Epitaxial p-type SiC as a self-driven photocathode for water splitting

Solar-to-hydrogen conversion efficiencies of water-splitting photochathodes using epitaxially grown p-type 4H-, 6H- and 3C-SiC were estimated in a two-electrode system without applying any external bias. By using electrode materials with small oxygen overpotentials as counter electrodes, the photocurrent became comparable to that observed in a three-electrode system with a suitable bias. Estimated efficiencies seem to depend on the bandgap of the SiC polytypes. For the 3C-SiC, the obtained efficiency was 0.38%, which is so far the highest value reported for SiC. We confirmed that the hydrogen volumes estimated from the photocurrent were almost the same as actual volumes observed by gas chromatography.     

Review of study on solid particle solar receivers

The solid particle solar receiver (SPSR) is a direct absorption central receiver that uses solid particles enclosed in a cavity to absorb concentrated solar radiation. The SPSR is a candidate for applications of solar energy in a thermo-chemical water-splitting process to produce hydrogen. This paper presents a review of the study on SPSRs, including the idea originality, design concepts, advantages and disadvantages, the solid particle identification, a conceptual design in Sandia National Laboratories and detailed studies performed on this design. The geometry, particle size, calculating domain selection, the wind effect, the aerowindow and other factors which influence the cavity efficiency have been studied and the results are presented.     

Performance assessment of a low-cost,scalable 0.5 kW alkaline zero-gap electrolyser

The performance of a six-cell zero-gap electrolyser with an active area of 300 cm² was analysed. The device featured a new design of flowplate that employed spot-welding in order to eliminate machining costs. Direct resistance measurements were made, and computer simulations performed to confirm the sub milli-ohm resistance of the flowplate design. An electrolyser test-rig was constructed to permit performance characterisation with various electrolytes and membranes at varying temperatures, and versus a comparable finite-gap design. The results were fitted to a simplified four-parameter model which permitted quantitative comparison, and performance projection up to a 100 kW device. The highest performance achieved was 84% efficiency with 6 M KOH at 65 °C and 400 mA/cm², and the cell voltage was still below 2 V at 800 mA/cm². The total material cost to build a 0.5 kW electrolyser is under 50 GBP (70 USD).     

Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle based on cerium oxides

A new thermochemical cycle for H₂ production based on CeO₂/Ce₂O₃ oxides has been successfully demonstrated. It consists of two chemical steps: (1) reduction, 2CeO₂ → Ce₂O₃ + 0.5O₂; (2) hydrolysis, Ce₂O₃ + H₂O → 2CeO₂ + H₂. The thermal reduction of Ce(IV) to Ce(III) (endothermic step) is performed in a solar reactor featuring a controlled inert atmosphere. The feasibility of this first step has been demonstrated and the operating conditions have been defined (T = 2000 °C, P = 100–200 mbar). The hydrogen generation step (water-splitting with Ce(III) oxide) is studied in a fixed bed reactor and the reaction is complete with a fast kinetic in the studied temperature range 400–600 °C. The recovered Ce(IV) oxide is then recycled in first step. In this process, water is the only material input and heat is the only energy input. The only outputs are hydrogen and oxygen, and these two gases are obtained in different steps avoiding a high temperature energy consuming gas-phase separation. Furthermore, pure hydrogen is produced (it is not contaminated by carbon products like CO, CO₂), thus it can be used directly in fuel cells. The results have shown that the cerium oxide two-step thermochemical cycle is a promising process for hydrogen production.     

Mixed-dimensional niobium disulfide-graphene foam heterostructures as an efficient catalyst for hydrogen production

Besides developing a large number of catalysts for hydrogen evolution reaction (HER) in alkaline electrolytes, its conversion efficiency remained low. Herein, we have developed mixed-dimensional heterostructures of niobium disulfide (NbS₂) with graphene foam grown on nickel foam (NbS₂-Gr-NF). The strong lateral fusion results in activating the catalytic sites of NbS₂, the three-dimensional substrate provides easy access of electrolyte to active sites and increased electrochemically active surface area, while enhanced conductivity provides faster transfer of electrons to and from active sites. Therefore, NbS₂-Gr-NF heterostructures resulted in an exceptionally high current density of 500 mA cm⁻² at a very low overpotential of 306 mV in 1 M KOH solution and even can achieve the current density values of 914 mAcm⁻² at 338 mV only at a slight increase in overpotential (32 mV). Moreover, a Tafel value of ~72 mV dec⁻¹ confirms that as-developed heterostructure provides fast reaction kinetics where the reaction is mainly controlled by the Volmer step. Achieving such high current density at a faster rate with high stability makes NbS₂-Gr-NF heterostructures a potential candidate for water-splitting, especially in alkaline electrolytes.     

