Research progress of photoelectrochemical conversion of CO₂ to C₂₊ products

INTRODUCTION

The hazards of climate change to humans and the environment necessitate transitioning from our current fossil fuel economy to an economy based on renewable and carbon-neutral energy technologies. The urgency to reduce primary greenhouse gas (CO₂) emissions while continuing to provide fuel for the growing global population remains one of the most significant technological challenges of our time. Photoelectrochemical technology not only maximizes the advantages of photocatalysis and electrocatalysis, but their synergy also sparks a different kind of brilliance. Utilizing photoelectrochemistry (PEC) technology to convert CO₂ into fuel represents a new, clean, and green approach to using CO₂^[1-3], which is increasingly attracting the interest of researchers [Figure 1]. Similar to other types of catalysis, the PEC conversion of CO₂ also involves multiple-step processes, resulting in a wide range of products such as C₁ (CO, CH₄, HCOOH, etc.) and C₂₊ products (C₂H₄, C₂H₆, CH₃COOH, etc.). Currently, most reported CO₂ conversion products are C₁ products because the generation of C₂₊ products involves a C-C coupling process, which is more difficult to obtain than C₁ products. But, compared with C₁ products, C₂₊ products have higher added value, and research on directly converting CO₂ into C₂₊ products holds significant significance. To date, several review articles have covered the overall research status on PEC CO₂ conversion^[1-7], the influence of cocatalysts^[8-10], the design of the PEC cathodes^[11-15], the impact of PEC parameters^[16,17], and the study of catalysts^[18-20]. However, no individual summaries have been on converting CO₂ into C₂₊ products using the PEC technology. Hence, firstly, this paper aims to briefly introduce the basic principles of CO₂ conversion into C₂₊ products. Secondly, it summarizes the catalysts and reactor types reported thus far for obtaining C₂₊ products. Thirdly, it emphasizes the design strategies of catalysts currently available for generating or enhancing C₂₊ selectivity. Finally, it concludes with a summary of the current work and prospects for future research.

Figure 1. The role of light and electricity in photoelectrochemical CO₂ reduction and the number of publications on PEC CO₂ conversion were obtained using keyword search (photoelectric and CO₂). Data was extracted from the Web of Science in May 2024. PEC: Photoelectrochemistry.

FUNDAMENTAL OF PHOTOELECTROCHEMICAL CO₂ REDUCTION

The PEC reduction of CO₂ involves light absorption, charge carrier transfer, CO₂ adsorption and activation, surface reactions, and product desorption. The energy input for PEC catalysis is provided by light and electricity. Light plays a role in generating high-energy photoelectrons for CO₂ reduction, generating photoelectric voltage to reduce energy consumption to some extent and lower economic costs, and producing heat to promote the progress of the reaction. Electricity plays a role in introducing an additional bias voltage to accelerate charge carrier separation and increase local charge density. By adjusting the bias voltage, the CO₂ reduction pathway can be altered to enhance selectivity.

Additionally, the supplementation of voltage can enable semiconductor materials that were originally not viable for photocatalytic CO₂ conversion to become feasible [Figure 1]. The synergistic effects of light and electricity can effectively reduce the activation energy for CO₂ reduction, but the study of the photoelectric synergistic effect is still lacking^[21,22]. Furthermore, the contribution and distinction between photogenerated and voltage-provided electrons have not yet been reported.

Based on current reports, the mechanism for converting CO₂ into C₂₊ involves first converting CO₂ into ^*CO intermediates, followed by the generation of various intermediates of C₂₊ products through C-C coupling and multi-proton-coupled electron transfer (PCET), as shown by the equations for different pathways (Route 1-6)^[23]. However, the competitive adsorption of ^*H_ad and ^*CO on monocomponent catalysts hinders the enhancement of PCET and C-C reaction kinetics. In order to break the linear scaling relationship between the adsorption of ^*H_ad/^*CO intermediates and improve water dissociation on auxiliary sites or CO₂-to-CO production, tandem catalysts consisting of multicomponents have been developed. These catalysts can achieve higher coverage of ^*H_ad and ^*CO, which is more conducive to the generation of C₂₊ products. However, to our knowledge, while there are detailed reports on achieving C₂₊ products through serial pathways in the electrocatalytic conversion of CO₂^[24], their application in the field of photoelectrocatalytic CO₂ conversion has not been observed.

