Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry

Single-Molecule Fluorescence Imaging of Energy-Related Catalytic Reactions

The pressing challenges of the energy crisis and environmental problems necessitate the pursuit of efficient and sustainable energy conversion technologies, wherein catalytic processes play a vital role in addressing these issues. Single-molecule fluorescence microscopy (SMFM) offers a transformative approach to understanding various catalytic reactions by enabling real-time visualization of molecular adsorption, diffusion, and transformation on catalytic surfaces. The unprecedented insights into the spatial distribution of active sites, catalytic heterogeneity, and the dynamics of key intermediates result in single- or subparticle level structure-property relations, thereby offering insightful perspectives for catalyst design and mechanistic understanding of energy-related catalytic processes. In this review, we provide an overview of the recent progress in using SMFM for investigating energy-related catalytic reactions. The advancement in SMFM imaging techniques for investigating nonfluorescent chemical processes is also highlighted. Finally, we conclude the review by commenting on the current challenges and prospects in advancing SMFM in energy research. We hope that the capable SMFM imaging techniques and insights will promote the development and realistic application of various energy-related catalytic reactions, together with inspiring researchers to explore the power of SMFM in other applications.

Modulation of interface structure on titanium-based metal-organic frameworks heterojunctions for boosting photocatalytic carbon dioxide reduction

Rational regulation of interface structure in photocatalysts is a promising strategy to improve the photocatalytic performance of carbon dioxide (CO2) reduction. However, it remains a challenge to modulate the interface structure of multi-component heterojunctions. Herein, a strategy integrating heterojunction with facet engineering is developed to modulate the interface structure of metal-organic frameworks (MOF)-based heterojunctions. A series of core-shell UiO-66 (Zr-MOF)-loaded MIL-125 (Ti-MOF) heterojunctions with exposed specific facets were prepared to enhance the separation efficiency of photogenerated electrons-holes in CO2 photoreduction. Impressively, MIL-125to@UiO-66 with exposed {1 1 1} facet exhibits an excellent CO production rate (56.4 μmol g-1 h-1) and selectivity (99 %) under visible light irradiation without any photosensitizers/sacrificial agents, being 1.4 and 11.3 times higher than individual MIL-125to and UiO-66, respectively. The type-II heterojunction significantly enhances the separation of photogenerated electrons-holes in physical space. The photogenerated electrons migrate from Zr in UiO-66 to Ti in MIL-125to, promoting a spatial synergy between CO2 reduction on MIL-125to and H2O oxidation on UiO-66. Compared with MIL-125rd@UiO-66 with exposed {1 1 0} facet and MIL-125ds@UiO-66 with exposed {0 0 1} facet, MIL-125to@UiO-66 with exposed {1 1 1} facet improves the exposure of surface-active Ti sites, thereby enhancing the adsorption/activation of CO2 to generate the *COOH intermediate. This work provides an effective strategy for designing MOF-based heterojunction photocatalysts to improve photocatalytic performance.

Single-Atom Catalysts for Converting CO2 into High Value-Added Products: From Photocatalysis and Electrocatalysis to Photoelectrocatalysis

Converting CO2 into valuable products via photo-, electro-, and photoelectrocatalysis offers the possibility of simultaneously mitigating global warming and energy shortages. Single-atom catalysts (SACs) have garnered significant interest from researchers owing to their optimal atom use, suitable coordination environments, distinctive electronic structures, and highly dispersed active sites. This work offers a thorough examination of the progress of research on SACs for photocatalytic, electrocatalytic, and photoelectrocatalytic conversion of carbon dioxide. The fundamental concepts of photo-, electro-, and photoelectrocatalytic reduction of CO2 are briefly described, respectively. Second, the preparation approaches and characterization techniques of SACs are summarized, with a focus on how to increase the single-atom loading rate and achieve scale-up preparation. Finally, the specific applications of SACs for photo-, electro-, and photoelectrocatalytic conversion of CO2 are discussed, and the future development of SACs in the field of CO2 catalytic reduction is summarized and prospected. Herein, the aim is to provide guidance and insights for the systematic design of SACs used in CO2 reduction reactions, serving as a reference for the further advancement of photo-, electro-, and photoelectrocatalytic reduction of CO2.

