Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction

Unraveling the Synergy of Interfacial Engineering in In Situ Prepared NiO/NdNiO3 for ppb-Level SO2 Sensing: Mechanistic and First-Principles Insights

Interfacial engineering of semiconductor metal oxides offers a plethora of features to overcome the limitations of chemiresistive gas sensors, thereby increasing their practical viability. Herein, the SO2 sensing characteristics of NiO are modulated through the incorporation of NdNiO3, via a facile in situ synthesis of NiO/NdNiO3 nanostructures that significantly enhance the sensor performance. To this end, systematic control of the Nd/Ni molar ratio is employed during the synthesis of NiO/NdNiO3, enabling the regulation of structural properties and interfacial interactions. The optimized NiO/NdNiO3-based sensor demonstrates superior SO2 detection at 140 °C, outperforming pristine NiO, owing to tunable charge carrier dynamics at the heterointerface during gas adsorption. The sensor showcases an extensive dynamic response range from 450 ppb to 200 ppm and an impressive detection limit (320 ppb), along with remarkable selectivity and excellent stability. First-principles calculations reveal NiO and NdNiO3 play distinct roles in SO2 adsorption, with NiO functioning as the receptor, selectively interacting with SO2 through dissociated oxygen, and NdNiO3 serving as the transducer, facilitating signal conversion by inhibiting oxygen dissociation. Additionally, the designed portable, threshold-triggered sensor prototype, integrating the developed NiO/NdNiO3 sensor with enhanced SO2 detection, presents a promising avenue for applications in industrial and environmental monitoring.

Efficient Alkaline Freshwater/Seawater Hydrogen Production via Heterogeneous N-Doped FeMoO4/Mo2N Rod-Shaped Electrocatalysts

Durable and efficient Fe-based electrocatalysts in alkaline freshwater/seawater electrolysis is highly desirable but persists a significant challenge. Herein, we report a durable and robust heterogenous nitrogen-doped FeMoO4/Mo2N rod-shaped catalyst on nickel foam (denoted NF@FMO/MN) affording hydrogen evolution reaction (HER) low overpotentials of 23/29 mV@10 mA cm-2 and 112/159 mV@100 mA cm-2 in both alkaline freshwater/seawater electrolytes, respectively. These results are significantly superior to the pristine FeMoO4 catalyst. Theoretical calculations consistently reveals that the combination of N-FeMoO4 and Mo2N effectively reduces water activation energy barrier, modulates the sluggish water-dissociation kinetics and accelerates the hydrogen adsorption process for efficient HER. The enhanced HER performance of the as-designed NF@FMO/MN catalyst is attributed to the in situ hetero-interfacial engineering between N-doped FeMoO4 and Mo2N. This present work nurtures the progress of FeMo-based electrocatalysts in alkaline freshwater/seawater electrolysis.

Screened d-p Orbital Hybridization in Turing Structure of Confined Nickel for Sulfion Oxidation Accelerated Hydrogen Production

The sulfion oxidation reaction (SOR) could offer an energy-efficient and tech-economically favorable alternative to the oxygen evolution reaction (OER) for H2 production. Transition metal (TM) based catalysts have been considered promising candidates for SOR but suffer from limited activity due to the excessive bond strength from TM-S2- d-p orbit coupling. Herein, we propose a feasible strategy of screening direct d-p orbit hybridization between TM and S2- by constructing the Turing structure composed of lamellar stacking carbon-confined nickel nanosheets. The optimized p-p orbit coupling between electron-injected carbon and S2- enables exceptional catalytic activity and stability for sulfion degradation and energy-efficient yet value-added H2 production. Specifically, it achieves a current density of 500 mA cm-2 at an ultralow potential of 0.67 V vs. RHE for alkaline SOR. Theoretical calculations indicate that the electron transfer from Ni imparts metallicity and a higher p-band center to carbon shells, thereby contributing to optimized p-p orbit hybridization and a thermodynamically favorable stepwise sulfion degradation. Practically, a two-electrode flow cell achieves an industrial current density of 1 A cm-2 at an unprecedented low voltage of 0.91 V while maintaining stability for over 300 hours, and exhibits high productivities of 3.83 and 0.32 kg h-1 m-2 for sulfur and H2, respectively.

