Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a photoelectrochemical cell: the concept of the Photofuelcell: a review of a re-emerging research field

Organic Upgrading through Photoelectrochemical Reactions: Toward Higher Profits

Aqueous photoelectrochemical (PEC) cells have long been considered a promising technology to convert solar energy into hydrogen. However, the solar-to-H2 (STH) efficiency and cost-effectiveness of PEC water splitting are significantly limited by sluggish oxygen evolution reaction (OER) kinetics and the low economic value of the produced O2 , hindering the practical commercialization of PEC cells. Recently, organic upgrading PEC reactions, especially for alternative OERs, have received tremendous attention, which improves not only the STH efficiency but also the economic effectiveness of the overall reaction. In this review, PEC reaction fundamentals and reactant-product cost analysis of organic upgrading reactions are briefly reviewed, recent advances made in organic upgrading reactions, which are categorized by their reactant substrates, such as methanol, ethanol, glycol, glycerol, and complex hydrocarbons, are then summarized and discussed. Finally, the current status, further outlooks, and challenges toward industrial applications are discussed.

Sustainable degradation of pollutants, generation of electricity and hydrogen evolution via photocatalytic fuel cells: An Inclusive Review

Environmental pollution and energy crisis have recently become one of the major global concerns. Insincere discharge of massive amount of organic and inorganic wastes into the aqueous bodies causes serious impact on our environment. However, these organic substances are significant sources of carbon and energy that could be sustainably utilized rather than being discarded. Photocatalytic fuel cell (PFC) is a smart and novel energy conversion device that has the ability to achieve dual benefits: degrading the organic contaminants and simultaneously generating electricity, thereby helping in environmental remediation. This article presents a detailed study of the recent advancements in the development of PFC systems and focuses on the fundamental working principles of PFCs. The degradation of various common organic and inorganic contaminants including dyes and antibiotics with simultaneous power generation and hydrogen evolution has been outlined. The impact of various operational factors on the PFC activity has also been briefly discussed. Moreover, it provides an overview of the design guidelines of the different PFC systems that has been developed recently. It also includes a mention of the materials employed for the construction of the photo electrodes and highlights the major limitations and relevant research scopes that are anticipated to be of interest in the days to come. The review is intended to serve as a handy resource for researchers and budding scientists opting to work in this area of PFC devices.

Synergistic effect of silver NPs immobilized on Fe3O4@L-proline magnetic nanocomposite toward the photocatalytic degradation of Victoria blue and reduction of organic pollutants

The surface of magnetite (Fe₃O₄) nanoparticles was subject to modification through the incorporation of L-proline (LP) by simple co-precipitation method in which silver nanoparticles were deposited by in situ method, thereby yielding the Fe₃O₄@LP-Ag nanocatalyst. The fabricated nanocatalyst was characterized using an array of techniques including Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), Brunauer-Emmett-Teller (BET), and UV-Vis spectroscopy. The results evince that the immobilization of LP on the Fe₃O₄ magnetic support facilitated the dispersion and stabilization of Ag NPs. The SPION@LP-Ag nanophotocatalyst exhibited exceptional catalytic efficiency facilitating the reduction of MO, MB, p-NP, p-NA, NB, and CR in the presence of NaBH₄. The rate constants obtained from the pseudo-first-order equation were 0.78, 0.41, 0.34, 0.27, 0.45, 0.44, and 0.34 min^-1 for CR, p-NP, NB, MB, MO, and p-NA, respectively. Additionally, the Langmuir-Hinshelwood model was deemed the most probable mechanism for catalytic reduction. The novelty of this study lies in the use of L-proline immobilized on Fe₃O₄ MNPs as a stabilizing agent for the in-situ deposition of silver nanoparticles, resulting in the synthesis of Fe₃O₄@LP-Ag nanocatalyst. This nanocatalyst exhibits high catalytic efficacy for the reduction of multiple organic pollutants and azo dyes, which can be attributed to the synergistic effects between the magnetic support and the catalytic activity of the silver nanoparticles. The easy recyclability and low cost of the Fe₃O₄@LP-Ag nanocatalyst further enhance its potential application in environmental remediation.

