Water Splitting System Research Articles

Enhanced photoelectrocatalytic hydrogen evolution using off-stoichiometry La0.43FeOy films

In this work, the influence of the stoichiometry of LaxFeOy films on their activity for H2 production by photoelectrochemical (PEC) water splitting was investigated. Thin LaxFeOy films were deposited on electrodes by magnetron co-sputtering and their composition was controlled by adjusting the power applied on metallic iron and lanthanum targets. The highest photoactivity was observed for La0.43FeOy films. The crystallization of these films and their photocatalytic activity were further improved by oxidation in an external furnace at 650 °C for 2 h. Transmission electron microscopy (TEM) analyses confirm the formation of LaFeO3 crystallites in the film and show a thin continuous polycrystalline Fe2O3 layer at the surface. The presence of Fe2O3 at the upper surface originates from an exsolution mechanism typically observed for non-stoichiometric perovskites. The improvement of the photocatalytic properties by adjusting the stoichiometry, the oxidation temperature and the film thickness was confirmed by H2 production measurements performed by mass spectrometry. We observe a large increase in H2 production rate (103%) by comparison with a stoichiometric LaFeO3 thin film. The improvement in catalytic performances for these non-stoichiometric films composed of earth abundant, low-cost and non-toxic elements, make them of high interest materials for photoelectrodes in PEC water splitting systems.

Charge Separated One-Dimensional Hybrid Cobalt/Nickel Phosphonate Frameworks: A Facile Approach to Design Bifunctional Electrocatalyst for Oxygen Evolution and Hydrogen Evolution Reactions.

Two new organoamine templated one-dimensional transition metal phosphonate compounds are synthesized, and their bifunctional electrocatalytic activities are examined in highly alkaline and acidic media. Compared with state-of-the-art materials, the cobalt phosphonate system is a new fabrication of sustainable and highly efficient catalysts toward electrochemical water splitting systems.

Efficient preparation of Ni-M (M = Fe, Co, Mo) bimetallic oxides layer on Ni nanorod arrays for electrocatalytic oxygen evolution

Fabrication of economic and high-performance electrodes for electrocatalytic oxygen evolution reaction (OER) accounts for a crucial issue associated with developing powerful and practical water splitting systems. In this work, free-standing Ni/Ni-M (M = Fe, Co, Mo) bimetallic oxides core/shell nanorod arrays (Ni/Ni-M NRAs) were prepared through electroless deposition of transition metal species on black nickel sheet (nickel nanorod arrays (Ni NRAs)) followed by electrochemical oxidation. All three types of Ni/Ni-M NRAs demonstrated enhanced electrocatalytic activity toward oxygen evolution reactions (OER). Especially, Ni/Ni-Fe NRAs electrode exhibit small onset potential of 1.535 V at current density of 10 mA∙cm−2. In contrast, the OER durability of these three samples was distinct. At 500 mV constant overpotential, the current density loss in OER of Ni/Ni-Fe NRAs was merely 13.5% for a period of 20000 s; but Ni/Ni-Mo and Ni/Ni-Co NRAs had almost disappeared catalytic activity under the identical conditions. According to many reports, the results were different for the superior OER stability of Ni-based bimetallic catalysts. Electrochemical analysis revealed that the NRAs structure dramatically improves charge transfer efficiency and electrochemically active surface area (ECSA). The present study might provide a new insight to design and fabricate more practical and high-performance Ni-based electrodes for OER.