Ohmic Contact Heterostructures Immobilized Pt Single Atoms for Boosting Alkaline Hydrogen Evolution Reaction

Pt single-atoms catalysts have been widely confirmed as ideal electrocatalysts for high-efficiency hydrogen evolution reaction (HER), but their activity and durability at high current density remain great challenges, especially in alkaline media. Herein, a unique Ohmic contact heterostructure is fabricated by integrating Ni and NiO to immobilize Pt single-atoms (Ni-NiO-Pt) via Pt-O4 coordination for boosting the alkaline HER. Owing to transient high temperature and pressure in the laser ablation process, Ohmic contact heterojunctions are constructed at the interfaces between metal Ni core and nanoporous semiconducting NiO shell with adequate oxygen vacancies. The large work function difference triggers the electron transfer from Ni to Pt-decorated NiO, which dramatically eliminates the electron conduction impedance and regulates the charge redistribution. Density functional theory calculation unveils that the multiple regulations of energy barrier and charge redistribution on Ohmic contact endow Ni-NiO-Pt with outstanding electrical conductivity and favorable hydrogen binding energy. Consequently, Ni-NiO-Pt displays superior alkaline HER performances with an overpotential of 23.54 mV at 10 mA cm-2 and protruding durability for 75 h at 500 mA cm-2, drastically outperforming commercial Pt/C and most reported HER electrocatalysts. The immobilization of Pt single-atoms on Ohmic contact opens up an avenue toward the rational design of high-efficiency electrocatalysts.

Oxalate-derived porous C-doped NiO with amorphous-crystalline heterophase for supercapacitors

Constructing amorphous/crystalline heterophase structure with high porosity is a promising strategy to effectively tailor the physicochemical properties of electrode materials and further improve the electrochemical performance of supercapacitors. Here, the porous C-doped NiO (C-NiO) with amorphous/crystalline heterophase grown on NF was prepared using NF as Ni source via a self-sacrificial template method. Calcining the self-sacrificial NiC2O4 template at a suitable temperature (400 °C) was beneficial to the formation of porous heterophase structure with abundant cavities and cracks, resulting in high electrical conductivity and rich ion/electron-transport channels. The density functional theory (DFT) calculations further verified that in-situ C-doping could modulate the electronic structure and enhance the OH- adsorption capability. The unique porous amorphous/crystalline heterophase structure greatly accelerated electrons/ions transfer and Faradaic reaction kinetic, which effectively improved the charge storage. The C-NiO calcined at 400 °C (C-NiO(400)) displayed a markedly enhanced specific charge, outstanding rate property and excellent cycling stability. Furthermore, the hybrid supercapacitor assembled by C-NiO(400) and active carbon achieved a high energy density of 49.0 Wh kg-1 at 800 W kg-1 and excellent cycle stability (90.9 % retention at 5 A/g after 10 000 cycles). This work provided a new strategy for designing amorphous/crystalline heterophase electrode materials in high-performance energy storage.

Deciphering the Electronic Coupling Dynamics of Laser-induced Ru/Cu Electrocatalyst for Dual-Side Hydrogen Production and Formic Acid Co-synthesis via DFT Analysis

Herein, a straightforward approach using pulsed laser technology to synthesize selective hexagonal-close-packed (hcp) Ru nanoparticles attached to Cu nanospheres (Ru/Cu) as bifunctional electrocatalyst for catalyzing the hydrogen evolution reaction (HER) and formaldehyde oxidation reaction (FOR) are reported. Initially, Ru-doped CuO flakes are synthesized using a coprecipitation method followed by transformation into Ru/Cu composites through a strategy involving pulsed laser irradiation in liquid. Specifically, the optimized Ru/Cu-4 composite not only demonstrates a low overpotential of 182 mV at 10 mA·cm-2 for the HER but also an ultralow working potential of 0.078 V (versus reversible hydrogen electrode) for the FOR at the same current density. Remarkably, the FOR∥HER-coupled electrolyzer employing the Ru/Cu-4∥Ru/Cu-4 system achieves H2 production at both electrodes with a cell voltage of 0.42 V at 10 mA·cm-2 while co-synthesizing formic acid. Furthermore, density functional theory analyses elucidate that the superior activity of the Ru/Cu composite originates from optimized adsorption energies of reactive species on the catalyst surfaces during the HER and FOR, facilitated by the synergistic coupling between Ru and Cu. This study presents an alternative strategy for synthesizing highly effective electrocatalytic materials for use in energy-efficient H2 production with the cosynthesis of value-added chemicals suitable for practical applications.

