Tunable Syngas Production from CO2and H2O in an Aqueous Photoelectrochemical Cell

Silicon photocathode functionalized with osmium complex catalyst for selective catalytic conversion of CO2 to methane

Solar-driven CO2 reduction to yield high-value chemicals presents an appealing avenue for combating climate change, yet achieving selective production of specific products remains a significant challenge. We showcase two osmium complexes, przpOs, and trzpOs, as CO2 reduction catalysts for selective CO2-to-methane conversion. Kinetically, the przpOs and trzpOs exhibit high CO2 reduction catalytic rate constants of 0.544 and 6.41 s-1, respectively. Under AM1.5 G irradiation, the optimal Si/TiO2/trzpOs have CH4 as the main product and >90% Faradaic efficiency, reaching -14.11 mA cm-2 photocurrent density at 0.0 VRHE. Density functional theory calculations reveal that the N atoms on the bipyrazole and triazole ligands effectively stabilize the CO2-adduct intermediates, which tend to be further hydrogenated to produce CH4, leading to their ultrahigh CO2-to-CH4 selectivity. These results are comparable to cutting-edge Si-based photocathodes for CO2 reduction, revealing a vast research potential in employing molecular catalysts for the photoelectrochemical conversion of CO2 to methane.

Solar-Driven Sustainability: III-V Semiconductor for Green Energy Production Technologies

Long-term societal prosperity depends on addressing the world's energy and environmental problems, and photocatalysis has emerged as a viable remedy. Improving the efficiency of photocatalytic processes is fundamentally achieved by optimizing the effective utilization of solar energy and enhancing the efficient separation of photogenerated charges. It has been demonstrated that the fabrication of III-V semiconductor-based photocatalysts is effective in increasing solar light absorption, long-term stability, large-scale production and promoting charge transfer. This focused review explores on the current developments in III-V semiconductor materials for solar-powered photocatalytic systems. The review explores on various subjects, including the advancement of III-V semiconductors, photocatalytic mechanisms, and their uses in H2 conversion, CO2 reduction, environmental remediation, and photocatalytic oxidation and reduction reactions. In order to design heterostructures, the review delves into basic concepts including solar light absorption and effective charge separation. It also highlights significant advancements in green energy systems for water splitting, emphasizing the significance of establishing eco-friendly systems for CO2 reduction and hydrogen production. The main purpose is to produce hydrogen through sustainable and ecologically friendly energy conversion. The review intends to foster the development of greener and more sustainable energy source by encouraging researchers and developers to focus on practical applications and advancements in solar-powered photocatalysis.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Light-Induced Dynamic Activation of Copper/Silicon Interface for Highly Selective Carbon Dioxide Reduction

Numerous studies have shown a fact that phase transformation and/or reconstruction are likely to occur and play crucial roles in electrochemical scenarios. Nevertheless, a decisive factor behind the diverse photoelectrochemical activity and selectivity of various copper/silicon photoelectrodes is still largely debated and missing in the community, especially the possibly dynamic behaviors of metal catalyst/semiconductor interface. Herein, through in situ X-ray absorption spectroscopy and transmission electron microscope, a model system of Cu nanocrystals with well-defined facets on black p-type silicon (BSi) is unprecedentedly demonstrated to reveal the dynamic phase transformation of forming irreversible silicide at Cu nanocrystal-BSi interface during photoelectrocatalysis, which is validated to originate from the atomic interdiffusion between Cu and Si driven by light-induced dynamic activation process. Significantly, the adaptive junction at Cu-Si interface is activated by an expansion of interatomic Cu-Cu distance for CO2 electroreduction, which efficiently restricts the C-C coupling pathway but strengthens the bonding with key intermediate of *CHO for CH4 yield, resulting in a remarkable 16-fold improvement in the product ratio of CH4/C2 products and an intriguing selectivity switch. This work offers new insights into dynamic structural transformations of metal/semiconductor junction and design of highly efficient catalysts toward photosynthesis.

