Water-splitting Devices Research Articles

(Invited) Solar Fuels Produced by Compound Semiconductors

Energy crisis is a broad and complex global topic. Natural resources such as gas and oil are in limited in supply. Development towards renewable resources is one of the most important technologies in the world. Currently, photocatalytic and photoelectrochemical (PEC) water splitting devices under the irradiation of sunlight have received much attention for the production of renewable hydrogen from water. Solar energy conversion and storage through photoelectrolysis of water using semiconductors as both light absorber and energy converter to store solar energy in simple chemical bond, H2, become highly desirable approaches to solving the energy shortage challenge. We focus on the effort to develop an efficient GaAs-based PEC water splitting device. The multifunctional designs for compound semiconductor photoelectrodes provide the potential for the future development in the renewable energy market.

Solar-Driven Water Splitting at 13.8% Solar-to-Hydrogen Efficiency by an Earth-Abundant Electrolyzer

We present the synthesis and characterization of an efficient and low cost solar-driven electrolyzer consisting of Earth-abundant materials. The trimetallic NiFeMo electrocatalyst takes the shape of nanometer-sized flakes anchored to a fully carbon-based current collector comprising a nitrogen-doped carbon nanotube network, which in turn is grown on a carbon fiber paper support. This catalyst electrode contains solely Earth-abundant materials, and the carbon fiber support renders it effective despite a low metal content. Notably, a bifunctional catalyst–electrode pair exhibits a low total overpotential of 450 mV to drive a full water-splitting reaction at a current density of 10 mA cm–2 and a measured hydrogen Faradaic efficiency of ∼100%. We combine the catalyst–electrode pair with solution-processed perovskite solar cells to form a lightweight solar-driven water-splitting device with a high peak solar-to-fuel conversion efficiency of 13.8%.

Storage batteries in photovoltaic–electrochemical device for solar hydrogen production

Hydrogen produced by water electrolysis, and electrochemical batteries are widely considered as primary routes for the long- and short-term storage of photovoltaic (PV) energy. At the same time fast power ramps and idle periods in PV power generation may cause degradation of water splitting electrochemical (EC) cells. Implementation of batteries in PV-EC systems is a viable option for smoothening out intermittence of PV power. Notably, the spreading of PV energy over the diurnal cycle reduces power of the EC cell and thus its overpotential loss. We study these potential advantages theoretically and experimentally for a simple parallel connected combination of PV, EC, and battery cells (PV-EC-B) operated without power management electronics. We show feasibility of the unaided operation of PV-EC-B device in a relevant duty cycle and explore how PV-EC-B system can operate at higher solar-to-hydrogen efficiency than the equivalent reference PV-EC system despite the losses caused by the battery. • Battery cell implemented in photovoltaic-assisted water splitting device. • Feasibility of continuous water splitting without power electronics. • Spreading of light energy over diurnal cycle reduces overpotential in water splitting. • Improved solar-to-hydrogen efficiency despite battery losses.

Density Functional Theory for Electrocatalysis

It is a considerably promising strategy to produce fuels and high‐value chemicals through an electrochemical conversion process in the green and sustainable energy systems. Catalysts for electrocatalytic reactions, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (NRR), carbon dioxide reduction reaction (CO2RR), play a significant role in the advanced energy conversion technologies, such as water splitting devices, fuel cells, and rechargeable metal‐air batteries. Developing low‐cost and highly efficient electrocatalysts is closely related to establishing the composition–structure–activity relationships and fundamental understanding of catalytic mechanisms. Density functional theory (DFT) is emerging as an important computational tool that can provide insights into the relationship between the electrochemical performances and physical/chemical properties of catalysts. This article presents a review on the progress of the DFT, and the computational simulations, within the framework of DFT, for the electrocatalytic processes, as well as the computational designs and virtual screenings of new electrocatalysts. Some useful descriptors and analysis tools for evaluating the electrocatalytic performances are highlighted, including formation energies, d‐band model, scaling relation, eg orbital occupation, and free energies of adsorption. Furthermore, the remaining questions and perspectives for the development of DFT for electrocatalysis are also proposed.