Route 1:

Route 2:

Route 3:

Route 4:

Route 5:

Route 6:

CATALYSTS AND REACTORS OF PHOTOELECTROCHEMICAL CO₂ UPGRADING TO C₂₊

Hitherto, catalyst research for the PEC conversion of CO₂ into C₂₊ products is focused on photocathodes, primarily using Cu-based catalysts^[21,24-29] [Table 1]. Cu-based species have a high binding affinity for ^*CO intermediates, making them active sites for CO₂ reduction and the generation of C₂₊ chemical species. However, the high energy barriers associated with C-C coupling and PCET processes still pose challenges for Cu-based catalysts regarding high overpotential and low selectivity towards C₂₊ products. In addition to Cu-based catalysts, reasonable structural design of some non-Cu-based catalysts enables higher selectivity towards C₂₊ products. Table 2 lists the selectivity of some non-Cu-based catalysts in photoelectrocatalytic CO₂ conversion. It shows that the non-Cu-based catalysts are mainly composite materials, including TiO₂, ZnO, WO₃, etc., with more negative potentials compared to Cu-based catalysts. Regardless of Cu-based or non-Cu-based catalysts, ethanol, acetic acid, and ethylene are the main C₂₊ products; the yield of C₂₊ products is relatively low (typically at the μmol level); the Faradaic efficiencies cover a wide range, and the faradaic efficiency for a single C₂₊ product can reach up to 80%. The selectivity of C₂₊ products is primarily enhanced by adjusting the synthesis process of materials, doping with heteroatoms, and compositing with other metals/metal oxides/non-metallic materials to alter the morphology, band structure, and local electron density of the electrode materials.

In addition to catalysts, the selection of reactors also significantly influences the generation of C₂₊ products. The light source, membrane characteristics, electrolyte, manufacturing materials, wall thickness, operational modes (batch or flow), heat exchange, phase requirements (gas-solid, liquid-solid, or gas-liquid-solid), CO₂ solubility, and flow characteristics of feed gases and electrolytes can affect catalytic performance^[54,55]. Currently, reactors used for CO₂ PEC conversion are primarily modified from electrochemical CO₂ reduction reactors and can be divided into H-type and flow electrolysis cells [Figure 2]^[56-58]. The H-type electrolysis cell is a relatively simple device that belongs to a batch reactor. It separates the anode and cathode using ion exchange membranes. However, its significant Ohmic resistance between the electrodes and limited CO₂ solubility have hindered its development. The low current density (< 100 mA·cm^-2) achieved in H-type cells does not meet industrial requirements. Furthermore, alkaline electrolytes are more conducive to suppressing H₂ reduction and enhancing C-C coupling. However, alkaline electrolytes are not feasible in aqueous CO₂ reduction conducted in H-type electrolysis cells. Nevertheless, the gas diffusion electrode (GDE) of the flow electrolysis cell separates the CO₂ flow field from the liquid electrolyte, allowing the use of high-concentration alkaline electrolytes.

Figure 2. PEC reactor for CO₂ conversion (A) H-type cell (batch reactor)^[54]; (B) flow cell (continuous reactor). PEC: Photoelectrochemistry.

