Facile Synthesis of Organic-Inorganic Hybrid Heterojunctions of Glycolated Conjugated Polymer-TiO2-X for Efficient Photocatalytic Hydrogen Evolution

The utilization of the organic-inorganic hybrid photocatalysts for water splitting has gained significant attention due to their ability to combine the advantages of both materials and generate synergistic effects. However, they are still far from practical application due to the limited understanding of the interactions between these two components and the complexity of their preparation process. Herein, a facial approach by combining a glycolated conjugated polymer with a TiO2-X mesoporous sphere to prepare high-efficiency hybrid photocatalysts is presented. The functionalization of conjugated polymers with hydrophilic oligo (ethylene glycol) side chains can not only facilitate the dispersion of conjugated polymers in water but also promote the interaction with TiO2-X forming stable heterojunction nanoparticles. An apparent quantum yield of 53.3% at 365 nm and a hydrogen evolution rate of 35.7 mmol h-1 g-1 is achieved by the photocatalyst in the presence of Pt co-catalyst. Advanced photophysical studies based on femtosecond transient absorption spectroscopy and in situ, XPS analyses reveal the charge transfer mechanism at type II heterojunction interfaces. This work shows the promising prospect of glycolated polymers in the construction of hybrid heterojunctions for photocatalytic hydrogen production and offers a deep understanding of high photocatalytic performance by such heterojunction photocatalysts.

Metal-Free Photocatalytic CO2 Reduction to CH4 and H2 O2 under Non-sacrificial Ambient Conditions

Photocatalytic CO2 reduction to CH4 requires photosensitizers and sacrificial agents to provide sufficient electrons and protons through metal-based photocatalysts, and the separation of CH4 from by-product O2 has poor applications. Herein, we successfully synthesize a metal-free photocatalyst of a novel electron-acceptor 4,5,9,10-pyrenetetrone (PT), to our best knowledge, this is the first time that metal-free catalyst achieves non-sacrificial photocatalytic CO2 to CH4 and easily separable H2 O2 . This photocatalyst offers CH4 product of 10.6 μmol ⋅ g-1  ⋅ h-1 under non-sacrificial ambient conditions (room temperature, and only water), which is two orders of magnitude higher than that of the reported metal-free photocatalysts. Comprehensive in situ characterizations and calculations reveal a multi-step reaction mechanism, in which the long-lived oxygen-centered radical in the excited PT provides as a site for CO2 activation, resulting in a stabilized cyclic carbonate intermediate with a lower formation energy. This key intermediate is thermodynamically crucial for the subsequent reduction to CH4 product with the electronic selectivity of up to 90 %. The work provides fresh insights on the economic viability of photocatalytic CO2 reduction to easily separable CH4 in non-sacrificial and metal-free conditions.

Assessment of the technological viability of photoelectrochemical devices for oxygen and fuel production on Moon and Mars

Human deep space exploration is presented with multiple challenges, such as the reliable, efficient and sustainable operation of life support systems. The production and recycling of oxygen, carbon dioxide (CO₂) and fuels are hereby key, as a resource resupply will not be possible. Photoelectrochemical (PEC) devices are investigated for the light-assisted production of hydrogen and carbon-based fuels from CO₂ within the green energy transition on Earth. Their monolithic design and the sole reliance on solar energy makes them attractive for applications in space. Here, we establish the framework to evaluate PEC device performances on Moon and Mars. We present a refined Martian solar irradiance spectrum and establish the thermodynamic and realistic efficiency limits of solar-driven lunar water-splitting and Martian carbon dioxide reduction (CO₂R) devices. Finally, we discuss the technological viability of PEC devices in space by assessing the performance combined with solar concentrator devices and explore their fabrication via in-situ resource utilization.