Cu-nanocluster-loaded N-doped porous graphitic carbon for electrochemical CO2 reduction towards syngas generation

In this study, a novel electrocatalyst, namely Cu/N-pg-C derived from Cu-doped ZIF-8, was investigated for making syngas products with various H2/CO ratios. Different ratios of the electrocatalytic syngas products CO and H2 could be selected by adjusting the applied potential and hence tuning the transfer of electrons from N-doped graphitic carbon to the well-dispersed Cu nanoclusters.

Underwater Bubble Manipulation on Surfaces with Patterned Regions with Infused Lubricants

The flexible manipulation of underwater gas bubbles on solid substrates has attracted considerable research interest from scientists in the fields of water electrolysis, bubble microreactions, drug delivery, and heat transfer. Inspired by the oxygen-binding mechanisms of aquatic organisms, scientists have designed a series of interfacial materials for use in collecting gases, detecting and grading bubbles, and conducting microbubble reactions. Aerophilic surfaces are commonly used in underwater bubble manipulation platforms due to their excellent gas-trapping properties. However, during bubble transport, some of the bubbles are retained in the rough structure of the aerophilic surface and cause gas loss, which in the long run reduces the gas transport function. In addition, the aerophilic surface is prone to failure in high-humidity and high-pressure underwater environments. The lubricant-infused surface features an oil layer that remains stable on a rough substrate and is immiscible with water. Additionally, the bubbles are transported over the oil layer without causing losses other than those dissolved in water. These attributes make it more favorable than the aerophilic surface. Inspired by the unique properties of Nepenthes and cactus spines, we developed a patterned slippery surface [patterned lubricant-infused surface (PLIS)] through laser etching and ammonia etching that facilitates the coexistence of superaerophobic and aerophilic surfaces. The PLIS executes bubble capture utilizing a difference in wettability measuring 78°, transports bubbles through Laplace force and buoyancy, and regulates bubble release by restricting the contact area on the PLIS. The PLIS can be prepared rapidly and affordably in just about an hour, and its potential for large-scale production is high. Following tests for shear, acid and alkali resistance, and corrosion resistance, the PLIS exhibited impressive weathering resistance and appears to have potential for application in some extreme environments.

Nanoscale Metal Particle Modified Single-Atom Catalyst: Synthesis, Characterization, and Application

Single-atom catalysts (SACs) have attracted considerable attention in heterogeneous catalysis because of their well-defined active sites, maximum atomic utilization efficiency, and unique unsaturated coordinated structures. However, their effectiveness is limited to reactions requiring active sites containing multiple metal atoms. Furthermore, the loading amounts of single-atom sites must be restricted to prevent aggregation, which can adversely affect the catalytic performance despite the high activity of the individual atoms. The introduction of nanoscale metal particles (NMPs) into SACs (NMP-SACs) has proven to be an efficient approach for improving their catalytic performance. A comprehensive review is urgently needed to systematically introduce the synthesis, characterization, and application of NMP-SACs and the mechanisms behind their superior catalytic performance. This review first presents and classifies the different mechanisms through which NMPs enhance the performance of SACs. It then summarizes the currently reported synthetic strategies and state-of-the-art characterization techniques of NMP-SACs. Moreover, their application in electro/thermo/photocatalysis, and the reasons for their superior performance are discussed. Finally, the challenges and perspectives of NMP-SACs for the future design of advanced catalysts are addressed.

Enhancing photoelectrochemical CO2 reduction with silicon photonic crystals

The effectiveness of silicon (Si) and silicon-based materials in catalyzing photoelectrochemistry (PEC) CO2 reduction is limited by poor visible light absorption. In this study, we prepared two-dimensional (2D) silicon-based photonic crystals (SiPCs) with circular dielectric pillars arranged in a square array to amplify the absorption of light within the wavelength of approximately 450 nm. By investigating five sets of n + p SiPCs with varying dielectric pillar sizes and periodicity while maintaining consistent filling ratios, our findings showed improved photocurrent densities and a notable shift in product selectivity towards CH4 (around 25% Faradaic Efficiency). Additionally, we integrated platinum nanoparticles, which further enhanced the photocurrent without impacting the enhanced light absorption effect of SiPCs. These results not only validate the crucial role of SiPCs in enhancing light absorption and improving PEC performance but also suggest a promising approach towards efficient and selective PEC CO2 reduction.