Type-II GeC/ZnTe heterostructure with high-efficiency of photoelectrochemical water splitting

Efficient carrier separation and suitable band edge position are necessary conditions for photocatalytic water splitting. Based on density functional theory, the electronic properties and optical performance of the GeC/ZnTe vdW heterostructure are systematically explored. The heterostructure with inherent type-II band arrangement is conducive to constantly separate the hole–electron pairs, thus improving the utilization of solar energy. Meanwhile, an excellent optical absorption coefficient has been proved in the heterostructure with large carrier mobility. Within the strain range of –4% to +3%, the heterostructure possesses appropriate band edges for photocatalytic water splitting and the light absorption performance is obviously improved. High solar-to-hydrogen efficiency (26.81%) means that the heterostructure can effectively convert photon energy into water splitting. These findings predict the possibility of GeC/ZnTe heterostructures as photocatalysts for water splitting and other optoelectronic devices.

A novel iron-based composite flocculant for enhanced wastewater treatment and upcycling hazardous sludge into trifunctional electrocatalyst

Sewage sludge, an unavoidable by-product of industrial wastewater treatment, is facing multitudinous challenges in drawing a cost-effective compromise between environmental and economic acceptance. The aim of this research is to produce high-quality energy materials by utilizing hazardous waster as precursors. A novel inorganic-organic composite flocculant was first manufactured and used as an efficient binary-coagulant for enhanced dyes removal and Fe-containing textile sludge (TS-Fe) production from dyeing wastewater. Then, three-dimensional interconnected hierarchical porous iron-nitrogen doped carbon materials (TS-Fe-N-C) were fabricated by pyrolysis of TS-Fe to realize the resource recovery. Upon accurate control the composition of TS-Fe-N-C, the optimal TS-Fe-N-C-0.23 has excellent performance for oxygen reduction reaction (half-wave potential of 0.837 V), oxygen evolution reaction (overpotential of 0.328 V at 10 mA cm-2), and hydrogen evolution reaction (overpotential of 0.144 V at 10 mA cm-2). Moreover, Zn-air battery and water splitting device based on TS-Fe-N-C-0.23 electrodes displayed satisfactory energy density (770.0 mAh g-1), power density (120.5 mW cm-2), and low water splitting voltage (1.70 V @ 10 mA cm-2). This study suggests that rational design of flocs by conventional wastewater treatment technologies is an effective strategy for optimizing the composition of sludge and boosting the additional value to the sludge-derived biochar product.

MoS2||CoP heterostructure loaded on N, P-doped carbon as an efficient trifunctional catalyst for oxygen reduction, oxygen evolution, and hydrogen evolution reaction

Exploring multifunctional electrocatalysts is crucial for the development of energy conversion and storage equipments, such as fuel cells, water splitting devices and zinc-air batteries. Herein, we provide a rational design whereby the cobalt phosphide particles are introduced into molybdenum sulfide nanosheets to form a heterostructure (MoS2||CoP) through the ultrasonic method and calcination. Subsequently, N, P-doped carbon (NPC) is obtained synchronously. The as-prepared MoS2||CoP/NPC demonstrates highly effective multifunctional catalytic performance for oxygen evolution and hydrogen evolution reaction at lower overpotential, as well as oxygen reduction reaction at high half-wave potential. What this reveals is higher power density and superior stability in zinc-air battery. The excellent electrocatalytic activity of MoS2||CoP/NPC may be attributed to the presence of the MoS2||CoP heterostructure, as well as N, P-doped carbon coupled with a high percentage of pyridinic-N. This work proposes a novel and facile strategy to prepare the heterostructure compound and serves as a good reference for constructing efficient and low-cost electrocatalysts.