Water-Splitting Artificial Leaf Based on a Triple-Junction Silicon Solar Cell: One-Step Fabrication through Photoinduced Deposition of Catalysts and Electrochemical Operando Monitoring

Solar hydrogen generation via water splitting using a monolithic photoelectrochemical cell, also called artificial leaf, could be a powerful technology to accelerate the transition from fossil to sustainable energy sources. Identification of scalable methods for the fabrication of monolithic devices and gaining insights into their operating mode to identify solutions to improve performance and stability represent great challenges. Herein, we report on the one-step fabrication of a CoWO|ITO|3jn-a-Si|Steel|CoWS monolithic device via the simple photoinduced deposition of CoWO and CoWS as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalyst layers, respectively, onto an illuminated ITO|3jn-a-Si|Steel solar cell using a single-deposition bath containing the [Co(WS₄)₂]^2- complex. In a pH 7 phosphate buffer solution, the best device achieved a solar-to-hydrogen conversion yield of 1.9%. Evolution of the catalyst layers and that of the 3jn-a-Si light-harvesting core during the operation of the monolithic device are examined by conventional tools such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and inductively coupled plasma optical emission spectroscopy (ICP-OES) together with a bipotentiostat measurement. We demonstrate that the device performance degrades due to the partial dissolution of the catalyst. Still, this degradation is healable by simply adding [Co(WS₄)₂]^2- to the operating solution. However, modifications on the protecting indium-doped tin oxide (ITO) layer are shown to initiate irreversible degradation of the 3jn-a-Si light-harvesting core, resulting in a 10-fold decrease of the performances of the monolithic device.

Electrospun Donor/Acceptor Nanofibers for Efficient Photocatalytic Hydrogen Evolution

We prepared a series of one-dimensional conjugated-material-based nanofibers with different morphologies and donor/acceptor (D/A) compositions by electrospinning for efficient photocatalytic hydrogen evolution. It was found that homogeneous D/A heterojunction nanofibers can be obtained by electrospinning, and the donor/acceptor ratio can be easily controlled. Compared with the single-component-based nanofibers, the D/A-based nanofibers showed a 34-fold increase in photocatalytic efficiency, attributed to the enhanced exciton dissociation in the nanofibrillar body. In addition, the photocatalytic activity of these nanofibers can be easily optimized by modulating the diameter. The results show that the diameter of the nanofibers can be conveniently controlled by the electrospinning feed rate, and the photocatalytic effect increases with decreasing fiber diameter. Consequently, the nanofibers with the smallest diameter exhibit the most efficient photocatalytic hydrogen evolution, with the highest release rate of 24.38 mmol/(gh). This work provides preliminary evidence of the advantages of the electrospinning strategy in the construction of D/A nanofibers with controlled morphology and donor/acceptor composition, enabling efficient hydrogen evolution.

Photoelectrochemistry of Self-Limiting Electrodeposition of Ni Film onto GaAs

Gallium arsenide (GaAs) provides a suitable bandgap (1.43 eV) for solar spectrum absorption and allows a larger photovoltage compared to silicon, suggesting great potential as a photoanode toward water splitting. Photocorrosion under water oxidation condition, however, leads to decomposition or the formation of an insulating oxide layer, which limits the photoelectrochemical performance and stability of GaAs. In this work, a self-limiting electrodeposition method of Ni on GaAs is reported to either generate ultra-thin continuous film or nanoislands with high particle density by controlling deposition time. The self-limiting growth mechanism is validated by potential transients, X-ray photoelectron spectroscopy composition and depth profile measurements. This deposition method exhibits a rapid nucleation, forms an initial metallic layer followed by a hydroxide/oxyhydroxide nanofilm on the GaAs surface and is independent of layer thickness versus deposition time when coalescence is reached. A photocurrent up to 8.9 mA cm^-2 with a photovoltage of 0.11 V is obtained for continuous ultrathin films, while a photocurrent density of 9.2 mA cm^-2 with a photovoltage of 0.50 V is reached for the discontinuous nanoislands layers in an aqueous solution containing the reversible redox couple K₃ Fe(CN)₆ /K₄ Fe(CN)₆ .

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol^–1. The 1e^– electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl^– from 1. 1-OPh, which exhibits an entropically favored intramolecular Re–O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1 and Mn-1OH display a selective reduction of CO₂ to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re–O bond and a preference for phenolate-based reactivity with CO₂ slightly inhibit the electrocatalytic reduction of CO₂ to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.