Tunable CO2-to-syngas conversion via strong electronic coupling in S-scheme ZnGa2O4/g-C3N4 photocatalysts

The conversion of CO2 into syngas, a mixture of CO and H2, via photocatalytic reduction, is a promising approach towards achieving a sustainable carbon economy. However, the evolution of highly adjustable syngas, particularly without the use of sacrifice reagents or additional cocatalysts, remains a significant challenge. In this study, a step-scheme (S-scheme) 0D ZnGa2O4 nanodots (∼7 nm) rooted g-C3N4 nanosheets (denoted as ZnGa2O4/C3N4) heterojunction photocatalyst was synthesized vis a facial in-situ growth strategy for efficient CO2-to-syngas conversion. Both experimental and theoretical studies have demonstrated that the polymeric nature of g-C3N4 and highly distributed ZnGa2O4 nanodots synergistically contribute to a strong interaction between metal oxide and C3N4 support. Furthermore, the desirable S-scheme heterojunction in ZnGa2O4/C3N4 efficiently promotes charge separation, enabling strong photoredox ability. As a result, the S-scheme ZnGa2O4/C3N4 exhibited remarkable activity and selectivity in photochemical conversion of CO2 into syngas, with a syngas production rate of up to 103.3 μ mol g-1 h-1, even in the absence of sacrificial agents and cocatalyst. Impressively, the CO/H2 ratio of syngas can be tunable within a wide range from 1:4 to 2:1. This work exemplifies the effectiveness of a meticulously designed S-scheme heterojunction photocatalyst for CO2-to-syngas conversion with adjustable composition, thus paving the way for new possibilities in sustainable energy conversion and utilization.

Recent Progress and Perspectives on Photocathode Materials for CO2 Catalytic Reduction

The continuous consumption of fossil energy and excessive emissions of carbon dioxide (CO₂) have caused a serious energy crisis and led to the greenhouse effect. Using natural resources to convert CO₂ into fuel or high-value chemicals is considered to be an effective solution. Photoelectrochemical (PEC) catalysis utilizes abundant solar energy resources, combined with the advantages of photocatalysis (PC) and electrocatalysis (EC), to achieve efficient CO₂ conversion. In this review, the basic principles and evaluation criteria, of PEC catalytic reduction to CO₂ (PEC CO₂RR), are introduced. Next, the recent research progress on typical kinds of photocathode materials for CO₂ reduction are reviewed, and the structure-function relationships between material composition/structure and activity/selectivity are discussed. Finally, the possible catalytic mechanisms and the challenges of using PEC to reduce CO₂ are proposed.

Bias-Free Solar-Driven Syngas Production: A Fe2 O3 Photoanode Featuring Single-Atom Cobalt Integrated with a Silver-Palladium Cathode

Photoelectrochemical syngas production from aqueous CO₂ is a promising technique for carbon capture and utilization. Herein, we demonstrate the efficient and tunable syngas production by integrating a single-atom cobalt-catalyst-decorated α-Fe₂ O₃ photoanode with a bimetallic Ag/Pd alloy cathode. A record syngas production activity of 81.9 μmol cm^-2  h^-1 (CO/H₂ ratio: ≈1 : 1) was achieved under artificial sunlight (AM 1.5 G) with an excellent durability. Systematic studies reveal that the Co single atoms effectively extract the holes from Fe₂ O₃ photoanodes and serve as active sites for promoting oxygen evolution. Simultaneously, the Pd and Ag atoms in bimetallic cathodes selectively adsorb CO₂ and protons for facilitating CO production. Further incorporation with a photovoltaic, to allow solar light (>600 nm) to be utilized, yields a bias-free CO₂ reduction device with solar-to-CO and solar-to-H₂ conversion efficiencies up to 1.33 and 1.36 %, respectively.

