Solar carbon fuel via photoelectrochemistry

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Tuning the morphology, stability and optical properties of CsSnBr3 nanocrystals through bismuth doping for visible-light-driven applications

In this investigation, we have demonstrated the synthesis of lead-free CsSnBr3 (CSB) and 5 mol% bismuth (Bi) doped CSB (CSB'B) nanocrystals, with a stable cubic perovskite structure following a facile hot injection technique. The Bi substitution in CSB was found to play a vital role in reducing the size of the nanocrystals significantly, from 316 ± 93 to 87 ± 22 nm. Additionally, Bi doping has inhibited the oxidation of Sn2+ of CSB perovskite. A reduction in the optical band gap from 1.89 to 1.73 eV was observed for CSB'B and the PL intensity was quenched due to the introduction of the Bi3+ dopant. To demonstrate one of the visible-light-driven applications of the nanocrystals, photodegradation experiments were carried out as a test case. Interestingly, under UV-vis irradiation, the degradation efficiency of CSB'B was roughly one order lower than that of P25 titania nanoparticles; however, it was almost five times higher when driven by visible light under identical conditions. The water stability of CSB'B perovskite and suppression of the oxidative degradation of Sn were confirmed through XRD and XPS analyses after photocatalysis. Moreover, by employing experimental parameters, DFT-based first-principles calculations were performed, which demonstrated an excellent qualitative agreement between experimental and theoretical outcomes. The as-synthesized Bi-doped CSB might be a stable halide perovskite with potential in visible-light-driven applications.

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.

V-Porphyrins Encapsulated or Supported on Siliceous Materials: Synthesis, Characterization, and Photoelectrochemical Properties

Metalloporphyrin-containing mesoporous materials, named VTPP@SBA, were prepared via a simple anchoring of vanadyl porphyrin (5,10,15,20-Tetraphenyl-21H,23H-porphine vanadium(IV) oxide) through a SBA-15-type mesoporous material. For comparison, vanadyl porphyrin was also impregnated on SiO₂ (VTPP/SiO₂). The characterization results of catalysts by XRD, FTIR, DR-UV-vis, and EPR confirm the incorporation of vanadyl porphyrin within the mesoporous SBA-15. These catalysts have also been studied using electrochemical and photoelectrochemical methods. Impedance measurements confirmed that supporting the porphyrin in silica improved the electrical conductivity of samples. In fact, when using mesoporous silica, current densities associated with oxidation/reduction processes appreciably increased, implying an enhancement in charge transfer processes and, therefore, in electrochemical performance. All samples presented n-type semiconductivity and provided an interesting photoelectrocatalytic response upon illumination, especially silica-supported porphyrins. This is the first time that V-porphyrin-derived materials have been tested for photoelectrochemical applications, showing good potential for this use.

Conversion of carbon dioxide to methanol: A comprehensive review

This review explains the various methods of conversion of Carbon dioxide (CO₂) to methanol by using homogenous, heterogeneous catalysts through hydrogenation, photochemical, electrochemical, and photo-electrochemical techniques. Since, CO₂ is the major contributor to global warming, its utilization for the production of fuels and chemicals is one of the best ways to save our environment in a sustainable manner. However, as the CO₂ is very stable and less reactive, a proper method and catalyst development is most important to break the CO₂ bond to produce valuable chemicals like methanol. Litertaure says the catalyt types, ratio and it surface structure along with the temperature and pressure are the most controlling parameters to optimize the process for the production of methanol from CO₂. This article explains about the various controlling parameters of synthesis of Methanol from CO₂ along with the advantages and drawbacks of each process. The mechanism of each synthesis process in presence of metal supported catalyst is described. Basically the activity of Cu supported catalyst and its stability based on the activity for the methanol synthesis from CO₂ through various methods is critically described.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Advancing Photoelectrochemical Energy Conversion through Atomic Design of Catalysts

Powered by inexhaustible solar energy, photoelectrochemical (PEC) hydrogen/ammonia production and reduction of carbon dioxide to high added-value chemicals in eco-friendly and mild conditions provide a highly attractive solution to carbon neutrality. Recently, substantial advances have been achieved in PEC systems by improving light absorption and charge separation/transfer in PEC devices. However, less attention is given to the atomic design of photoelectrocatalysts to facilitate the final catalytic reactions occurring at photoelectrode surface, which largely limits the overall photo-to-energy conversion of PEC system. Fundamental catalytic mechanisms and recent progress in atomic design of PEC materials are comprehensively reviewed by engineering of defect, dopant, facet, strain, and single atom to enhance the activity and selectivity. Finally, the emerging challenges and research directions in design of PEC systems for future photo-to-energy conversions are proposed.