Interfacial Engineering of Nickel Oxide-Perovskite Interface with Amino Acid Complexed NiO to Improve Perovskite Solar Cell Performance

The interface between NiO and perovskite in inverted perovskite solar cells (PSCs) is a major factor that can limit device performance due to defects and inappropriate redox reactions, which cause nonradiative recombination and decrease in open-circuit voltage (VOC). In the present study, a novel approach is used for the first time, where an amino acid (glycine (Gly), alanine (Ala), and aminobutyric acid (ABA))-complexed NiO are used as interface modifiers to eliminate defect sites and hydroxyl groups from the surface of NiO. The Ala-complexed NiO suppresses interfacial non-radiative recombination, improves the perovskite layer quality and better energy band alignment with the perovskite, resulting in improved charge transfer and reduced recombination. The incorporation of the Ala-complexed NiO leads to a PCE of 20.27% with enhanced stability under the conditions of ambient air, light soaking, and heating to 85 °C, as it retains over 82%, 85%, and 61% of its initial PCE after 1000, 500, and 350 h, respectively. The low-temperature technique also leads to the fabrication of a NiO thin film that is suitable for flexible PSCs. The Ala-complexed NiO is fabricated on the flexible substrate and achieved 17.12% efficiency while retaining 71% of initial PCE after 5,000 bending.

Superwetting Nanofluids of NiOx-Nanocrystals/CsBr Solution for Fabricating Quality NiOx-CsPbBr3 Gradient Hybrid Film in Carbon-Based Perovskite Solar Cells

The wettability of precursor solution on substrates is the critical factor for fabricating quality film. In this work, superwetting nanofluids (NFs) of non-stoichiometric nickel oxide (NiOx) nanocrystals (NCs)-CsBr solution are first utilized to fabricate quality NiOx-CsPbBr3 hybrid film with gradient-distributed NiOx NCs in the upper part for constructing hole transport ladder in carbon-based perovskite solar cells (C-PSCs). As anticipated, the crystalline properties (improved crystalline grain diameters and reduced impurity phase) and hole extraction/transport of the NiOx-CsPbBr3 hybrid film are improved after incorporating NiOx NCs into CsPbBr3. This originates from the superb wettability of NiOx-CsBr NFs on substrates and the excellent hole-transport properties of NiOx. Consequently, the C-PSCs with the structure of FTO/SnO2/NiOx-CsPbBr3/C displays a power conversion efficiency of 10.07%, resulting in a 23.6% improvement as compared with the pristine CsPbBr3 cell. This work opens up a promising strategy to improve the absorber layer in PSCs by incorporating NCs into perovskite layers through the use of the superwettability of NFs and by composition gradient engineering.

Molybdenum oxide/nickel molybdenum oxide heterostructures hybridized active platinum co-catalyst toward superb-efficiency water splitting catalysis

A new catalyst has been developed that utilizes molybdenum oxide (MoO3)/nickel molybdenum oxide (NiMoO4) heterostructured nanorods coupled with Pt ultrafine nanoparticles for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) toward industrial-grade water splitting. This catalyst has been synthesized using a versatile approach and has shown to perform better than noble-metals catalysts, such as Pt/C and RuO2, at industrial-grade current level (≥1000 mA·cm-2). When used simultaneously as a cathode and anode, the proposed material yields 10 mA·cm-2 at a remarkably small cell voltage of 1.55 V and has shown extraordinary durability for over 50 h. Density functional theory (DFT) calculations have proved that the combination of MoO3 and NiMoO4 creates a metallic heterostructure with outstanding charge transfer ability. The DFT calculations have also shown that the excellent chemical coupling effect between the MoO3/NiMoO4 and Pt synergistically optimize the charge transfer capability and Gibbs free energies of intermediate species, leading to remarkably speeding up the reaction kinetics of water electrolysis.