Precise Engineering of Asymmetric Tri-Active Sites by Symbiotic Strategy for Photocontrolled Directional Reforming of Biomass

Sunlight-driven production of high-value chemicals from renewable resources represents a pivotal driver toward achieving sustainable energy supply. However, fundamental barriers include inadequate use of light energy and insufficient understanding of reactive oxygen species (ROS) regulating mechanisms in photocatalytic processes. To address this, a novel symbiotic strategy for the design of Cux/TiO2 single-atom catalysts (SACs) supported by density functional theory (DFT) calculations was proposed. The developed catalyst achieved nearly 100% conversion and selectivity for the directional photooxidative transformation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) or 2,5-furandicarboxylic acid (FDCA) under both vis-light and UV-vis light conditions. Importantly, compared to previous works, this catalyst exhibited the highest photooxidation activity reported to date while effectively suppressing the over-oxidation of HMF to CO2. Mechanistic investigations revealed that rational construction of Cu single-atoms (SAs) could effectively create the asymmetric Cu-Ov-Ti structure, which significantly enhanced the activation of O2 and HMF, facilitating generation of oxygen vacancy (Ov) and Ti3+. Furthermore, Cu SAs served as hole (h+) extractors in the photooxidation process, promoting rapid charge carrier transfer and ROS formation. The applicability of this developed strategy was further demonstrated for photooxidative conversion of various bio-feedstocks, including HMF and alcoholic substrates, indicating its great potential for harnessing light energy for sustainable valorization of biomass into high-value chemicals.

Visible-Light-Driven CO₂ Reduction Using Imidazole-Based Metal-Organic Frameworks as Heterogeneous Photocatalysts

The development of robust, efficient, and cost-effective heterogeneous photocatalysts for visible light-driven CO2 reduction continues to be a significant challenge in the quest for sustainable energy solutions. As a result, increasing attention is being directed towards the exploration of high-performance photocatalysts capable of converting CO2 into valuable chemical feedstocks. In context to this, Imidazolate Frameworks Potsdam (IFPs), a class of metal-organic frameworks (MOFs), can be a promising candidate for CO2 photoreduction due to their ease of synthesis, use of low-cost, earth-abundant metals, and high chemical and thermal stability. In this study, we report the solvothermal synthesis of Zn(II)- and Co(II)-based IFPs, specifically IFP-1(Zn) and IFP-5(Co), for photocatalytic CO2 reduction. Moreover, we demonstrate the enhanced photocatalytic activity of redox-innocent Zn-based IFP-1 by partially substituting Zn(II) with redox-active Co(II) in IFP-1(Zn), resulting in the formation of a bimetallic photocatalyst, IFP-1(Zn/Co). The metal-exchanged IFP-1(Zn/Co) exhibited significantly improved CO evolution (637 μmol g-1 in 1 hour), compared to the pristine IFP-1(Zn) (29 μmol g-1). Notably, among all the prepared photocatalysts, IFP-5(Co) outperformed both the systems, achieving a CO evolution of 1174 μmol g-1 within 1 hour, due to the presence of catalytic cobalt sites. In addition, through the combination of photophysical and electrochemical studies, along with DFT calculations, we have proposed a plausible mechanism for the photocatalytic CO2 reduction.

Photoirradiation-enhanced behavior via morphological manipulation of CoFe2O4/g-C3N4 heterojunction for supercapacitor and CO2 reduction

Regulating the morphology of graphitic carbon nitride (g-C3N4, CN) and constructing CoFe2O4/g-C3N4 (CFO/CN) heterojunctions were adopted in the photocatalytic energy storage and photocatalytic CO2 reduction (PCR). CFO/CNS had outstanding light response ability, while CFO/CNT possessed excellent charge transfer ability. Consequently, CFO/CNT electrode exhibited the highest specific capacitance without light, CFO/CNS electrode showed the most obvious photo-enhanced capacitance behavior with an increase by 21.05 % under light. This was ascribed to the generation and separation of photo-generated carriers, promoting oxidation/reduction reactions. And in PCR, the electron consumption rates of four CFO/CN heterojunctions were CFO/CNT > CFO/BCN > CFO/MCN > CFO/CNS. CFO/CNT presented the highest photocatalytic activity, attributing to the strong redox ability and photo-enhanced electron transfer. This strategy of utilizing CFO/CN heterojunctions to construct photo-enhanced supercapacitor electrodes and photocatalytic CO2 reduction catalysts provided new ideas for energy conversion and storage.

