Water Reduction Reaction Research Articles

Effects of water injection on reaction temperature and hydrogen production in horizontal co-axial underground coal gasification

Underground coal gasification (UCG) is process of directly recovering energy as combustible gases such as hydrogen and carbon monoxide by combusting unmined coal resources in situ. During UCG process, the temperature in the gasification zone can exceed 1,300 °C, raising concerns about the potential melting of the steel pipe for oxidant injection. To control the temperature in the gasification zone, the use of water injection as an injection agent can be an option. Injecting water during UCG process serves two purposes: it decreases the temperature in the reaction zone by the endothermic effect of water, and it enhances the production of H2 by the reduction reaction of water. However, injecting excessive amounts of water may lead to a significant decrease in the temperature in the reaction zone, consequently cannot maintain the temperature range required for the UCG reaction. This study discusses the effects of water injection on the temperature of the gasification zone and the product gas by developing a chemical reaction model of UCG using COMSOL Multiphysics® software. The model was developed based on the temperature and produced gas results obtained from laboratory-scale artificial coal seam UCG experiments. Our findings reveal that water injection significantly influences the gasification process, controlling temperature in the reaction zone and promoting H2 production through steam gasification and water-gas shift reactions. Moreover, under the experiment and analysis conditions of this study, it is revealed that water injection up to an H2O/O2 molar ratio of 3.9 can effectively control the temperature of the gasification zone with enhancing H2 production.

Simultaneous efficient production of hydrogen peroxide, urea and H2 by bifunctional Cu-doped TiO2 electrocatalyst from CO2, N2 and water

The production of three separated, high-value chemicals (H2O2, urea and H2) from CO2, N2, and H2O represents a significant challenge. It is a prospective and innovative way to simultaneously produce these chemicals in an anion exchange membrane (AEM) electrolyzer, which couples anodic 2e- water oxidation (2e-WOR) and cathodic CO2, N2 and water reduction reaction (CNWRR), but has not yet been realized. Here, we found that bifunctional CuxTi1-xO2-y catalysts can be employed in the electrochemical production of H2O2, urea and H2 from CO2, N2 and water, and the optimized Cu0.12Ti0.88O2-y catalyst exhibits an excellent productivity of 11.8 μmol cm−2 min−1, 0.3 mg cm-2h−1 and 0.82 mmol cm-2h−1 for H2O2, urea and H2 at 4.0 V (50 mA cm−2) in AEM electrolyzer, respectively. Theoretical calculations reveal that the superior performance of the 2e-WOR and CNWRR is attributed to the Cu doping effect, which not only promotes the coupling of *OH but also optimizes the adsorption energy barriers of COOH* and NNH*. It is worth noting that the AEM electrolyzer assembled by bifunctional Cu0.12Ti0.88O2-y reaches a current density of 50 mA cm−2 with a cell voltage of 4.0 V and exhibits excellent stability of 50-hour continuous operation.

Construction of Ru/MnO–Mn7C3/N-doped carbon sheets for boosting electrocatalytic hydrogen generation

Suffering from their low conductivity, Mn-base oxides are commonly inactive in H–OH activation. Generally, tailoring the coordinate environment of materials could induce variation in the atomic arrangement and further influence their chemical activity. Herein, we propose a feasible strategy to mediate the electronic structure of Mn-based oxides by facile hydrothermal method and thermal reduction route. Compared with MnO2 sheets, the as-synthesized Ru/MnO–Mn7C3/N-doped carbon sheets (Ru/MnO–Mn7C3/NC) exhibit the higher activity for both acidic (73 mV) and alkaline water reduction reaction (44 mV@10 mA/cm2). Benefitting from unique structures, Ru/MnO–Mn7C3/NC sheets show the better freshwater/seawater oxidation reactivity, exceeding RuO2. The strong interaction of MnO–Mn7C3 heterostructures, Ru, and N-doped carbon endows Ru/MnO–Mn7C3/NC with the good freshwater and seawater dissociation performance, outperforming most of the reported Mn-based oxides.