A self-supporting bifunctional catalyst electrode made of amorphous and porous CoP3 nanoneedle array: exhaling during overall water splitting

The current bottleneck that clean energy cannot be fully utilized is that grid-level energy storage technology is still expensive. To become the energy storage link for clean energy usage, it is necessary to find electrodes with low cost and high energy conversion efficiency for hydrogen production by water splitting. In this article, we provide a strategy to make an electrode with the excellent catalyst of amorphous and porous CoP3, which is shaped into hollow needles. The porous structure fully exposes the active sites, amorphization modulates the electronic environment and accelerates the gas desorption process. The geometric gradient of the electrode's needle shape leads to Laplace force acting on the catalytic process: gas attached on the surface can be driven to coalesce and detach due to the conversion of excess surface energy into kinetic energy. The general consideration about dehydrogenation overpotential and degassing overpotential makes our final electrodes as both anode and cathode of overall water-splitting device exhibit an excellent catalytic performance with a partial current density over 1 A cm−2, even better than the asymmetrical precious metals benchmark Pt/C || RuO2 pair.

Recent Progress in the Development of Advanced Functionalized Electrodes for Oxygen Evolution Reaction: An Overview.

Presently, the global energy demand for increasing clean and green energy consumption lies in the development of low-cost, sustainable, economically viable and eco-friendly natured electrochemical conversion process, which is a significant advancement in different morphological types of advanced electrocatalysts to promote their electrocatalytic properties. Herein, we overviewed the recent advancements in oxygen evolution reactions (OERs), including easy electrode fabrication and significant action in water-splitting devices. To date, various synthetic approaches and modern characterization techniques have effectively been anticipated for upgraded OER activity. Moreover, the discussed electrode catalysts have emerged as the most hopeful constituents and received massive appreciation in OER with low overpotential and long-term cyclic stability. This review article broadly confers the recent progress research in OER, the general mechanistic approaches, challenges to enhance the catalytic performances and future directions for the scientific community.

Two-Dimensional In2X2X' (X and X' = S, Se, and Te) Monolayers with an Intrinsic Electric Field for High-Performance Photocatalytic and Piezoelectric Applications.

Two-dimensional (2D) materials with excellent photocatalytic properties and unique piezoelectric response have attracted great attention. However, these characters are rare for traditional 2D structures. With an intrinsic electric field, the Janus 2D materials show great promise in photocatalytic and out-of-plane piezoelectric applications. Herein, we show that Janus In2X2X' (X and X' = S, Se, and Te) monolayers are desirable in the overall water splitting and piezoelectric devices. Comprehensive investigations reveal that the band gaps of these Janus monolayers are from 0.34 to 2.27 eV. With proper band edge positions, strong solar absorption, fast transfer and efficient separation of carriers, and high solar to hydrogen (STH) efficiencies (reaching 37.70%), eight members of them stand out. Besides, the electrons and holes have sufficient driving forces in the process of redox reaction. The piezoelectric response for in- and out-of-plane is superior for all monolayers. These compelling features make them suitable for photocatalysts, sensors, actuators, and energy conversion devices.

Efficient electrocatalytic water splitting by bimetallic cobalt iron boride nanoparticles with controlled electronic structure

Developing an efficient bifunctional catalyst for Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting technology is very attractive for clean energy. Here, a new Co-Fe-B ternary catalyst with improved crystallinity is successfully synthesized by combining the chemical reduction and subsequent solid-state reaction method. Synchrotron-based X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) indicate the electronic structure redistribution is favor for the improved performance. The overpotential is only 129 mV and 280 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline condition, the corresponding Tafel slope is 67.3 mV dec−1 and 38.9 mV dec−1. Density functional theory calculations distinguish that the ternary crystalline Co-Fe-B catalysts are thermodynamically favorable for HER and OER. The actual active species of the ternary catalyst in OER is the CoOOH and FeOOH as indicated in ex situ Raman spectra. The present work may introduce promising crystallinity borides material for the anode and cathode of water splitting device.