Gas diffusion TiO2 photoanode for photocatalytic fuel cell towards simultaneous VOCs degradation and electricity generation

In this work, a photocatalytic fuel cell (PFC) with a gas diffusion TiO₂ photoanode is proposed to directly convert chemical energy contained in volatile organic compounds into electricity by using solar energy. The gas diffusion TiO₂ photoanode is prepared by coating TiO₂ nanoparticles onto Ti mesh, whose intrinsic porous structure allows for gaseous pollutants to directly transfer inside the photoanode and thereby enhances mass transport. The feasibility of the developed gas diffusion photoanode is demonstrated by degrading toluene as a model gaseous pollutant. It is shown that the newly-developed PFC yields better electricity generation and toluene removal efficiency due to the enhanced mass transport of toluene and the eliminated interference of gas bubbles. The short-circuit current density and maximum power density of the PFC with a gas diffusion TiO₂ photoanode (0.1 mA/cm² and 0.02 mW/cm²) are about 3.3 times and 4 times as those of the bubbling PFC (0.03 mA/cm² and 0.005 mW/cm²), respectively. Both the discharging performance and toluene removal efficiency increase with increasing the light intensity and electrolyte concentration, while there exists an optimal gas flow rate leading to the best performance. The present work provides an innovative strategy for clean processing of volatile organic compounds while recycling the contained chemical energy.

Converting synthetic azo dye and real textile wastewater into clean energy by using synthesized CuO/C as photocathode in dual-photoelectrode photocatalytic fuel cell

Cathode in photocatalytic fuel cell (PFC) plays a crucial role in degradation of organic contaminants. In this study, synthesized copper oxide (CuO) was loaded on carbon plate and used as photocathode in PFC for degradation of synthetic azo dye Reactive Black 5 (RB5) and real textile wastewater. Morphology and structural phase of the synthesized CuO were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Several operating parameters had been investigated such as light irradiation, initial dye concentration, and pH of azo dye solution within 6 h of irradiation time. The lowest initial concentration of RB5 (10 mg L^-1) achieved 100% color removal compared to the highest initial concentration (40 mg L^-1) which only achieved 77.1% color removal within 6 h of irradiation time. The influence of external resistance was significant in electricity generation but trivial in dye degradation efficiency. The external resistance of 6000 Ω yielded highest maximum power density, with P_max of 0.2631 μW cm^-2, followed by 1000 Ω (0.2196 μW cm^-2) and 8000 Ω (0.1587 μW cm^-2), respectively. The real textile wastewater with dilution ratio (DR) 1:6 yielded the highest energy conversion efficiency, η (3.62%), followed by DR 1:4 (3.19%) and DR 1:2 (1.96%), respectively.

Synergy of photocatalysis and fuel cells: A chronological review on efficient designs, potential materials and emerging applications

The rising demand of energy and lack of clean water are two major concerns of modern world. Renewable energy sources are the only way out in order to provide energy in a sustainable manner for the ever-increasing demands of the society. A renewable energy source which can also provide clean water will be of immense interest and that is where Photocatalytic Fuel Cells (PFCs) exactly fit in. PFCs hold the ability to produce electric power with simultaneous photocatalytic degradation of pollutants on exposure to light. Different strategies, including conventional Photoelectrochemical cell design, have been technically upgraded to exploit the advantage of PFCs and to widen their applicability. Parallel to the research on design, researchers have put an immense effort into developing materials/composites for electrodes and their unique properties. The efficient strategies and potential materials have opened up a new horizon of applications for PFCs. Recent research reports reveal this persistently broadening arena which includes hydrogen and hydrogen peroxide generation, carbon dioxide and heavy metal reduction and even sensor applications. The review reported here consolidates all the aspects of various design strategies, materials and applications of PFCs. The review provides an overall understanding of PFC systems, which possess the potential to be a marvellous renewable source of energy with a handful of simultaneous applications. The review is a read to the scientific community and early researchers interested in working on PFC systems.