Single-Atom-Induced Hybridization States Promote the Direct Trapping of Hot Carriers by Reactants for Photocatalysis

Single-atom manipulation has emerged as an effective strategy for enhancing the photocatalytic efficiency. However, the mechanism of photogenerated carrier dynamics under single-atom modulation remains unclear. Combining first-principles calculations and non-adiabatic molecular dynamics simulations, we systematically studied carrier transfer and recombination in the oxygen reduction reaction of single-atom-doped C3N4 systems. Unlike the conventional two-step process, where single atoms trap photogenerated carriers before transferring them to reactants, our findings reveal a direct one-step electron transfer process, where single-atom-induced hybridization states facilitate the direct trapping of hot carriers by reactants from photocatalysts. Specifically, photogenerated electron transfer time through the one-step process is 237 and 325 fs for Sb and Cu single-atom-doped systems, respectively, considerably faster than the two-step process (hundreds of picoseconds). Moreover, these systems exhibit a nanosecond-level photogenerated carrier lifetime, driving a high photocatalytic efficiency. This study elucidates the carrier dynamics in single-atom photocatalysts, facilitating the screening of high-performance photocatalysts.

Single atoms meeting 2D materials: an excellent configuration for photocatalysis

Photocatalysis has problems such as low light absorption efficiency and rapid recombination of photogenerated electron-hole pairs. Many studies have been conducted to improve these issues. This review encapsulates the progress and applications of two pioneering research fields in catalysis: single-atom and two-dimensional (2D) material catalysts. The advent of this new type of catalysts, which integrates single atoms onto 2D materials, has seen remarkable growth in recent years, offering distinctive advantages. The article delves into the array of synthesis methods employed for loading single atoms onto 2D materials, including the wet chemical approach, atomic layer deposition technique, and thermal decomposition method. A highlight of the review is the superior attributes of single-atom catalysts supported on 2D materials (SACs-2D) in photocatalysis, such as extending the light absorption wavelength range, enhancing the efficiency of photogenerated electron-hole pair separation, and accelerating redox kinetics. The review meticulously examines the diverse applications of SACs-2D photocatalysis, which encompass water splitting for hydrogen generation, carbon dioxide reduction, degradation of organic pollutants, nitrogen fixation and hydrogen peroxide synthesis. These applications demonstrate the potential of SACs-2D materials in addressing pressing environmental and energy challenges. Finally, this article evaluates the current state of this burgeoning field, discussing the opportunities and challenges ahead.

Emerging ZnO Semiconductors for Photocatalytic CO2 Reduction to Methanol

Carbon recycling is poised to emerge as a prominent trend for mitigating severe climate change and meeting the rising demand for energy. Converting carbon dioxide (CO2) into green energy and valuable feedstocks through photocatalytic CO2 reduction (PCCR) offers a promising solution to global warming and energy needs. Among all semiconductors, zinc oxide (ZnO) has garnered considerable interest due to its ecofriendly nature, biocompatibility, abundance, exceptional semiconducting and optical properties, cost-effectiveness, easy synthesis, and durability. This review thoroughly discusses recent advances in mechanistic insights, fundamental principles, experimental parameters, and modulation of ZnO catalysts for direct PCCR to C1 products (methanol). Various ZnO modification techniques are explored, including atomic size regulation, synthesis strategies, morphology manipulation, doping with cocatalysts, defect engineering, incorporation of plasmonic metals, and single atom modulation to boost its photocatalytic performance. Additionally, the review highlights the importance of photoreactor design, reactor types, geometries, operating modes, and phases. Future research endeavors should prioritize the development of cost-effective catalyst immobilization methods for solid-liquid separation and catalyst recycling, while emphasizing the use of abundant and non-toxic materials to ensure environmental sustainability and economic viability. Finally, the review outlines key challenges and proposes novel directions for further enhancing ZnO-based photocatalytic CO2 conversion processes.

Radioisotope thermoelectric generators (RTGs): a review of current challenges and future applications

Nuclear power sources can be effectively employed to generate renewable energy as a counter to global reliance on fossil fuels and increasing energy demands. Nuclear radiation can be utilized in numerous ways to produce energy. Along with their use as fuel for nuclear power plants, the decay process of radioisotopes can also be used to create electrical energy. A radioisotope thermoelectric generator (RTG) is one such example, as it is tasked to convert the decay energy of radioisotopes into heat energy, which is later converted into electrical power using the Seebeck effect. Radioisotopes have high energy densities and certain isotopes can generate considerable heat energy for prolonged timescales. As such, RTGs have found applications in powering interplanetary exploration and interstellar space missions due to their long half-life. They are also commonly used in remote regions of the earth where frequent replacement, charging and maintenance of power sources is difficult. In this review, we will discuss the working principle and architecture of RTGs, challenges faced in further development and potential applications of this technology.