The flow electrolysis cell is a continuous operation reactor where gases and electrolytes can be continuously supplied to the electrolytic cell. GDEs are used as the electrodes, which greatly overcome the limitations of CO₂ solubility. However, a certain distance between the anode and cathode still exists, resulting in significant Ohmic resistance. The membrane-electrode reactor integrates the anode, cathode, and membrane assembly, significantly reducing the distance between the electrodes and exhibiting improved catalytic performance. However, it lacks a reference electrode, making it challenging to obtain accurate working electrode potentials. Although membrane-electrode reactors have found wide applications in electrochemical CO₂ reduction, there are few reports on their use in photoelectrochemical CO₂ conversion^[37]. In addition, the state-of-the-art electrocatalytic CO₂ reduction performance achieved from flow electrolysis cells and membrane-electrode electrolyzers can almost produce CO and formic acid at industrial current densities. However, producing single high-value C₂₊ products, such as ethylene and ethanol, remains challenging. As for photoelectrochemical CO₂ reduction, the current densities used in the electrolytic cells have a significant gap compared to industrial current densities. It is worth mentioning that most of the recent research employs self-designed, assembled, or customized CO₂ photoelectrochemical reactors. No universally recognized standard exists for the configuration and operation of CO₂ electrolyzers. Different studies use distinct reactor structures, components, and catalysts, making it difficult to directly compare results and select optimal cathodic catalysts.

STRATEGIES TO IMPROVE THE SELECTIVITY OF C₂₊ PRODUCTS

The photoelectrochemical conversion of CO₂ into C₂₊ products is highly challenging. The entire process involves multiple aspects: absorption of light to obtain electron-hole pairs, followed by charge separation and transfer to the surface of the photocathode. At the same time, CO₂ adsorption is necessary for reducing CO₂ to form C₁ radicals and intermediates, and the stability of intermediates and C-C coupling is crucial. Moreover, the production of C₂₊ products requires more electrons compared to C₁ products. These processes indicate that improving light absorption, enhancing CO₂ adsorption capacity, increasing charge separation and transfer efficiency, and constructing suitable active sites are essential for obtaining high-content C₂₊ products. In addition, the reactor configuration, operating conditions, and electrode assembly also significantly influence catalytic performance. A continuous flow PEC reactor can produce more C₂-C₃ products compared to batch reactors^[58], and selecting an appropriate voltage is beneficial for the generation of C₂₊ products^[17]; appropriate electrode assembly can increase CO₂ adsorption, suppress hydrogen production, and facilitate C₂₊ product generation^[40]. However, there is a lack of research on the impact of reactor configuration, operating conditions, and electrode assembly on the obtained C₂₊ products. Besides, the current yield of C₂₊ products from CO₂ conversion via photoelectrocatalysis is generally low (often below 50 µmol·cm^-2·h^-1), with correspondingly low current densities (below 10 mA·cm^-2); many studies do not report CO₂ conversion rates or product yields^[25]. According to the reaction process, conversion rates and yields can be enhanced by adjusting reactor and reaction parameters (such as applying pressure) and modifying the properties of the photoelectrode. Specifically, improving light absorption capacity (through morphology control and elemental doping, etc.), enhancing charge carrier separation (via auxiliary metal addition and defect or heterojunction construction, etc.), and promoting surface reactions (by depositing a thin bilayer of Sustainion/Nafion, etc.) can contribute to these enhancements^[4]. Therefore, this section mainly summarizes the research progress in improving the selectivity of C₂₊ products from four aspects: electrode material morphology design, adjustment of band structure, construction of heterojunctions, and design of active sites.

Morphology

The morphology significantly influences the selectivity of CO₂ PEC reduction products [Figure 3]. Current research suggests that C₂₊ products can be acquired by constructing specific spatial structures. Wang et al. found that three-dimensional (3D) nanorod-shaped zinc oxide exhibited the optimal spatial environment compared to amorphous block-, mushroom-, and ball cactus-like structures, facilitating C-C coupling to produce C₂ products (ethanol and acetic acid)^[44]. Furthermore, their research group discovered that porous PEC based on a 3D hierarchical structure of covalent organic frameworks (COFs) could effectively enhance CO₂ adsorption and the rate of C-C coupling to generate C₂₊ products^[30]. Hollow catalysts exhibited different selectivity on the inner and outer surfaces due to the increased collision probability of intermediates within the closed cavity, achieving a high selectivity of C₂₊ products (67.0%) on the inner surface^[49]. In addition, by simulating the morphology of thylakoids in plant cells, the Si@WO₃-nanosheet (NS) catalyst with a 2D-3D structure is advantageous for achieving C-C coupling^[53].