Interfacial electronic modulation of CoP-CoO p-p type heterojunction for enhancing oxygen evolution reaction

Heterojunction can effectively improve oxygen evolution reaction (OER) activity by regulating the interfacial electronic structure of catalysts. However, p-p type heterojunction OER catalysts have obtained less attention, and the corresponding catalytic mechanisms are unclear either. Herein, the self-supported CoP-CoO p-p type heterojunction arrays are fabricated on carbon cloth substrate (CoP-CoO/CC). Band structure analysis shows that the formation of p-p heterojunction can drive the electrons from CoO to flow into CoP. This electronic modulation contributes to positively charged regions on the CoO and enhances the OH− adsorption during OER, proven by X-ray photoelectron spectroscopy and methanol molecular detection, respectively. As a result, the CoP-CoO/CC electrode only needs 210 mV overpotential to drive a current density of 10 mA cm−2 in an alkaline medium, superior to the most reported OER catalysts. Additionally, the CoP-CoO/CC also exhibits an ideal hydrogen evolution reaction response, and a water splitting system has been successfully constructed which can drive a 10 mA cm−2 within 1.65 V. This study supplies insight for catalytic origins p-p type heterojunctions OER catalyst, which provides a reference value for the efficient and reasonable design of heterojunction catalysts.

Graphene Quantum Dots Improved "Caterpillar"-like TiO2 for Highly Efficient Photocatalytic Hydrogen Production.

Photocatalytic water splitting for hydrogen production via heterojunction provides a convenient approach to solve the world crises of energy supply. Herein, graphene quantum dots modified TiO2 hybrids (TiO2-GQDs) with a “caterpillar”-like structure exhibit stronger light absorption in the visible region and an enhanced hydrogen production capacity of about 3.5-fold compared to the pristine TiO2 caterpillar. These results inferred that the addition of GQDs drastically promotes the interfacial electron transfer from GQDs to TiO2 through C–O–Ti bonds via the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional groups in GQDs. Using a “caterpillar”-like structure are expected to provide a new platform for the development of highly efficient solar-driven water splitting systems based on nanocomposite photocatalyst.

Rational Design of Photoelectrodes for the Fully Integrated Polymer Electrode Membrane–Photoelectrochemical Water-Splitting System: A Case Study of Bismuth Vanadate

Photoelectrochemical (PEC) reactors based on polymer electrolyte membrane (PEM) electrolyzers are an attractive alternative to improve scalability compared to conventional monolithic devices. To introduce narrow band gap photoabsorbers such as BiVO4 in PEM-PEC system requires cost-effective and scalable deposition techniques beyond those previously demonstrated on monolithic FTO-coated glass substrates, followed by the preparation of membrane electrode assemblies. Herein, we address the significant challenges in coating narrow band gap metal-oxides on porous substrates as suitable photoelectrodes for the PEM-PEC configuration. In particular, we demonstrate the deposition and integration of W-doped BiVO4 on porous conductive substrates by a simple, cost-effective, and scalable deposition based on the SILAR (successive ionic layer adsorption and reaction) technique. The resultant W-doped BiVO4 photoanode exhibits a photocurrent density of 2.1 mA·cm–2, @1.23V vs RHE, the highest reported so far for the BiVO4 on any porous substrates. Furthermore, we integrated the BiVO4 on the PEM-PEC reactor to demonstrate the solar hydrogen production from ambient air with humidity as the only water source, retaining 1.55 mA·cm–2, @1.23V vs RHE. The concept provides insights into the features necessary for the successful development of materials suitable for the PEM-PEC tandem configuration reactors and the gas-phase operation of the reactor, which is a promising approach for low-cost, large-scale solar hydrogen production.

In situ integration of Fe3N@Co4N@CoFe alloy nanoparticles as efficient and stable electrocatalyst for overall water splitting

Developing highly efficient and stable electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is critical for overall water splitting to solve the shortage of fossil fuel and environment pollution, but it is full of obstacles and challenges. Fe3N@Co4N@CoFe alloy nanoparticles were prepared by using an in situ coupling method. The obtained catalyst showed a satisfactory OER performance in alkaline solution with a low overpotential of 225 mV at 10 mA cm−2 current density and a small Tafel slope of 48 mV/dec. Meanwhile, the HER performance was also improved in alkaline solution after cyclic voltammetry (CV) cycles of OER due to the formation of CoFe oxyhydroxides, which required an overpotential of 135 mV to achieve 10 mA cm−2 current density and a Tafel slope of 109 mV/dec. More importantly, the obtained Fe3N@Co4N@CoFe alloy electrode and Fe3N@Co4N@CoFe alloy-CV electrode (treated through CV cycles of OER) were used as anode and cathode to assemble a two-electrode system for overall water splitting. The constructed system displayed an impressive competency with a voltage of 1.59 V at 10 mA cm−2 current density. The prepared electrocatalysts demonstrated superb long-term stability under 50 mA cm−2 current density for OER, HER and overall water splitting.

