Owing to their favorable features such as efficient optoelectronic properties, low cost, and simple synthesis method, metal–halide perovskites can be considered a notable material for next-generation devices
. Metal–halide perovskites have a general formula of ABX3, where A is a monovalent organic or inorganic cation (e.g., CH
), and X is a halide ion. Depending on the cations in the A-site of the perovskite structure, these compounds are divided into organometallic halide perovskites (OHPs) and inorganic halide perovskites (IHPs)
. Metal–halide perovskites display high absorption coefficients and high exciton emission and tunable energy bands at ambient temperature. Also, these perovskites are described as favorite direct-bandgap semiconductor compounds
. MAPbI
are the main types of these perovskites and are the most widely applied materials in photovoltaic applications due to their high absorption coefficient and high electron and hole mobility
. In particular, these classes of hybrid metal–halide perovskites are well developed for various scalable solution-based deposition techniques, with a promising pathway toward the sustainable generation of renewable energies
. In spite of their outstanding features, the metal–halide perovskites have serious problems in practical applications because of their lack of stability.
The electrochemical reduction of CO
2 is a promising and outstanding method that leads to diminishing greenhouse gas emissions by allowing renewable energy storage in different chemical forms. This procedure presents exceptional benefits; for instance, the EC method can employ some environmentally friendly energy sources like solar energy, and it can be carried out at atmospheric temperature and pressure. Meanwhile, through altering the electrolyte and applied voltage, the reaction conditions and products can be controlled
[51]. Nevertheless, CO
2 is a stable and inert linear molecule that requires an appropriate electrocatalyst to assist in breaking the C=O bond. The EC process can yield a diversity of products, including formic acid, formate, oxalic acid or oxalate, carbon monoxide, formaldehyde, methanol, methane, ethanol, ethylene, and other products. Solid-oxide fuel cells (SOFCs) and organic- or inorganic-materials-based electrodes collected with typical electrochemical cells are usually utilized for CO
2 conversion in high-temperature and low-temperature methods, respectively.
Nowadays, in order to reduce the disadvantages and promote the benefits of electrochemical and photocatalytic techniques for efficient CO
2 reduction, the combination of both technologies is being investigated. In the photoelectrochemical technique (PEC), which is also known as artificial photosynthesis because it imitates nature’s energy cycle, CO
2 reduces into various products at ambient pressure and temperature by harvesting light energy. The PEC CO
2 reduction offers many advantages such as environmental compatibility, great selectivity, economic viability, and the use of solar as a renewable energy source. In PEC systems, photocathodes and anodes are p-type and n-type semiconductors, respectively. Whenever light is illuminated, electrons are formed at the conduction band (CB) and holes are generated at the valence band (VB), leading to band bending at the connection point of the p-type and n-type electrodes. To improve the CO
2 reduction efficiency, band bending is crucial to create discrete electrons and holes at the electrodes and the electrolyte interface
[9]. In the PEC CO
2RR process in aqueous solution (pH = 7), various intermediates are produced. The reactions below show the creation of electrons and protons that lead to the formation of various products
[52].
4. Perovskites for CO2 Reduction
4.1. Direct Electrochemical CO2 Reduction
An active and selective CO
2 reduction reaction (CO
2RR) is an important step in recycling excess CO
2 into renewable forms of carbon and building an energy-efficient society. Direct electrochemical CO
2RRs can proceed under ambient conditions and have been extensively studied to understand the fundamentals of catalysis and to develop efficient catalysts for practical applications
[53]. Li et al. showed, for the first time, that a Sr
2Fe
1.5Mo
0.5O
6−δ (SFM) ceramic fuel-reforming electrode with a cubic perovskite structure can be used as an electrocatalyst to electrolyze pure CO
2 and convert it into CO without using syn-gases such as H
2 and CO in solid-oxide electrolysis cells
[54]. Higher electrochemical efficiencies are demonstrated for single SFM cathodes using pure CO
2 as the feed gas compared with those reported for simple oxide–ceramic electrodes. The electrocatalytic properties of the SFM cathode for CO
2RR in solid-oxide electrolysis cells were tested by supported cells based on the double-layer electrolyte, containing an LSGM (La
0.9Sr
0.1Ga
0.8Mg
0.2O
3−δ) electrolyte and an LDC (La
0.4Ce
0.6O
2−δ) barrier layer with a thickness of ~230 μm and 5 μm, respectively. Owing to the improved conductivity and greater electrocatalytic activity at raised temperatures, higher current densities and faster CO
2RR rates were obtained by increasing the operational temperature. Applied temperatures of 650, 700, 750, and 800 °C under a voltage of 1.5 V led to current densities of 0.26, 0.33, 0.49, and 0.71 A·cm
−2, respectively. The achieved current density of 0.71 A·cm
−2 was greater than almost all of the reported results for electrolyzing CO
2 using stable oxide electrodes at 800 °C. For CO
2 electrolysis under harsh conditions of pure CO
2 without a shielding gas, a relatively stable efficiency was obtained, and the current density reached above 1 A cm
−2; furthermore, the faradaic efficiency (FE) was above 95%, which was hardly ever attained in the literature but very desirable for commercial use. By using a SFM−Sm
0.2Ce
0.8O
2−δ composite cathode in 1.5 V and different temperatures of 650, 700, 750, and 800 °C, current densities of 0.40, 0.54, 0.75, and 1.09 A cm
−2 were attained, respectively. Using the SFM−Sm
0.2Ce
0.8O
2−δ composite cathode, the electrocatalytic activity was considerably developed, and the current density showed a growth of about 53.5% in a practical voltage of 1.5 V at 800 °C. These results signified that the SFM ceramics are favorable catalysts for CO
2 electroreduction.
