Water Splitting Reaction Research Articles

Synthesis of diverse stable MOFs and their electro catalytic capabilities towards desulfurization, water splitting and various nitrophenol reduction reactions

Metal–organic frameworks are a novel class of crystalline porous materials with enormous application potential, consisting of metal nodes and organic linkers.

Bulk-independent surface oxide composition controls the electrochemical performance of high-entropy alloys.

Multi-element alloys and high-entropy alloys show promising electrocatalytic behavior for water splitting and other catalytic reactions, due to their highly tunable composition. While preparation and synthesis of these materials are thoroughly investigated, the true reactive surface composition is still not well understood, as it may significantly differ from the bulk composition. Precise knowledge and understanding of resulting surface composition is crucial for effective control of the electrocatalytic performance. In this work, low energy ion scattering spectroscopy was applied to determine the surface oxide composition of a series of Ni-based multi-metallic alloys with Mn, Fe, Co, and Cr under alkaline, neutral and acidic conditions. The composition of the surface oxide was investigated with sub-nanometer depth resolution. In electrochemical tests, good catalytic activity was found for the oxygen evolution reaction, although a strong dependence on the selected reaction conditions was observed. The surface composition under OER conditions deviates significantly from the bulk composition. No significant benefit of high entropy alloying compared with binary or ternary alloys concerning catalytic OER performance was found.

Role of reduced graphene oxide in boosting visible-light-driven photocatalytic activity of BiVO4 nanostructures

Although monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising photocatalyst due to its low band gap (Eg = 2.4-2.6 eV), significant visible light absorption, and its valence band potential is positive enough for water splitting and pollutants degradation, it has some drawbacks hindering its sole usage in photocatalysis. These drawbacks include low surface conductivity, fast electron-hole (e-/h+) pair recombination, low surface area, and low solubility in the aqueous medium. Therefore, m-BiVO4 is composited with reduced graphene oxide (r-GO) to mitigate these drawbacks. r-GO has an extremely large surface area, a high electrical conductivity and can accept and trap electrons from m-BiVO4 via its delocalized conjugated 𝜋-system. Such traps lengthen the electron / hole (e-/h+) pair lifetime on m-BiVO4 increasing the photocatalytic reactions efficiency on its surface. In addition, the presence of oxygen-containing groups on r-GO helps in anchoring m-BiVO4 particles on the r-GO layer so the m-BiVO4 particles are more dispersed and display a larger surface area. These oxygenated groups ease the solubilization of anchored m-BiVO4 particles in water by forming hydrogen bonds. In this mini-review, m-BiVO4–r-GO composite applications in photocatalytic water splitting, pollutants degradation, and other reactions will be briefly discussed. Generally, these composites showed remarkable results in reactions that rely on the valence band holes of m-BiVO4, whereas the reactions that depend on conduction band electrons required morphology and size modification for the m-BiVO4 before its compositing with r-GO.

Advances and challenges in MXene-based electrocatalysts: unlocking the potential for sustainable energy conversion.

MXenes, a novel class of two-dimensional materials, have garnered significant attention for their promising electrocatalytic properties in various energy conversion applications such as water splitting, fuel cells, metal-air batteries, and nitrogen reduction reactions. Their excellent electrical conductivity, high specific surface area, and versatile surface chemistry enable exceptional catalytic performance. This review highlights recent advancements in the design and application strategies of MXenes as electrocatalysts, focusing on key reactions including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and nitrogen reduction reaction (NRR). We discuss the tunability of MXenes' layered structures and surface properties through surface modification, MXene lattice substitution, defect and morphology engineering, and heterostructure construction. Despite the considerable progress, MXenes face challenges such as restacking during catalysis, stability issues, and difficulties in large-scale production. Addressing these challenges through innovative engineering approaches and advancing industrial synthesis techniques is crucial for the broader application of MXene-based materials. Our review underscores the potential of MXenes in transforming electrocatalytic processes and highlights future research directions to optimize their catalytic efficiency and stability.

Two-dimensional direct Z-scheme AlN/GaS-SiP heterojunctions enhance photocatalytic hydrogen production from water: a DFT study.