Core-shell cobalt particles Co@CoO loaded on nitrogen-doped graphene for photocatalytic water-splitting

It is of great significance to explore a bifunctional catalyst that can produce both hydrogen and oxygen to accelerate the development of water-splitting technology. In this work, Co@CoO/NG was obtained via calcinating ZIF-67 and in-situ preparation process, which exhibited excellent performance (water oxidation AQE 10.22% at λ = 450 nm and oxygen production rate 543198 μmol g⁻¹ h⁻¹ and hydrogen production rate 330 μmol⁻¹ g⁻¹ h⁻¹). A comprehensive analysis of SEM, XRD, TEM, UV–vis, EIS, and PL showed that Co@CoO/NG-7 prepared has a perfect skeleton and more crystal defects, which can provide more reactive sites. The core-shell structure Co@CoO has a synergistic effect with graphene, which is beneficial to the light absorption, separation of photo-generated charges. Meanwhile, cyclic experiments of water oxidation and water reduction showed that the catalyst exhibited high stability during the reaction process. This study has provided a universal strategy to design efficient bifunctional catalyst for water-splitting.     

Photocurrent generation properties of Histag-photosystem II immobilized on nanostructured gold electrode

It is well known that photosystem II (PSII) can produce electrons, oxygen, and protons simultaneously via a water-splitting photoreaction. These photochemical properties are expected to exhibit photoconductive function for PSII. In the present study, we have first observed a stable photocurrent due to the photoexcitation of PSII and the subsequent water-splitting reaction, by successfully immobilizing PSII on the self-assembled monolayer (SAM) of a nickel-nitrilotriacetic acid complex (Ni-NTA) prepared on a gold surface via a polyhistidine tag (Histag) as a linker molecule. We have further succeeded in the fabrication of PSII-gold nanoparticle multistructures on the surface of gold electrode, and significant enhancement of photocurrents was achieved due to increased number of immobilized PSII.     

InSe based Janus Ⅵ-Ⅲ-Ⅳ-Ⅴ monolayers as water-splitting photocatalysts: Role of vacuum level difference

Two-dimensional Janus monolayers have great potential to solve the increasing energy crisis and environmental pollution. In this work, we mainly study the structural, electronic and photocatalytic properties of InSe based Janus Ⅵ-Ⅲ-Ⅳ-Ⅴ monolayers, i.e. InSe-MQ (M = Si, Ge, Sn; Q = P, As) monolayers via first-principles calculations. Theoretical results reveal that they satisfy dynamic, energetic and mechanic stability. Their band gaps range from 1.76 to 2.28 eV, so that abundant visible light could be absorbed. The band edges of single-layer InSe-MQ except InSe–SiP straddle the redox potential of water, and thus the InSe-MQ monolayers except InSe–SiP are promising water-splitting photocatalysts at pH = 0. Particularly, most of InSe-MQ monolayers still have water splitting ability at less acid conditions with higher pH values. On the other hand, our theoretical results also indicate the band edges of group Ⅵ-Ⅲ-Ⅳ monolayers are to a large extent modified as the vacuum level difference between top and bottom surfaces is included, which has been neglected before and firmly collaborated by recalculations on the band arrangements of explored GaS-SnP and InS–SnP monolayers. Overall, this work not only defines several water-splitting photocatalysts but also paves the way to obtain reliable photocatalytic properties of the large group Ⅵ-Ⅲ-Ⅳ-Ⅴ (Ⅲ = Al, Ga, In; Ⅵ = O, S, Se, Te; Ⅳ = Si, Ge, Sn; Ⅴ = N, P, As) monolayers.     

Hydrogen evolution reaction electrocatalysis trends of confined gallium phosphide with substitutional defects

The integration of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gives better prediction of the system properties towards the applications like water-splitting and gas storage capacity/mechanism. The pursuit of generating low-cost and effective catalyst for such purposes has motivated the material scientists and researchers to design and study the novel nanostructured materials both theoretically and experimentally. We have utilized the well-established state-of-the-art density functional theory (DFT) for envisaging the HER activity of the two-dimensionally confined Gallium Phosphide (GaP). The effect of substitutional defect caused by foreign atoms like boron and nitrogen on the structural, electronic and adsorption properties of the GaP nanowire is analyzed by incorporating the van der Waals dispersion correction. The energy differences and the contributions of the individual atomic species to the electronic energy states have been observed by computing the electronic density of states. Introduction of the defect in the system significantly modifies the electronic and adsorption properties of the system. The results suggest GaP to be highly active for hydrogen adsorption which further gets pronounced by introducing boron defect in the system. The results on adsorption energy and Gibbs free energy stipulating better adsorptive nature for hydrogen give confidence to utilize GaP as an HER catalyst by further tuning the adsorption response by means of defect engineering. In a nut-shell, we assert the dependence of material properties that are very sensitive to defects and the cause root beneath this response can serve as a blueprint for designing prominent materials for HER based applications.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.