A novel design of urea-assisted hydrogen production in electrochemical-chemical decoupled self-circulating systems.

In traditional water electrolysis processes, the oxidation and reduction reactions of water are coupled in both time and space, which presents significant challenges. Here, we propose an optimized design for an electrochemical-chemical self-circulating decoupled system. This system uses the continuous Ni2+/Ni3+ redox process on nickel hydroxide electrode sheets to stepwise couple the urea oxidation-assisted hydrogen production system, separating the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) into two distinct steps: electrochemical and chemical reactions. In the first electrochemical step, water is reduced at the cathode to produce hydrogen, while the single-electron electrochemical oxidation of Ni(OH)2 at the anode generates NiOOH. Then, in the second chemical reaction step, NiOOH spontaneously oxidizes urea, causing its decomposition and simultaneously reducing back to the Ni(OH)2 state. We concurrently investigated the effects of temperature and OH-concentration on the spontaneous oxidation of urea. At 80 °C and with a 1 M KOH concentration containing 50 mg of urea solution, the NiOOH electrode successfully catalyzed the spontaneous decomposition of urea, achieving conversion rate of 100% and faradaic efficiency of 98%.

Electrochemical Amalgamation Optimization as Part of the Development of the OREATE Process

Oxide reduction by electrochemical amalgamation and thermal extraction (OREATE) is an electrochemical process in which metal oxides are converted to highly pure metals. This process that is currently being developed at LANL involves the dissolution and chlorination of the metal oxide into an acidic medium. This metal cation is then electrochemically amalgamated via reduction using a mercury pool cathode. Finally, mercury's high vapor pressure allows an easy thermal extraction process to obtain the metal. In addition to its ability to electrochemically form amalgams, mercury is used because it has a high hydrogen evolution overpotential, allowing for a higher current efficiency for reduction of the metals in acidic solution. As a critical step of the OREATE process, the electrochemical amalgamation reaction must be optimized in order to advance the overall performance.In this work, we studied the electrochemical amalgamation of cerium through anodic stripping voltammetry (ASV) experiments using a silver-based mercury film electrode (SBMFE). The SBMFE allows for the electrochemical amalgamation to be optimized at a smaller scale than the mercury pool, which not only minimizes the amount of mercury needed, but also drastically reduces the experiment time. This study included several different experimental parameters such as the pH of the buffered solution, concentration of cerium, reduction potential, and temperature. For each parameter, the reaction rate and current efficiency were evaluated to determine the optimal conditions for the electrochemical amalgamation reaction.The results showed that the pH of the buffered solution must be 3.5 or lower to avoid the formation of cerium hydroxides, which can passivate the surface of the mercury electrode. However, an increasingly acidic solution promotes the hydrogen evolution reaction, which directly competes with the electrochemical reduction of cerium, decreasing the overall current efficiency. Likewise, it has been demonstrated that an increase of the cerium concentration in solution drops the current efficiency of the electrochemical amalgamation reaction. This effect can be explained by the fact that cerium is a small cation with a +3 charge that leads to a high degree of solvation. Therefore, as the cerium concentration in solution increases, more water molecules are brought to the electrode surface during electrochemical amalgamation. This favors the water reduction reaction that increases hydrogen evolution and lowers current efficiency. However, increasing the concentration of cerium enhances the reaction rate of the electrochemical amalgamation, resulting in more cerium entering the mercury film. Furthermore, as the reduction potential is moved towards more negative potentials, the reaction rate increases. However, a more negative reduction potential promotes a faster hydrogen evolution reaction, which diminishes the current efficiency. Finally, it has been demonstrated that temperature increases the reaction rate, though it does not have a significant effect on the current efficiency.In conclusion, a variety of system parameters were explored for their effect on current efficiency and reaction rate of the electrochemical amalgamation reaction. This allowed for the determination of the optimal parameters to be used during the electrochemical amalgamation in the large-scale OREATE process.

Penternary Wurtzitic Nitrides Li1-xZnxGe2-xGaxN3: Powder Synthesis, Crystal Structure, and Potentiality as a Solar-Active Photocatalyst.