MOF-derived CoP-nitrogen-doped carbon@NiFeP nanoflakes as an efficient and durable electrocatalyst with multiple catalytically active sites for OER, HER, ORR and rechargeable zinc-air batteries

Highly active, long-lasting, and low-cost nanostructured catalysts with efficient oxygen evolution and oxygen reduction reactions (OER and ORR) are critical for achieving high-performance zinc-air batteries. Herein, we developed CoP-nitrogen-doped carbon@NiFeP nanoflakes (CoP-NC@NFP), derived from MOF enriched with multiple active sites, for multifunctional water splitting and zinc-air battery applications. The experimental results revealed that the multiple active catalytic sites of CoP-NC@NFP were responsible for the excellent charge-transfer kinetics and electrocatalytic performance with respect to water splitting. This performance is comparable to that of precious metal catalysts in alkaline electrolytes (OER: overpotential of 270 mV; HER: overpotential of 162 mV; ORR: Tafel slope of 46 mV dec−1; overall water splitting device: cell voltage of 1.57 V at 10 mA cm−2) with excellent electrochemical durability. Additionally, the structural stability of the OER and the HER durability of the CoP-NC@NFP electrocatalyst were confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) studies. Most impressively, zinc-air batteries (ZABs) assembled with CoP-NC@NFP as the air–cathode exhibit exceptionally high power density of 93 mW cm−2 and prolonged operational stability over 200 h compared with a ZAB equipped with a benchmark air–cathode. The outcome of this study opens a practical possibility for the preparation of efficient multifunctional catalysts free of noble metals for clean energy production and storage.

Er3+ doped titania photoanode for enhanced performance of photo-electrochemical water splitting devices

The origin of enhanced performance of properties of erbium (Er3+)-doped TiO2 photoanodes used in photo-electrochemical cell (PEC) water-splitting devices and power-conversion efficiency in DSSC devices is presented. Particularly, the dielectric measurements of Er3+-TiO2 conclude that the optimised concentration of Er (0.2% mol)-TiO2 has a better conductivity compared to undoped TiO2. The fabricated PEC devices with Er (0.2% mol)-doped TiO2 photoanode exhibited the higher current density of (9 μA-cm−2) at 1.2 V vs RHE potential which is ≈50% higher than that measured for PEC with undoped-TiO2. This enhancement is corroborated with the EIS measurements presenting lower charge transfer resistance values for devices with Er (0.2% mol)-TiO2.

Phosphorus doped nickel selenide for full device water splitting

The lack of high active and stable electrocatalysts has impeded the development of electrochemical water splitting device, which is promising technique for renewable energy conversion system. Here, we report a one-step protocol to synthesize P doped NiSe2 (P-NiSe2) by selenylation process derived from nickel foam with assistant of NaH2PO2 and Se powder. The P-NiSe2 could be directly used as working electrode and shows the superior electrochemical activity, offering current density of 10 mA cm−2 with overpotential of 270 mV for OER and 71 mV for HER. The enhanced electrochemical activity can be ascribed to the P atom doping. The P atom doping leads to the high valence state of Ni active sites, which have high catalytic ability towards OER. Moreover, the P doping makes the d-band center of Ni atoms in P-NiSe2 move close to Fermi level, facilitating the HER kinetics with respect to proton adsorption and hydrogen desorption. When employed P-NiSe2 as both anodic and cathodic electrode in alkaline water electrolyzer, a current density of 10 mA cm−2 can be achieved at 1.58 V. Our work highlights the importance of P doping in determining the surface electron configuration for full device water splitting and the facile synthesis protocol would be promising for realistic applications.