Efficient Oxygen Vacancy Defect Engineering for Enhancing Visible-Light Photocatalytic Performance over SnO2-x Ultrafine Nanocrystals

Regardless of its good electron-transfer ability and chemical stability, pure Zn₂SnO₄ (ZSO) still has intrinsic deficiencies of a narrow spectral response region, poor absorption ability, and high photo-activated carrier recombination rate. Aiming to overcome the deficiencies above-mentioned, we designed a facile hydrothermal route for etching ZSO nanoparticles in a dilute acetic acid solution, through which efficient oxygen vacancy defect engineering was accomplished and SnO_2-x nanocrystals were obtained with an ultrafine particle size. In comparison with the untreated ZSO nanoparticles, the specific surface area of SnO_2-x nanocrystals was substantially enlarged, subsequently leading to the notable augmentation of active sites for the photo-degradation reaction. Aside from the above, it is worth noting that SnO_2-x nanocrystals were endowed with a broad spectral response, enhancing light absorption capacity and the photo-activated carrier transfer rate with the aid of oxygen vacancy defect engineering. Accordingly, SnO_2-x nanocrystals exhibited significantly enhanced photoactivity toward the degradation of the organic dye rhodamine B (RhB), which could be imputed to the synergistic effect of increasing active sites, intensified visible-light harvesting, and the separation rate of the photo-activated charge carrier caused by the oxygen vacancy defect engineering. In addition, these findings will inspire us to open up a novel pathway to design and prepare oxide compound photocatalysts modified by oxygen vacancy defects in pursuing excellent visible-light photoactivity.

Photocatalytic Fuel Cells for Simultaneous Wastewater Treatment and Power Generation: Mechanisms, Challenges, and Future Prospects

Technological advancement is accompanied by excessive consumption of fossil fuels and affluent uses of chemical substances in many sectors, including transportation and manufacturing companies, and so on. Being an exhaustible resource, the excessive use of fossil fuels and of chemical substances may lead to a serious energy crisis in the long run, and it may additionally impose environmental pollution. Attempts have been made in the solution of such serious issues from every nook and corner. Nonetheless, no method has been found to be a panacea in waste water treatment and subsequent beneficiaries. One of the attempts in the solution to such issues is the application of photocatalytic technology, which could serve as a dual function in environmental remediation and clean energy production. A photocatalytic fuel cell is a tool developed for the recovery of energy from organic wastes. A rational cell construction needs the fabrication of photoelectrodes, the design of a photoanode and a photocathode chamber, in addition to an ion-transport membrane for pollution treatment and electricity generation. In this review, comprehensive fundamental assessments and recent developments in the design of photocatalytic fuel cells, their applications, future prospects, and challenges are covered.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Enhancement of the electron transfer and ion transport phenomena in microbial fuel cells containing humic acid-modified bioanodes

Although microbial fuel cells (MFCs) are an attractive alternative to environmental remediation and power generation, there are still significant limitations related to power density and coulombic efficiency. Previous works have shown that the addition of humic acids (HA, a type of organic matter contained in soils and composting-by-products), improves the fuel to electricity conversion at the porous bioanodes (ba)|anolyte junction. In this work, MFCs having HA-modified bioanodes (MFC/baHA) were prepared and electrochemically analyzed utilizing discharge curves (cell potential vs current density plots) and electrochemical impedance spectroscopy (EIS). This investigation was motivated by looking for a deeper understanding of the functional effects of HA molecules on the operation of baHA-containing Microbial Fuel Cells (MFC/baHA). Our results revealed that the modification of bioanodes with HA molecules decreases the activation energy of the acetate ion oxidation, increasing by a factor of three the consumption rate of this fuel at the baHA|anolyte interface, and enhancing the diffusive transport of these ions through the pores of the baHA permeated by the anolyte.

Photoelectrochemical Oxidation in Ambient Conditions Using Earth-Abundant Hematite Anode: A Green Route for the Synthesis of Biobased Polymer Building Blocks

This study demonstrates the use of a photoelectrochemical device comprising earth-abundant hematite photoanode for the oxidation of 5-hydroxymethylfurfural (5-HMF), a versatile bio-based platform chemical, under ambient conditions in the presence of an electron mediator. The results obtained in this study showed that the hematite photoanode, upon doping with fluorine, can oxidize water even at lower pH (4.5 and 9.0). For 5-HMF oxidation, three different pH conditions were investigated, and complete oxidation to 2,5-furandicarboxylic acid (FDCA) via 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was achieved at pH above 12. At lower pH, the oxidation followed another route via 2,5-diformylfuran (DFF), yielding 5-formyl-2-furancarboxylic acid (FFCA) as the main product. Using the oxidized intermediates as substrates showed DFF to be most efficiently oxidized to FDCA. We also show that, at pH 4.5, the addition of the laccase enzyme promoted the oxidation of 5-HMF to FFCA.

