Dense Crystalline/Amorphous Phosphides/Oxides Interfacial Sites for Enhanced Industrial-Level Large Current Density Seawater Oxidation

Anchoring Ni(OH)2-CeOx Heterostructure on FeOOH-Modified Nickel-Mesh for Efficient Alkaline Water-Splitting Performance with Improved Stability under Quasi-Industrial Conditions

Developing low-cost and industrially viable electrode materials for efficient water-splitting performance and constructing intrinsically active materials with abundant active sites is still challenging. In this study, a self-supported porous network Ni(OH)2-CeOx heterostructure layer on a FeOOH-modified Ni-mesh (NiCe/Fe@NM) electrode is successfully prepared by a facile, scalable two-electrode electrodeposition strategy for overall alkaline water splitting. The optimized NiCe0.05/Fe@NM catalyst reaches a current density of 100 mA cm-2 at an overpotential of 163 and 262 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1.0 m KOH with excellent stability. Additionally, NiCe0.05/Fe@NM demonstrates exceptional HER performance in alkaline seawater, requiring only 148 mV overpotential at 100 mA cm-2. Under real water splitting conditions, NiCe0.05/Fe@NM requires only 1.701 V to achieve 100 mA cm-2 with robust stability over 1000 h in an alkaline medium. The remarkable water-splitting performance and stability of the NiCe0.05/Fe@NM catalyst result from a synergistic combination of factors, including well-optimized surface and electronic structures facilitated by an optimal Ce ratio, rapid reaction kinetics, a superhydrophilic/superaerophobic interface, and enhanced intrinsic catalytic activity. This study presents a simple two-electrode electrodeposition method for the scalable production of self-supported electrocatalysts, paving the way for their practical application in industrial water-splitting processes.

Strong Interface Coupling Enables Stability of Amorphous Meta-Stable State in CoS/Ni3S2 for Efficient Oxygen Evolution

Rational design of heterostructure catalysts through phase engineering strategy plays a critical role in heightening the electrocatalytic performance of catalysts. Herein, a novel amorphous/crystalline (a/c) heterostructure (a-CoS/Ni3S2) is manufactured by a facile hydrothermal sulfurization method. Strikingly, the interface coupling between amorphous phase (a-CoS) and crystalline phase (Ni3S2) in a-CoS/Ni3S2 is much stronger than that between crystalline phase (c-CoS) and crystalline phase (Ni3S2) in crystalline/crystalline (c/c) heterostructure (c-CoS/Ni3S2) as control sample, which makes the meta-stable amorphous structure more stable. Meanwhile, a-CoS/Ni3S2 has more S vacancies (Sv) than c-CoS/Ni3S2 because of the presence of an amorphous phase. Eventually, for the oxygen evolution reaction (OER), the a-CoS/Ni3S2 exhibits a significantly lower overpotential of 192 mV at 10 mA cm-2 compared to the c-CoS/Ni3S2 (242 mV). An exceptionally low cell voltage of 1.51 V is required to achieve a current density of 50 mA cm-2 for overall water splitting in the assembled cell (a-CoS/Ni3S2 || Pt/C). Theoretical calculations reveal that more charges transfer from a-CoS to Ni3S2 in a-CoS/Ni3S2 than in c-CoS/Ni3S2, which promotes the enhancement of OER activity. This work will bring into play a fabrication strategy of a/c catalysts and the understanding of the catalytic mechanism of a/c heterostructures.

Constructing Amorphous-Crystalline Interfacial Bifunctional Site Island-Sea Synergy by Morphology Engineering Boosts Alkaline Seawater Hydrogen Evolution

The development of efficient and durable non-precious hydrogen evolution reaction (HER) catalysts for scaling up alkaline water/seawater electrolysis is highly desirable but challenging. Amorphous-crystalline (A-C) heterostructures have garnered attention due to their unusual atomic arrangements at hetero-interfaces, highly exposed active sites, and excellent stability. Here, a heterogeneous synthesis strategy for constructing A-C non-homogeneous interfacial centers of electrocatalysts on nanocages is presented. Isolated PdCo clusters on nanoscale islands in conjunction with Co3S4 A-C, functioning as a bifunctional site "island-sea" synergy, enable the dynamic confinement design of metal active atoms, resulting in excellent HER catalytic activity and durability. The hierarchical structure of hollow porous nanocages and nanoclusters, along with their large surface area and multi-dimensional A-C boundaries and defects, provides the catalyst with abundant active centers. Theoretical calculations demonstrate that the combination of PdCo and Co3S4 regulates the redistribution of interface electrons effectively, promoting the sluggish water-dissociation kinetics at the cluster Co sites. Additionally, PdCo-Co3S4 heterostructure nanocages exhibit outstanding HER activity in alkaline seawater and long-term stability for 100 h, which can be powered by commercial silicon solar cells. This finding significantly advances the development of alkaline seawater electrolysis for large-scale hydrogen production.