Durable and efficient photoelectrochemical water splitting using TiO2 and 3C–SiC single crystals in a tandem structure

Hydrogen can be generated by the photolysis of water with semiconductors as photocatalysts. The most important characteristics of water splitting are its durability and efficiency. Furthermore, TiO 2 and 3C–SiC are durable photocatalysts. Because of their different band gaps and conductivity types, a tandem structure combined with these materials functions as an efficient and durable solar water splitting system. It was observed that the photocurrent from the tandem structure was higher at low pH, as noted by the pH dependence of the photocurrent at the TiO 2 photoanode. The photocurrent from the tandem structure had a light intensity dependence similar to the photocurrent from the TiO 2 photoanode. Therefore, the performance of the tandem structure appears to be limited by the photocurrents from the TiO 2 photoanode. We obtained a maximum applied bias photon-to-current conversion efficiency of approximately 0.74% from the tandem structure with no additional bias applied. We also confirmed the presence of a stable photocurrent from the tandem structure for over ~100 days. •The pH dependence and the irradiation light intensity dependence of TiO 2 , 3C–SiC single crystals and the tandem structure combined were characterized. •The lower the pH, the higher the irradiation light intensity, the better the performance of TiO 2 , 3C–SiC single crystals and the tandem structure. •The measured value of the hydrogen generation is 85% of the calculated value which means that hydrogen is almost completely generated in the actual hydrogen and oxygen generation experiment. •The tandem structure shows a 100 days' long-term durability which is pretty longer than most of the photoelectrodes and photocatalysts.

Plasma‐Assisted Synthesis of Co3O4‐Based Electrocatalysts on Ni Foam Substrates for the Oxygen Evolution Reaction (Adv. Mater. Interfaces 18/2021)

Co3O4-Based Electrocatalysts on Ni Foam The development of active and cheap materials to trigger oxygen evolution is of utmost importance in water splitting systems to yield molecular hydrogen. In article number 2100763, Alberto Gasparotto, Davide Barreca, and co-workers report that plasma-fabricated Co3O4-based nanostructures on porous metal scaffolds yield an overpotential of ≈350 mV and a Tafel slope of 60 mV/dec, opening the door to nano-engineering of low-cost electrocatalysts based on earth-abundant and non-precious materials.

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy.

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO4) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO4 with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO4 and BiVO4/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (kh+0) to electron (ke-0) via the photocatalyst of BiVO4/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO4 under illumination. In this case, the ratio of the rate constants kh+0/ke-0 stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuOx onto BiVO4 further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.

FeTiO3 Perovskite Nanoparticles for Efficient Electrochemical Water Splitting

The use of water splitting has been investigated as a good alternate for storing electrical energy. While the general interest in developing non-toxic, high-performance, and economically feasible catalysts for oxygen evolution reaction (OER) is noteworthy, there is also significant interest in water splitting research. Recently, perovskite-type oxides have performed as an alternative to non-precious metal catalysts and can act as a new class of effective catalysts in water splitting systems. Herein, a perovskite-structured FeTiO3 was prepared via a facile one-step solvothermal method using ionic liquid as templates. The results of structural and morphological studies have supported the formation of FeTiO3 perovskite. Furthermore, FeTiO3 perovskite demonstrated OER activity with a lower onset potential of 1.45 V vs. RHE and Tafel slope value of 0.133 V.dec−1 at 1 M KOH solution using mercury/mercurous oxide (Hg/HgO) were used as working electrodes.