We developed a new penternary wurtzitic nitride system Li1-xZnxGe2-xGaxN3 (0 ≤ x ≤ 1) by hybridizing LiGe2N3 and ZnGeGaN3. Fairly stoichiometric fine powder samples were synthesized by the reduction-nitridation process at 900 °C. While the end member LiGe2N3 possessed a relatively large band gap of 4.16 eV, the band gap of the developed penternary system varied in a broad range of 3.81 to 3.10 eV, showing promising responsivity to the solar spectrum. The crystal structure of LiGe2N3 was precisely determined by time-of-flight neutron powder diffraction for the first time, revealing the complete ordering of Li and Ge in the Cmc21 structure. The structural evolution from completely ordered LiGe2N3 to fully disordered ZnGeGaN3 was quantitatively analyzed by Rietveld refinement based on a partially disordered Cmc21 model, and the obtained results were also supported by 71Ga solid-state NMR spectroscopy. The synthesized Li1-xZnxGe2-xGaxN3 powder samples exhibited photocatalytic activities for the water reduction and oxidation reactions under solar light irradiation, with the H2 evolution rate of 0.3-59.0 μmol/h and the O2 evolution rate of 3.1-296.2 μmol/h, depending on the composition. Stable solar hydrogen generation of up to 48 h was demonstrated by the x = 0.80 sample, with the total amount of H2 evolved over 1.6 mmol and an external quantum efficiency of 2.1%.

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives.

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Photovoltaic-driven Ni(ii)/Ni(iii) redox mediator for the valorization of PET plastic waste with hydrogen production.

Electrocatalytic valorization of PET plastic waste provides an appealing route by converting intermittent renewable energy into valuable chemicals and high-energy fuels. Normally, anodic PET hydrolysate oxidation and cathodic water reduction reactions occur simultaneously in the same time and space, which increases the challenges for product separation and operational conditions. Although these problems can be addressed by utilizing membranes or diaphragms, the parasitic cell resistance and high overall cost severely restrict their future application. Herein, we introduce a Ni(ii)/Ni(iii) redox mediator to decouple these reactions into two independent processes: an electrochemical process for water reduction to produce hydrogen fuel assisted by the oxidation of the Ni(OH)2 electrode into the NiOOH counterpart, followed subsequently by a spontaneous chemical process for the valorization of PET hydrolysate to produce formic acid with a high faradaic efficiency of ∼96% by the oxidized NiOOH electrode. This decoupling strategy enables the electrochemical valorization of PET plastic waste in a membrane-free system to produce high-value formic acid and high-purity hydrogen production. This study provides an appealing route to facilitate the transformation process of PET plastic waste into high-value products with high efficiency, low cost and high purity.

Theoretical insights into the effect of surface structure and anion ordering on the properties of SrTaO2N for photocatalytic water splitting

AbstractAs one of the representative perovskite‐type oxynitride photocatalysts, SrTaO2N has the ability to split water in the visible‐light region. It was found that the surface modification and interfacial design of SrTaO2N‐based materials are closely related to the photocatalytic activity, but the microscopic mechanisms of these experimental phenomena are not well understood. In this work, we have utilized density functional theory (DFT) calculations to investigate the effect of anion ordering and exposed terminations on the electronic structures, optical absorption, water adsorption and the mechanisms of water oxidation and reduction reactions of SrTaO2N. Our results indicate that cis configurations are more stable than trans configurations. The anion ordering has an important effect on the band gap and optical absorption coefficient. The terminations with exposed Ta atoms are more stable and have bigger work functions than those with exposed Sr atoms possibly due to the bonding ionicity and surface dipoles. The dissociative adsorption of water is energetically more favorable than the molecular adsorption on most surfaces. The highly active sites of hydrogen evolution reaction (HER) are the exposed nonmetal atoms. Terminations with exposed Sr and N atoms have lower overpotentials (0.70–0.77 V) of oxygen evolution reaction (OER) than others. They are comparable to the calculated results of common photocatalysts, such as Co3O4 and TiO2. This study sheds light on the relationship between the termination structure with different anion orders and the photocatalysis‐related properties of SrTaO2N at a molecular level, which provides guidance for constructing highly active photocatalytic materials.