Photocatalytic water splitting for hydrogen production offers a feasible solution to the problems of energy shortages and environmental pollution. However, its low photocatalytic efficiency limits the application of this technology in real world scenarios. In this study, a two-dimensional AlN/PSi-GaS-I van der Waals heterojunction is constructed and the properties of water photolysis are studied based on first-principles calculations. The results demonstrate that AlN/PSi-GaS-I exhibits exceptional photocatalytic performance with good stability, a narrow bandgap, appropriate band-edge position, a broader light absorption range and efficient separation of photogenerated electron-hole pairs. Moreover, the Gibbs free energies of different intermediates throughout the entire reaction process are calculated based on type-II and Z-scheme reaction mechanisms. By comparing the free energy barriers of the two pathways, it is observed that the Z-scheme reaction pathway has a lower energy barrier. Consequently, it can be concluded that AlN/PSi-GaS-I belongs to the direct Z-scheme heterojunction. These findings suggest that AlN/PSi-GaS-I exhibits an enhanced redox capacity, efficiently driving the water splitting reaction. More excitingly, the AlN/PSi-GaS-I can undergo spontaneous photocatalytic reactions under acidic conditions when provided with adequate optical driving force. This study not only proves that AlN/PSi-GaS-I is a promising high-efficiency photocatalyst for water splitting, but also describes a method for determining direct Z-scheme heterojunctions, which offers theoretical guidance for the design of efficient and stable photocatalysts.

Computational study based prediction of new photocatalysts for water splitting by systematic manipulation of MXene surfaces

Photocatalytic water splitting reactions (HER and OER) on the asymmetric surfaces of Janus MXenes with the help of intrinsic electric field (Ei).

Rational design of 2D Janus P3m1 M2N3 (M = Cu, Zr, and Hf) and their surface-functionalized derivatives: ferromagnetic, piezoelectric, and photocatalytic properties.

In this study, first-principles calculations were employed to rationally design two-dimensional (2D) Janus transition metal nitrides of P3m1 M2N3 phases, where M is a d-block element (Sc-Zn, Y-Cd, Hf-Hg). Among the 29 examined 2D M2N3, three 2D phases, namely P3m1 Cu2N3, Zr2N3, and Hf2N3, exhibit excellent thermodynamic, dynamic, mechanical, and thermal stabilities. These novel Janus 2D materials exhibit ferromagnetic metallic and half-metallic behavior. The related 2D Janus surface-functionalized derivatives, Cu2N3H, Cu2N3F, Cu2N3Cl, Zr2N3H, Hf2N3H, and Hf2N3F, are all dynamically stable. The 2D Janus P3m1 phases of Zr2N3H, Hf2N3H, and Hf2N3F, all with M in the +IV oxidation state, act as semiconductors in the visible region, with energy band gaps of 2.26-2.70 eV at the HSE06 level of theory. On the other hand, the 2D Janus P3m1 Cu2N3X phases (where X = H, F, and Cl) are ferromagnetic half-metals. Additionally, it has been unveiled that there are high hole mobilities (∼6 × 103 cm2 V-1 s-1) derived from the moderate deformation potential and effective mass in the 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases. Uniaxial strain engineering has demonstrated the outstanding in-plane piezoelectric properties of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F with high d11 values (∼99.91 pm V-1). Furthermore, the desirable band-edge alignments and high anisotropic carrier mobilities of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases indicate their potential as visible light-driven photocatalysts for water splitting reactions on different facets. These properties render 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases promising for use in optoelectronics, piezoelectric sensing, and photocatalysis applications.

1,2-Bis-(diphenylphosphino)ethane (dppe) appended cobalt(iii)-dithiocarbamates as single source precursors of cobalt sulfide/cobalt phosphide composites: apt electrocatalytic materials for water splitting reactions

Three new heteroleptic Co(iii)dppe dithiocarbamates were synthesized and employed as single source precursors of cobalt sulfide/cobalt phosphide nanocomposites that were used as electrocatalysts for the OER and HER.

Effect of electrolyte concentration on bubble evolution and mass transfer characteristics on surface of photoelectrode