Tin Sulfide/Gallium Oxide Heterojunctions for Solar Water Splitting

Tin (II) sulfide (SnS) is a promising semiconductor material for next‐generation solar energy conversion due to its favorable bandgap, elemental abundance, low toxicity, and low cost. A major challenge, however, lie in the low open circuit voltages that are typically obtained in SnS‐based devices. Herein, a low‐cost solution‐phase deposition technique is used to prepare SnS thin films and investigate different junction materials (Ga2O3 and In2S3) to improve the photovoltage in SnS‐based water splitting photocathodes. Molecular inks are prepared by dissolving SnS powder in solvent mixtures of ethylenediamine and 1,2‐ethanedithiol. SnS thin films are then successfully deposited by spin coating the inks onto substrates, followed by a heat treatment at 350°C in an inert atmosphere. With a photoelectrode based on a SnS/Ga2O3 heterojunction, an onset potential of +0.25 V versus reversible hydrogen electrode (RHE) is achieved for photoelectrochemical hydrogen evolution in pH 7 phosphate buffer, which is until now the earliest onset potential (highest photovoltage) among nontoxic replacements to CdS junctions in SnS‐based water splitting systems.

A facile one-pot room-temperature growth of self-supported ultrathin rhodium-iridium nanosheets as high-efficiency electrocatalysts for hydrogen evolution reaction

The development of low-cost and high-efficiency electrocatalysts is very important for electrocatalytic hydrogen evolution reaction (HER) in water splitting system. Herein, ultrathin rhodium-iridium nanosheets were facilely in-situ grown on nickel foam (RhIr NSs/NF) by a one-pot aqueous strategy at room temperature. The sheet-like structures with the film thickness of 78 nm were identified by scanning electron microscopy and transmission electron microscopy. The catalyst showed greatly high HER features in both 1.0 M KOH and 0.5 M H2SO4 with the overpotentials of 15 and 14 mV to achieve 10 mA cm−2, respectively, surpassing most Pt-free catalysts. Also, the RhIr NSs/NF exhibited amazing catalytic stability during the long-term operation. This study offers a facile and rational pathway for design and synthesis of advanced HER electrocatalysts for energy conversion devices.

A sustainable molybdenum oxysulphide-cobalt phosphate photocatalyst for effectual solar-driven water splitting

IntroductionHydrogen is considered as a clean alternative green energy future fuel. Since the Honda-Fujishima effect for photoelectrochemical water splitting is known, there has been a substantial boost in this field. Numerous photocatalysts based on metals, semiconductors, and organic-inorganic hybrid-systems have been proposed. Several factors limit their efficiency, e.g., a stable PEC-WS setup, absorbing visible light, well-aligned band energy for charge transfer, electrons and holes, and their separation to avoid recombination and limited water redox reactions. Metallic doping and impregnation of stable and efficient co-catalysts such as Pt, Ag, and Au showed enhanced PEC-WS. We used Cobalt-based co-catalyst with molybdenum oxysulfide photocatalyst for effectual solar-driven water splitting. ObjectivesTo develop photocatalysts for efficient PEC processes capable of absorbing sufficient visible light, good band energy for effective charge transfer, inexpensive, significant solar-to-chemical energy conversion efficiencies. Above all, it is developing such PEC-WS systems that will be commercially viable for renewable energy resources. MethodsWe prepared Molybdenum oxysulphide-cobalt phosphate photocatalyst for PEC-WS through a facile hydrothermal route using ammonium heptamolybdate, thiourea, and metallic Cobalt precursors. ResultsAn effectual photocatalyst is produced for solar-driven water splitting. The conformal morphology of MoOxSy-CoPi nanoflowers is a significant feature, as observed under FE-SEM and HR-TEM. XRD confirmed the degree of purity and orthorhombic crystal structure of MoOxSy-CoPi. EDX and XPS identify the elemental compositions and corresponding oxidation states of each atom. A 2.44 eV band-gap energy is calculated for MoOxSy-CoPi from the diffused reflectance spectrum. Photo- Electrochemical Studies (PEC) under 1-SUN solar irradiation revealed 7-8 folds enhanced photocurrent (∼ 3.5 mA/cm2) generated from MoOxSy-CoPi/FTO in comparison to Co-PI/FTO (∼ 0.5 mA/cm2) and MoOxSy-/FTO respectively, within 0.5 M Na2SO4 electrolyte (@pH=7) and standard three electrodes electrochemical cell. ConclusionOur results showed MoOxSy-CoPi as promising photocatalyst material for improved solar-driven photoelectrochemical water splitting system.