SrSnO
3 nanowires (NWs) with a single 1D nanostructure and cubic perovskite phase have been reported by Pi et al.
[55]. An adapted setup of a gas-tight two-chamber with a proton-exchange membrane (PEM) as the separator was designed for electrochemical investigation. An Ag/AgCl electrode, carbon-fiber-paper-modified SrSnO
3 nanowires in the cathodic chamber, and a Pt wire in the anodic chamber were used as the reference electrode (RE), working electrode (WE), and counter electrode (CE), respectively. When applied to CO
2ER catalysis, these prepared perovskite NWs show outstanding selectivity and catalytic activity in formate production, with a high FE (~80%) and remarkable current density over a wide potential range, making them outstanding electrocatalysts for the CO
2 electroreduction to formate. Consequently, great selectivity (~80%), a large current density (21.6 mA cm
−2), and a notable durability of at least 10 h were obtained, making a new class of perovskite NWs with potential applications for CO
2ER and beyond. In comparison, bulk SrSnO
3 and SnO
2 nanoparticles (NPs) show very low activity and/or selectivity for CO
2ER.
Various nanostructures based on transition-metal nitrides have been examined for electroreduction reactions; however, a very limited number of these materials have been verified for CO
2 reduction reactions. For instance, Ni
3N was employed for the CO
2 reduction to CO
[56]. Yin et al. prepared perovskites based on copper (I) nitride (Cu
3N) nanocubes as a new catalyst for the selective CO
2 reduction to ethylene (C
2H
4) under ambient conditions
[57]. These samples were synthesized by the nitridation of copper (II) hydroxide nanowires and were converted into multigrain nanowires to prove their great CO
2 reduction selectivity toward C
2 products, possibly due to the semiconductor properties of Cu
3N. These samples, with perovskite-related ReO
3 structures, can convert CO
2 into ethylene with a faradaic efficiency of about 60% and a Cu mass activity of around 34 A/g in a 0.1 M KHCO
3 electrolyte solution at −1.6 V compared with a reversible hydrogen electrode (RHE). Notably, in the products of the gas phase with a molar ratio of >2000 for C
2H
4/CH
4, the Cu
3N catalyst suspends CH
4 formation, and the highest selectivity is realized for CO
2RR by Cu-based catalysis.
Two bismuth-based halide perovskite composites, Cs
3Bi
2Br
9/C and Cs
2AgBiBr
6/C, were prepared for electrochemical CO
2 reduction to HCOOH by Wang et al.
[58]. They carried out the electrochemical analyses in acidic media (pH = 2.5) and could directly convert intermediates to formic acid instead of formate. In acidic electrolysis, the two Cs
3Bi
2Br
9/C and Cs
2AgBiBr
6/C composites can successfully inhibit structural degradation. A great FE of 92% at −0.95 V vs. RHE with a current density (j
HCOOH) of 133.7 mAcm
−2 was obtained by Cs
3Bi
2Br
9/C, while the Cs
2AgBiBr
6/C show a lower FE (68%) at −1.15 V vs. RHE. More investigation and analyses confirm that the existence of more Bi atoms in Cs
3Bi
2Br
9 is a crucial factor to more favorably generate OCHO* intermediates than Cs
2AgBiBr
6/C, accelerating the fabrication of HCOOH. Moreover, at −0.95 V over 20 h of electrolysis, for Cs
3Bi
2Br
9/C, the current density and FE (~92%) showed no evident decline, while the FE of HCOOH in Cs
2AgBiBr
6/C clearly deteriorated from 68% to 50% at −1.15 V after 10 h. The difference in stability between the two composites could be due to the faster charge transfer and faster increase in the CO
2 conversion ratio for Cs
3Bi
2Br
9 than Cs
2AgBiBr
6, leading to a reduction in the degradation effect and an enhancement in durability in an aqueous electrolyte.