In the photoelectrochemical water splitting reaction system, bubbles will cover the reaction area on the photoelectrode surface, affecting the reaction impedance and gas-liquid mass transfer. A laser irradiation system is built and it is coupled with an electrochemical workstation and high-speed microscopic imaging system. The evolution behavior and mass transfer characteristics of single O<sub>2</sub> bubble on the TiO<sub>2</sub> photoelectrode are studied at different electrolyte concentrations (Na<sub>2</sub>SO<sub>4</sub>, 0.1–2.0 mol/L). With the increase of electrolyte concentration from 0.1 mol/L to 2.0 mol/L, the solution resistance and bubble additional resistance decrease, and the overpotential in the stable growth stage of bubble decreases from 0.113 V to –0.089 V. The bubble will cause the fluctuation of overpotential in the nucleation, growth and detachment stages, which is consistent with the impedance change caused by the change of dissolved oxygen concentration in the liquid phase. By analyzing the correlation between gas evolution efficiency and bubble coverage, it is found that the increase of electrolyte concentration will lead the bubble coverage and gas evolution efficiency to decrease simultaneously. By calculating the Sherwood dimensionless number, the results show that the total convective mass transfer coefficient increases with the electrolyte concentration increasing. Single-phase natural convection plays a dominant role in the process of gas product transfer, and its mass transfer coefficient is one order of magnitude larger than that of bubble-induced convection. In summary, by adjusting the electrolyte concentration, the bubble on the gas evolution photoelectrode surface can be effectively removed and the mass transfer of the system can be optimized, which is of great significance in improving the efficiency of photoelectrochemical water splitting.

Long-wavelength photoresponsive gallium zinc oxynitride for efficient oxygen evolution and Z-scheme water splitting reactions

Long-wavelength photoresponsive GaN:ZnO efficiently driving the oxygen evolution reaction expands the possibilities for effectively harnessing solar energy through water splitting.

A Cu loaded NiFe layered double hydroxide bifunctional electrocatalyst with a coupled interface structure for both the nitrate reduction reaction and oxygen evolution reaction

We present an interface coupling strategy using Cu nanoclusters and NiFe LDH nanosheets to form a heterostructure electrocatalyst (Cu/NiFe LDH) and apply it as both NO3−RR and OER bifunctional electrodes under ambient conditions.

Insights of water-to-hydrogen conversion from thermodynamics

<p>Water-to-hydrogen can be achieved using a variety of driving energy sources, including thermal, electrical, or photo energy. While methods for hydrogen production in specific energy driving scenarios have been extensively studied, a comprehensive theory to explain the conversion of various energies into hydrogen is still lacking. This study provides a novel exergy-based perspective on hydrogen production methods, revealing that the thermodynamic infeasible water splitting process is derived from insufficient exergy input relative to the reaction exergy requirement. Enhancing the exergy input beyond the reaction exergy requirement can break through chemical equilibrium and enable the reaction to proceed. Providing high exergy-to-energy ratios of energy sources such as electrical, photo, and chemical energy for thermochemical water splitting reactions can reduce the thermal exergy demand for hydrogen production, thus facilitating water-to-hydrogen conversion at lower temperatures. By applying this new insight to coupled photochemical- and thermochemical water splitting reactions, equilibrium conversion rates corresponding to solar spectra with different wavelengths are obtained. The highest water-to-hydrogen conversion rate is achieved by the solar spectrum at a wavelength of about 451nm. The appropriate wavelength region for high water-to-hydrogen conversion is identified. This study also identifies the theoretical conversion limit of photochemical water splitting, providing insights into the potential improvements of current experiments. More importantly, our work offers a unified thermodynamic framework for understanding hydrogen production methods and presents a theoretical basis for reducing reaction temperature and enhancing conversion rate.</p>

Built-in electrophilic/nucleophilic domain of nitrogen-doped carbon nanofiber-confined Ni2P/Ni3N nanoparticles for efficient urea-containing water-splitting reactions.

Transferring urea-containing waste water to clean hydrogen energy has received increasing attention, while challenges are still faced in the sluggish catalytic kinetics of urea oxidation. Herein, a novel hybrid catalyst of Ni2P/Ni3N embedded in nitrogen-doped carbon nanofiber (Ni2P/Ni3N/NCNF) is developed for energy-relevant urea-containing water-splitting reactions. The built-in electrophilic/nucleophilic domain resulting from the electron transfer from Ni2P to Ni3N accelerates the formation of high-valent active Ni species and promotes favourable urea molecule adsorption. A spectral study and theoretical analysis reveal that the negatively shifted Ni d-band centre in Ni2P/Ni3N/NCNF weakens the adsorption of intermediate CO2 and facilitates its desorption, thereby improving the urea oxidation reaction kinetics. The overall reaction process is also optimized by minimizing the energy barrier of the rate-determining step. Following the stability test, the surface reconstruction of the pre-catalyst is discussed, where an amorphous layer of NiOOH as the real active phase is formed on the surface/interface of Ni2P/Ni3N for urea oxidation. Benefiting from these characteristics, a high current density of 151.11 mA cm-2 at 1.54 V vs. RHE is obtained for urea oxidation catalysed by Ni2P/Ni3N/NCNF, exceeding that of most of the similar catalysts. A low cell voltage of 1.39 V is required to reach 10 mA cm-2 for urea electrolysis, which is about 200 mV less than that of the general water electrolysis. The current work will be helpful for the development of advanced catalysts and their application in the urea-containing waste water transfer to clean hydrogen energy.