Visible light-driven novel Bi2Ti2O7/CaTiO3 composite photocatalyst with enhanced photocatalytic activity towards NO removal

Photocatalysis is regarded as a promising technology for removal of nitrogen oxide (NO), however, the low photocatalytic efficiencies under visible light irradiation and the deactivation of the photocatalyst are as yet the significant issues that should be addressed. In this work, visible-light-driven Bi₂Ti₂O₇/CaTiO₃ heterojunction composites were synthesized by a facile in-situ hydrothermal method. The Bi₂Ti₂O₇/CaTiO₃ composites displayed superior visible light photocatalytic activity than pure CaTiO₃ and pure Bi₂Ti₂O₇ in the removal of NO at the 600 ppb level in air. Among all the composites, Bi₂Ti₂O₇/CaTiO₃ containing 20 wt% Bi₂Ti₂O₇ exhibited the best photocatalytic activity, achieving a maximum removal efficiency of 59%. The improved photocatalytic performance is mainly attributed to the strong visible-light-absorbing ability, the presence of an appropriate density of oxygen vacancy defects and the formation of heterojunction between CaTiO₃ and Bi₂Ti₂O₇, resulting in an efficient charge separation at the interface as proven by photoluminescence (PL) and photo-induced current measurements. According to trapping experiments and spin-trapping ESR analysis, the ^•O₂^- and h⁺ are the principal reactive species involved in the photocatalytic NO removal. In addition, the as-obtained Bi₂Ti₂O₇/CaTiO₃ composite showed good chemical stability, which is beneficial for practical applications in air pollution removal.

Bio-Hydrogen Production from Wastewater: A Comparative Study of Low Energy Intensive Production Processes

Billions of litres of wastewater are produced daily from domestic and industrial areas, and whilst wastewater is often perceived as a problem, it has the potential to be viewed as a rich source for resources and energy. Wastewater contains between four and five times more energy than is required to treat it, and is a potential source of bio-hydrogen—a clean energy vector, a feedstock chemical and a fuel, widely recognised to have a role in the decarbonisation of the future energy system. This paper investigates sustainable, low-energy intensive routes for hydrogen production from wastewater, critically analysing five technologies, namely photo-fermentation, dark fermentation, photocatalysis, microbial photo electrochemical processes and microbial electrolysis cells (MECs). The paper compares key parameters influencing H2 production yield, such as pH, temperature and reactor design, summarises the state of the art in each area, and highlights the scale-up technical challenges. In addition to H2 production, these processes can be used for partial wastewater remediation, providing at least 45% reduction in chemical oxygen demand (COD), and are suitable for integration into existing wastewater treatment plants. Key advancements in lab-based research are included, highlighting the potential for each technology to contribute to the development of clean energy. Whilst there have been efforts to scale dark fermentation, electro and photo chemical technologies are still at the early stages of development (Technology Readiness Levels below 4); therefore, pilot plants and demonstrators sited at wastewater treatment facilities are needed to assess commercial viability. As such, a multidisciplinary approach is needed to overcome the current barriers to implementation, integrating expertise in engineering, chemistry and microbiology with the commercial experience of both water and energy sectors. The review concludes by highlighting MECs as a promising technology, due to excellent system modularity, good hydrogen yield (3.6–7.9 L/L/d from synthetic wastewater) and the potential to remove up to 80% COD from influent streams.