A nanosized LaInO
3 perovskite electrocatalyst was effectively prepared by a simple hydrothermal–calcination method and applied in the electrocatalytic reduction of CO
2 to formate by Zhu et al.
[59]. This synthesized electrocatalyst displayed superb activity and excellent selectivity in the CO
2RR. In an aqueous electrolyte solution, CO
2 has low solubility, so its reduction reaction is rigorously restricted in the H-cell, and the current density was very low. So, the flow cell was applied to carry out the CO
2RR. Thus, to solve the problem of solubility and the inadequate mass transfer of CO
2 in an aqueous solution, a suitable setup was designed by the CO
2 diffusion and electrolyte flow-channel-flanked gas-diffusion electrode. The results showed that the current density and faradaic efficiency of LaInO
3 in the flow cell was higher than in the H-cell. Electrochemical tests showed that the FE of formate reached up 91.4% (−1.1 V vs. RHE) and the current density of formate was 116.8 mA cm
−2 (−1.1 V vs. RHE).
4.2. Photoelectrochemical CO2 Reduction
Making advances in electrocatalysts and photoelectrocatalytic systems for the selective and efficient production of a target product is a substantial challenge
[60][61][62]. Indeed, perovskite structures with interesting photoconversion and redox properties are always interesting for photoelectrochemical concepts. An In
0.4Bi
0.6 alloy-coated CH
3NH
3PbI
3-based photocathode has been demonstrated to produce selective CO
2 reduction with nearly 100% faradaic efficiency (FE) for formic acid production in aqueous solution by Chen et al.
[63]. Their examinations revealed that In
0.4Bi
0.6 and In
0.6Bi
0.2Sn
0.2 alloys led to more electrocatalytic activity for the selective production of HCOOH as a result of the presence of the In
5Bi
3 phases. The HCOO*, as the intermediate in this procedure, was better stabilized on the In
5Bi
3 phases than on other alloys. At an applied voltage of −1.2 to −1.3 V versus RHE, the highest FE (95−98%) for the production of HCOOH was achieved by the In
0.6Bi
0.2Sn
0.2 and In
0.4Bi
0.6 alloys. Next, a favorable method that included coating the perovskite halide with an In
0.4Bi
0.6 alloy was used. With the In
0.4Bi
0.6/CH
3NH
3PbI
3 photocathode, at a low applied potential of −0.52 V vs. RHE under AM 1.5G irradiation, almost 100% FE for formic acid formation could be attained, signifying a 680 mV positive shift relative to the potential detected for the In
0.4Bi
0.6 electrode. At −0.6 V vs. RHE, the faradaic efficiency remained at nearly 100% for at least 1.5 h, and a photo-assisted electrocatalysis efficiency of 7.2% was determined. This photoelectrocatalyst showed a high selectivity for CO
2 reduction to HCOOH.
LSV analysis was applied to examine the samples for their PEC performances in a cell containing propylene carbonate solution saturated with CO2 in 0.1 M tetrabutyl ammonium hexafluorophosphate (TBAPF6). Compared with the CH3NH3PbBr3 quantum dots, GO/CH3NH3PbBr3 showed an increase in photocurrents for the photoelectrochemical CO2 reduction, as the maximum variation of photocurrent was observed at ca. −0.6 V. The photocurrent response of the as-synthesized samples was investigated at −0.6 V. This voltage was selected from the LSV results. An increase in the photocurrent response was observed for GO/CH3NH3PbBr3, while CH3NH3PbBr3 showed a decaying photocurrent, suggesting that graphene oxide acts as a stabilizer to preserve CH3NH3PbBr3 QDs. Graphene oxide can protect CH3NH3PbBr3 quantum dots against degradation by organic solvents and can also act as a charge-transfer intermediate to efficiently distinguish generated electrons and holes. The as-prepared catalyst exhibits an outstanding yield for CO2 conversion to CO (1.05 μmol cm−2 h−1). The chronoamperometric analysis was applied to examine the effect of GO on the stability of CH3NH3PbBr3. The photocurrent of CH3NH3PbBr3 remained stable for about 1800 s. After introducing GO, the photocathode produced a long photocurrent for about 6000 s because of the protective effect of GO.