CuInS2 Photocathodes with Atomic Gradation-Controlled (Ta,Mo)x(O,S)y Passivation Layers for Efficient Photoelectrochemical H2 Production

An atomic gradient passivation layer, (Ta,Mo)_x(O,S)_y, is designed to improve the charge transportation and photoelectrochemical activity of CuInS₂-based photoelectrodes. We found that Mo spontaneously diffused to the a-TaO_x layer during e-beam evaporation. This result indicates that the gradient profile of MoO_x/TaO_x is formed in the sublayer of (Ta,Mo)_x(O,S)_y. To understand the atomic-gradation effects of the (Ta,Mo)_x(O,S)_y passive layer, the composition and (photo)electrochemical properties have been characterized in detail. When this atomic gradient-passive layer is applied to CuInS₂-based photocathodes, promising photocurrent and onset potential are seen without using Pt cocatalysts. This is one of the highest activities among reported CuInS₂ photocathodes, which are not combined with noble metal cocatalysts. Excellent photoelectrochemical activity of the photoelectrode can be mainly achieved by (1) the electron transient time improved due to the conductive Mo-incorporated TaO_x layer and (2) the boosted electrocatalytic activity by Mo_x(O,S)_y formation.

Simultaneous removal of organic pollutants and heavy metals in wastewater by photoelectrocatalysis: A review

As a powerful technique by combining photocatalysis with electrochemistry, photoelectrocatalysis has been extensively explored to simultaneously remove mixed pollutants of organic and heavy metal in wastewater in the past decade. In the photoelectrocatalytic system, the bias potential can remarkably promote the oxidation of organic pollutants on the photoanode by suppressing the recombination of photogenerated electron-hole pairs and extending the lifetime of photogenerated holes. Meanwhile, some photogenerated electrons are driven by the bias potential to the cathode to reduce heavy metals. In this review, we summarize the research advances in photoelectrocatalytic treatment of organic-heavy metal mixed pollution systems under UV light, visible light and sunlight. We demonstrate the main operation variables affecting the photoelectrocatalytic removal processes of organic pollutants and heavy metals. The problems for utilization of solar energy in photoelectrocatalysis are discussed. Finally, this review proposes the perspectives for future development of photoelectrocatalysis to industrial applications.

Optically and Electrocatalytically Decoupled Si Photocathodes with a Porous Carbon Nitride Catalyst for Nitrogen Reduction with Over 61.8% Faradaic Efficiency

The photoelectrochemical (PEC) approach is attractive as a promising route for the nitrogen reduction reaction (NRR) toward ammonia (NH₃ ) synthesis. However, the challenges in synergistic management of optical, electrical, and catalytic properties have limited the efficiency of PEC NRR devices. Herein, to enhance light-harvesting, carrier separation/transport, and the catalytic reactions, a concept of decoupling light-harvesting and electrocatalysis by employing a cascade n⁺ np⁺ -Si photocathode is implemented. Such a decoupling design not only abolishes the parasitic light blocking but also concurrently improves the optical and electrical properties of the n⁺ np⁺ -Si photocathode without compromising the efficiency. Experimental and density functional theory studies reveal that the porous architecture and N-vacancies promote N₂ adsorption of the Au/porous carbon nitride (PCN) catalyst. Impressively, an n⁺ np⁺ -Si photocathode integrating the Au/PCN catalyst exhibits an outstanding PEC NRR performance with maximum Faradaic efficiency (FE) of 61.8% and NH₃ production yield of 13.8 µg h^-1 cm^-2 at -0.10 V versus reversible hydrogen electrode (RHE), which is the highest FE at low applied potential ever reported for the PEC NRR.

Grain Boundary Engineering of Cu-Ag Thin-Film Catalysts for Selective (Photo)Electrochemical CO2 Reduction to CO and CH4

We investigated the relationship between grain boundary (GB) oxidation of Cu-Ag thin-film catalysts and selectivity of the (photo)electrochemical CO₂ reduction reaction (CO₂ RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH₄. However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu-Ag catalysts. The optimized Cu (100 nm)-Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FE_CO = 79.8%) and CH₄ (FE_CH4 = 59.3%) at a reductive potential of -1.0 and -1.4 V_RHE, respectively. Moreover, the Cu-Ag thin film was used as a cocatalyst for photo-electrochemical CO₂ reduction by patterning the Cu-Ag thin film and a SiO₂ passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH₄ under light illumination (100 mW/cm²).