Tunable green syngas generation from CO2 and H2O with sunlight as the only energy input

The carbon-neutral synthesis of syngas from CO₂ and H₂O powered by solar energy holds grand promise for solving critical issues such as global warming and the energy crisis. Here we report photochemical reduction of CO₂ with H₂O into syngas using core/shell Au@Cr₂O₃ dual cocatalyst-decorated multistacked InGaN/GaN nanowires (NWs) with sunlight as the only energy input. First-principle density functional theory calculations revealed that Au and Cr₂O₃ are synergetic in deforming the linear CO₂ molecule to a bent state with an O-C-O angle of 116.5°, thus significantly reducing the energy barrier of CO₂RR compared with that over a single component of Au or Cr₂O₃. Hydrogen evolution reaction was promoted by the same cocatalyst simultaneously. By combining the cooperative catalytic properties of Au@Cr₂O₃ with the distinguished optoelectronic virtues of the multistacked InGaN NW semiconductor, the developed photocatalyst demonstrated high syngas activity of 1.08 mol/g_cat/h with widely tunable H₂/CO ratios between 1.6 and 9.2 under concentrated solar light illumination. Nearly stoichiometric oxygen was evolved from water splitting at a rate of 0.57 mol/g_cat/h, and isotopic testing confirmed that syngas originated from CO₂RR. The solar-to-syngas energy efficiency approached 0.89% during overall CO₂ reduction coupled with water splitting. The work paves a way for carbon-neutral synthesis of syngas with the sole inputs of CO₂, H₂O, and solar light.

Hybrid Metal-Organic Frameworks Encapsulated Hybrid Ni-Doped CdS Nanoparticles for Visible-Light-Driven CO2 Reduction

The photocatalytic production of syngas from CO₂ and water is an attractive and straightforward way for both solar energy storage and sustainable development. Here, we combined the hybrid shell of a bimetallic metal-organic framework (MOF) Zn/Co-zeolitic imidazolate framework (ZIF) and the hybrid photoactive center of Ni-doped CdS nanoparticles (Ni@CdS) to construct a new "2 + 2" photocatalysis system Ni@CdS⊂Zn/Co-ZIF through a facile self-assembly process, which exhibited a double-synergic effect for visible light harvesting and CO₂ conversion, leading to one of the highest photocatalytic syngas production rates and excellent recyclability. The H₂/CO of syngas ratios can be readily adjusted by controlling the ratio of Zn/Co in the hybrid MOF shell.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

Adjustable Underwater Gas Transportation Using Bioinspired Superhydrophobic Elastic String

Dynamic and precise manipulation of the gas flow in a liquid environment through a facile and reliable approach is of great importance for directional gas transportation and multiphase chemical reactions. In this research, elastic superhydrophobic strings were prepared by a one-step, non-fluorinated dip-coating strategy. The surface-treatment string demonstrated a good superaerophilicity underwater. By simply elongating or shortening superaerophilic strings, the gas flux underwater was precisely manipulated in a gas-siphon underwater experiment. The result reveals that a large strain of the treated string induces a low gas flow, and a rope woven with more strings results in a larger range of gas flow regulation. The elastic superhydrophobic/superaerophilic string was utilized to adjust the reaction time of carbon dioxide and sodium hydroxide aqueous solution successfully. Furthermore, in a wet oxidation experiment for treating simulated flue gas composed of nitric oxide (NO), nitrogen and oxygen, superhydrophobic and stretched strings with a strain of 200% demonstrated a 7.9% higher NO removal efficiency than that of untreated strings. Interestingly, NO removal efficiency can be regulated by mechanical stretching of gas-conducting strings. We believe that this facile and low-cost approach provides a valid method of on-demand manipulation of the gas flow for underwater gas transportation.

