Water Splitting Technology Research Articles

Overall water splitting: What’s next?

Photocatalytic overall water splitting reaction driven by sunlight has been studied as a means of producing renewable solar hydrogen on a large scale. The practicality of this technology will ultimately depend on the cost of the produced solar hydrogen. This cost, in turn, is determined by various factors such as solar-to-hydrogen energy conversion efficiency, scalability, construction/operating costs and the service life of such systems. This article both summarizes the current state of development of photocatalytic overall water splitting technology and presents issues that must be addressed in the future to allow such systems to have real-world utility.

Analysis of XGaO3 (X = Ba and Cs) cubic based perovskite materials for photocatalytic water splitting applications: a DFT study

Energy conversion has become an important technology for meeting energy production and consumption in the modern era. Water splitting and solar cell technologies are projected to close the gap between demand and consumption. Therefore, XGaO3 (X = Ba and Cs) compounds having characteristics i.e., electrical, optical, mechanical, and structural are depicted by using a density functional theory (DFT) based CASTEP software with ultrasoft pseudo-potential plane-wave and Generalized Gradient Approximation and Perdew Burke Ernzerhof exchange correlation functional (GGA-PBE). According to the findings, all of these compounds have a cubic “pm3m” structure with space group 221. The CsGaO3 and BaGaO3 have direct and indirect band gaps, with respect to electronic band-structure recreations. Density of states like total density of states (TDOS) and partial density of states (PDOS) commend the extent of localization of electrons in numerous bands. The optical properties of these compounds are explored by adjusting dispersion curve/relation for theoretical dielectric function (DF) scale to the corresponding peaks. As a result, these materials could be used to consume light in the visible zone via photo catalysis. CsGaO3 in combination with BaGaO3 can produce effective results, so these compounds have a remarkable potential application for sensing and water splitting.

NiS Protective Layer for Repelling Chloride Ion Effectively for Water Oxidation on Photocatalytic Seawater Splitting

Due to the increase of global power use, the shortage of energy supply and global warming have become the main obstacles to social and economic development. Today’s society is gradually developing cleaner and greener energy represented by solar energy to alleviate the current energy shortage. Among them, semiconductor photocatalytic water splitting technology is one of the most important methods. However, global freshwater resources are in short supply; the design and synthesis of proper photocatalysts for seawater splitting has been regarded as one of the most powerful approaches solving the dilemma of a shortage of freshwater resources and utilization of solar energy. Based on that, the quaternion Pt/Co3O4–NiS/STO composites for photocatalytic water splitting in a seawater system were successfully synthesized by a simple hydrothermal method and photodeposition method. Noticeably, the loading of Pt and Co3O4 cocatalysts greatly improved the separation efficiency of photogenerated electrons and holes. At the same time, the surface modification of NiS made the photocatalysts able to resist the toxicity of a large number of chloride ions in seawater. The oxygen yield for seawater splitting was nearly twice that of untreated samples and about half of the amount of that in pure water. Our work introduced an ideal modification material structure to demonstrate the feasibility of seawater splitting, which would guide other photocatalytic seawater splitting systems in the future.

Composition-controlled chemical bath deposition of Fe-doped NiO microflowers for boosting oxygen evolution reaction

Water electrolysis for green hydrogen production is gaining tremendous attention in the quest towards sustainable energy sources. At the heart of water splitting technology are the electrocatalysts, which facilitate the two half-cell reactions, i.e., the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with the latter being the most thermodynamically uphill. Herein, we managed to fabricate Ni1-xFexO microflowers (μFs) with varying % of Fe doping (0 < x < 0.36) via an easy chemical bath deposition (CBD) method. The as-synthesized μFs drop-casted on graphene paper (GP) are then applied as electrocatalysts for OER. Compared to contrast catalysts, the electrocatalyst with xFe = 0.1 exhibits a lower overpotential of 297 mV at a current density of 10 mA cm−2, Tafel slope of 44 mV dec−1 and unprecedented turnover frequency of 4.6 s−1 at 300 mV. It is believed that this remarkable electrochemical performance mainly stems from the synergistic effects of Ni and Fe species, working in harmony to enhance charge transfer kinetics and intrinsic activity of the catalyst. This work provides a promising avenue for developing cost-effective and highly active electrocatalysts as advanced electrodes for energy related applications.