(Invited) Contribution of Oxygen Reduction to Mg Corrosion: Computational Modelling

The magnesium corrosion mechanisms are still a matter of discussions. Independent from that there is agreement that the cathodic behaviour is based on the superposition of water and oxygen reduction reactions [1]. Accessing the amount of cathodic reaction contributions to the entire corrosion current density via experiment is challenging [2,3]. In a recent work of Wang et al. [4] a combined experimental and computational approach was utilized to explore the superposition of cathodic reactions during Mg corrosion in aqueous electrolytes for different iron impurity situations. The study shows significant variation of percentual contribution of oxygen reduction to the cathodic current for the studied purity levels.In this work a computational modelling approach is presented accordingly. It is based on the numerical model developed by Höche [5] and enables virtual local electrochemical testing and computational assessment of (electro-)chemical system and transport descriptors. Respective simulation results show, how corrosion and testing system parameters change the measured oxygen approach curves. Subsequently a multi-parameter calibration enabled the quantification of oxygen reduction reaction to the entire corrosion process. The reproduction of experimental results and strategies for model improvement are discussed. REFERENCES: [1] Höche, D., Blawert, C., Lamaka, S. V., Scharnagl, N., Mendis, C., & Zheludkevich, M. L., The effect of iron re-deposition on the corrosion of impurity-containing magnesium. Physical Chemistry Chemical Physics, 18-2 (2016), 1279-1291.[2] Yasakau, K. A., Höche, D., Lamaka, S. L., Ferreira, M. G., & Zheludkevich, M. L., Kelvin microprobe analytics on iron-enriched corroded magnesium surface. Corrosion, 73-55 (2017), 583-595.[3] E.L. Silva, S.V. Lamaka, D. Mei, M.L. Zheludkevich, The Reduction of Dissolved Oxygen During Magnesium Corrosion, ChemistryOpen, 7 (2018) 664-668.[4] C. Wang, W. Xu, D. Höche, M.L. Zheludkevich, S.V. Lamaka, Exploring the contribution of oxygen reduction reaction to Mg corrosion by modeling assisted local analysis, Journal of Magnesium and Alloys, 11 (2023) 100-109.[5] Höche D., Simulation of corrosion product deposit layer growth on bare magnesium galvanically coupled to aluminum. Journal of the electrochemical society, 162-1 (2014), C1. ACKNOWLEDGEMENTS: C. Wang, W. Xu, thank China Scholarship Council for the funding, grants no. 201806310128 and 201908510177

N-Heterocyclic Carbene Based Nanolayer for Copper Film Oxidation Mitigation

The wide use of copper is limited by its rapid oxidation. Main oxidation mitigation approaches involve alloying or surface passivation technologies. However, surface alloying often modifies the physical properties of copper, while surface passivation is characterized by limited thermal and chemical stability. Herein, we demonstrate an electrochemical approach for surface-anchoring of an N-heterocyclic carbene (NHC) nanolayer on a copper electrode by electro-deposition of alkyne-functionalized imidazolium cations. Water reduction reaction generated a high concentration of hydroxide ions that induced deprotonation of imidazolium cations and self-assembly of NHCs on the copper electrode. In addition, alkyne group deprotonation enabled on-surface polymerization by coupling surface-anchored and solvated NHCs, which resulted in a 2 nm thick NHC-nanolayer. Copper film coated with a NHC-nanolayer demonstrated high oxidation resistance at elevated temperatures and under alkaline conditions.The ease of preparation and well-controlled growth process of electrochemically-induced NHC nanolayer make it an easily-applicable method for large-scale coating to provide thin and effective passivation layer for copper surfaces. Moreover, the electro-induced mechanism of NHC-nanolayer formation makes it possible to selectively deposit the protective layer on conducting copper wires without changing the optical properties of the entire device. These advantages make the presented technology highly suitable for applications that require high transparency, such as solar cells and electroluminescence devices.