Treasure-bowl style bifunctional site in cerium-tungsten hetero-clusters for superior solar-driven hydrogen production

Electrochemical water splitting powered by renewable energy sources hold potential for clean hydrogen production. However, there is still persistent challenges such as low solar-to-hydrogen conversion efficiency and sluggish oxygen evolution reactions. Here, we address the poor kinetics by studying and strengthening the coupling between Ce and W, and concurrently establishing Ce-W bi-atomic clusters on P,N-doped carbon (WN/WC-CeO2-x@PNC) with a "treasure-bowl" style. The bifunctional active sites are established using a novel and effective self-sacrificial strategy involving in situ induced defect formation. In addition, by altering the coupling of the W(d)-N(p) and W(d)-Ce(f) orbitals in the WN/WC-CeO2-x supramolecular clusters, we are able to disrupt the linear relationship between the binding energies of reaction intermediates, a key to obtain high catalytic performance for transition metals. Through the confinement of the WN/WC-CeO2-x composite hetero-clusters within the sub-nanometre spaces of hollow nano-bowl-shaped carbon reactors, a stable and efficient hydrogen production via water electrolysis could be achieved. When assembled together with a solar GaAs triple junction solar cell, a solar-to-hydrogen conversion efficiency of 18.92% in alkaline media could be realized. We show that the key to establish noble metal free catalysts with high efficiency lies in the fine-tuning of the metal-metal interface, forming regions with near optimal adsorption energies for the reaction intermediates participating in water electrolysis.

Real Active Site Identification of Co/Co3O4 Anchoring Ni-MOF Nanosheets with Fast OER Kinetics for Overall Water Splitting

Doping metals and constructing heterostructures are pivotal strategies to enhance the electrocatalytic activity of metal-organic frameworks (MOFs). Nevertheless, effectively designing MOF-based catalysts that incorporate both doping and multiphase interfaces poses a significant challenge. In this study, a one-step Co-doped and Co3O4-modified Ni-MOF catalyst (named Ni NDC-Co/CP) with a thickness of approximately 5.0 nm was synthesized by a solvothermal-assisted etching growth strategy. Studies indicate that the formation of the Co-O-Ni-O-Co bond in Ni NDC-Co/CP was found to facilitate charge density redistribution more effectively than the Co-O-Ni bimetallic synergistic effect in NiCo NDC/CP. The designating Ni NDC-Co/CP achieved superior oxygen evolution reaction (OER) activity (245 mV @ 10 mA cm-2) and robust long stability (100 h @ 100 mA cm-2) in 1.0 M KOH. Furthermore, the Ni NDC-Co/CP(+)||Pt/C/CP(-) displays pregnant overall water splitting performance, achieving a current density of 10 mA cm-2 at an ultralow voltage of 1.52 V, which is significantly lower than that of commercial electrolyzer using Pt/C and IrO2 electrode materials. In situ Raman spectroscopy elucidated the transformation of Ni NDC-Co to Ni(Co)OOH under an electric field. This study introduces a novel approach for the rational design of MOF-based OER electrocatalysts.

Laser-Induced Preparation of Anderson-Type Polyoxometalate-Derived Sulfide/Oxide Electrocatalysts for Electrochemical Water Oxidation

Developing cost-effective and high-active electrocatalysts is vital to enhance the electrocatalytic performance for oxygen evolution reaction (OER). However, traditional pyrolysis methods require complicated procedures, exact temperatures, and long reaction times, leading to high costs and low yields of electrocatalysts in potential industrial applications. Herein, a rapid and economic laser-induced preparation strategy is proposed to synthesize three bimetallic sulfide/oxide composites (MMoOS, M=Fe, Co, and Ni) on a nickel foam (NF) substrate. A focused CO2 laser with high energy is applied to decompose Anderson-type polyoxometalate (POM)-based precursors, enabling the creation of abundant heteropore and defective structures in the MMoOS composites that have multi-components of MS/Mo4O11/MoS2. Remarkably, owing to the structural interactions between the active species, FeMoOS shows superior electrocatalytic performance for OER in an alkaline medium, exhibiting a low overpotential of 240 mV at 50 mA cm-2, a small Tafel slope of 79 mV dec-1, and good durability for 80 h. Physical characterizations after OER imply that partially dissolved Mo-based species and new-formed NiO/NiOOH can effectively uncover abundant active sites, fasten charge transfer, and modify defective structures. This work provides a rapid laser-induced irradiation method for the synthesis of POM-derived nanocomposites as promoted electrocatalysts.