Recent Advances of Modified Ni (Co, Fe)-Based LDH 2D Materials for Water Splitting.

Water splitting technology is an efficient approach to produce hydrogen (H2) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H2 with O2. The efficiency of H2 production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO2, and IrO2. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

The three-dimensionally ordered microporous CaTiO3 coupling Zn0.3Cd0.7S quantum dots for simultaneously enhanced photocatalytic H2 production and glucose conversion

Glucose conversion assisted photocatalytic water splitting technology to simultaneously produce H2 and high value-added chemicals is a promising method for alleviating the energy shortage and environmental crisis. In this work, we constructing type II heterojunction by in-situ coupling Zn0.3Cd0.7S quantum dots (ZCS QDs) on three-dimensionally ordered microporous CaTiO3 (3DOM CTO) for photocatalytic H2 production and glucose conversion. The DFT calculations demonstrate that substitution of Zn on the Cd site improves the separation and transmission of photogenerated carriers. Therefore, 3DOM CTO-ZCS composite exhibits best H2 production performance (2.81 mmol g-1h−1) and highest apparent quantum efficiency (AQY) (5.56 %) at 365 nm, which are about 47 and 18 times that of CTO nanoparticles (NPs). The improved catalytic performance ascribed to not only good mass diffusion and exchange, highly efficient light harvesting of 3DOM structure, but also the efficient charges separation of type Ⅱ heterojunction. The investigation on photocatalytic mechanism indicates that the glucose is mainly converted to gluconic acid and lactic acid, and the control reaction step is gluconic acid to lactic acid. The selectivity for gluconic acid on 3DOM CTO-ZCS is 85.65 %. Our work here proposes a green sustainable method to achieve highly efficient H2 production and selective conversion of glucose to gluconic acid.

Recent Advances of Oxygen Carriers for Hydrogen Production via Chemical Looping Water-Splitting

Hydrogen is an important green energy source and chemical raw material for various industrial processes. At present, the major technique of hydrogen production is steam methane reforming (SMR), which suffers from high energy penalties and enormous CO2 emissions. As an alternative, chemical looping water-splitting (CLWS) technology represents an energy-efficient and environmentally friendly method for hydrogen production. The key to CLWS lies in the selection of suitable oxygen carriers (OCs) that hold outstanding sintering resistance, structural reversibility, and capability to release lattice oxygen and deoxygenate the steam for hydrogen generation. Described herein are the recent advances in designing OCs, including simple metal oxides (e.g., Fe, Zn, Ce, and Ti-based metal oxides) and composite metal oxides (e.g., perovskite, spinel, and garnets), for different CLWS processes with emphasis on the crucial parameters that determine their redox performance and future challenges.

Dual Integrating Oxygen and Sulphur on Surface of CoTe Nanorods Triggers Enhanced Oxygen Evolution Reaction.

The bottleneck of large-scale implementation of electrocatalytic water-splitting technology lies in lacking inexpensive, efficient, and durable catalysts to accelerate the sluggish oxygen evolution reaction kinetics. Owing to more metallic features, transition metal telluride (TMT) with good electronic conductivity holds promising potential as an ideal type of electrocatalysts for oxygen evolution reaction (OER), whereas most TMTs reported up to now still show unsatisfactory OER performance that is far below corresponding sulfide and selenide counterparts. Here, the activation and stabilization of cobalt telluride (CoTe) nanoarrays toward OER through dual integration of sulfur (S) doping and surface oxidization is reported. The as-synthesized CoO@S-CoTe catalyst exhibits a low overpotential of only 246mV at 10mAcm-2 and a long-term stability of more than 36h, outperforming commercial RuO2 and other reported telluride-based OER catalysts. The combined experimental and theoretical results reveal that the enhanced OER performance stems from increased active sites exposure, improved charge transfer ability, and optimized electronic state. This work will provide a valuable guidance to release the catalytic potential of telluride-based OER catalysts via interface modulating engineering.