Unlocking the Potential of NiV-LDH@Mn2O3 Heterostructure via Band gap Modulation for Enhanced Water Splitting

Overall water-splitting reactions provide significant challenges for the rational design and manufacture of electrocatalysts with exceptional capabilities and long-term durability. NiV-LDH is a promising electrocatalyst for OER, however its low...

A first-principles prediction of the structural, electronic, transport and photocatalytic properties of GaGeX3 (X = S, Se, Te) monolayers.

The discovery of new 2D materials with superior properties motivates scientists to make breakthroughs in various applications. In this study, using calculations based on density functional theory (DFT), we have comprehensively investigated the geometrical characteristics and stability of GaGeX3 monolayers (X = S, Se, or Te), determining their electronic and transport properties, and some essential optical and photocatalytic properties. AIMD simulations show that these materials are highly structurally and thermodynamically stable. Notably, the GaGeSe3 monolayer is a semiconductor with a band gap of 1.9 eV and has a high photon absorption coefficient of up to 1.1 × 105 cm-1 in the visible region. The calculated solar-to-hydrogen conversion efficiency of the GaGeSe3 monolayer is 11.33%, which is relatively high compared to some published 2D materials. Furthermore, the electronic conductivity of the GaGeSe3 monolayer is 790.65 cm2 V-1 s-1. Our findings suggest that the GaGeSe3 monolayer is a new promising catalyst for the solar water-splitting reaction to give hydrogen and a potential new 2D material for electrical devices with high electron mobility.

Promoting water-splitting reaction on TiO2/gCN with Pd/SrO cocatalysts: H2 evolution in the absence of a sacrificial reagent

This study investigates Pd–SrO@TiO2/gCN photocatalysts for eco-friendly H2 generation through water splitting.

Electrocatalytic activity of tungsten carbide hybrids with two different MOFs for water splitting: a comparative analysis.

Conventional energy resources are diminishing, and environmental pollution is constantly increasing because of the excessive use of fossil fuels to sustain the ever-increasing population and industrialization. This has raised concerns regarding a sustainable future. In the pursuit of addressing sustainability in industrial processes and energy systems, the production of green hydrogen is considered a promising and crucial solution to meet the growing energy demands. Water-splitting is one of the most effective technologies for producing clean and carbon-neutral hydrogen. Water-splitting is a scientifically emerging application, but it is commercially limited due to its economic non-viability. The sluggish kinetics and the high overpotential needed for the water-splitting reactions (HER and OER) have encouraged the scientific community to design electrocatalysts that address the concerns of low activity, efficiency and stability. Designing a hybrid catalyst using metal-organic frameworks (MOFs) with transition metal carbides can be a suitable approach to address the deficiencies of conventional water-splitting catalysts. In this study, we have designed and fabricated an electrocatalyst of tungsten carbide (WC) with two different MOFs (Zr-based and Fe-based) and explored their electrocatalytic activity for hydrogen generation in an alkaline medium. It should be noted that hybrids of tungsten carbide with a zirconia MOF (UiO-66) showed better electrocatalytic activity with low overpotentials of 104 mV (HER) and 152 mV (OER) at a current density of 10 mA cm-2. This superior activity of WC with the Zr-MOF in comparison to the Fe-MOF is due to the synergistic effect of Zr present in UiO-66 grown on the WC matrix. Moreover, UiO-66 provides a larger electrocatalytic active surface area, so available active sites are more in UiO-66 as compared to the Fe-MOF. These findings set the stage for the systematic development and production of bi-functional hybrid catalysts with the potential to be utilized in water-splitting processes.