Photo-electrochemical Osmotic System Enables Simultaneous Metal Recovery and Electricity Generation from Wastewater

Global depletion of natural resources provides an impetus for developing low-cost, environmentally benign technologies for the recovery of valuable resources from wastewater. In this study, we present an autonomous photo-electrochemical osmotic system (PECOS) that can recover a wide range of metals from simulated metal-laden wastewater with sunlight illumination while generating electricity. The PECOS comprises a draw solution chamber with a nickel nanoparticle-functionalized titanium nanowire (Ni-TiNA) photoanode, a feed solution chamber containing synthetic wastewater with an immersed carbon fiber cathode, and a forward osmosis (FO) membrane mounted between the chambers as a separator. Using a Na₂-EDTA anolyte as a draw solution at neutral pH, we demonstrate that a sunlit PECOS achieves copper recovery at a rate of 51 g h^-1 per m^-2 of membrane area from simulated copper-laden wastewater while simultaneously producing a maximum power density of 228 mW m^-2. Moreover, because of the osmotic pressure difference generated by the photo-electrochemical reactions, the PECOS reduces the wastewater volume by extracting fresh water through the FO membrane at a water flux of 0.84 L m^-2 h^-1. We further demonstrate the feasibility of the PECOS in recovering diverse metals from a simulated metal-laden industrial wastewater under sunlight irradiation. Our proof-of-concept PECOS prototype provides a sustainable technological solution that leverages sunlight in an electrochemical osmotic system to recover multiple resources from wastewater.

Efficient solar hydrogen production coupled with organics degradation by a hybrid tandem photocatalytic fuel cell using a silicon-doped TiO2 nanorod array with enhanced electronic properties

A novel, unassisted, hybrid tandem photocatalytic fuel cell (HTPFC) is constructed by adhering a silicon solar cell (SSC) to the back of a highly-active silicon-doped TiO₂ nanorod array (STNR) for efficient solar hydrogen production coupled with organic compound degradation. The STNR with vertically arranged nanorods is prepared by a facile hydrothermal method and has improved charge transport properties and donor density due to the homogenously distributed silicon in the TiO₂ matrix. As a result, the STNR has a notably enhanced photocurrent density that is as high as ˜0.76 mA cm^-2 at 0.2 V vs Ag/AgCl, which is ˜271% of the photocurrent density of undoped sample. By combining the intriguing features of the STNR and SSC, the HTPFC shows a superior performance for tetracycline degradation and hydrogen production, with a removal ratio of 94.3% after 1.5 h of operation and an average hydrogen generation rate of ˜28.8 μmol h^-1 cm^-2. Compared to conventional PFCs, HTPFCs have improved light absorption and charge transfer, owing to the synergistic effect between the STNR and SSC. The results also indicate that the HTPFC is highly flexible, adaptable, and stable when treating wastewaters with various organics, and a wide range of pH values and salinities.

Electrode Materials Engineering in Electrocatalytic CO2 Reduction: Energy Input and Conversion Efficiency

Electrocatalytic CO₂ reduction (ECR) is a promising technology to simultaneously alleviate CO₂ -caused climate hazards and ever-increasing energy demands, as it can utilize CO₂ in the atmosphere to provide the required feedstocks for industrial production and daily life. In recent years, substantial progress in ECR systems has been achieved by the exploitation of various novel electrode materials. The anodic materials and cathodic catalysts that have, respectively, led to high-efficiency energy input and effective heterogenous catalytic conversion in ECR systems are comprehensively reviewed. Based on the differences in the nature of energy sources and the role of materials used at the anode, the fundamentals of ECR systems, including photo-anode-assisted ECR systems and bio-anode-assisted ECR systems, are explained in detail. Additionally, the cathodic reaction mechanisms and pathways of ECR are described along with a discussion of different design strategies for cathode catalysts to enhance conversion efficiency and selectivity. The emerging challenges and some perspective on both anode materials and cathodic catalysts are also outlined for better development of ECR systems.

Investigation of Electrochemical Oxidation Behaviors and Mechanism of Single-Crystal Silicon (100) Wafer under Potentiostatic Mode

Electrochemical oxidation (ECO) has been used widely to oxidize single crystal Si wafers. Aiming at optimizing the ECO assisted machining methods, the oxidation behaviors of single- crystal silicon (100) wafer under potentiostatic mode are experimentally investigated. It is shown that the Si wafer can be electrochemically oxidized and the oxidized film thickness reaches to 239.6 nanometers in 20 min. The hardness of the oxidized surface is reduced by more than 50 percent of the original surface. The results indicate that the oxide thickness and the hardness can be controlled by changing the voltage. Based on the experimental findings, a hypothesis on the ECO mechanism under potentiostatic mode was proposed to explain the fluctuations of current density under specific applied voltage. The occurrence of the multiple peaks in the current density curve during the oxidation process is due to the formation of discharge channels, which was initiated from the defects at the interface between the oxide bottom and the substrate. This breaks the electrical isolation and leads to the discontinuous growth of the electrochemical oxide layer. The present work contributes to the fundamental understanding of the ECO behaviors for the single-crystal Si (100) wafer under potentiostatic mode.