Direct Z-scheme construction of C2N/Mg(OH)2 heterojunction and first-principles investigation of photocatalytic water splitting

Efficient water-splitting photocatalysts are pivotal in enhancing solar energy utilization to its maximum potential. Using first-principles calculations, the current study systematically explores the heterojunction's electron, carrier transfer, and optical properties. The observations indicate that the C2N/Mg(OH)2 heterojunction manifests a staggered band configuration, culminating in the accomplishment of a Z-type charge transfer paradigm. This mechanism promotes the recombination of e-h pairs within the C2N/Mg(OH)2 heterojunction, leading to the prolonged retention of photogenerated electrons in the Mg(OH)2 layer and photogenerated holes in the C2N layer. Consequently, this enables a sustained water reduction and oxidation reaction. The C2N/Mg(OH)2 heterojunction exhibits oxidation-reduction potentials that cross those of aqueous solutions under acidic (pH = 0) and neutral (pH = 7) conditions, thereby facilitating the photocatalytic process of water decomposition. Moreover, the optical absorption coefficient of the heterojunction surpasses that of a single two-dimensional material. Therefore, the direct Z-type C2N/Mg(OH)2 heterojunction holds promising potential as an exceptional photocatalyst for water decomposition.

Hole Transport Layer in Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising approach to produce green and renewable hydrogen fuel, alleviating the CO2 emissions, air pollution, and energy crisis. However, the efficiency is limited by the recombination of photogenerated carriers and the losses of holes, resulting in a mismatch between the rates of water oxidation and reduction reactions. This article starts with a discussion of the principle of photoelectrochemical water splitting, highlighting the role and importance of holes, and then summarizes the development of the hole transport layer, with a focus on the classification of the hole transport layer, the structure and properties of common hole transport materials, and the construction and improvement of the hole transport layer. Finally, it is concluded with a summary and perspective of strategies for the future development of the hole transport layer.

Electrostatic Self-Assembled Synthesis of Amorphous/Crystalline g-C3N4 Homo-Junction for Efficient Photocatalytic H2 Production with Simultaneous Antibiotic Degradation.

g-C3N4 has been regarded as a promising photocatalyst for photo-reforming antibiotics for H2 production but still suffers from its high charge recombination, which has been proven to be solvable by constructing a g-C3N4 homo-junction. However, those reported methods based on uncontrollable calcination for preparing a g-C3N4 homo-junction are difficult to reproduce. Herein, an amorphous/crystalline g-C3N4 homo-junction (ACN/CCN) was successfully synthesized via the electrostatic self-assembly attachment of negatively charged crystalline g-C3N4 nanorods (CCN) on positively charged amorphous g-C3N4 sheets (ACN). All the ACN/CCN samples displayed much higher photo-reforming of antibiotics for H2 production ability than that of pristine ACN and CCN. In particular, ACN/CCN-2 with the optimal ratio exhibited the best photocatalytic performance, with a H2 evolution rate of 162.5 μmol·g-1·h-1 and simultaneous consecutive ciprofloxacin (CIP) degradation under light irradiation for 4 h. The UV-vis diffuse reflectance spectra (DRS), photoluminescence (PL), and electrochemical results revealed that a homo-junction is formed in ACN/CCN due to the difference in the band arrangement of ACN and CCN, which effectively suppressed the charge recombination and then led to those above significantly enhanced photocatalytic activity. Moreover, H2 was generated from the water reduction reaction with a photogenerated electron (e-), and CIP was degraded via a photogenerated hole (h+). ACN/CCN exhibited adequate photostability and reusability for photocatalytic H2 production with simultaneous CIP degradation. This work provides a new idea for rationally designing and preparing homo-junction photocatalysts to achieve the dual purpose of chemical energy production and environmental treatment.