Beyond the Platinum Era─Scalable Preparation and Electrochemical Activation of TaS2 Flakes.

Among 2D materials, transition-metal dichalcogenides (TMDCs) of group 5 metals recently have attracted substantial interest due to their superior electrocatalytic activity toward hydrogen evolution reaction (HER). However, a straightforward and efficient synthesis of the TMDCs which can be easily scaled up is missing. Herein, we report an innovative, simple, and scalable method for tantalum disulfide (TaS2) synthesis, involving CS2 as a sulfurizing agent and Ta2O5 as a metal precursor. The structure of the created TaS2 flakes was analyzed by Raman, XRD, XPS, SEM, and HRTEM techniques. It was demonstrated that a tuning between 1T (metallic) and 3R (semiconductor) TaS2 phases can be accomplished by varying the reaction conditions. The created materials were tested for HER, and the electrocatalytic activity of both phases was significantly enhanced by electrochemical self-activation, up to that comparable with the Pt one. The final values of the Tafel slopes of activated TaS2 were found to be 35 and 43 mV/dec for 3R-TaS2 and 1T-TaS2, respectively, with the corresponding overpotentials of 63 and 109 mV required to reach a current density of 10 mA/cm2. We also investigated the mechanism of flake activation, which can be attributed to the changes in the flake morphology and surface chemistry. Our work provides a scalable and simple synthesis method to produce transition-metal sulfides which could replace the platinum catalyst in water splitting technology.

The Mo as electron trapper and donor to modulate the electron of CoP2 in phosphating process

To prepare bifunctional electrocatalyst towards HER and OER is extremely important for promoting the development of electrochemical water splitting technology. Herein, the element doping method is employed to tune the electron environment of cobalt phosphide (CoP). The Mo-doped CoP supported on carbon cloth (CC) is constructed by solvothermal and annealing method. The effect of Mo on the electron modulation of CoP during different phosphating time was studied carefully. It is noted that the Mo play an important role in tuning the electron state of Co and P elements which can trap the electron and was reduced to low valence, then transfer the electron to Co and P. With increasing the phosphating time, the electron transfer phenomenon between Mo and CoP is obvious. Benefiting from the electron engineering of Mo, Co and P as well as thin and wrinkle sheets structure, the optimal electrocatalyst only requires 39 mV and 251 mV to deliver 10 mA cm−2 for HER and OER, respectively. Also, as for the whole water splitting performance, it delivers 10 mA cm−2 at cell voltage of 1.56 V. Importantly, Faraday efficiency of the optimal catalyst achieves 99.9% for HER due to the tuned electron state of Co and P, high ECSA and low Rct.

Entropy-Stabilized Multicomponent Porous Spinel Nanowires of NiFeXO4 (X = Fe, Ni, Al, Mo, Co, Cr) for Efficient and Durable Electrocatalytic Oxygen Evolution Reaction in Alkaline Medium.

Cost-effective electrochemical water splitting technology hinges on the development of efficient and durable catalysts for oxygen evolution reaction (OER). Spinel oxides (formula: AxB3-xO4) are structurally stable for real applications. Much effort has been devoted to improving the catalytic activity. Here, we report a eutectic dealloying strategy to activate the porous spinel NiFe2O4 nanowires with up to four metal cation substitutions. As-obtained spinel NiFeXO4 (X = Fe, Ni, Al, Mo, Co, Cr) delivers a benchmark current density of 10 mA·cm-2 at an overpotential of only 195 mV, outperforming most spinel phase OER electrocatalysts and comparable to the state-of-the-art NiFe hydroxides. It is stable for over 115 h of electrolysis. Aberration-corrected transmission electron microscopy, high-resolution electron energy loss spectroscopy, and atomic-scale strain mappings reveal that the multivalent cation substitutions result in substantial lattice distortion and significant electronic coupling of metal 3d and O 2p orbitals for increased covalency. Further theoretical calculations suggest that the NiFeXO4 are stabilized by the high configurational entropy, and their synergy favors the absorption of H2O molecules and lowers the adsorption energy barrier of the OOH* intermediate. The improved intrinsic activity together with the highly nanoporous structures contribute to the appealing apparent catalytic performances. The work demonstrates an effective approach for the synthesis of stable multicomponent spinel oxides and highlights the effectiveness of the multication substitution strategy for producing highly durable and active spinel catalysts, which meet multiplexed structure and superior property requirements in practical applications.