Sulfur-doped graphitic carbon nitride incorporated bismuth oxychloride/Cobalt based type-II heterojunction as a highly stable material for photoelectrochemical water splitting

Cobalt incorporated sulfur-doped graphitic carbon nitride with bismuth oxychloride (Co/S-gC₃N₄/BiOCl) heterojunction is prepared by an ultrasonically assisted hydrothermal treatment. The heterojunction materials have employed in photoelectrochemical (PEC) water splitting. The PEC activity and stability of the materials are promoted by constructing an interface between the visible light active semiconductor photocatalyst and cocatalysts. The photocurrent density of Co-9% S-gC₃N₄/BiOCl has attained 393.0 μA cm^-2 at 1.23 V vs. RHE, which is 7-fold larger than BiOCl and ~3-fold higher than 9% S-gC₃N₄/BiOCl. The enhanced PEC activity can be attributed to the improved electron-hole charge separation and the boosted charge transfer is confirmed by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) analysis. The fabricated Co/S-gC₃N₄/BiOCl nanohybrid material has exhibited high stability of up to 10,800 s (3 h) at 1.23 V vs. RHE during PEC water splitting reaction and the obtained photo-conversion efficiency is 3.7-fold greater than S-gC₃N₄/BiOCl and 17-fold higher than BiOCl. The FESEM and HRTEM images have revealed the formation of heterojunction interface between S-gC₃N₄ and BiOCl and the elemental mapping has confirmed the presence of cobalt over S-gC₃N₄/BiOCl. The heterojunction interface has facilitated the photo-excited charge separation and transport across the electrode/electrolyte interface and also the flat-band potential, which is confirmed by Mott-Schottky analysis.

Recent Developments in the Use of Heterogeneous Semiconductor Photocatalyst Based Materials for a Visible-Light-Induced Water-Splitting System—A Brief Review

Visible-light-driven photoelectrochemical (PEC) and photocatalytic water splitting systems featuring heterogeneous semiconductor photocatalysts (oxynitrides, oxysulfides, organophotocatalysts) signify an environmentally friendly and promising approach for the manufacturing of renewable hydrogen fuel. Semiconducting electrode materials as the main constituents in the PEC water splitting system have substantial effects on the device’s solar-to-hydrogen (STH) conversion efficiency. Given the complication of the photocatalysis and photoelectrolysis methods, it is indispensable to include the different electrocatalytic materials for advancing visible-light-driven water splitting, considered a difficult challenge. Heterogeneous semiconductor-based materials with narrower bandgaps (2.5 to 1.9 eV), equivalent to the theoretical STH efficiencies ranging from 9.3% to 20.9%, are recognized as new types of photoabsorbents to engage as photoelectrodes for PEC water oxidation and have fascinated much consideration. Herein, we spotlight mainly on heterogenous semiconductor-based photoanode materials for PEC water splitting. Different heterogeneous photocatalysts based materials are emphasized in different groups, such as oxynitrides, oxysulfides, and organic solids. Lastly, the design approach and future developments regarding heterogeneous photocatalysts oxide electrodes for PEC applications and photocatalytic applications are also discussed.

The application and improvement of TiO2 (titanate) based nanomaterials for the photoelectrochemical conversion of CO2 and N2 into useful products

In this review, we describe the photoelectrochemical (PEC) transformation of atmospheric species such as carbon dioxide (CO₂) and nitrogen (N₂) into useful industrial products on TiO₂ and TiO₂ composite photoelectrodes.

Integrated nano-architectured photocatalysts for photochemical CO2 reduction

Recent advances in nanotechnology, especially the development of integrated nanostructured materials, have offered unprecedented opportunities for photocatalytic CO2 reduction. Compared to bulk semiconductor photocatalysts, most of these nanostructured photocatalysts offer at least one advantage in areas such as photogenerated carrier kinetics, light absorption, and active surface area, supporting improved photochemical reaction efficiencies. In this review, we briefly cover the cutting-edge research activities in the area of integrated nanostructured catalysts for photochemical CO2 reduction, including aqueous and gas-phase reactions. Primarily explored are the basic principles of tailor-made nanostructured composite photocatalysts and how nanostructuring influences photochemical performance. Specifically, we summarize the recent developments related to integrated nanostructured materials for photocatalytic CO2 reduction, mainly in the following five categories: carbon-based nano-architectures, metal-organic frameworks, covalent-organic frameworks, conjugated porous polymers, and layered double hydroxide-based inorganic hybrids. Besides the technical aspects of nanostructure-enhanced catalytic performance in photochemical CO2 reduction, some future research trends and promising strategies are addressed.