Figure 3. (A) SEM image of 3D nanorod ZnO^[44]. Copyright 2018 Springer; (B) COFs-based porous photoelectrochemical with 3D hierarchical structure for CO₂ reduction^[30]. Copyright 2020 Elsevier; (C) TEM image of CdS/V_S-MoS₂ and (D) catalytic performance of CdS/V_S-MoS₂-PS^[49]. Copyright 2023 Elsevier. SEM: Scanning electron microscopy; 3D: three-dimensional; COFs: covalent organic frameworks; TEM: transmission electron microscopy.

Band structure

The band structure of electrode materials significantly influences light utilization and the types of CO₂ reduction products. A narrower band gap is beneficial for broadening the absorption range of light. In contrast, adjustable conduction band positions facilitate the control of product selectivity [Figure 4]. The position of the conduction band is related to the reduction capability of the catalyst, playing a crucial role in the photoelectrocatalytic reduction of CO₂ and serving as a thermodynamic factor influencing the availability of C₂₊ products. Adjusting the conduction band to the appropriate position is beneficial for improving the acquisition of certain C₂₊ products. Han et al. achieved improved selectivity of CO₂ reduction products by depositing metals such as Pd, Pt, Ni, Au, etc., to modulate the conduction band potentials of the TiO₂@ZnO/FTO^[45]. Besides doping with metals, doping with non-metals can alter the conduction band position. Yu et al. designed and prepared a novel P-doped WO₃ catalyst where P doping elevated the conduction band position of the WO₃ electrode from -0.13 to -0.54 V [V vs. normal hydrogen electrode (NHE)], achieving an acetic acid yield of 4.7 μmol/(L·h·cm²) at a conduction band position of -0.38 V (V vs. NHE)^[48].

Figure 4. (A) Band structure of samples; (B) CO₂ PEC performance of the samples^[45]. Copyright 2018 Elsevier. PEC: Photoelectrochemistry.

Heterojunction

The built-in electric field of heterojunctions facilitates carrier separation, accelerates charge transfer, and enhances the velocity of intermediates, resulting in a high probability of C-C coupling and, consequently, higher C₂₊ product yields^[31,53] [Figure 5]. Lu et al. constructed 0D/1D CuFeO₂/CuO nanowire heterojunction arrays. They achieved a higher Faradaic efficiency (66.73%) for ethanol production at -0.6 V vs. Ag/AgCl, mainly attributed to the rapid separation of photogenerated electrons and holes in the heterojunction material and the transfer of electrons from CuFeO₂ to CuO, enabling highly content of C₂₊ products (ethanol)^[39]. Xu et al. fabricated 2D TiO₂/Ti₃CN MXene heterojunctions, where the Pd@TOCN-200 photoelectrode exhibited the highest ethanol production rate (14 µM·cm^-2·h^-1) due to the enhanced CO₂ adsorption and favorable C-C coupling environment provided by the 2D TiO₂/Ti₃CN heterojunction structure^[50].

Figure 5. (A) Band structure of CuO-CuFeO₂; (B) CO₂ PEC performance of samples at -1.0 V vs. Ag/AgCl^[39]. Copyright 2023 Elsevier; (C) Band structure of Pd@TOCN; (D) CO₂ PEC performance of samples^[50]. Copyright 2023 Elsevier; (E) Band structure of Si@WO₃ and (F) CO₂ PEC performance of samples^[53]. Copyright 2023 Elsevier. PEC: Photoelectrochemistry.

