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).