Surficial Reconstruction of Pt-Ni/NiS and Its Effect on Electrocatalytic Hydrogen Evolution in Alkaline Medium

Cyclic voltammetry pretreatment of Pt-based electrocatalysts has been proven to be a normal activation process on achieving the optimal alkaline hydrogen evolution performance. Until now, the congruent relationship between the microstructural evolution and performance improvement during this process has rarely been reported. Herein, when the in situ transmission electron microscopy and in situ Raman analyses are employed, a self-reconstruction process from crystalline NiS into amorphous nickel hydroxide hydrate [Ni(OH)2-x·H2O, where x ≈ 0.3] on the surface of platinum-nickel nanowires has first been captured, which is the critical water dissociation active site to offer a sufficient proton supply. Furthermore, such a surficial reconstruction triggers an increase in the current density from -2.3 to -38.8 mA/cm2 (at -70 mV), which is nearly 17 times. These observations point to the fact that it is essential to consider the fundamental mechanisms of hydrogen evolution on the active sites when the process is scaled up.

Boosting Efficient Alkaline Hydrogen Evolution Reaction of CoFe-Layered Double Hydroxides Nanosheets via Co-Coordination Mechanism of W-Doping and Oxygen Defect Engineering

While surface defects and heteroatom doping exhibit promising potential in augmenting the electrocatalytic hydrogen evolution reaction (HER), their performance remains unable to rival that of the costly Pt-based catalysts. Yet, the concurrent modification of catalysts by integrating both approaches stands as a promising strategy to effectively address the aforementioned limitation. In this work, tungsten dopants are introduced into self-supported CoFe-layered double hydroxides (LDH) on nickel foam using a hydrothermal method, and oxygen vacancies (Ov) are further introduced through calcination. The analysis results demonstrated that tungsten doping reduces the Ov formation energy of CoFeW-LDH. The Ov acted as oxophilic sites, facilitating water adsorption and dissociation, and reducing the barrier for cleaving HO─H bonds from 0.64 to 0.14 eV. Additionally, Ov regulated the electronic structure of CoFeW-LDH to endow optimized hydrogen binding ability on tungsten atoms, thereby accelerating alkaline Volmer and Heyrovsky reaction kinetics. Specifically, the abundance of Ov induced a transition of tungsten from a six-coordinated to highly active four-coordinated structure, which becomes the active site for HER. Consequently, an ultra-low overpotential of 41 mV at 10 mA cm-2, and a low Tafel slope of 35 mV dec-1 are achieved. These findings offer crucial insights for the design of efficient HER electrocatalysts.

Unveiling the Synergistic Role of Frustrated Lewis Pairs in Carbon-Encapsulated Ni/NiOx Photothermal Cocatalyst for Enhanced Photocatalytic Hydrogen Production

The development of high-density and closely spaced frustrated Lewis pairs (FLPs) is crucial for enhancing catalyst activity and accelerating reaction rates. However, constructing efficient FLPs by breaking classical Lewis bonds poses a significant challenge. Here, this work has made a pivotal discovery regarding the Jahn-Teller effect during the formation of grain boundaries in carbon-encapsulated Ni/NiOx (Ni/NiOx@C). This effect facilitates the formation of high-density O (VO) and Ni (VNi) vacancy sites with different charge polarities, specifically FLP-VO-C basic sites and FLP-VNi-C acidic sites. The synergistic interaction between FLP-VO-C and FLP-VNi-C sites not only reduces energy barriers for water adsorption and splitting, but also induces a strong photothermal effect. This mutually reinforcing effect contributes to the exceptional performance of Ni/NiOx@C as a cocatalyst in photothermal-assisted photocatalytic hydrogen production. Notably, the Ni/NiOx@C/g-C3N4 (NOCC) composite photocatalyst exhibits remarkable hydrogen production activity with a rate of 10.7 mmol g-1 h-1, surpassing that of the Pt cocatalyst by 1.76 times. Moreover, the NOCC achieves an impressive apparent quantum yield of 40.78% at a wavelength of 380 nm. This work paves the way for designing novel defect-state multiphase cocatalysts with high-density and adjacent FLP sites, which hold promise for enhancing various catalytic reactions.