Engineering cobalt molybdate nanosheet arrays with phosphorus-modified nickel as heterogeneous electrodes for highly-active energy-saving water splitting

Electrochemical urea electrolysis has been regarded as a promising strategy to replace traditional water-splitting technology to achieve hydrogen fuel due to its cost savings and high energy efficiency. Designing efficient bifunctional electrocatalysts easily is important but still faces significant challenges. Herein, an interface engineering strategy is used to construct a hybrid material by coupling cobalt molybdate (CoMoO4) nanosheet arrays with phosphorus-modified nickel (P-Ni) particles on copper foam (P-Ni@CoMoO4/CF) through the hydrothermal and in-situ electrodeposition process. Benefiting from the abundant catalytic active sites, low charge transfer resistance, and synergistic coupling effect, the optimal P-Ni@CoMoO4/CF electrocatalyst presents a superior bifunctional activity for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). In detail, a small overpotential of 125 mV and a low potential of 1.36 V is required to attain the current density of 100 mA cm−2 for HER and UOR, respectively. In the process of urea electrolysis, the P-Ni@CoMoO4/CF-based electrolyzer provides a current density of 100 mA cm−2 with an overall voltage of 1.50 V, about 170 mV less than that in a traditional water electrolyzer. The high performance of P-Ni@CoMoO4/CF outperforms many recently reported electrodes, suggesting its promising application in energy-saving hydrogen production. Our work proposes a novel idea for the rational design and exploitation of low-cost and robust bifunctional electrodes for electrocatalysis.

Ultrafast Combustion Synthesis of Robust and Efficient Electrocatalysts for High-Current-Density Water Oxidation.

The scalable production of inexpensive, efficient, and robust catalysts for oxygen evolution reaction (OER) that can deliver high current densities at low potentials is critical for the industrial implementation of water splitting technology. Herein, a series of metal oxides coupled with Fe2O3 are in situ grown on iron foam massively via an ultrafast combustion approach for a few seconds. Benefiting from the three-dimensional nanosheet array framework and the heterojunction structure, the self-supporting electrodes with abundant active centers can regulate mass transport and electronic structure for prompting OER activity at high current density. The optimized Ni(OH)2/Fe2O3 with robust structure can deliver a high current density of 1000 mA cm-2 at the overpotential as low as 271 mV in 1.0 M KOH for up to 1500 h. Theoretical calculation demonstrates that the strong electronic modulation plays a crucial part in the hybrid by optimizing the adsorption energy of the intermediate, thereby enhancing the efficiency of oxygen evolution. This work proposes a method to construct cheap and robust catalysts for practical application in energy conversion and storage.

Energy storage and water splitting applications of self-grown Na2O-NiCl2 upright standing nanoplates: a process of 3D nickel surface modification using seawater.