Enhanced electrocatalytic overall water splitting over novel one-pot synthesized Ru–MoO3-x and Fe3O4–NiFe layered double hydroxide on Ni foam

Herein, two novel electrocatalysts are simultaneously grown on Ni foam (NF) through a one-step hydrothermal method. The as-prepared Ru–MoO3-x NF exhibits good performance for HER with low overpotentials of 12 mV at 10 mA cm−2 and 49 mV at 100 mA cm−2, a low Tafel slope of 32.96 mV dec−1, as well as an excellent durability at least lasting 17 h in alkaline media. The Fe3O4–NiFe LDH NF (layered double hydroxide nickel foam) ultrathin nanosheets electrode shows a low OER overpotential of 216 mV at 10 mA cm−2 and a small Tafel slope of 42.42 mV dec−1. Moreover, a two-electrode water splitting device is fabricated with Ru–MoO3-x NF as cathode and Fe3O4–NiFe LDH NF as anode, it only requires 1.46 V at 10 mA cm−2 and exhibits a long term durability without obvious loss of activity after continuously operating for 180 h. The superior catalytic behavior should be originated from the open heterostructure and oxygen vacancies with high intrinsic activity, abundant exposed active sites, and fast charge transfer. This work provides a new strategy to design heterostructure and transition metal oxides catalysts with improved catalytic activity and durability by a facile route for overall water splitting or other applications.

An efficient and durable trifunctional electrocatalyst for zinc–air batteries driven overall water splitting

Constructing more active and durable trifunctional electrocatalysts is key for boosting overall water splitting and metal–air battery efficiency. Herein, we developed a trifunctional electrocatalyst of ultrafine Pt nanoparticles anchored on CoS2-N-doped reduced graphene oxide (Pt@CoS2-NrGO). Owing to its more Pt active sites with rapid ion/electron transport ability, the Pt@CoS2-NrGO shows excellent trifunctional activities towards HER (ƞ10 = 39 mV), OER (ƞ10 = 235 mV) ORR (E1/2 = 0.85 V vs. RHE) and water splitting device of Pt@CoS2-NrGO||Pt@CoS2-NrGO achieved cell voltage of 1.48 V at 10 mA cm−2, which is better than Pt-C||RuO2. Finally, we employed Pt@CoS2-NrGO as air cathode for zinc–air battery to display a power density of 114 mW cm-2 and durability of 55 h, outperforming than Pt-C + RuO2 based zinc–air batteries. For practical aspects, Pt@CoS2-NrGO based zinc–air batteries were connected to overall water splitting device to produce H2 and O2 gases for hydrogen fuel cell.

Kesterite Cu2ZnSnS4 thin-film solar water-splitting photovoltaics for solar seawater desalination

Kesterite photovoltaic materials of Cu 2 ZnSnS 4 (CZTS) thin films present excellent photoelectrochemical (PEC) properties in solar water-splitting devices. It is found that photoelectrodes, including CZTS for solar water splitting, are suitable for application in solar seawater desalination due to the similarity of their operation principle and work conditions to photoelectrodes/solutions. Here we report the application of a CZTS-based photocathode from solar water splitting to solar seawater desalination. With the TiO 2 surface modification layer, the CZTS-based photocathode shows superior stability in the progress of seawater desalination and maintains desalination performance above 95% after testing for almost 500 h. Moreover, the device driven by solar energy does not require an additional electrical supply. The combination of redox flow and ion exchange membrane (IEM) for seawater desalination is a highly efficient and promising water treatment technique that can effectively alleviate the shortage of freshwater resources. Solar water-splitting photocathode with promising solar desalination performance Unbiased and stable solar desalination for almost 500 h is presented Intersection of research between water splitting and solar desalination The application of solar water-splitting photocathode for solar desalination Many regions have a shortage of freshwater, and the situation is getting worse with time. Here, Li et al. obtain freshwater from seawater using solar energy by exploiting a solar water-splitting photocathode.

Photocorrosion of WO3 Photoanodes in Different Electrolytes.