Active site

The active site is a critical part of catalytic reactions and directly determines the distribution of products. They change product distribution by affecting the adsorption strength of products and intermediates and promote the coupling of ^*CO with ^*CO by adjusting the local electron density gradient, facilitating C-C coupling, and enhancing the acquisition of C₂₊ products. In the PEC CO₂ reduction reaction, active sites composed of multimetallic species^{[31,33,35,36,52]}, oxygen vacancies/sulfur vacancies^[34,51], certain specific functional groups (such as adjacent imine groups)^[47], and metal-nonmetal junctions^[38] are favorable for CO₂ adsorption and C-C coupling, promoting the generation of C₂₊ products [Figure 6]. Among them, Lu et al. doped Re into CuO and combined it with Ti nanotubes. Under solar light and -0.5 V vs. saturated calomel electrode (SCE) conditions, the product was almost entirely ethanol (1.9 µM·h^-1). The above results were attributed to high-valence rhenium as an active site for hydrogenation reactions. Its doping promoted the alcoholization process and facilitated C-C coupling^[33]. Zhou et al. adjusted the surface sulfur vacancies of Cu₂ZnSnS₄ (CZTS)/CdS by annealing in different atmospheres, which were active sites for CO adsorption and followed C-C coupling. By rational tuning, they achieved higher ethanol selectivity^[34]. In addition, Wang et al. introduced N into CuO to obtain Cu-N active components. These components possessed asymmetric d-p orbitals, which could reduce the activation energy of C-C coupling and induce local charge redistribution, thereby improving the material’s conductivity. They achieved a 15.2% C₂ product selectivity in a water solution at 0.2 V vs. reversible hydrogen electrode (RHE)^[38].

Figure 6. (A) Schematic mechanism of Re-doped CuO/TiO₂-NTs for CO₂ reduction^[33]. Copyright 2021 Elsevier; (B) The scheme for product generation of TiO₂-NT/GNR-metal electrodes^[35]. Copyright 2021 Elsevier; (C) The corresponding CO₂ conversion yields of CZTS/CdS (HA) and CZTS/CdS (HN)^[34]. Copyright 2021 Wiley; (D) CO₂ PEC performance of Pd/R-TiO₂/TiN-x^[51]. Copyright 2023 Elsevier; (E) Schematic mechanism of EY+S-TiO₂@GS for CO₂ reduction^[47]. Copyright 2020 Elsevier; and (F) Schematic mechanism of CuN_x/CuO for the CO₂ PEC reduction^[38]. Copyright 2022 Elsevier. PEC: Photoelectrochemistry.

CONCLUSION AND PROSPECTS

Currently, research on PEC conversion of CO₂ to C₂₊ products primarily focuses on developing photoelectrodes. The catalysts used are mainly Cu-based, and the major C₂₊ products obtained are ethanol, acetic acid, and ethylene. The involved reactors are predominantly flow reactors. Adjusting the catalyst morphology, band structure, and active sites can improve light absorption, enhance charge carrier transfer, strengthen CO₂/CO^* adsorption, and provide a favorable C-C coupling environment, achieving high selectivity for C₂₊ products. Although some progress has been made in the research on PEC CO₂ conversion to C₂₊ products, there is currently limited reporting on products containing three or more carbon atoms. The types of catalysts used are also limited, and their yields are relatively low (typically at the level of μmol). The reactors used in studies are mostly self-designed, assembled, or customized. There is no recognized standard for the configuration and operation of CO₂ reactors, and it is difficult to selectively optimize cathode catalysts by directly comparing different research reports. The current density employed in electrolytic cells differs significantly from industrial-scale requirements, making industrialization challenging. The present reaction pathway is not yet clear, and there is still insufficient research on the specific reaction mechanisms involved in catalysts. Additionally, research on photoelectric synergies is lacking. Therefore, increasing efforts in reactor development and establishing design standards, exploring and developing cost-effective and high-performing catalysts, and conducting in-depth research to enhance catalytic performance and understand reaction mechanisms are the main directions for future investigations.