The recent trend in research fosters the use of abundant seawater for modifying metal surfaces as electrode materials for energy generation, storage, transport, and water-splitting technologies. Economic and ecofriendly seawater is used as a solvent for modifying the surface of 3D nickel-foam (NiF) to Na2O-NiCl2@NiF as an electrode material in electrochemical supercapacitors and water-splitting electrocatalysis applications. The phase of the as-obtained Na2O-NiCl2 is confirmed from the proposed reaction mechanism, followed various physical measurement tests such as X-ray photoelectron spectroscopy and Fourier transform infrared analysis. The formation of Na2O-NiCl2 is caused by a high operation temperature and pressure of seawater solvent, the presence of lone pair electrons on oxygen, and more reactivity of Na for combining with dissolved oxygen than the lone-pair free Cl (towards Ni). In addition to exceptional HER and OER electrocatalytic activities, i.e., 146.3 mV cm-2 and 217 mV cm-2 at a scan rate of 5 mV s-1 to attain the 10 mA cm-2 current density, the Na2O-NiCl2 has demonstrated moderate energy storage ability with considerable durability, i.e., 2533 F g-1 specific capacitance at 3 A g-1 current density even after 2000 redox cycles. The as-assembled Na2O-NiCl2//Na2O-NiCl2 symmetric electrochemical supercapacitor device has ignited a "CNED" panel consisting of nearly forty LEDs with full brightness, offering applied importance in home appliances. In nutshell, seawater-modified metal surfaces can be used for energy storage and water-splitting applications.

Interfacial microenvironment modulation enhancing catalytic kinetics of CoNiP@NiFe LDH heterostructures for highly efficient oxygen evolution reaction.

The development of highly active and robust OER catalysts is the key to address the constraints on the efficiency of electrocatalytic water splitting technology. Herein, CoNi-pristine was synthesized by a simple hydrothermal method, further phosphorylation treatment and construction of heterojunctions to synthesize efficient oxygen evolution catalysts. The OER catalytic performance of the material was greatly enhanced by the advantages of proper self-supporting 3D morphology, formation of heterogeneous interfaces and the synergistic effect of CoNiP and NiFe LDH. In 1 M KOH, CoNiP@NiFe LDH/NF only requires an overpotential of 207 mV to reach a current density of 10 mA cm-2 and operates at high current densities for more than 120 h without significant decay. It provides assistance for the rational design of interface-engineered heterostructures based on the synthesis of OER catalysts with high catalytic activity.

Developments and Challenges of Catalytic Materials for Green Hydrogen Production

Water splitting coupled to renewable power systems is an attractive way to generate green hydrogen and achieve zero carbon emissions, and represents a strategic technology to meet the high demand of carbon-neutral development. Catalysts essentially determine the efficiency and cost of water splitting technologies, and are a class of key materials for green hydrogen production. In this review, we summarize the catalyst developments for the mainstream green hydrogen production technologies, including water electrolysis, water photolysis, and photoelectrocatalytic water splitting. We first present basic catalytic mechanisms of these water splitting pathways, as well as emphasize their current research status and challenges for practical application. We subsequently introduce the recent progress in representative catalysts and design strategies toward these photo(electro)catalytic technologies, paying particular attention to water electrolysis, including alkaline water electrolyzer (AWE), proton exchange membrane water electrolyzer (PEMWE), anion-exchange membrane water electrolyzer (AEMWE) and solid oxide electrolysis cell (SOEC). Finally, we propose future prospects to develop more desirable catalysts for green hydrogen production at a large scale.

1-D arrays of porous Mn0.21Co2.79O4 nanoneedles with an enhanced electrocatalytic activity toward the oxygen evolution reaction.

Developing effective electrocatalysts for the oxygen evolution reaction (OER) that are highly efficient, abundantly available, inexpensive, and environmentally friendly is critical to improving the overall efficiency of water splitting and the large-scale development of water splitting technologies. We, herein, introduce a facile synthetic strategy for depositing the self-supported arrays of 1D-porous nanoneedles of a manganese cobalt oxide (Mn0.21Co2.79O4: MCO) thin film demonstrating an enhanced electrocatalytic activity for OER in an alkaline electrolyte. For this, an MCO film was synthesized via thermal treatment of a hydroxycarbonate film obtained from a hydrothermal route. The deposited films were characterized through scanning electron microscopy (SEM), X-ray diffractometry (XRD), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). In contrast to a similar 1D-array of a pristine Co3O4 (CO) nanoneedle film, the MCO film exhibits a remarkably enhanced electrocatalytic performance in the OER with an 85 mV lower overpotential for the benchmark current density of 10 mA cm-2. In addition, the MCO film also demonstrates long-term electrochemical stability for the OER in 1.0 M KOH aqueous electrolyte.