Understanding and Tuning the Effects of H2O on Catalytic CO and CO2 Hydrogenation

Catalytic COx (CO and CO2) hydrogenation to valued chemicals is one of the promising approaches to address challenges in energy, environment, and climate change. H2O is an inevitable side product in these reactions, where its existence and effect are often ignored. In fact, H2O significantly influences the catalytic active centers, reaction mechanism, and catalytic performance, preventing us from a definitive and deep understanding on the structure-performance relationship of the authentic catalysts. It is necessary, although challenging, to clarify its effect and provide practical strategies to tune the concentration and distribution of H2O to optimize its influence. In this review, we focus on how H2O in COx hydrogenation induces the structural evolution of catalysts and assists in the catalytic processes, as well as efforts to understand the underlying mechanism. We summarize and discuss some representative tuning strategies for realizing the rapid removal or local enrichment of H2O around the catalysts, along with brief techno-economic analysis and life cycle assessment. These fundamental understandings and strategies are further extended to the reactions of CO and CO2 reduction under an external field (light, electricity, and plasma). We also present suggestions and prospects for deciphering and controlling the effect of H2O in practical applications.

Outstanding CO2 Photoreduction in Single-Atom Thulium Modified Carbon Nitride

CO2 reduction photocatalysts are favorable for obtaining renewable energy. Enriched active sites and effective photogenerated-carriers separation are keys for improving CO2 photo-reduction. A thulium (Tm) single atom tailoring strategy introducing carbon vacancies in porous tubular graphitic carbon nitride (g-C3N4) surpassing the ever-reported g-C3N4 based photocatalysts, with 199.47 µmol g-1 h-1 CO yield, 96.8% CO selectivity, 0.84% apparent quantum efficiency and excellent photocatalytic stability, is implemented in this work. Results revealed that in-plane Tm sites and interlayer-bridged Tm-N charge transfer channels significantly enhanced the aggregation/transfer of photogenerated electrons thus promoting CO2 adsorption/activation and contributing to *COOH intermediates formation. Meanwhile, Tm atoms and carbon vacancies both benefit for rich active sites and enhanced photogenerated-charge separation, thus optimizing reaction pathway and leading to excellent CO2 photo-reduction. This work not only provides guidelines for CO2 photo-reduction catalysts design but also offers mechanistic insights into single-atom based photocatalysts for solar fuel production.

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Integration of Co Single Atoms and Ni Clusters on Defect-Rich ZrO2 for Strong Photothermal Coupling Boosts Photocatalytic CO2 Reduction

We report a solvothermal method for the synthesis of an oxygen vacancy-enriched ZrO2 photocatalyst with Co single atoms and Ni clusters immobilized on the surface. This catalyst presents superior performance for the reduction of CO2 in H2O vapor, with a CO yield reaching 663.84 μmol g-1 h-1 and a selectivity of 99.52%. The total solar-to-chemical energy conversion efficiency is up to 0.372‰, which is among the highest reported values. The success, on one hand, depends on the Co single atoms and Ni clusters for both extended spectrum absorption and serving as dual-active centers for CO2 reduction and H2O dissociation, respectively; on the other hand, this is attributed to the enhanced photoelectric and thermal effect induced by concentrated solar irradiation. We demonstrate that an intermediate impurity state is formed by the hybridization of the d-orbital of single-atom Co with the molecular orbital of H2O, enabling visible-light-driven excitation over the catalyst. In addition, Ni clusters play a crucial role in altering the adsorption configuration of CO2, with the localized surface plasmon resonance effect enhancing the activation and dissociation of CO2 induced by visible-near-infrared light. This study provides valuable insights into the synergistic effect of the dual cocatalyst toward both efficient photothermal coupling and surface redox reactions for solar CO2 reduction.

Suppressing the Shuttle Effect of Aqueous Zinc-Iodine Batteries: Progress and Prospects

Aqueous zinc-iodine batteries are considered to be one of the most promising devices for future electrical energy storage due to their low cost, high safety, high theoretical specific capacity, and multivalent properties. However, the shuttle effect currently faced by zinc-iodine batteries causes the loss of cathode active material and corrosion of the zinc anodes, limiting the large-scale application of zinc-iodine batteries. In this paper, the electrochemical processes of iodine conversion and the zinc anode, as well as the induced mechanism of the shuttle effect, are introduced from the basic configuration of the aqueous zinc-iodine battery. Then, the inhibition strategy of the shuttle effect is summarized from four aspects: the design of cathode materials, electrolyte regulation, the modification of the separator, and anode protection. Finally, the current status of aqueous zinc-iodine batteries is analyzed and recommendations and perspectives are presented. This review is expected to deepen the understanding of aqueous zinc-iodide batteries and is expected to guide the design of high-performance aqueous zinc-iodide batteries.