Crystallographic Effects of GaN Nanostructures in Photoelectrochemical Reaction

Tuning the surface structure of the photoelectrode provides one of the most effective ways to address the critical challenges in artificial photosynthesis, such as efficiency, stability, and product selectivity, for which gallium nitride (GaN) nanowires have shown great promise. In the GaN wurtzite crystal structure, polar, semipolar, and nonpolar planes coexist and exhibit very different structural, electronic, and chemical properties. Here, through a comprehensive study of the photoelectrochemical performance of GaN photocathodes in the form of films and nanowires with controlled surface polarities we show that significant photoelectrochemical activity can be observed when the nonpolar surfaces are exposed in the electrolyte, whereas little or no activity is measured from the GaN polar c-plane surfaces. The atomic origin of this fundamental difference is further revealed through density functional theory calculations. This study provides guideline on crystal facet engineering of metal-nitride photo(electro)catalysts for a broad range of artificial photosynthesis chemical reactions.

Electrically Induced Underwater Superaerophilicity/Superaerophobicity Switching on Polypyrrole-Coated Mesh Films for Selective Bubble Permeation

Recently, materials with controllable superwettability have attracted much attention. However, almost all studies focused on controlling wetting of water and oil; research on underwater gas bubble wetting control is still rare. Herein, we report a mesh film prepared by coating polypyrrole (PPy) film on Ti mesh. Briefly, the film mesh is underwater superaerophilic when PPy is doped with perfluorooctanesulfonate ions (PFOS^- ), and becomes underwater superaerophobic as the PFOS^- are removed. The transition of the wettability can be triggered by electrical stimuli, which is attributed to the cooperative effect between the rough structure and chemical components variation. The controllable wettability allows adjustable bubble permeation. It can be envisioned that the film will provide potential applications in the future, such as underwater bubble capture/release and microfluidic devices.

Enhanced syngas production from CO2 photoreduction over CoPd alloy modified NiAl-LDH under visible light

We reported the modification of a NiAl-layered double hydroxide (LDH) by loading well-dispersed CoPd alloys through a NaBH₄ reduction method. The modified NiAl-LDH achieved 14.5-fold and 2.1-fold improvements of the H₂ and CO evolution rates and an applicable ratio of H₂/CO (nearly 1 : 1) under visible light (λ > 420 nm). This study revealed the potential of alloys to adjust the H₂/CO ratio and enhance syngas production for LDHs for the first time.

CuS-Decorated GaN Nanowires on Silicon Photocathodes for Converting CO2 Mixture Gas to HCOOH

Hybrid materials consisting of semiconductors and cocatalysts have been widely used for photoelectrochemical (PEC) conversion of CO₂ gas to value-added chemicals such as formic acid (HCOOH). To date, however, the rational design of catalytic architecture enabling the reduction of real CO₂ gas to chemical has remained a grand challenge. Here, we report a unique photocathode consisting of CuS-decorated GaN nanowires (NWs) integrated on planar silicon (Si) for the conversion of H₂S-containing CO₂ mixture gas to HCOOH. It was discovered that H₂S impurity in the modeled industrial CO₂ gas could lead to the spontaneous transformation of Cu to CuS NPs, which resulted in significantly increased faradaic efficiency of HCOOH generation. The CuS/GaN/Si photocathode exhibited superior faradaic efficiency of HCOOH = 70.2% and partial current density = 7.07 mA/cm² at -1.0 V_RHE under AM1.5G 1 sun illumination. To our knowledge, this is the first demonstration that impurity mixed in the CO₂ gas can enhance, rather than degrade, the performance of the PEC CO₂ reduction reaction.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO₂ reduction, and N₂ reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

In situ chloride-mediated synthesis of TiO2 thin film photoanode with enhanced photoelectrochemical activity for carbamazepine oxidation coupled with simultaneous cathodic H2 production and CO2 conversion to fuels

This study investigated the simultaneous photoelectrochemical (PEC) degradation of carbamazepine (CBZ), reduction of CO₂ and production of H₂ using a TiO₂ thin film as photoanode and Ag plate as cathode. The photoanode was fabricated using sequential hydrothermal and calcination processes. The use of chloride during the hydrothermal process enhanced formation of oxygen vacancies and defects on the TiO₂ surface. Calcination not only further strengthened those features but also enhanced the crystallinity and anatase/rutile ratio, endowing the TiO₂ photoanode with superior PEC capacity. Characterization of physicochemical and PEC properties revealed that photogenerated electrons-holes were rapidly generated and efficiently separated on the TiO₂ surface during the PEC process. Hydroxyl radicals were the main active species responsible for anodic oxidation of carbamazepine, while hydrogen radicals and carbon dioxide radical anions mediated CO₂ reduction and H₂ production in the cathodic process. This work confirms the suitability of the prepared TiO₂ photoanode for PEC degradation of organic pollutants coupled with CO₂ reduction and H₂ production.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW-1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV-Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction

Photoelectrochemical CO₂ reduction into syngas (a mixture of CO and H₂) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H₂-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H₂ ratios (0–10) under one-sun illumination. Moreover, unassisted solar CO₂ reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

•

Combined experimental and theoretical investigations were performed

•

A record high applied bias photon-to-current efficiency of 1.88% was achieved

•

The CO/H₂ ratio in the syngas product can be controllably tuned in a wide range

•

Unassisted syngas generation was proved in a tandem photoelectrochemical cell

Double-shelled Cu2O/MnOx mesoporous hollow structure for CO2 photoreduction with enhanced stability and activity

Photocatalytic CO₂ reduction reaction (CRR) represents a prospective route for the clean utilization of greenhouse gas CO₂ and solar energy, and cuprous oxide (Cu₂O) is a favourable material for the CRR to avoid excess generation of hydrogen through the competitive hydrogen evolution reaction (HER). However, the application of Cu₂O-based photocatalysts is limited by their poor stability and low activity, which result from the self-corrosion by photogenerated holes. Here we construct a double-shelled Cu₂O/MnO_x mesoporous hollow structure (D-CMH) to enhance the stability and activity of Cu₂O-based photocatalysts. Because of the thin shells, which can shorten the diffusion distance of charge carriers, and the oxidation cocatalyst MnO_x, which can efficiently attract holes, the photogenerated holes can be immediately removed from Cu₂O and react with reactants. Additionally, the D-CMH can also provide enhanced efficiency of charge separation, prolonged path of light scattering and reflection and enlarged surface area for active sites, which result in the initial activity enhanced by 7.1 times and stability enhanced by 11.2 times compared with benchmark Cu₂O nanoparticles.

On factors limiting the performance of photoelectrochemical CO2 reduction

The photoelectrochemical CO₂ reduction reaction (PEC-CO2RR) is a promising artificial photosynthetic system for storing solar energy as the energy of chemical bonds and stabilizing the atmospheric CO₂ level. An applicable PEC-CO2RR is expected to have broad light absorption, high selectivity to a single product, and high solar to fuel efficiency. However, the PEC-CO2RR still faces challenges from complex reaction pathways, obstructed mass transfer, and large photovoltage requirements. The goal of this perspective is to point out some of the limitations of PEC-CO2RR to a practical application. In brief, we discuss the basic concepts of PEC-CO2RR and summarize state-of-the-art progress. Moreover, we highlight the remaining challenges to both science and engineering and propose the key steps in developing a fully functional PEC-CO2RR system. Finally, an ideal PEC-CO2RR system is proposed for future studies, which is essentially wireless and combines the advantages of minimized polarization loss and broad light absorption.

Highly efficient binary copper-iron catalyst for photoelectrochemical carbon dioxide reduction toward methane

A rational design of an electrocatalyst presents a promising avenue for solar fuels synthesis from carbon dioxide (CO₂) fixation but is extremely challenging. Herein, we use density functional theory calculations to study an inexpensive binary copper-iron catalyst for photoelectrochemical CO₂ reduction toward methane. The calculations of reaction energetics suggest that Cu and Fe in the binary system can work in synergy to significantly deform the linear configuration of CO₂ and reduce the high energy barrier by stabilizing the reaction intermediates, thus spontaneously favoring CO₂ activation and conversion for methane synthesis. Experimentally, the designed CuFe catalyst exhibits a high current density of -38.3 mA⋅cm^-2 using industry-ready silicon photoelectrodes with an impressive methane Faradaic efficiency of up to 51%, leading to a distinct turnover frequency of 2,176 h^-1 under air mass 1.5 global (AM 1.5G) one-sun illumination.