Electrochemical CO2 Reduction to CO Catalyzed by 2D Nanostructures

Electrochemical CO2 reduction towards value-added chemical feedstocks has been extensively studied in recent years to resolve the energy and environmental problems. The practical application of electrochemical CO2 reduction technology requires a cost-effective, highly efficient, and robust catalyst. To date, vigorous research have been carried out to increase the proficiency of electrocatalysts. In recent years, two-dimensional (2D) graphene and transition metal chalcogenides (TMCs) have displayed excellent activity towards CO2 reduction. This review focuses on the recent progress of 2D graphene and TMCs for selective electrochemical CO2 reduction into CO.

Electrostatic promotion of the catalyst activity for ammonia photosynthesis upon a robust affordable nanostructured uni-electrodic photodevice/reactor

The catalytic ability of the uni-electrodic photoelectrochemical system to synthesize ammonia can be electrostatically boosted by applying a non-faradaic potential bias to the photodevice/reactor or adding a promoter species into the reaction medium.

Water as a tuneable solvent: a perspective

Water is the most sustainable solvent, but its polarity limits the solubility of non-polar solutes. Confining water in hydrophobic nanopores could be a way to modulate water solvent properties and enable using water as tuneable solvent (WaTuSo).

Prospects for Hermetic Sealing of Scaled-Up Photoelectrochemical Hydrogen Generators for Reliable and Risk Free Operation

Photo-electrochemical (PEC) systems have the potential to contribute to de-carbonation of the global energy supply because solar energy can be directly converted to hydrogen, which can be burnt without the release of greenhouse gases. However, meaningful deployment of PEC technology in the global energy system, even when highly efficient scaled up devices become available, shall only be a reality when their safe and reliable operation can be guaranteed over several years of service life. The first part of this review discusses the importance of hermetic sealing of up scaled PEC device provided by the casing and sealing joints from a reliability and risk perspective. The second part of the review presents a survey of fully functional devices and early stage demonstrators and uses this to establish the extent to which the state of the art in PEC device design address the issue of hermetic sealing. The survey revealed that current material choices and sealing techniques are still unsuitable for scale–up and commercialization. Accordingly, we examined possible synergies with related photovoltaic and electrochemical devices that have been commericalised, and derived therefrom, recommendations for future research routes that could accelerate the development of hermetic seals of PEC devices.

Fundamentals and applications of photocatalytic CO2 methanation

The extraction and combustion of fossil natural gas, consisting primarily of methane, generates vast amounts of greenhouse gases that contribute to climate change. However, as a result of recent research efforts, “solar methane” can now be produced through the photocatalytic conversion of carbon dioxide and water to methane and oxygen. This approach could play an integral role in realizing a sustainable energy economy by closing the carbon cycle and enabling the efficient storage and transportation of intermittent solar energy within the chemical bonds of methane molecules. In this article, we explore the latest research and development activities involving the light-assisted conversion of carbon dioxide to methane.

While natural gas and fossil fuels power human activities, increasing concerns over fuel reserves and environmental impacts require finding alternative, renewable resources. Here, authors review the fundamental science and progress on solar-powered conversion of carbon dioxide to methane.

Design of Heterostructured Hollow Photocatalysts for Solar-to-Chemical Energy Conversion

Direct conversion of solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues. Fabrication of highly active photocatalysts is of great significance for the practical applications of efficient solar-to-chemical energy conversion systems. Among various photocatalytic materials, semiconductor-based heterostructured photocatalysts with hollow features show distinct advantages. Recent research efforts on rational design of heterostructured hollow photocatalysts toward photocatalytic water splitting and CO₂ reduction are presented. First, both single-shelled and multishelled heterostructured photocatalysts are surveyed. Then, heterostructured hollow photocatalysts with tube-like and frame-like morphologies are discussed. It is intended that further innovative works on the material design of high-performance photocatalysts for solar energy utilization can be inspired.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.