Recent progress in hierarchical nanostructures for Ni-based industrial-level OER catalysts

Exploring efficient and low-cost oxygen evolution reaction (OER) electrocatalysts reaching the industrial level current density is crucial for hydrogen production via water electrolysis. In this feature article, we summarize the recent progress in hierarchical nanostructures for the industrial-level OER. The contents mainly concern (i) the design of a hierarchical structure; (ii) a Ni-based hierarchical structure for the industrial current density OER; and (iii) the surface reconstruction of the hierarchical structure during the OER process. The work provides valuable guidance and insights for the manufacture of hierarchical nanomaterials and devices for industrial applications.

Microzone-Acidification-Driven Degradation Mechanism of the NiFe-Based Anode in Seawater Electrolysis

The anode stability is critical for efficient and reliable seawater electrolyzers. Herein, a NiFe-based film catalyst was prepared by anodic oxidation to serve as a model electrode, which exhibited a satisfactory oxygen evolution performance in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) with an overpotential of 348 mV at 100 mA cm-2 and a long-term stability of over 100 h. After that, the effects of the current density and bulk pH of the electrolyte on its stability were evaluated. It was found that the electrode stability was sensitive to electrolysis conditions, failing at 20 mA cm-2 in 0.1 M KOH + 0.5 M NaCl but over 500 mA cm-2 in 0.5 M KOH + 0.5 M NaCl. The electrode dissolved, and some precipitates immediately formed at the region very close to the electrode surface during the electrolysis. This can be ascribed to the pH difference between the electrode/electrolyte interface and the bulk electrolyte under anodic polarization. In other words, the microzone acidification accelerates the corrosion of the electrode by Cl-, thus affecting the electrode stability. The operational performances of the electrode under different electrolysis conditions were classified to further analyze the degradation behavior, which resulted in three regions corresponding to the stable oxygen evolution, violent dissolution-precipitation, and complete passivation processes, respectively. Thereby increasing the bulk pH could alleviate the microzone acidification and improve the stability of the anode at high current densities. Overall, this study provides new insights into understanding the degradation mechanism of NiFe-based catalysts and offers electrolyte engineering strategies for the application of anodes.

Interfacial Se-O Bonds Modulating Spatial Charge Distribution in FeSe2/Nb:Fe2O3 with Rapid Hole Extraction for Efficient Photoelectrochemical Water Oxidation

Surface engineering is an effective strategy to improve the photoelectrochemical (PEC) catalytic activity of hematite, and the defect states with abundant coordinative unsaturation atoms can serve as anchoring sites for constructing intimate connections between semiconductors. On this basis, we anchored an ultrathin FeSe2 layer on Nb5+-doped Fe2O3 (FeSe2/Nb:Fe2O3) via interfacial Se-O chemical bonds to tune the surface potential. Density functional theory (DFT) calculations indicate that amorphous FeSe2 decoration could generate electron delocalization over the composite photoanodes so that the electron mobility was improved to a large extent. Furthermore, electrons could be transferred via the newly formed Se-O bonds at the interface and holes were collected at the surface of electrode for PEC water oxidation. The desired charge redistribution is in favor of suppressing charge recombination and extracting effective holes. Later, work function calculations and Mott-Schottky (M-S) plots demonstrate that a type-II heterojunction was formed in FeSe2/Nb:Fe2O3, which further expedited carrier separation. Except for spatial carrier modulation, the amorphous FeSe2 layer also provided abundant active sites for intermediates adsorption according to the d band center results. In consequence, the target photoanodes attained an improved photocurrent density of 2.42 mA cm-2 at 1.23 V versus the reversible hydrogen electrode (RHE), 2.5 times as that of the bare Fe2O3. This study proposed a defect-anchoring method to grow a close-connected layer via interfacial chemical bonds and revealed the spatial charge distribution effects of FeSe2 on Nb:Fe2O3, giving insights into rational designation in composite photoanodes.

Materials Design and System Innovation for Direct and Indirect Seawater Electrolysis

Green hydrogen production from renewably powered water electrolysis is considered as an ideal approach to decarbonizing the energy and industry sectors. Given the high-cost supply of ultra-high-purity water, as well as the mismatched distribution of water sources and renewable energies, combining seawater electrolysis with coastal solar/offshore wind power is attracting increasing interest for large-scale green hydrogen production. However, various impurities in seawater lead to corrosive and toxic halides, hydroxide precipitation, and physical blocking, which will significantly degrade catalysts, electrodes, and membranes, thus shortening the stable service life of electrolyzers. To accelerate the development of seawater electrolysis, it is crucial to widen the working potential gap between oxygen evolution and chlorine evolution reactions and develop flexible and highly efficient seawater purification technologies. In this review, we comprehensively discuss present challenges, research efforts, and design principles for direct/indirect seawater electrolysis from the aspects of materials engineering and system innovation. Further opportunities in developing efficient and stable catalysts, advanced membranes, and integrated electrolyzers are highlighted for green hydrogen production from both seawater and low-grade water sources.