Abstract
The realization of a complete techno-economy through a significant carbon dioxide (CO2) reduction in the atmosphere has been explored in various ways. CO2 reduction reactions (CO2RRs) can be induced using sustainable energy, including electric and solar energy, using systems such as electrochemical (EC) CO2RR and photoelectrochemical (PEC) systems. This study summarizes various fabrication strategies for non-noble metal, copper-based, and metal-organic framework-based catalysts with excellent FE for target carbon compounds, and for noble metals with low overvoltages. Even though EC and PEC systems exhibit high energy-conversion efficiency using excellent catalysts, they are not completely bias-free operations because they require external power. Therefore, photovoltaics, which can overcome the limitations of these systems, have been introduced. The utilization of silicon and perovskite solar cells for photovaltaics-assisted EC (PV-EC) and photovaltaics-assisted PEC (PV-PEC) CO2RR systems are cost efficient, and the III-V semiconductor photoabsorbers achieved high solar-to-carbon efficiency. This review focuses on all the members composed of PV-EC and PV-PEC CO2RR systems and then summarizes the special cell configurations, including the tandem and stacked structures. Moreover, current problems such as a low energy conversion rate, expensive PV, theoretical limitations, and scale-up to industrialization are discussed with the suggested direction.
KEYWORDS
Catalysts, electrochemical CO2 reduction reaction, photoelectrochemical CO2 reduction reaction, photovoltaic cell
1 INTRODUCTION
Since the 19th century, the acceleration of industrial development has led to the enormous combustion of fossil fuels.[1] The excessive emission of carbon dioxide (CO2) generated during the burning of fossil fuels accumulates in the atmosphere and traps greenhouse gases that cause not only abnormal climates such as cyclones, floods, and droughts, but also anthropological problems.[2-3] Therefore, there is an urgent need to find a variety of sustainable and renewable energy sources to replace fossil fuels.[4,5] Currently, to deal with this problem, electrochemical (EC) and photoelectrochemical (PEC) reactions with the CO2 reduction reaction (CO2RR) are considered as efficient potential solutions owing to their high energy efficiency and low cost.[6-8] CO2RR not only reduces the concentration of CO2 in the atmosphere but also produces additional value-added carbon compounds.[4,9] However, CO2molecules are extremely inert gases in the atmosphere owing to the thermochemically stable activity of the C=O bond; therefore, it is imperative to utilize efficient catalysts.[10,11]Moreover, photovoltaics (PV), using sustainable solar energy, is capable of serving additional power to the CO2RR system in combination with EC and PEC systems, which are known as photovoltaic-electrochemical (PV-EC) and photovoltaic-photoelectrochemical (PV-PEC) systems, respectively.[12] These systems require 2.6 V to carry out the oxygen evolution reaction (OER) at the anode and CO2RR at the cathode, which can be supplied by a solar cell.[13] Therefore, designing efficient OER and CO2RR catalysts, as well as innovative PV device systems, is essential for achieving high solar-to-carbon (STC) energy conversion efficiency.[14,15] As depicted in Scheme 1, we organized advances in the PV-assisted CO2RR for techno-economy.
In this review, the current challenges of PV-EC and PV-PEC cell systems, such as theoretical limitations due to the inherent semiconductor bandgap, inhomogeneous uniformity owing to increased active area, and efficiency degradation, are discussed. In particular, we focus on EC and PEC catalysts because carbon compounds are manipulated during CO2RR through sequential electron and charge transfer,[16] determined by the free Gibbs energy of CO2 absorption and the desorption energy of the products between the interface of the CO2 molecules and the catalyst electrode.[15,17,18] Although noble metals are not competitive in terms of industry, gold (Au) and silver (Ag) have many advantages, including low overpotential and high Faradaic efficiency (FE) over a wide range of potentials for the formation of CO from CO2,[19-21] thus, noble metals can serve as cathode parts of PV-EC and PV-PEC because of their advantages. Furthermore, non-noble, metal-organic framework (MOFs)-based, and single-atom-based catalysts as state-of-the-art catalysts[22-24] have been described owing to their abundant reserves, unique morphology, reconstruction, superior electron-charge transfer, excellent utilization, and tunable band gap energy.[25] In addition, only copper (Cu) is feasible among the diverse elements to produce C2+carbon compounds via continuous C-C coupling under CO2RR. [26,27] Therefore, we briefly explain intensive efforts to attain high selectivity for the desired carbon compounds by manipulating the surface facets of Cu, restructuring the morphology of Cu nanostructures, and causing synergistic effects from heterogeneous catalysts.[28-30] To accomplish a complete techno-economy, PV provides new possibilities to convert STC efficiency with minimal energy consumption by constructing self-contained PV-integrated CO2RR reactors.[31]Therefore, the performance of the PV-EC and PV-PEC type of CO2RR system that combines various types of PV including Si-based,[32]perovskite-based,[33] and GaAs-based[34] as well as tandem structures is introduced by analyzing the current/current density– voltage (I/J-V ) characteristics.[35]
2 ELECTROCHEMICAL CO2RR
Among all the CO2 conversion methods, the electrochemical reduction of CO2 is particularly attractive because of its advantages, such as an easy operating system and controllable process.[36-37] Currently, various types of electrocatalysts such as metals,[38-39] metals oxides,[40] alloys,[41] and single-atom catalysts,[42-44] have been studied to improve the performance of the CO2RR. Electrocatalysts are divided into four groups: noble metals, non-noble metals, copper (Cu)-based, and single-atom catalysts (SAC).