Toward practical solar hydrogen production - an artificial photosynthetic leaf-to-farm challenge

Solar water splitting is a promising approach to transform sunlight into renewable, sustainable and green hydrogen energy. There are three representative ways of transforming solar radiation into molecular hydrogen, which are the photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-electrolysis (PV-EC) routes. Having the future perspective of green hydrogen economy in mind, this review article discusses devices and systems for solar-to-hydrogen production including comparison of the above solar water splitting systems. The focus is placed on a critical assessment of the key components needed to scale up PEC water splitting systems such as materials efficiency, cost, elemental abundancy, stability, fuel separation, device operability, cell architecture, and techno-economic aspects of the systems. The review follows a stepwise approach and provides (i) a summary of the basic principles and photocatalytic materials employed for PEC water splitting, (ii) an extensive discussion of technologies, procedures, and system designs, and (iii) an introduction to international demonstration projects, and the development of benchmarked devices and large-scale prototype systems. The task of scaling up of laboratory overall water splitting devices to practical systems may be called "an artificial photosynthetic leaf-to-farm challenge".

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

A microfluidic photoelectrochemical cell for solar-driven CO2 conversion into liquid fuels with CuO-based photocathodes

Schematic representation of photoelectrochemical CO₂ reduction set-up.

The effect of covalently bonded aryl layers on the band bending and electron density of SnO2 surfaces probed by synchrotron X-ray photoelectron spectroscopy

A downward to upward surface band bending change can be induced by grafted 4-(trifluoromethyl)phenyl groups on SnO₂.

A methodological review on material growth and synthesis of solar-driven water splitting photoelectrochemical cells

As a renewable and sustainable energy source and an alternative to fossil fuels, solar-driven water splitting with photoelectrochemical (PEC) cell is a promising approach to obtain hydrogen fuel with its near-zero carbon emission pathway by transforming incident sunlight, the most abundant energy source. Because of its importance and future prospects, a number of architectures with their own features have been formed by various synthesis and growth methods. Because the materials themselves are one of the most dominant components, they determine the solar-to-hydrogen efficiency of the PEC cells. Thus, several representative PEC cells were reviewed by categorizing them as per synthesis and/or growth methods such as physical vapor deposition, chemical vapor deposition, electrochemical deposition, etc. This review provides researchers with an overview and acts as a guide for research on solar-driven water splitting PEC cells.

Solar-driven PEC cell is a promising approach to obtain hydrogen with near-zero carbon emission pathway. In this article, PEC cell was reviewed as per growth/synthesis methods. This review provides an overview and a guide for research on PEC cell.

Advancing semiconductor-electrocatalyst systems: application of surface transformation films and nanosphere lithography

Photoelectrochemical (PEC) cells offer the possibility of carbon-neutral solar fuel production through artificial photosynthesis. The pursued design involves technologically advanced III-V semiconductor absorbers coupled via an interfacial film to an electrocatalyst layer. These systems have been prepared by in situ surface transformations in electrochemical environments. High activity nanostructured electrocatalysts are required for an efficiently operating cell, optimized in their optical and electrical properties. We demonstrate that shadow nanosphere lithography (SNL) is an auspicious tool to systematically create three-dimensional electrocatalyst nanostructures on the semiconductor photoelectrode through controlling their morphology and optical properties. First results are demonstrated by means of the photoelectrochemical production of hydrogen on p-type InP photocathodes where hitherto applied photoelectrodeposition and SNL-deposited Rh electrocatalysts are compared based on their J-V and spectroscopic behavior. We show that smaller polystyrene particle masks achieve higher defect nanostructures of rhodium on the photoelectrode which leads to a higher catalytic activity and larger short circuit currents. Structural analyses including HRSEM and the analysis of the photoelectrode surface composition by using photoelectron spectroscopy support and complement the photoelectrochemical observations. The optical performance is further compared to theoretical models of the nanostructured photoelectrodes on light scattering and propagation.

Strategies for stable water splitting via protected photoelectrodes

Photoelectrochemical (PEC) solar-fuel conversion is a promising approach to provide clean and storable fuel (e.g., hydrogen and methanol) directly from sunlight, water and CO₂. However, major challenges still have to be overcome before commercialization can be achieved. One of the largest barriers to overcome is to achieve a stable PEC reaction in either strongly basic or acidic electrolytes without degradation of the semiconductor photoelectrodes. In this work, we discuss fundamental aspects of protection strategies for achieving stable solid/liquid interfaces. We then analyse the charge transfer mechanism through the protection layers for both photoanodes and photocathodes. In addition, we review protection layer approaches and their stabilities for a wide variety of experimental photoelectrodes for water reduction. Finally, we discuss key aspects which should be addressed in continued work on realizing stable and practical PEC solar water splitting systems.