Nitrogen-doped carbon dot anchored 1-D WO3 for enhanced solar water splitting: A nano surface imaging evidence of charge separation and accumulation

Combining WO3 with suitable materials to form heterojunction is essential to overcome the limitations of WO3 to enhance its photoelectrochemical (PEC) water splitting activity. Moreover, a clear understanding of photo-response and charge behavior of materials could lead to the rational design of efficient photoelectrodes. Given this, an efficient strategy is applied to fabricate WO3 heterojunction with nitrogen-doped carbon dots (NCDs) and in-depth characterization to investigate the surface charge dynamics using nano imaging in a relation to the enhanced PEC water splitting activity. The optimized NCDs loading to the WO3 NRs exhibited the enhanced photocurrent density of 1.54 mA cm−2 at 1.23V vs RHE under AM 1.5 G illumination, highest IPCE of ~82 % (at 308.32 nm). The Kelvin probe force microscopy and electrostatic force microscopy reveal that after loading NCDs to the WO3, a relatively smooth charge transport has been observed, which improves the PEC. Furthermore, this work demonstrates the effect of photogenerated charges caused by the NCDs that assist in enhancing the increased photocurrent, hydrogen production efficiency, and stability of the PEC water splitting system. Significantly, the nano imaging characterization utilized in this work could be extended to various photoanodes to study the surface charge dynamics.

Controlled preparation of hollow Zn0.3Cd0.7S nanospheres modified by NiS1.97 nanosheets for superior photocatalytic hydrogen production

Developing durable and efficient photocatalysts for H2 evolution is highly desirable to expedite current research on solar-chemical energy conversion. In this work, a novel photocatalytic H2 evolution system based on Zn0.3Cd0.7S/NiS1.97 nanocomposite was rationally designed for the first time. In this advanced composite structure, NiS1.97 nanosheets as a co-catalyst were intimately coupled to the inner surface of the hollow spherical Zn0.3Cd0.7S. The construction of the hollow spherical shell shortened the distance of charge migration to the surface site and increased the multiple absorption of incident light. The introduction of NiS1.97 nanosheets increased the light absorption capacity of the composite system and also greatly improved the separation and migration behavior of photo-generated carriers due to its narrower band gap and relatively low conduction band position, which had been confirmed by DRS, EIS and PL. As a result, the hollow Zn0.3Cd0.7S/NiS1.97 composite material exhibited excellent photocatalytic activity. At the loading amount of NiS1.97 up to 15 at.%, the hollow Zn0.3Cd0.7S/NiS1.97 composite exhibited the best photocatalytic activity with a corresponding H2 production rate of 22.637 mmol g-1h−1, which was 1.42 times and 1.85 times that of hollow Zn0.3Cd0.7S and solid Zn0.3Cd0.7S, respectively. Moreover, this novel catalyst also displayed a long-term stability without apparent debasement in H2 evolution activity. It is expected that this work could provide new inspiration to the design and development of other highly active photocatalytic systems for water splitting.

Iron, rhodium-codoped Ni2P nanosheets arrays supported on nickel foam as an efficient bifunctional electrocatalyst for overall water splitting

To enhance the overall water splitting efficiency, it is widely attractive yet challenging to develop low price, abundance and efficient bifunctional electrocatalysts towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, Fe,Rh-codoped Ni2P nanosheets arrays were in situ anchored on three-dimension (3D) Ni foam under hydrothermal condition and successive phosphorization, denoted as Fe,Rh-Ni2P/NF for simplicity. The unique nanosheets arrays effectively enriched the active sites with easy accessibility. By virtue of the unique sheet-like arrays and 3D porous conductive substrate, the prepared Fe,Rh-Ni2P/NF showed the low overpotentials of 226 mV at 30 mA cm−2 towards the OER and 73 mV at 10 mA cm−2 for the HER. Moreover, the electrocatalyst effectively worked as anode and cathode for overall water splitting system, showing a small voltage of 1.62 V to drive a current density of 10 mA cm−2. The present work provides alternative option for fabricating advanced catalysts in electrocatalysis and energy devices.