Co2P nanowire arrays anchored on a 3D porous reduced graphene oxide matrix embedded in nickel foam for a high-efficiency hydrogen evolution reaction.

Regulating the structural and interfacial properties of transition metal phosphides (TMPs) by coupling carbon-based materials with large surface areas to enhance hydrogen evolution reaction (HER) performance presents significant progress for water splitting technology. Herein, we constructed a composite substrate of a three-dimensional porous graphene oxide matrix (3D-GO) embedded in nickel foam (NF) to grow a Co2P electrocatalyst. Well-defined gladiolus-like Co2P nanowire arrays tightly anchored on the substrate show enhanced electrochemical characteristics for the hydrogen evolution reaction (HER) based on the promoting roles of 3D porous reduced GO (3D-rGO) derived from 3D-GO, which promotes the dispersion of active components, improves the rate of electron transfer, and facilitates the transport of water molecules. As a result, the obtained Co2P@3D-rGO/NF electrode exhibits superior HER activity in 1.0 M KOH media, achieving overpotentials of 36.5 and 264.7 mV at current densities of 10 and 100 mA cm-2, respectively. The electrode also has a low Tafel slope of 55.5 mV dec-1, a large electrochemical surface area, and small charge-transfer resistance, further revealing its mechanism of high intrinsic activity. Moreover, the electrode exhibits excellent HER stability and durability without surface morphology and chemical state changes.

Investigation of polyethylene glycol (PEG) assisted solvothermal synthesis of CuCoO2 nanosheets for efficient oxygen evolution reaction.

Water splitting to produce hydrogen is known as an effective way to alleviate the energy crisis, but the slow kinetics of the oxygen evolution reaction (OER) has been seriously restricting the development of water splitting technology. Therefore, low cost and high efficiency OER electrocatalysts have become substitutes for traditional noble metal-based catalysts. In this work, CuCoO2 nanosheets (denoted by CCO2) were successfully synthesized under the regulation of surfactants and a solvent polyethylene glycol (PEG) by a solvothermal route using Cu-BTC and Co(NO3)2·6H2O as reactants. The experimental results confirmed that PEG addition could further reduce significantly the crystal size of the CCO2 nanosheets, i.e., the size was about 150 nm and the thickness was 13 nm. The Ni@CCO2 electrode exhibits outstanding OER performance in 1.0 M KOH electrolyte, which shows the overpotential at 10 mA cm-2 is 378 mV, and the Tafel slope is 85 mV dec-1. Moreover, the CCO2 nanosheets exhibit good structural and compositional stability after the 18 h constant current OER test. Therefore, this work may offer a novel insight into enhancing the OER performance of CuCoO2 catalysts by decreasing their crystal size, and using a solvothermal route.

Development of inorganic-based nano-electrocatalysts for highly efficient water electrolysis

Polymer electrolyte membrane water electrolysis (PEMWE) is regarded as commercial electrochemical water-splitting technology to produce clean and sustainable hydrogen fuel from renewable energy resources. But current PEMWE system requires the noble metal-based metal, such as Pt, Ir or Ru, as electrocatalysts, which limits the large-scale commercialization of PEMWE. Thus, developing cost-effective and highly active electrocatalysts is important to produce hydrogen fuel on a large scale. In this paper, we introduce our recent works to develop inorganic-based noble-metal reduction or noble-metal-free nano-electrocatalysts for highly efficient water electrolysis through various facile synthetic strategies. First is Pt/MoC hybrid nanoparticles by hybridization of MoC and Pt nanoparticles for noble-metal-reduced HER electrocatalysts. Second is nickel and phosphorus substituted (Co1-xNix)(S1-yPy)2/Graphene ((Co1-xNix)(S1-yPy)2/G) 3D nano-architecture, which consist of self-assembly of 2D nanosheet, by structure-engineering and composition-engineering. Thrid is CoP nanosheets which are self-supported on carbon fabric current collector.