Escalating the synergism on CdZnS via Ag2S/Cu2S co-catalysts: Boosts hydrogen evolution from water splitting under sunlight

Photocatalytic hydrogen production through water splitting is one of the effective ways to fulfill future energy demands. In current study, Ag and Cu were simultaneously incorporated into CdZnS through hydrothermal reaction to synthesize 1 % Ag2S/Cu2S codoped-CdZnS. The catalysts were characterized via XRD, FT-IR, SEM, EDX, UV–Vis/DRS, XPS, BET, PL and EIS techniques. It has been observed that 1 % Ag2S/Cu2S is optimized amount of dopants that enables CdZnS to produce hydrogen with 9.76 mmol g−1h−1 rate. This amount of H2 is 3 times higher than that of bare CdZnS (2.91 mmol g−1h−1). Photocatalytic activity shown by 1 % Ag2S/Cu2S-CdZnS is due the incorporation of Ag and Cu which exist in the form of Ag2S and Cu2S clusters respectively and causes synergistic effect during photoreaction. During the photoreactions, oxidation half reactions occur at Ag2S, while reduction half reaction at Cu2S (i.e. conversion of H+ ions to H2). On the basis of the results, it can be concluded that Cu2S acts as reduction co-catalyst while Ag2S serves as oxidation co-catalyst in water reduction reaction. This work could offer efficient and low cost photocatalysts for hydrogen production from the water splitting reactions.

In situ infrared, Raman and X-ray spectroscopy for the mechanistic understanding of hydrogen evolution reaction

Hydrogen production by water reduction reactions has received considerable attention because hydrogen is considered a clean-energy carrier, key for a sustainable energy future. Computational methods have been widely used to study the reaction mechanism of the hydrogen evolution reaction (HER), but the calculation results need to be supported by experimental results and direct evidence to confirm the mechanistic insights. In this review, we discuss the fundamental principles of the in situ spectroscopic strategy and a theoretical model for a mechanistic understanding of the HER. In addition, we investigate recent studies by in situ Fourier transform infrared (FTIR), Raman spectroscopy, and X-ray absorption spectroscopy (XAS) and cover new findings that occur at the catalyst–electrolyte interface during HER. These spectroscopic strategies provide practical ways to elucidate catalyst phase, reaction intermediate, catalyst-electrolyte interface, intermediate binding energy, metal valency state, and coordination environment during HER.

Visible light sensitized graphitic carbon nitride based semiconducting materials for photoelectrochemical water splitting

Semiconductor based photoelectrochemical water splitting technology directly harvests solar energy into fuels with environment friendly byproducts. The semiconducting material implemented in this technology should be highly photoactive, chemically stable, nontoxic, inexpensive and environmentally benevolent. Even though, various semiconducting materials have been implemented for this purpose, graphitic carbon nitride (GCN) has emerged as an efficient electrode material for water splitting through photoelectrochemical (PEC) route to generate sustainable hydrogen energy owing to its appropriate band position for water reduction reaction, low cost, visible light response, good chemical stability and non-toxicity. However, the practical application of GCN is hindered for PEC water splitting because of numerous challenges, such as its poor electrical conduction ability and high recombination rate. In this paper, we address the fundamental principles for PEC splitting of water with respect to GCN. The broad spectrum of modification strategies for the improvement of GCN based photoelectrodes including heteroelement doping, heterojunction construction, defect engineering and morphology control are also explored. These modification strategies made GCN an efficient material for photoelectrochemical water splitting. The maximum photocurrent density was reported as 320µA cm−2. Conclusively, the crucial challenges and probable solutions for forthcoming advancement in PEC water splitting by GCN based photoelectrodes are highlighted.

