises the energetic efficiency. Therefore suppressing the proton reduction to hydrogen without compromising carbon dioxide reduction by the use of catalyst is the major target. 
Of all the valuable carbon dioxide reduction products, formic acid (HCOOH) is a very important chemical and functions as precursors for value-added chemicals and feedstocks for fuels. Demand for this formic acid keeps increasing recently in paper production, pharmaceutical synthesis as well as some traditional applications such as animal feeds additive and textile finishing. In addition, formic acid has been proposed as a promising fuel for hydrogen storage and fuel cells. Therefore electroreduction of carbon dioxide to formic acid appears to have the best chance for practical development of economically and technically viable processes since formic acid can be obtained with high selectivity in aqueous system. The conversion of carbon dioxide into formic acid follows the equation
Fig. 1 shows a typical setup for electrocatalytic reduction of carbon dioxide. The device is filled up with aqueous electrolyte such as potassium bicarbonate containing saturated carbon dioxide. The current collector loaded with catalyst serves as the working electrode in the three-electrode system. During the electrochemical reduction process, negative potential is applied on the working electrode and therefore electrons were driven from the counter electrode to the working electrode. Once the electrons reaches the working electrode, both protons and dissolved carbon dioxide molecules in the electrolyte compete for the electron to be reduced. 
Various cheap transition metals including lead, tin, and indium can be used as effective catalysts for carbon dioxide reduction into formic acid with high selectivity.  It is concluded that carbon dioxide species have stronger binding effect with these transition metal surfaces compared with protons. As a result, the reduction of protons to hydrogen is inhibited. 
To conclude, electrochemical reduction of carbon dioxide into low-carbon fuels is an important development and research subject for limiting the net emissions of greenhouse gas, carbon dioxide. Moreover, it provides the option of generating valuable fuels to solve our energy storage issues using undesirable carbon dioxide. By using the cheap transition metals as catalysts, hydrogen evolution could be suppresses and thus efficiency of carbon dioxide conversion is greatly improved. Nevertheless, it seems that the maturity of carbon dioxide electrochemical reduction technology to produce useful fuels is still far from reaching the requirements for commercialization, because of several major technological bottlenecks, including limited product selectivity, low catalyst activity and poor catalyst stability.
© Zheng Liang. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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