catalysts Review Advances in Clean Fuel Ethanol Production from Electro-, Photo- and Photoelectro-Catalytic CO2 Reduction Yanfang Song 1, Wei Chen 1,* , Wei Wei 1,2,* and Yuhan Sun 1,2,3,* 1 CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China; [email protected] 2 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China 3 Shanghai Institute of Clean Technology, Shanghai 201620, China * Correspondence: [email protected] (W.C.); [email protected] (W.W.); [email protected] (Y.S.); Tel.: +86-21-20350954 (W.C.) Received: 22 October 2020; Accepted: 3 November 2020; Published: 5 November 2020 Abstract: Using renewable energy to convert CO2 to a clean fuel ethanol can not only reduce carbon emission through the utilization of CO2 as feedstock, but also store renewable energy as the widely used chemical and high-energy-density fuel, being considered as a perfect strategy to address current environment and energy issues. Developing efficient electrocatalysts, photocatalysts, and photoelectrocatalysts for CO2 reduction is the most crucial keystone for achieving this goal. Considerable progresses in CO2-based ethanol production have been made over the past decades. This review provides the general principles and summarizes the latest advancements in electrocatalytic, photocatalytic and photoelectrocatalytic CO2 conversion to ethanol. Furthermore, the main challenges and proposed future prospects are illustrated for further developments in clean fuel ethanol production. Keywords: CO2 reduction; ethanol; electrolysis; photocatalysis; photoelectrolysis 1. Introduction With the fast development of the economy and society, the ever-increasing demand for energy all over the world while the limited fossil fuel resources lead to an aggravated energy crisis [1,2]. The huge consumption of fossil fuels causes the constantly accumulating of CO2 in the atmosphere. By May 2020, the concentration of atmospheric CO2 reached another record of 412.69 parts per million (ppm) [3], far exceeding the upper safety limit of 350 ppm, which may cause disastrous environmental consequences such as global warming, polar glacier melting, rising sea level, etc. [4]. On the other hand, the renewable energy sources from wind, sun, etc., have been rapidly developed in recent years. Unfortunately, the power from these renewable energy sources cannot be integrated into the electric grid well due to the intrinsic inferiorities of instability and anti-peak-load regulating, resulting in the huge waste and development limitation [5]. An ideal strategy to solve the energy and environmental problems is to convert CO2 into fuels and value-added chemicals using renewable electricity and/or solar energy. Such a strategy can not only reduce the concentration of atmospheric CO2 through the utilization of CO2 as feedstock, but also store renewable energy as fuels and useful chemicals, thus relieving our dependency on fossil fuels [6–8]. Powered by renewable electricity and/or solar energy, CO2 can be reduced to clean fuels, such as carbon monoxide (CO), methane, formic acid, methanol, ethanol, etc. [9,10]. By contrast, ethanol, a kind of clean and renewable liquid fuel with a higher heating value of 1366.8 kJ mol 1, is a preferred product. − · − With a higher energy density, easier to store and transport than that of gas products, ethanol has also Catalysts 2020, 10, 1287; doi:10.3390/catal10111287 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1287 2 of 25 Catalysts 2020, 10, x FOR PEER REVIEW 2 of 25 been considered as one of the optimal candidate fuels that substitute or supplement fossils in many applicationssupplement [fossils11]. Ethanol in many is theapplications most used [11]. and largestEthanol additive is the most to gasoline, used and and largest can be additive seamlessly to accessedgasoline, byand the can widest be seamlessly energy infrastructures. accessed by the Furthermore, widest energy ethanol infrastructures. is also an important Furthermore, and ethanol widely usedis also common an important chemical and feedstock widely used for organiccommon chemicals chemical and feedstock medical for disinfectant. organic chemicals Large-scale and medical ethanol productiondisinfectant. to dateLarge-scale is mainly ethanol based on production the fermentation to date of agriculturalis mainly carbohydratesbased on the such fermentation as cane sugar of andagricultural cornstarch. carbohydrates However, it such seems as thatcane nature sugar and cannot cornstarch. provide bothHowever, food andit seems fuel forthat a nature still-growing cannot andprovide increasingly both food energy-hungry and fuel for a worldstill-growing population and increasingly in the near future energy-hungry Therefore, world