Modifying Surface Termination by Bidentate Chelating Strategy Enables 13.77% Efficient Kesterite Solar Cells

Surfaces display discontinuities in the kesterite-based polycrystalline films can produce large defect densities, including strained and dangling bonds. These physical defects tend to introduce electronic defects and surface states, which can greatly promote nonradiative recombination of electron-hole pairs and damage device performance. Here, an effective chelation strategy is reported to suppress these harmful physical defects related to unterminated Cu, Zn, and Sn sites by modifying the surface of Cu2ZnSn(S,Se)4 (CZTSSe) films with sodium diethyldithiocarbamate (NaDDTC). The conjoint theoretical calculations and experimental results reveal that the NaDDTC molecules can be coordinate to surface metal sites of CZTSSe films via robust bidentate chelating interactions, effectively reducing surface undercoordinated defects and passivating the electron trap states. Consequently, the solar cell efficiency of the NaDDTC-treated device is increased to as high as 13.77% under 100 mW cm-2 illumination, with significant improvement in fill factor and open-circuit voltage. This surface chelation strategy provides strong surface termination and defect passivation for further development and application of kesterite-based photovoltaics.

Stabilizing electron-rich Ni single-atoms on black phosphorus nanosheets boosts photocatalytic carbon dioxide reduction

The development of unique single-atom catalysts with electron-rich feature is essential to promoting the photocatalytic CO2 reduction, yet remains a big challenge. Here, a conceptionally new single-atom catalyst constructed from atomically dispersed Ni-P3 species on black phosphorus (BP) nanosheets (BP-Ni) is synthesized for realizing highly efficient visible-light-driven CO2 reduction when trapping photogenerated electrons from homogeneous light absorbers in the presence of triethanolamine as the sacrificial agent. Both the experimental and theoretical calculation data reveal that the Ni-P3 species on BP nanosheets own the electron-rich feature that can improve the photogenerated charge separation efficiency and lower the activation barrier of CO2 conversion. This unique feature makes BP-Ni exhibit the much higher activity as cocatalyst in the photocatalytic CO2 reduction than BP nanosheets. The BP-Ni can also be applied as a cocatalyst for enhanced photocatalytic CO2 reduction after combining with CdSe/S colloidal crystal photocatalyst. The present study offers valuable inspirations for the design and construction of effective catalytic sites toward photocatalytic CO2 reduction reactions.

Cu-Based Materials for Enhanced C2+ Product Selectivity in Photo-/Electro-Catalytic CO2 Reduction: Challenges and Prospects

Carbon dioxide conversion into valuable products using photocatalysis and electrocatalysis is an effective approach to mitigate global environmental issues and the energy shortages. Among the materials utilized for catalytic reduction of CO2, Cu-based materials are highly advantageous owing to their widespread availability, cost-effectiveness, and environmental sustainability. Furthermore, Cu-based materials demonstrate interesting abilities in the adsorption and activation of carbon dioxide, allowing the formation of C2+ compounds through C-C coupling process. Herein, the basic principles of photocatalytic CO2 reduction reactions (PCO2RR) and electrocatalytic CO2 reduction reaction (ECO2RR) and the pathways for the generation C2+ products are introduced. This review categorizes Cu-based materials into different groups including Cu metal, Cu oxides, Cu alloys, and Cu SACs, Cu heterojunctions based on their catalytic applications. The relationship between the Cu surfaces and their efficiency in both PCO2RR and ECO2RR is emphasized. Through a review of recent studies on PCO2RR and ECO2RR using Cu-based catalysts, the focus is on understanding the underlying reasons for the enhanced selectivity toward C2+ products. Finally, the opportunities and challenges associated with Cu-based materials in the CO2 catalytic reduction applications are presented, along with research directions that can guide for the design of highly active and selective Cu-based materials for CO2 reduction processes in the future.