NiFeOx decorated Ge-hematite/perovskite for an efficient water splitting system

To boost the photoelectrochemical water oxidation performance of hematite photoanodes, high temperature annealing has been widely applied to enhance crystallinity, to improve the interface between the hematite-substrate interface, and to introduce tin-dopants from the substrate. However, when using additional dopants, the interaction between the unintentional tin and intentional dopant is poorly understood. Here, using germanium, we investigate how tin diffusion affects overall photoelectrochemical performance in germanium:tin co-doped systems. After revealing that germanium is a better dopant than tin, we develop a facile germanium-doping method which suppresses tin diffusion from the fluorine doped tin oxide substrate, significantly improving hematite performance. The NiFeOx@Ge-PH photoanode shows a photocurrent density of 4.6 mA cm−2 at 1.23 VRHE with a low turn-on voltage. After combining with a perovskite solar cell, our tandem system achieves 4.8% solar-to-hydrogen conversion efficiency (3.9 mA cm−2 in NiFeOx@Ge-PH/perovskite solar water splitting system). Our work provides important insights on a promising diagnostic tool for future co-doping system design.

Template-free synthesis of 1D hollow Fe doped CoP nanoneedles as highly activity electrocatalysts for overall water splitting

Development of low cost and high efficiency electrocatalysts for water splitting systems to produce renewable hydrogen energy is still a significant requirement. The engineering of nanostructure and element doping are effective methods to further improve the performance of catalysts. Nonmetal (such as N, P, S) doping has been extensively investigated, while the report of metal doping is relatively few. Herein, Fe doped CoP 1D hollow nanoneedles on carbon cloth (CC) are designed and fabricated by a hydrothermal method and subsequent phosphorization procedure. The conversion of Fe doped Co-hydroxide@CC to Fe–CoP can produce large number of nanopores, which are closely connected to each other, and form hollow structures within the nanoneedles. Benefiting from the effective Fe doping and the particular hollow nanoneedle structure, the obtained Fe–CoP@CC demonstrates good electrocatalytic activity for hydrogen evolution reaction (HER) both in alkaline and acidic solution, affording a current density of 10 mA cm−2 at overpotential of 49 mV and 80 mV, respectively. Moreover, the two-electrode electrolyzer with Fe–CoP@CC as both the cathode and anode catalyst achieve a current density of 10 mA cm−2 at a cell voltage of 1.58 V in 1.0 M KOH solution. The results illustrate that the obtained hollow Fe–CoP@CC nanoneedles can serve as an efficient catalyst for overall water splitting.

Pore morphology evolution and atom distribution of doped Fe2O3 foams developed by freeze-casting after redox cycling

Chemical looping water splitting systems operate at relatively high temperatures (450-800 degree C) to produce, purify, or store hydrogen by the cyclic reduction and oxidation (redox) of a solid oxygen carrier. Therefore, to improve long-term operation, it is necessary to develop highly stable oxygen carriers with large specific surface areas. In this work, highly interconnected doped Fe2O3 foams are fabricated through the freeze-casting technique, and the aid of a submicrometric camphene-based suspension to prevent Fe sintering and pore clogging during redox operation. The influence of the dopant elements (Al and Ce) over the pore morphology evolution, and redox performances are examined. The use of an Fe2O3 porous structure with initial pore size above 100 microns shows a significant reduction of the sample densification, and the addition of Al2O3 by the co-precipitation process proves to be beneficial in preventing the generation of a core-shell structure following redox processing.