Use of Chalcogenide-Semiconductor-Sensitized Titania to Directly Charge a Vanadium Redox Battery

Unmediated charging of a battery using solar radiation is a very attractive project of solar energy conversion and storage. In the present work, solar energy was converted into electricity using a photocatalytic fuel cell operating with a chalcogenide-semiconductor-sensitized nanoparticulate titania photoanode and an air-cathode functioning by oxygen reduction. This cell produced sufficient energy to directly charge a vanadium redox battery functioning with a VOSO₄ electrolyte and carbon paper electrodes. The whole system is characterized by ease of construction and simplicity of conception; therefore, it satisfies conditions for practical applications.

Important Considerations in Plasmon-Enhanced Electrochemical Conversion at Voltage-Biased Electrodes

In this perspective we compare plasmon-enhanced electrochemical conversion (PEEC) with photoelectrochemistry (PEC). PEEC is the oxidation or reduction of a reactant at the illuminated surface of a plasmonic metal (or other conductive material) while a potential bias is applied. PEC uses solar light to generate photoexcited electron-hole pairs to drive an electrochemical reaction at a biased or unbiased semiconductor photoelectrode. The mechanism of photoexcitation of charge carriers is different between PEEC and PEC. Here we explore how this difference affects the response of PEEC and PEC systems to changes in light, temperature, and surface morphology of the photoelectrode.

Photoelectrocatalytic Hydrogen Peroxide Production Using Nanoparticulate WO3 as Photocatalyst and Glycerol or Ethanol as Sacrificial Agents

Photoelectrochemical production of hydrogen peroxide was studied by using a cell functioning with a WO3 photoanode and an air breathing cathode made of carbon cloth with a hydrophobic layer of carbon black. The photoanode functioned in the absence of any sacrificial agent by water splitting, but the produced photocurrent was doubled in the presence of glycerol or ethanol. Hydrogen peroxide production was monitored in all cases, mainly in the presence of glycerol. The presence or absence of the organic fuel affected only the obtained photocurrent. The Faradaic efficiency for hydrogen peroxide production was the same in all cases, mounting up to 74%. The duplication of the photocurrent in the presence of biomass derivatives such as glycerol or ethanol and the fact that WO3 absorbed light in a substantial range of the visible spectrum promotes the presently studied system as a sustainable source of hydrogen peroxide production.

Photoelectrocatalytic Hydrogen Production Using a TiO2/WO3 Bilayer Photocatalyst in the Presence of Ethanol as a Fuel

Photoelectrocatalytic hydrogen production was studied by using a photoelectrochemical cell where the photoanode was made by depositing on FTO electrodes either a nanoparticulate WO3 film alone or a bilayer film made of nanoparticulate WO3 at the bottom covered with a nanoparticulate TiO2 film on the top. Both the electric current and the hydrogen produced by the photoelectrocatalysis cell substantially increased by adding the top titania layer. The presence of this layer did not affect the current-voltage characteristics of the cell (besides the increase of the current density). This was an indication that the flow of electrons in the combined semiconductor photoanode was through the WO3 layer. The increase of the current was mainly attributed to the passivation of the surface recombination sites on WO3 contributing to the limitation of charge recombination mechanisms. In addition, the top titania layer may have contributed to photon absorption by back scattering of light and thus by enhancement of light absorption by WO3. Relatively high charge densities were recorded, owing both to the improvement of the photoanode by the combined photocatalyst and to the presence of ethanol as the sacrificial agent (fuel), which affected the recorded current by “current doubling” phenomena. Hydrogen was produced under electric bias using a simple cathode electrode made of carbon paper carrying carbon black as the electrocatalyst. This electrode gave a Faradaic efficiency of 58% for hydrogen production.