Synthesis and characterization of Zn2GeO4/Mg-MOF-74 composites with enhanced photocatalytic activity for CO2 reduction

Zn₂GeO₄/Mg-MOF-74 composites are explored for artificial CO₂ phototransformation under mild conditions.

Computational insights into charge transfer across functionalized semiconductor surfaces

Photoelectrochemical water-splitting is a promising carbon-free fuel production method for producing H₂ and O₂ gas from liquid water. These cells are typically composed of at least one semiconductor photoelectrode which is prone to degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet stabilization techniques are often accompanied by a decrease in photoelectrode performance. However, the impact of surface modification on charge transport and its consequence on performance is still lacking, creating a roadblock for further improvements. In this review, we discuss how density functional theory and finite-element device simulations are reliable tools for providing insight into charge transport across modified photoelectrodes.

Electrochemical CO2 Reduction at Glassy Carbon Electrodes Functionalized by MnI and ReI Organometallic Complexes

The catalytic activities towards electrochemical CO₂ reduction of two new rhenium and manganese complexes, namely fac-Mn(apbpy)(CO)₃ Br (1) and fac-Re(apbpy)(CO)₃ Cl (2) (apbpy=4-(4-aminophenyl)-2,2'-bipyridine), in both homogeneous and heterogeneous phases are compared. A glassy carbon electrode (GCE) surface has been functionalized with complexes 1 and 2 by two approaches: a) direct electrochemical oxidation of the amino group with formation of C-N bonds, and b) electrochemical reduction of the corresponding diazonium salts with formation of C-C bonds. The chemically modified GCEs show efficient conversion of CO₂ into CO, with turnover numbers (TONs) about 60 times higher than those of the corresponding catalysts in homogeneous solutions, and in a much shorter time.

Transparent Bipolar Membrane for Water Splitting Applications

This study describes the use of a benzimidazolium-based anion exchange membrane for creating bipolar membranes and the assessment of their suitability for solar-driven water splitting. Bipolar membranes were prepared by laminating anion exchange membrane with Nafion NR-211 membrane without modification of the interface. Under acidic and basic conditions, proton and hydroxide ion conductivities of 103 and 102 mS cm^-1 were obtained for Nafion and benzimidazolium-based membranes, respectively. The fabricated bipolar membranes have an average thickness of 90 μm and show high transmittance, up to 75% of the visible light. The findings suggest that the two membranes create a sharp hydrophilic interface with a space charge region of only a few nanometers, thereby generating a large electric field at the interface that enhances water dissociation.

The Chemical Route to a Carbon Dioxide Neutral World

Excessive CO₂ emissions in the atmosphere from anthropogenic activity can be divided into point sources and diffuse sources. The capture of CO₂ from flue gases of large industrial installations and its conversion into fuels and chemicals with fast catalytic processes seems technically possible. Some emerging technologies are already being demonstrated on an industrial scale. Others are still being tested on a laboratory or pilot scale. These emerging chemical technologies can be implemented in a time window ranging from 5 to 20 years. The massive amounts of energy needed for capturing processes and the conversion of CO₂ should come from low-carbon energy sources, such as tidal, geothermal, and nuclear energy, but also, mainly, from the sun. Synthetic methane gas that can be formed from CO₂ and hydrogen gas is an attractive renewable energy carrier with an existing distribution system. Methanol offers advantages as a liquid fuel and is also a building block for the chemical industry. CO₂ emissions from diffuse sources is a difficult problem to solve, particularly for CO₂ emissions from road, water, and air transport, but steady progress in the development of technology for capturing CO₂ from air is being made. It is impossible to ban carbon from the entire energy supply of mankind with the current technological knowledge, but a transition to a mixed carbon-hydrogen economy can reduce net CO₂ emissions and ultimately lead to a CO₂ -neutral world.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-R)(CO)3 Br] Complexes