A PEC system based on a Au-decorated ZnO@ZnTe@CdTe photocathode and CH
3NH
3PbI
3 perovskite tandem cell has been verified by Jang et al. as an unbiased solar-driven CO
2 reduction to CO
[64]. The designed cell disclosed a stable efficiency of solar-to-CO of above 0.35% with a faradaic efficiency of over 80% and a solar-to-fuel efficiency of more than 0.43%, with H
2 as a byproduct. This PEC cell showed the selective CO creation from CO
2 reduction using an operative photocathode−perovskite tandem device working in sunlight without an exterior bias. The overpotential was decreased by the ternary photocathode, and the CH
3NH
3PbI
3 perovskite generated a voltage that is sufficient to initiate the reduction of CO
2 without an exterior bias.
4.3. Photovoltaic–Electrochemical CO2 Reduction
Artificial photosynthesis, a technological device that uses solar irradiation as an energy source and water as an electron source to convert carbon dioxide into energy-dense organic compounds (fuels or other carbons for the chemical industry), is attractive in specific contexts
[65]. This can be achieved in an integrated cell by using photovoltaic (PV) cells to supply photoelectrons and holes to be used in an electrochemical cell (EC) to oxidize water at the anode and to reduce carbon dioxide at the cathode, creating a PV-EC reference system to reduce CO
2 to fuels. The PV-EC systems can be classified into two general configurations, namely monolithic and polylithic. In a monolithic configuration, the junction terminals of the photovoltaic cell is directly connected to the anode and cathode of the EC unit, and the direct-circuit (DC) photogenerated power is designed to be used in the electrocatalytic process without any external electrical connections and/or inversion, while in a polylithic configuration, the PV and EC cells work separately, and the electrical power of the PV cell can be used in the electrochemical cell usually after a DC/DC inversion. Generally, the polylithic PV-EC has significantly better controllability of product selectivity, e.g., increasing the applied voltage via the series connection of PV modules and easier matching of the operating current by using certain ratios of the PV and EC areas.
Huan et al. provided a new catalyst for CO
2 conversion to hydrocarbons, which exhibited a faradaic yield of 62% in product generation
[66]. This three-layer catalyst contains a metal Cu core protected by layers of CuO and Cu
2O. Remarkably, over a 60 min electrolysis at −0.95 V versus RHE, the structure of the CuO cathode changed. When joined with an advanced, low-cost perovskite photovoltaic minimodule, this PV-EC device achieves a solar-to-fuel value of 2.3% by combining it with a mini-module perovskite photovoltaic device recognized using two sequences of three perovskite photovoltaics with an active area of 0.25 cm
2 coupled in parallel, offering a yardstick for PV-EC systems that includes plentiful earth-abundant elements. To examine the CO
2-reduction performance on this system, current–voltage measurements of the modules were applied, which exposed a high photo-to-current efficiency (17.5%) at 2.45 V and 10.0 mA. A constant current of 6.0 ± 0.2 mA with a current density of ~18 mA·cm
−2 and a potential of 2.8 ± 0.02 (V) were proved by the connection of both systems. Over a 50 min electrolysis by using the gas chromatography (GC) technique and analyzing the reduction products, it was shown that the PV-EC catalyst generates a steady current.
5. Conclusions
The conversion of CO
2 to added-value chemicals or fuels is an attractive strategy for storing such a renewable source of energy into the form of chemical energy. The catalysis technology has received more and more attention for such a concept. Therefore, in parallel to more developed systems for electrochemical water splitting, the utility of innovative and advanced catalysts to develop critical performance factors, e.g., durability, conversion efficiency, and energy/power densities, must be explored in the specific concept of the CO
2RR. To achieve this purpose, the scientific community has been working on the construction of new materials that allow the design of advanced electrode systems for efficient CO
2 capture and reduction. Recently, perovskite-based catalysts have drawn a growing interest in various sectors from renewable energy to environmental processes. Perovskite oxides have shown more potential in EC CO
2RRs because of their oxygen vacancy sites and lattice distortions, which create more active sites for electrochemical CO
2 reduction. Moreover, perovskite oxides have more stability under harsh conditions like high temperature and can be used for solid-oxide fuel cells. Due to the presence of halide ions, halide perovskites show higher absorption coefficients and have more carrier mobilities, which make them more suitable for PEC and PV-EC CO
2 reduction
[67][68]. To achieve outstanding perovskite-based electrocatalysts, there are various strategies available, including developing the intrinsic activity and increasing available active sites by defect engineering on the perovskite surface, crystal structure regulation, tuning the electrical conductivity and synthesis process, and surface modification. High electron transfer of perovskites leads to the reduction of ohmic losses and the evolution of active sites, developing electrocatalytic performance. To increase electronic conductivity, some strategies such as using electrically conductive materials and doping agents are beneficial.