Platelike carbon-encapsulated nickel nanocrystals for efficient electrooxidation of 5-hydroxymethylfurfural

Platelike carbon-encapsulated nickel nanocrystals (Ni@C) were engineered as a high-performance electrocatalyst for the conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). This electrocatalyst demonstrated remarkable electrocatalytic performance in oxidizing HMF at a low potential, achieving 100% HMF conversion, 97.7% FDCA yield, and 97.4% Faraday efficiency (FE).

Revealing the role of a bridging oxygen in a carbon shell coated Ni interface for enhanced alkaline hydrogen oxidation reaction

Encapsulating metal nanoparticles inside carbon layers is a promising approach to simultaneously improving the activity and stability of electrocatalysts. The role of carbon layer shells, however, is not fully understood. Herein, we report a study of boron doped carbon layers coated on nickel nanoparticles (Ni@BC), which were used as a model catalyst to understand the role of a bridging oxygen in a carbon shell coated Ni interface for the improvement of the hydrogen oxidation reaction (HOR) activity using an alkaline electrolyte. Combining experimental results and density functional theory (DFT) calculations, we find that the electronic structure of Ni can be precisely tailored by Ni-O-C and Ni-O-B coordinated environments, leading to a volcano type correlation between the binding ability of the OH* adsorbate and HOR activity. The obtained Ni@BC with a optimized d-band center displays a remarkable HOR performance with a mass activity of 34.91 mA mgNi-1, as well as superior stability and CO tolerance. The findings reported in this work not only highlight the role of the OH* binding strength in alkaline HOR but also provide guidelines for the rational design of advanced carbon layers used to coat metal electrocatalysts.

Interfacial engineering of POM-stabilized Ni quantum dots on porous titanium mesh for high-rate and stable alkaline hydrogen production

The development of low-cost, high-efficiency, and stable electrocatalysts for the alkaline hydrogen evolution reaction (HER) is a key challenge because the alkaline HER kinetics is slowed by an additional water dissociation step. Herein, we report an interfacial engineering strategy for polyoxometalate (POM)-stabilized nickel (Ni) quantum dots decorated on the surface of porous titanium mesh (POMs-Ni@PTM) for high-rate and stable alkaline hydrogen production. Benefiting from the strong interfacial interactions among POMs, Ni atoms, and PTM substrates, as well as unique POM-Ni quantum dot structures, the optimized POMs-Ni@PTM electrocatalyst exhibits a remarkable alkaline HER performance with an overpotential (η10) of 30.1 mV to reach a current density of 10 mA cm-2, which is much better than those of bare Ni decorated porous titanium mesh (Ni@PTM) (η10 = 171.1 mV) and POM decorated porous titanium mesh (POMs@PTM) electrocatalysts (η10 = 493.6 mV), comparable to that of the commercial 20 wt% platinum/carbon (20% Pt/C) electrocatalyst (η10 = 20 mV). Moreover, the optimized POMs-Ni@PTM electrocatalyst demonstrates excellent stability under continuous alkaline water-splitting at a current density of ∼100 mA cm-2 for 100 h, demonstrating great potential for its practical application.

Metal Electrocatalysts for Hydrogen Production in Water Splitting

The rising demand for fossil fuels and the resulting pollution have raised environmental concerns about energy production. Undoubtedly, hydrogen is the best candidate for producing clean and sustainable energy now and in the future. Water splitting is a promising and efficient process for hydrogen production, where catalysts play a key role in the hydrogen evolution reaction (HER). HER electrocatalysis can be well performed by Pt with a low overpotential close to zero and a Tafel slope of about 30 mV dec-1. However, the main challenge in expanding the hydrogen production process is using efficient and inexpensive catalysts. Due to electrocatalytic activity and electrochemical stability, transition metal compounds are the best options for HER electrocatalysts. This study will focus on analyzing the current situation and recent advances in the design and development of nanostructured electrocatalysts for noble and non-noble metals in HER electrocatalysis. In general, strategies including doping, crystallization control, structural engineering, carbon nanomaterials, and increasing active sites by changing morphology are helpful to improve HER performance. Finally, the challenges and future perspectives in designing functional and stable electrocatalysts for HER in efficient hydrogen production from water-splitting electrolysis will be described.