Non-Interacting Ni and Fe Dual-Atom Pair Sites in N-Doped Carbon Catalysts for Efficient Concentrating Solar-Driven Photothermal CO2 Reduction

Solar-to-chemical energy conversion under weak solar irradiation is generally difficult to meet the heat demand of CO2 reduction. Herein, a new concentrated solar-driven photothermal system coupling a dual-metal single-atom catalyst (DSAC) with adjacent Ni-N4 and Fe-N4 pair sites is designed for boosting gas-solid CO2 reduction with H2 O under simulated solar irradiation, even under ambient sunlight. As expected, the (Ni, Fe)-N-C DSAC exhibits a superior photothermal catalytic performance for CO2 reduction to CO (86.16 μmol g-1  h-1 ), CH4 (135.35 μmol g-1  h-1 ) and CH3 OH (59.81 μmol g-1  h-1 ), which are equivalent to 1.70-fold, 1.27-fold and 1.23-fold higher than those of the Fe-N-C catalyst, respectively. Based on theoretical simulations, the Fermi level and d-band center of Fe atom is efficiently regulated in non-interacting Ni and Fe dual-atom pair sites with electronic interaction through electron orbital hybridization on (Ni, Fe)-N-C DSAC. Crucially, the distance between adjacent Ni and Fe atoms of the Ni-N-N-Fe configuration means that the additional Ni atom as a new active site contributes to the main *COOH and *HCO3 dissociation to optimize the corresponding energy barriers in the reaction process, leading to specific dual reaction pathways (COOH and HCO3 pathways) for solar-driven photothermal CO2 reduction to initial CO production.

Nickel hydroxide-decorating potassium-doped graphitic carbon nitride for boosting photocatalytic carbon dioxide reduction

In this study, nickel hydroxide (Ni(OH)2) was employed to modify potassium (K)-doped graphitic carbon nitride (g-C3N4, CN) for enhancing photocatalytic CO2 reduction. The light absorption and charge separation performances of CN were enhanced after modification. Experiments and theoretical calculations indicated that the loaded Ni(OH)2 could gather electrons, facilitate adsorption and activation of CO2. The optimized photocatalyst exhibited high CO2 reductive rate with CO and CH4 yields of 42.6 and 3.5 μmol g-1, respectively after 3 h irradiation in the presence of 0.5 mL water, which was 1.4 and 4.6 times higher than the yields on K-doped CN and Ni(OH)2-decorated CN, respectively. This work provides new thought for enhancing CO2 reductive performance of CN.

Progress on Single-Atom Photocatalysts for H2 Generation: Material Design, Catalytic Mechanism, and Perspectives

Solar energy utilization is of great significance to current challenges of the energy crisis and environmental pollution, which benefit the development of the global community to achieve carbon neutrality goals. Hydrogen energy is also treated as a good candidate for future energy supply since its combustion not only supplies high-density energy but also shows no pollution gas. In particular, photocatalytic water splitting has attracted increasing research as a promising method for H2 production. Recently, single-atom (SA) photocatalysts have been proposed as a potential solution to improve catalytic efficiency and lower the costs of photocatalytic water splitting for H2 generation. Owing to the maximized atom utilization rate, abundant surface active sites, and tunable coordination environment, SA photocatalysts have achieved significant progress. This review reviews developments of advanced SA photocatalysts for H2 generation regarding the different support materials. The recent progress of titanium dioxide, metal-organic frameworks, two-dimensional carbon materials, and red phosphorus supported SA photocatalysts are carefully discussed. In particular, the material designs, reaction mechanisms, modulation strategies, and perspectives are highlighted for realizing improved solar-to-energy efficiency and H2 generation rate. This work will supply significant references for future design and synthesis of advanced SA photocatalysts.

Recent Advances on Single-Atom Catalysts for Photocatalytic CO2 Reduction

The present energy crisis and environmental challenges may be efficiently resolved by converting carbon dioxide (CO2 ) into various useful carbon products. The development of more effective catalysts has been the main focus of current research on photocatalytic CO2 reduction. Due to their high atomic efficiency and superior catalytic activity, single-atom catalysts (SACs) have attracted considerable interest in catalytic CO2 conversion. This review discusses the current research developments, obstacles, and potential of SACs for photocatalytic CO2 reduction. And further, discusses the principle of photocatalytic carbon dioxide reduction. This work has compared and analyzed the effects of support materials and active site types in SACs on photocatalytic CO2 reduction performance. This work believes that by sharing these developments, some inspiration for the rational design and development of stable and effective photocatalytic CO2 reduction catalysts based on SACs can be provided.

Morphological and Coordination Modulations in Iridium Electrocatalyst for Robust and Stable Acidic OER Catalysis

Proton exchange membrane water splitting (PEMWS) technology has high-level current density, high operating pressure, small electrolyzer-size, integrity, flexibility, and has good adaptability to the volatility of wind power and photovoltaics, but the development of both active and high stability of the anode electrocatalyst in acidic environment is still a huge challenge, which seriously hinders the promotion and application of PEMWS. In recent years, researchers have made tremendous attempts in the development of high-quality active anode electrocatalyst, and we summarize some of the research progress made by our group in the design and synthesis of PEMWS anode electrocatalysts with different nanostructures, and makes full use of electrocatalytic activity points to increase the inherent activity of Iridium (Ir) sites, and provides optimization strategies for the long-term non-decay of catalysts under high anode potential in acidic environments. At this stage, these research advances are expected to facilitate the research and technological progress of PEMWS, and providing some research ideas and references for future research on efficient and inexpensive PEMWS anode electrocatalysts.