Photocorrosion of an n-type semiconductor is anticipated to be unfavorable if its decomposition potential is situated below its valence band-edge position. Tungsten trioxide (WO3) is generally considered as a stable photoanode for different photoelectrochemical (PEC) applications. Such oversimplified considerations ignore reactions with electrolytes added to the solvent. Moreover, kinetic effects are neglected. The fallacy of such approaches has been demonstrated in our previous study dealing with WO3 instability in H2SO4. In this work, in order to understand parameters influencing WO3 photocorrosion and to identify more suitable reaction environments, H2SO4, HClO4, HNO3, CH3O3SH, as electrolytes are considered. Model WO3 thin films are fabricated with a spray-coating process. Photoactivity of the samples is determined with a photoelectrochemical scanning flow cell. Photostability is measured in real time by coupling an inductively coupled plasma mass spectrometer to the scanning flow cell to determine the photoanode dissolution products. It is found that the photoactivity of the WO3 films increases as HNO3 < HClO4 ≈ H2SO4 < CH3O3SH, whereas the photostability exhibits the opposite trend. The differences observed in photocorrosion are explained considering stability of the electrolytes toward decomposition. This work demonstrates that electrolytes and their reactive intermediates clearly influence the photostability of photoelectrodes. Thus, the careful selection of the photoelectrode/electrolyte combination is of crucial importance in the design of a stable photoelectrochemical water-splitting device.

Arsenic carbide allotropes prediction: An efficient platform for hole-conductions, optical and photocatalysis applications

Based on the first-principle framework and particle swarm optimization approach, we have predicted two allotropes of arsenic carbide (AsC) monolayers consisting of an equimolar mixture of arsenic and carbon atoms with honeycomb structures, namely as puckered and buckled. Besides excellent stability, they present rich properties that could establish an efficient platform for hole-conductions, optical, and photocatalysis applications. The lower cohesive energy, absence of negative frequencies in the phonon dispersion curve, and ab initio molecular dynamics simulation prove both monolayers are thermodynamically, kinetically, and thermally stable at room temperature. Besides, electronic band structure calculations indicate that puckered and buckled configurations of AsC monolayers possess intrinsic band gaps of 1.27 and 1.78 eV. In particular, the absorption coefficient and bandgap of a puckered configuration are significantly tuned under the uniaxial strain and induces a transition from direct to the indirect gap. The appropriate band edge positions and strong absorption coefficient in the broad visible and ultraviolet light region make both monolayers a promising candidate for photocatalytic water-splitting and next-generation optoelectronic devices. The carrier mobility calculation exhibits both monolayers have the largest hole carrier mobility at room temperature, rendering it for a p-type field-effect transistor.

Carbonized Wood Decorated with Cobalt‐Nickel Binary Nanoparticles as a Low‐Cost and Efficient Electrode for Water Splitting

AbstractUsing an inexpensive and eco‐friendly wood substrate, herein, a one‐step calcination method is developed to deposit Co‐Ni binary nanoparticles into aligned wood channels and an effective carbonized wood (CW) electrode (termed as Co/Ni‐CW) is fabricated. Well distributed Co‐Ni nanoparticles are achieved by the coordination bonds between the hydroxyl groups on wood matrix and soaked metal cations. Subsequently, high‐temperature calcination promotes the nucleation of Co‐Ni nanoparticles and the formation of CW. With the uniform distribution of Co‐Ni nanoparticles and porous wood structure, not only is a high active surface area, but also the electron and mass diffusion pathways are enhanced. Thus, the as‐prepared Co/Ni‐CW affords the current density of 10 mA cm–2 at low overpotentials of 330 and 157 mV for oxygen and hydrogen evolution, respectively. Remarkably, when the wood‐based bifunctional electrocatalyst is used as both the anode and cathode, a low cell voltage of 1.64 V is required to reach the current density of 10 mA cm–2. Compared with most substrates used in bifunctional electrocatalysts, the abundance, low cost, eco‐friendliness, and easy operation of wood‐based catalysts allow for an active and scalable electrode for water splitting and many other energy storage devices.