The electrochemical behavior of fac-[Mn(pdbpy)(CO)₃ Br] (pdbpy=4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine) (1) in acetonitrile under Ar, and its catalytic performances for CO₂ reduction with added water, 2,2,2-trifluoroethanol (TFE), and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V versus the standard calomel electrode (SCE) in CO₂ -saturated acetonitrile, reveal that the process selectivity is extremely sensitive to the acid strength, producing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV/Vis) study under Ar and CO₂ atmospheres shows that 1 undergoes fast solvolysis; however, dimer formation in acetonitrile is suppressed, resulting in an atypical reduction mechanism in comparison with other reported Mn^I catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO₂ reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)₃ Br] (ptbpy=4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst (2), bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO₂ .

Enhanced photocatalytic CO2 reduction over Co-doped NH2-MIL-125(Ti) under visible light

Photocatalytic CO₂ reduction coupled with the selective oxidation of benzylic alcohols can be achieved over Co/NH₂-MIL-125(Ti) upon visible-light irradiation.

Improvement of Catalytic Activity by Nanofibrous CuInS2 for Electrochemical CO2 Reduction

The current study reports the application of chalcopyrite semiconductor CuInS₂ (CIS) nanofibers for the reduction of CO₂ to CO with a remarkable Faradaic efficiency of 77 ± 4%. Initially the synthesis of CuInS₂ nanofibers was carried out by adaptable electrospinning technique. To reduce the imperfection in the crystalline fiber, polyacrylonitrile (PAN) was selected as template polymer. Afterward, the desired chemical structure of nanofibers was achieved through sulfurization process. Making continuous CuInS₂ nanofibers on the cathode surface by the electrospinning method brings the advantages of being economical, environmentally safe, and versatile. The obtained nanofibers of well investigated size and diameter according to the SEM (scanning electron microscope) were used in electrochemical studies. An improvement of Faradaic efficiency was achieved with the catalytic active CuInS₂ in nanofibrous structure as compared to the solution processed CuInS₂. This underlines the important effect of the electrode fabrication on the catalytic performance. Being less contaminated as compared to solution processing, and having a well-defined composition and increased catalytically active area, the CuInS₂ nanofiber electrodes prepared by the electrospinning technique show a 4 times higher Faradaic efficiency. Furthermore, in this study, attention was paid to the stability of the CuInS₂ nanofiber electrodes. The electrochemical reduction of CO₂ to CO by using CIS nanofibers coated onto FTO electrodes was carried out for 10 h in total. The observed current density of 0.22 mA cm^-2 and the stability of CIS nanofiber electrodes are found to be competitive with other heterogeneous electrocatalysts. Hence, we believe that the fabrication and application of nanofibrous materials through the electrospinning technique might be of interest for electrocatalytic studies in CO₂ reduction.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Inhibition of Tafel Kinetics for Electrolytic Hydrogen Evolution on Isolated Micron Scale Electrocatalysts on Semiconductor Interfaces

Semiconductor-liquid junctions are ubiquitous in photoelectrochemical approaches to artificial photosynthesis. By analogy with the antennae and reaction centers in natural photosynthetic complexes, separating the light-absorbing semiconductor and electrocatalysts can improve catalytic efficiency. A catalytic layer can also impair the photovoltage-generating energetics of the electrode without appropriate microscopic organization of catalytically active area on the surface. Here, we have developed a method using high-speed X-ray phase contrast imaging to study in situ electrolytic bubble growth on semiconductor electrodes fabricated with isolated, micron-scale platinum electrocatalysts. X-rays are a nonperturbative probe by which gas evolution dynamics can be studied under conditions relevant to solar fuels applications. The self-limited growth of a bubble residing on the isolated electrocatalyst was measured by tracking the evolution of the gas-liquid boundary. Contrary to observations on macroscopic electrodes, bubble evolution on isolated, microscopic Pt pads on Si electrodes was insensitive to increasing overpotential. The persistence of the bubble causes mass transport limitations and inhibits the expected Tafel-like kinetics. The observed scaling of catalytic current densities with pad size implies that electrolysis is occurring predominantly on the perimeter of the active area.