Progress in photocatalytic CO2 reduction based on single-atom catalysts

Reduced CO2 emissions, conversion, and reuse are critical steps toward carbon peaking and carbon neutrality. Converting CO2 into high-value carbon-containing compounds or fuels may effectively address the energy shortage and environmental issues, which is consistent with the notion of sustainable development. Photocatalytic CO2 reduction processes have become one of the research focuses, where single-atom catalysts have demonstrated significant benefits owing to their excellent percentage of atom utilization. However, among the crucial challenges confronting contemporary research is the production of efficient, low-cost, and durable photocatalysts. In this paper, we offer a comprehensive overview of the study growth on single-atom catalysts for photocatalytic CO2 reduction reactions, describe several techniques for preparing single-atom catalysts, and discuss the advantages and disadvantages of single-atom catalysts and present the study findings of three single-atom photocatalysts with TiO2, g-C3N4 and MOFs materials as carriers based on the interaction between single atoms and carriers, and finally provide an outlook on the innovation of photocatalytic CO2 reduction reactions.

Thermal Effect on Photoelectrochemical Water Splitting Toward Highly Solar to Hydrogen Efficiency

Photoelectrochemical (PEC) hydrogen production is an emerging technology that uses renewable solar light aimed to establish a sustainable carbon-neutral society. The barriers to commercialization are low efficiency and high cost. To date, researchers have focused on materials and systems. However, recent studies have been conducted to utilize thermal effects in PEC hydrogen production. This Review provides a fresh perspective to utilize the thermal effects for PEC performance enhancement while delineating the underlying principles and equations associated with efficiency. The fundamentals of the thermal effect on the PEC system are summarized from various perspectives: kinetics, thermodynamics, and empirical equations. Based on this, materials are classified as plasmonic metals, quantum dot-based semiconductors, and photothermal organic materials, which have an inherent response to photothermal irradiation. Finally, the economic viability and challenges of these strategies for PEC are explained, which can pave the way for the future progress in the field.

Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO₂ reduction, water splitting, N₂ fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.

Recent progress of cocatalysts loaded on carbon nitride for selective photoreduction of CO2 to CH4

A photocatalytic system driven by solar light is one of the promising strategies for converting CO₂ into valuable energy. The reduction of CO₂ to CH₄ is widely studied since CH₄ has a high energy density as the main component of nonrenewable natural gas. Therefore, it is necessary to develop semiconductor materials with high photocatalytic activity and CH₄ selectivity. Graphitic carbon nitride (g-C₃N₄/CN) has attracted widespread attention for photocatalytic CO₂ reduction due to its excellent redox ability and visible light response. A hybrid system constructed by loading cocatalysts on g-C₃N₄ can significantly improve the yield of target products, and serve as a general platform to explore the mechanism of the CO₂ reduction reaction. Herein, we briefly introduce the theory of selective CO₂ photoreduction and the basic properties of cocatalysts. Then, several typical configurations and modification strategies of cocatalyst/CN systems for promoting CH₄ selective production are presented in detail. In particular, we systematically summarize the application of cocatalyst/CN composite photocatalysts in the selective reduction of CO₂ to methane, according to the classification of cocatalysts (monometal, bimetal, metal-based compound, and nanocarbon materials). Finally, the challenges and perspectives for developing cocatalyst/g-C₃N₄ systems with high CH₄ selectivity are presented to guide the rational design of catalysts with high performance in the future.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Topologically Porous Heterostructures for Photo/Photothermal Catalysis of Clean Energy Conversion

As a straightforward way to fix solar energy, photo/photothermal catalysis with semiconductor provides a promising way to settle the energy shortage and environmental crisis in many fields, especially in clean energy conversion. Topologically porous heterostructures (TPHs), featured with well-defined pores and mainly composed by the derivatives of some precursors with specific morphology, are a major part of hierarchical materials in photo/photothermal catalysis and provide a versatile platform to construct efficient photocatalysts for their enhanced light absorption, accelerated charges transfer, improved stability, and promoted mass transportation. Therefore, a comprehensive and timely review on the advantages and recent applications of the TPHs is of great importance to forecast the potential applications and research trend in the future. This review initially demonstrates the advantages of TPHs in photo/photothermal catalysis. Then the universal classifications and design strategies of TPHs are emphasized. Besides, the applications and mechanisms of photo/photothermal catalysis in hydrogen evolution from water splitting and CO_x hydrogenation over TPHs are carefully reviewed and highlighted. Finally, the challenges and perspectives of TPHs in photo/photothermal catalysis are also critically discussed.