Facile synthesis of CuO/CdS heterostructure photocatalyst for the effective degradation of dye under visible light

In this work, the photocatalytic property of p-type CuO was tailored by creating a heterojunction with n-type CdS. The CuO/CdS nanocomposite photocatalyst was synthesized by the ultrasound-assisted-wet-impregnation method and the physicochemical and optical properties of the catalysts were evaluated by using N₂ physisorption, X-Ray Diffraction (XRD),X-Ray Photoelectron Spectroscopy (XPS), Raman spectroscopy, Transmission electron microscopy (TEM), Energy dispersive X-Ray (EDX) mapping, Field Emission Scanning Electron Microscope (FE-SEM), UV-Vis and photoluminescence spectroscopy experiments. Detailed characterization revealed the formation of a nanocomposite with a remarkable improvement in the charge carrier (electron/hole) separation. The photocatalytic degradation efficiencies of CuO and CuO/CdS were investigated for different dyes, for instance, rhodamine B (RhB), methylene blue (MLB), methyl blue (MB) and methyl orange (MO) under visible light irradiation. The obtained dye degradation efficiencies were ~93%, ~75%, ~83% and ~80%, respectively. The quantum yield for RhB degradation under visible light was 6.5 × 10^-5. Reusability tests revealed that the CuO/CdS photocatalyst was recyclable up to four times. The possible mechanisms for the photocatalytic dye degradation over CuO/CdS nanocomposite were elucidated by utilizing various scavengers. Through these studies, it can be confirmed that the conduction band edges of CuO and CdS play a significant role in producing O₂^-. The produced O₂^- degraded the dye molecules in the bulk solution whereas the valence band position of CuO acted as the water oxidation site. In conclusion, the incorporation of CuO with CdS was demonstrated to be a viable strategy for the efficient photocatalytic degradation of dyes in aqueous solutions.

Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode

Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiO2-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO2, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.

Bimetal composites for photocatalytic reduction of CO2 to CO in the near-infrared region by the SPR effect

A major challenge in the field of photocatalytic carbon dioxide (CO₂) reduction is to design catalyst systems featuring high selectivity for CO production, long-term stability and a composition of Earth-abundant elements.

CO2 Reduction of Hybrid Cu2 O-Cu/Gas Diffusion Layer Electrodes and their Integration in a Cu-based Photoelectrocatalytic Cell

Cu₂ O/gas diffusion layer (GDL) electrodes prepared by electrodeposition were studied for the electrocatalytic reduction of CO₂ . The designed electrode was also tested in solar-light-induced CO₂ conversion in combination with a CuO/NtTiO₂ photoanode using a compact photoelectrocatalytic (PEC) cell. Both PEC cell electrodes were prepared using non-critical raw materials and low cost, easily scalable procedures. In the PEC experiments, a total carbon faradaic selectivity of about 90 % to formate and about 75 % to acetate was obtained after 24 h of operations without application of potential/current or using sacrificial agents. In electrocatalytic tests of CO₂ reduction at -1.5 V, the same electrode yielded high total faradaic selectivity (>95 %) but formed selectively formate (about 80 % selectivity) rather than acetate. The in situ transformation of the Cu₂ O/GDL electrode leads to the formation of a hybrid Cu₂ O-Cu/GDL system. Cyclic voltammetry data indicate that the potential and the presence of CO₂ (not only of HCO₃ ^- species) are both important elements in this transformation. Data also indicate that the surface concentration of CO₂ (or of its products of transformation) on the electrode is an important factor to determine performance in the conversion of CO₂ .

Recent Advances and Perspectives of Carbon-Based Nanostructures as Anode Materials for Li-ion Batteries

Rechargeable batteries are attractive power storage equipment for a broad diversity of applications. Lithium-ion (Li-ion) batteries are widely used the superior rechargeable battery in portable electronics. The increasing needs in portable electronic devices require improved Li-ion batteries with excellent results over many discharge-recharge cycles. One important approach to ensure the electrodes' integrity is by increasing the storage capacity of cathode and anode materials. This could be achieved using nanoscale-sized electrode materials. In the article, we review the recent advances and perspectives of carbon nanomaterials as anode material for Lithium-ion battery applications. The first section of the review presents the general introduction, industrial use, and working principles of Li-ion batteries. It also demonstrates the advantages and disadvantages of nanomaterials and challenges to utilize nanomaterials for Li-ion battery applications. The second section of the review describes the utilization of various carbon-based nanomaterials as anode materials for Li-ion battery applications. The last section presents the conclusion and future directions.

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

Schematic representation of photoelectrochemical CO₂ reduction set-up.