Comparative study of persulfate oxidants promoted photocatalytic fuel cell performance: Simultaneous dye removal and electricity generation

Introducing peroxymonosulfate (PMS) and peroxydisulfate (PDS) into the photocatalytic fuel cell (PFC) system were investigated by comparing the Reactive Brilliant Blue (KN-R) degradation and synchronous electricity production. The two persulfates (PS) themselves are strong oxidant, and could be activated and as electron sacrificial agent in the PFCs, facilitating the photoelectrocatalysis and expanding redox to the entire cell space. Hence, the two established PFC/PS systems manifested prominent cell performances, enhancing the KN-R decomposition and electric power production relative to the virgin PFC. Thereinto, the KN-R removal rate of PFC/PMS was faster than that of PFC/PDS, but an opposite trend appeared in the electricity generation. Besides, the cell performances of the two cooperative systems were evaluated at different operation conditions, including PS dosage, solution pH, and irradiation strength. Moreover, the dye elimination principle was explored by radicals scavenging experiment, and the consequence revealed that hydroxyl radical (HO^•), sulfate radical (SO₄^•-) and singlet oxygen were chief active species in the PFC/PMS, and HO^•, SO₄^•- and superoxide anion played the key roles in the PFC/PDS. Furthermore, the calculated economic indicator demonstrated that the economy of the two synergistic processes were greater than that of UV/PS and solo PFC, and the PFC/PDS was more cost-effective than PFC/PMS.

Enhancement of photoelectrochemical organics degradation and power generation by electrodeposited coatings of g-C3N4 and graphene on TiO2 nanotube arrays

New g-C₃N₄ coatings obtained via electropolymerization (EP) of melamine followed by a heat treatment and graphene oxide (GO) coatings based on combining GO sheets via EP of GO phenolic groups are used to improve the performance of photoanodes composed of TiO₂ nanotube arrays towards the photoelectrochemical (PEC) oxidation of methanol. This process, as examined in Na₂CO₃ solution (pH 11.4) for the two types of coatings and serving as a model for the degradation of an organic pollutant, demonstrates enhanced PEC performance as compared to that obtained using electrochemically reduced GO coatings. PEC oxidation currents obtained with 1 M methanol reach saturation at potentials as low as ∼-0.4 V vs. Ag/AgCl, with the highest saturation current density of ∼2.6 mA cm^-2 and photon-to-current efficiency of 52% as observed for the new TiO₂NTs/g-C₃N₄ photoanodes. Electrochemical impedance spectroscopy measurements for these photoanodes show a charge transfer resistance one order of magnitude lower than that obtained by the other types of coatings. This indicates an enhanced charge separation ability for the photogenerated electron-hole pairs and faster interfacial charge transfer between the electron donor (methanol) and acceptor (holes). It is also demonstrated that the process of organics degradation can be achieved not only via an applied potential but also in a galvanic photofuelcell with methanol and oxygen serving as the fuel and oxidant, respectively. The power densities achieved with the electrochemically prepared g-C₃N₄ photoanodes (∼0.5 mW cm^-2) are at least one order of magnitude higher than those reported for other TiO₂-based systems.

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e^--h⁺) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO₂ reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

Recent advances on photocatalytic fuel cell for environmental applications-The marriage of photocatalysis and fuel cells

Environmental pollution and energy crisis have become recent worldwide concerns. Huge amounts of organic wastes are discharged into water bodies, causing serious environmental pollution. Meanwhile, these organic compounds are important carbon and energy sources that could be utilized instead of being discarded. A smart design of a photocatalytic fuel cell (PFC) can achieve double benefits: it can degrade organic pollutants and at the same time generate energy. In this review article, we discuss recent progress in the development of PFC systems, and summarize the principles for constructing advanced PFC systems. We particularly focus on the rational design of electrode materials in terms of surface, morphology, facet, and interfacial reaction engineering. The impact of important operational parameters on PFC performance is further discussed in detail. We then discuss the major limitations and opportunities for future PFCs research. The development of smart and advanced PFC systems depends on highly interdisciplinary collaborations, which require concerted efforts from the communities of materials science, chemistry, engineering, and environmental science.