Toward a Comprehensive Understanding of Amorphous Photocatalysts: Fundamental Hypotheses and Applications in CO2 Photoreduction

In principle, photocatalytic activity can be precisely controlled with crystalline catalysts. However, an amorphous photocatalyst could be a viable candidate for CO₂ photoreduction to form value-added products. The amorphous phase is currently part of the crystalline material in several ongoing CO₂ photoreduction studies. Additionally, no study indicates the amorphous material required for overall CO₂ photoreduction. This perspective review article highlights fundamental assumptions that are necessary to gain insights and understand the effectiveness of amorphous photocatalysts for CO₂ photoreduction. We start with basic ideas and theories about these materials, including light harvesting, variable coordination number, and the interaction of CO₂ molecules with the amorphous catalytic surface. To understand the prospects of the amorphous photocatalyst, we explore machine learning with EXAFS. Furthermore, we discuss product selectivity and regeneration of photocatalysts in detail. Finally, we briefly review the work in progress on amorphous materials and compare it to that on crystalline ones.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO2 reduction reaction (CRR) is an effective strategy because it affords the conversion of CO2 into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Surface Electron Localization in Cu-MOF-Bonded Double-Heterojunction Cu2O Induces Highly Efficient Photocatalytic CO2 Reduction

Truncated octahedron Cu₂O (TOC) has attracted more attention for its suitable band gap and high carrier separation efficiency due to introduction of the facet heterojunction, but its practical drawback is still the instability caused by the irreversible disproportionation reaction (Cu₂O → Cu + CuO). Here, we design and fabricate the TOC/Cu-MOF (MOF: metal-organic framework) double-heterojunction structures with different Cu-MOF loadings. The introduced heterojunction between TOC and Cu-MOF not only produces a stable interface Cu^x+ bonding structure with the electronic states localized within the average collisional diameter of electrons 1.72 nm for TOC/2.1 wt %Cu-MOF as the active sites, but also promotes the surface energy level difference between the (100) and (111) facet heterojunctions. Meanwhile, the bonded Cu-MOF with a narrow bandgap effectively consumes holes by recombination with the photoexcited electrons from Cu-MOF itself. In our experiments, the TOC/Cu-MOF double heterostructure with a loading amount of 2.1 wt % Cu-MOF shows an optimal photocatalytic CO₂ reduction performance. The CO evolution rate reaches 23.01 μmol g^-1 h^-1, which is about 2.01 and 4.47 times larger than those of octahedral and hexahedral Cu₂O/Cu-MOF, respectively, and an excellent photostability is shown for four cycles with each cycle lasting for 4 h. Such a double heterostructure provides insight into highly efficient electron transfer and photostability in Cu₂O-related composite materials.

A Comprehensive Review on Graphitic Carbon Nitride for Carbon Dioxide Photoreduction

Inspired by natural photosynthesis, harnessing the wide range of natural solar energy and utilizing appropriate semiconductor-based catalysts to convert carbon dioxide into beneficial energy species, for example, CO, CH₄ , HCOOH, and CH₃ COH have been shown to be a sustainable and more environmentally friendly approach. Graphitic carbon nitride (g-C₃ N₄ ) has been regarded as a highly effective photocatalyst for the CO₂ reduction reaction, owing to its cost-effectiveness, high thermal and chemical stability, visible light absorption capability, and low toxicity. However, weaker electrical conductivity, fast recombination rate, smaller visible light absorption window, and reduced surface area make this catalytic material unsuitable for commercial photocatalytic applications. Therefore, certain procedures, including elemental doping, structural modulation, functional group adjustment of g-C₃ N₄ , the addition of metal complex motif, and others, may be used to improve its photocatalytic activity towards effective CO₂ reduction. This review has investigated the scientific community's perspectives on synthetic pathways and material optimization approaches used to increase the selectivity and efficiency of the g-C₃ N₄ -based hybrid structures, as well as their benefits and drawbacks on photocatalytic CO₂ reduction. Finally, the review concludes a comparative discussion and presents a promising picture of the future scope of the improvements.