CO2 conversionpopulation toin ethanolthe near driven future by Therefore, renewable CO energy2 conversion offers a goodto ethanol alternative driven (Scheme by renewable1). energy offers a good alternative (Scheme 1). Scheme 1. Schematic illustration of carbon recycling via CO2-to-ethanol conversion powered by Scheme 1. Schematic illustration of carbon recycling via CO2-to-ethanol conversion powered by renewable energy sources such as solar and wind. renewable energy sources such as solar and wind. According to the variety of renewable solar energy assistance, CO2-to-ethanol conversion can be According to the variety of renewable solar energy assistance, CO2-to-ethanol conversion can be divided into three major categories: electrocatalytic reduction by an electrolyzer powered by commercial divided into three major categories: electrocatalytic reduction by an electrolyzer powered by photovoltaic (PV) devices, photocatalytic reduction by an efficient photocatalyst, photoelectrocatalytic commercial photovoltaic (PV) devices, photocatalytic reduction by an efficient photocatalyst, reduction by a semiconducting photocathode and an electrolyzer [12]. Over the past decades, photoelectrocatalytic reduction by a semiconducting photocathode and an electrolyzer [12]. Over the numerous efforts have been devoted to researching the three kinds of CO2 reduction techniques for the past decades, numerous efforts have been devoted to researching the three kinds of CO2 reduction production of clean fuel ethanol [13–16]. Different from C1 products (CO, CH4, formate, methanol, techniques for the production of clean fuel ethanol [13–16]. Different from C1 products (CO, CH4, etc.), the multiple electron–proton transfers involved with ethanol production from CO2 have been formate, methanol, etc.), the multiple electron–proton transfers involved with ethanol production reported with low efficiency due to the kinetic barriers. Typically, multiple electron–proton transfer from CO2 have been reported with low efficiency due to the kinetic barriers. Typically, multiple steps must be orchestrated with their own associated activation energies, thus presenting kinetic electron–proton transfer steps must be orchestrated with their own associated activation energies, barriers to the forward reaction [12]. Therefore, efficient and robust electrocatalysts, photocatalysts thus presenting kinetic barriers to the forward reaction [12]. Therefore, efficient and robust and photoelectrocatalysts are required to promote this kinetically sluggish reduction process. electrocatalysts, photocatalysts and photoelectrocatalysts are required to promote this kinetically This review will focus on the most-studied catalysts and their corresponding catalytic systems sluggish reduction process. for the reduction of CO to ethanol in the categories of electrochemical, photochemical and This review will focus2 on the most-studied catalysts and their corresponding catalytic systems photoelectrochemical approaches. Since the catalytic activity and selectivity are mainly determined by for the reduction of CO2 to ethanol in the categories of electrochemical, photochemical and the structures and surface states of catalysts as well as the reaction conditions, the second section of this photoelectrochemical approaches. Since the catalytic activity and selectivity are mainly determined review provides the general principles of electroreduction, photoreduction and photoelectroreduction by the structures and surface states of catalysts as well as the reaction conditions, the second section of CO , as well as the theoretical foundation for ethanol production. Lastly, a short prospect is given of of this2 review provides the general principles of electroreduction, photoreduction and the challenges and new directions in the development of efficient CO2 reduction to solar fuel ethanol. photoelectroreduction of CO2, as well as the theoretical foundation for ethanol production. Lastly, a short prospect is given of the challenges and new directions in the development of efficient CO2 reduction to solar fuel ethanol. 2. Basic Principles of Clean Fuel Ethanol Production from CO2 2.1. CO2 Electroreduction Catalysts 2020, 10, 1287 3 of 25 2. Basic Principles of Clean Fuel Ethanol Production from CO2 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 25 2.1. CO2 Electroreduction Electroreduction of CO2 is commonly carried out in aa gas-tight,gas-tight, two-compartmenttwo-compartment electrolysiselectrolysis cell equipped with a working electrode, a counter el electrodeectrode and a proton exchange membrane as the separator (Scheme(Scheme2 A).2A). The The membrane membrane was was employed employed to restrict to restrict the transport