Colloidal silver diphosphide (AgP2) nanocrystals as low overpotential catalysts for CO2 reduction to tunable syngas

Production of syngas with tunable CO/H₂ ratio from renewable resources is an ideal way to provide a carbon-neutral feedstock for liquid fuel production. Ag is a benchmark electrocatalysts for CO₂-to-CO conversion but high overpotential limits the efficiency. We synthesize AgP₂ nanocrystals (NCs) with a greater than 3-fold reduction in overpotential for electrochemical CO₂-to-CO reduction compared to Ag and greatly enhanced stability. Density functional theory calculations reveal a significant energy barrier decrease in the formate intermediate formation step. In situ X-ray absorption spectroscopy (XAS) shows that a maximum Faradaic efficiency is achieved at an average silver valence state of +1.08 in AgP₂ NCs. A photocathode consisting of a n⁺p-Si wafer coated with ultrathin Al₂O₃ and AgP₂ NCs achieves an onset potential of 0.2 V vs. RHE for CO production and a partial photocurrent density for CO at −0.11 V vs. RHE (j_{−0.11, CO}) of −3.2 mA cm⁻².

Conversion of CO₂ into value-added chemicals by use of renewable energy is promising to achieve a carbon-neutral energy cycle. Here, the authors show that AgP₂ is a stable, selective and efficient syngas catalyst for solar-to-fuel conversion with a 3-fold lower overpotential compared to the benchmark Ag catalyst.

Photoelectrochemical CO2 Reduction with a Rhenium Organometallic Redox Mediator at Semiconductor/Aqueous Liquid Junction Interfaces

Electrochemical and photoelectrochemical CO₂ reductions were carried out with Re(bh-bipy)(CO)₃ (OH₂ ) cocatalysts in aqueous electrolytes. Competition between hydrogen evolution and CO₂ reduction was observed under (photo)electrochemical conditions for both glassy carbon and CuInS₂ electrodes. The partial current density for CO generation is limited even though the additional potential is applied. However, electrochemical hydrogen evolution was suppressed under photoelectrochemical conditions, and the selectivity and partial current density for CO were considerably increased when compared to the electrochemical reduction in an identical electrode/electrolyte system. This finding may provide insights into using semiconductor/liquid junctions for solar fuel devices to overcome the limitations of electrolysis systems with an external bias.

The Development of Cocatalysts for Photoelectrochemical CO2 Reduction

The ever-increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO₂ have led to a severe energy crisis and climate change. Photocatalytic reduction of CO₂ into fuels using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO₂ can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO₂ on the surface of a semiconductor-based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO₂ reduction over semiconductor-based photoelectrodes are described, and the basic principles of PEC CO₂ reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO₂ are also presented.

Photoelectrocatalytic reduction of CO2 to syngas over Ag nanoparticle modified p-Si nanowire arrays

The solar energy-driven reduction of CO₂ and H₂O to syngas (H₂/CO), an important platform to produce chemicals, is of significance for alleviating greenhouse gas emission and utilizing sustainable solar energy. Here, we report a facile method for the photoelectrocatalytic reduction of CO₂ and H₂O to syngas over an Ag nanoparticle (NP) modified p-Si nanowire array catalyst. The particle size of Ag significantly influences the activity of CO₂ reduction to CO. The H₂/CO molar ratio in reduction products can be tuned in the range from 1 to 4 by controlling the size of Ag NPs from 4.2 to 16 nm. The adsorption strength of CO on the catalyst was found to decline with the increase in the size of Ag NPs. The Ag NPs of 8.2 nm, which possess a moderate CO adsorption strength, exhibit the maximum production of CO with the H₂/CO ratio of 2/1.

MOF-derived Co1.11Te2 with half-metallic character for efficient photochemical conversion of CO2 under visible-light irradiation

Co_1.11Te₂ enwrapped in a carbon layer displayed high catalytic performance for photocatalytic CO₂ reduction. The strong electron transfer ability and the low energy barrier are the key factors that promote its high catalytic activity.