Nanostructured p-PbS/p-CuO sulfide/oxide bilayer heterojunction as a promising photoelectrode for hydrogen gas generation

Abstract Recently, the photoelectrochemical (PEC) water-splitting reaction for hydrogen (H2) production has been a competitive research route to realize clean and sustainable electric power. In this work, copper oxide (CuO) and PbS thin films were fabricated on commercial glass, respectively, using the techniques of successive ionic-layer adsorption and reaction and chemical bath deposition. These nanostructured thin films served successfully as photoelectrodes for the photogeneration of H2. In addition, a p-PbS/p-CuO bilayer system was also fabricated, and a remarkable boost in PEC efficiency was observed compared to pure CuO and PbS thin films. Optical examinations showed excellent absorbance properties of the p-PbS/p-CuO bilayer in the visible range, with a bandgap of ∼1.28 eV. X-ray diffraction analysis indicated a monoclinic CuO/cubic PbS crystalline structure with a particle size of ∼18 nm. The photocurrent density (J ph) values were obtained using a three-electrode electrochemical cell in 0.3 M Na2S2O3 electrolyte. The p-PbS/p-CuO photoelectrode demonstrated a J ph value of −0.390 mA cm−2, which is significantly higher than the values of −0.120 and −0.008 mA cm−2 for the pure PbS and CuO photoelectrodes, respectively. This improvement is attributed to the p-PbS/p-CuO oxide/sulfide bilayer heterojunction, which improved the visible light absorption and reduced the electron–hole (e–h) recombination. The effects of pH value, temperature light intensity, and wavelength were all additionally studied. Remarkably, the photoelectrodes were stable under a pH of ∼7, which makes them promising for H2 production using normal drinking/seawater. These findings confirm the ability of the prepared photoelectrodes to facilitate water splitting and H2 generation under various environmental, chemical, and illumination conditions.

Construction of Ni2P-MoC/Coal-Based Carbon Fiber Self-Supporting Catalysts for Enhanced Hydrogen Evolution.

Efficient and inexpensive electrocatalysts play an important role in the hydrogen evolution reaction (HER) of electrolytic water splitting. Herein, Ni2P-MoC/coal-based carbon fiber (Ni2P-MoC/C-CF) self-supporting catalysts were obtained by low-temperature phosphorization and high-temperature carbonization. The Mo source and oxidized coal were uniformly dispersed in the carbon support by electrospinning technology. A precursor of Ni was introduced by the impregnation method. The synergistic effect of MoC and Ni2P may reduce the strong hydrogen adsorption capacity of pure MoC and provide a fast hydrogen release process. In addition, the C-CFs prepared by electrospinning can not only prevent the agglomeration of MoC and Ni2P particles at a high temperature but also provide a self-supporting support for the catalyst. As a result, the catalytic performance of the HER was improved greatly, and a low overpotential of 112 mV at 10 mA cm-2 was exhibited stably by the Ni2P-MoC/C-CFs. This work not only converts coal into coal-based carbon materials but also provides a feasible pathway for the rational design of large-scale molded hydrogen electrocatalysts.

(Invited) Advances in Photon-Harvesting Technologies for Perovskite Absorbers and Water Splitting Reactions

Hybrid halide perovskites have made significant progress in power conversion efficiencies (> 25 %) and high stability achieved through chemical engineering of the AMX3 composition. Compositional engineering of perovskite precursor inks minimizes batch-to-batch variations and fluctuation in PCE values, however the understanding of perovskite inks and solution behavior remains a key step towards the exploitation of solution-processed perovskite devices. Herein, a comprehensive 207Pb nuclear magnetic resonance spectroscopy (NMR) study performed on various lead perovskite precursor inks will be presented. The NMR data provide fresh insights in identifying molecular species that are invariably and predominantly formed in precursor mixtures to coexist with larger networks and colloids. The solution chemistry of PbI2+I- is dominated by the type of connectivity as edge- face- and corner-sharing (PbI6)-octahedra and their dimensionality. The trends in the 207Pb chemical shifts revealed dynamic equilibria in the inks that can be controlled by stoichiometry, nature of the solvents and temperature. Furthermore, perovskite precursor ink development can be applied to fabricate devices beyond the planar device structures, such as direct electrospinning of the three major perovskite solar cell components, namely, photo absorber, hole, and electron transport materials, as continuous single triaxial fibers of μm radial dimensions. These perovskite fibers lay the foundation of materials engineering for fabricating tiny solar cells, which can either be woven into fabrics or incorporated as single fibers to power wearables and a variety of devices or sensors, forming the internet of things. They are also suited for tandem PV-PEC set-up for solar-assisted photoelectrochemical production of hydrogen by water-splitting reactions.