Enhanced photocatalytic hydrogen production of S-scheme TiO2/g-C3N4 heterojunction loaded with single-atom Ni

S-scheme heterostructure photocatalysts can achieve highly efficient solar energy utilization. Here, single-atom Ni species were deposited onto TiO2/g-C3N4 (TCN) composite photocatalyst with an S-scheme heterojunction for highly efficient photocatalytic water splitting to produce hydrogen. Under solar irradiation, it realized the hydrogen production activity of 134 µmol g–1 h–1, about 5 times higher than the TCN without atomic Ni. In-situ Kelvin probe force microscopy characterization and the density functional calculation certify that by forming the S-scheme heterojunction, the photo-excited electrons from the TiO2 combine with the photogenerated holes at the coupled g-C3N4 driven by a built-in electric field. More importantly, the single-atom Ni species stabilized the photogenerated electrons from the g-C3N4 could effectively enhance the charge separation between the holes on the valence band of TiO2 and electrons at the conduction band of g-C3N4. Meanwhile, the Ni atoms act as the surface catalytic centers for the water reduction reaction, which greatly improves the reactivity of the photocatalyst. The present work provides a new approach for developing noble metal-free heterojunctions for high-efficiency photocatalysis.

Hollow dodecahedron K3PW12O40/CdS core-shell S-scheme heterojunction for photocatalytic synergistic H2 evolution and benzyl alcohol oxidation

Simultaneous generation of clean energy, H2, and organic products holds immense potential in the realm of photocatalysis. The S-scheme heterojunction stands out for these dual-function applications due to its robust redox capacity, facilitating both the water reduction reaction and organic oxidation reactions. In this study a Keggin-type polymetallic oxide, H3PW12O40 hollow dodecahedron (KPW), was synthesized using a hydrothermal approach. Subsequently, cadmium sulfide (CdS) nanoparticles averaging 15 nm in size were integrated in situ onto the KPW shell, resulting in the creation of a core-shell KPW@CdS S-scheme heterojunction. This optimized composite showcased a hydrogen evolution rate of 18.7 mmol g−1 h−1, alongside a value-added product, benzaldehyde, with a yield of 17.5 mmol g−1 h−1 substantially surpassing the performance of standalone CdS. This S-scheme junction, featuring a pronounced internal electronic field, emerges between the KPW and CdS. It significantly enhances the segregation of photogenerated carriers while preserving formidable redox capability. Furthermore, the hollow structure augments light absorption and utility, and the core-shell architecture delivers dual reduction and oxidation sites. As a result, the interplay between the hollow core-shell configuration and the S-scheme mode intensifies the photocatalytic activity. This research provides an innovative approach to crafting hollow S-scheme heterojunctions, aiming to optimize photocatalytic redox reactions for effective solar energy utilization.

(Invited) In-Situ Time-Resolved Probe of Charge Carrier Dynamics at Planar Semiconductor Photoelectrode/Liquid Interface

Photoelectrochemistry is a promising approach for converting solar energy to storable chemical energy in chemical fuels. The efficiency of photoelectrochemcial devices depends critically on efficient transfer of charge carrier across the electrode liquid interface and the efficiency and selectivity of the catalytic reaction on electrode surface. Although transient absorption spectroscopy is a powerful tool for probing interfacial dynamics in high surface area electrodes, it is often not applicable on planar electrodes because the observed signal is often dominated by carrier dynamics in the bulk. In this talk, we discuss our recent effort in developing in situ time-resolved linear and nonlinear spectroscopic tools that can be used to probe carrier dynamics and reactions at planar electrode/liquid interfaces, focusing on transient reflection spectroscopy and electric field induced second harmonic generation (EFISH) spectroscopy. Compared to transient absorption, transient reflection spectroscopy is more sensitive to surface carrier density that can be more directly correlated to catalysis. We showed that transient reflectance change can be used to follow charge carrier dynamics and their amplitude can be directly correlated to efficiency of initial interfacial charge separation on p-GaP single crystals protected by TiO2 ALD layer. Our finding suggests that IPCE is determined by the product of the efficiency of initial ultrafast (< hundreds of ps) charge separation across the GaP/TiO2 interface and the efficiency of water reduction reaction on the slower time scale. Key loss pathways that limit these efficiencies are discussed. In a second approach, we showed the EFISH signal in a n-doped single crystal TiO2 can be used to probe the accumulation of photogenerated carriers at surface states and these trapped carriers are responsible for the water oxidation reaction.