Self-supporting hierarchical Co3O4-nanowires@NiO-nanosheets core-shell nanostructure on carbon foam to form efficient bifunctional electrocatalyst for overall water splitting

Designing cost-effective and robust bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desired in hydrogen production from overall water splitting, but still suffers great challenges due to the sluggish catalytic OER/HER kinetics. In this paper, a surface/defect engineering strategy is developed to synthesize three-dimensional (3D) carbon foam (CF)-supported unique hierarchical Co3O4 nanowires@NiO nanosheets core-shell nanostructured catalyst (NiO@Co3O4/CF) with rich oxygen-vacancies as a novel bifunctional catalyst for alkaline water splitting electrolysis. Benefited from the synergy of the 3D hierarchical core-shell structure and rich oxygen vacancies, the as-obtained NiO@Co3O4/CF shows both excellent OER (η200 = 325 mV, η500 = 374 mV) and HER (η10 = 104 mV) activities with low Tafel slopes (64.26 mV dec-1 for OER and 109.14 mV dec-1 for HER, respectively) and outstanding stability. A simple overall water splitting electrolysis cell assembled by this bifunctional NiO@Co3O4/CF as both OER and HER catalysts requires only a cell voltage of 1.53 V to obtain the current density of 10 mA cm-2 and displays a long-term stability. This work has successfully developed an approach for rational design and novel synthesis of metal oxide hybrids as bifunctional electrocatalysts with high activity and stability for overall water splitting.

Engineering the Crystal Facet of Monoclinic NiO for Efficient Catalytic Ozonation of Toluene

Modulating oxygen vacancies of catalysts through crystal facet engineering is an innovative strategy for boosting the activity for ozonation of catalytic volatile organic compounds (VOCs). In this work, three kinds of facet-engineered monoclinic NiO catalysts were successfully prepared and utilized for catalytic toluene ozonation (CTO). Density functional theory calculations revealed that Ni vacancies were more likely to form preferentially than O vacancies on the (110), (100), and (111) facets of monoclinic NiO due to the stronger Ni-vacancy formation ability, further affecting O-vacancy formation. Extensive characterizations demonstrated that Ni vacancies significantly promoted the formation of O vacancies and thus reactive oxygen species in the (111) facet of monoclinic NiO, among the three facets. The performance evaluation showed that the monoclinic NiO catalyst with a dominant (111) facet exhibits excellent performance for CTO, achieving a toluene conversion of ∼100% at 30 °C after reaction for 120 min under 30 ppm toluene, 210 ppm ozone, 45% relative humidity, and a space velocity of 120 000 h-1. This outperformed the previously reported noble/non-noble metal oxide catalysts used for CTO at room temperature. This study provided novel insight into the development of highly efficient facet-engineered catalysts for the elimination of catalytic VOCs.

Ni-CeO2 Heterostructure Promotes Hydrogen Evolution Reaction via Tuning of the O-H Bond Length of Adsorbed Water at the Electrolyte/Electrode Interface

Understanding the properties and structure of reactant water molecules at the electrolyte solution/electrode interface is relevant to know the mechanisms of hydrogen evolution reaction (HER). However, this approach has rarely been implemented due to the elusive local microenvironment in the vicinity of the catalyst. Taking the Ni-CeO2 heterostructure immobilized onto carbon paper (Ni-CeO2 /CP) as a model, the dynamic behavior of adsorbed intermediates during the reaction was measured by in situ surface-enhanced infrared absorption spectroscopy with attenuated total reflection configuration (ATR-SEIRAS). Theoretical calculations are used in combination to comprehend the potential causes of increased HER activity. The results show that the O-H bond of adsorbed water at the electrolyte solution/electrode interface becomes longer for promoting the dissociation of water and accelerating the kinetically slow Volmer step. In addition, forming the Ni-CeO2 heterostructure interface optimizes the hydrogen adsorption Gibbs free energy, thus increasing HER activity. Therefore, the Ni-CeO2 /CP electrode exhibits remarkably low HER overpotentials of 37 and 119 mV at 10 and 100 mA cm-2 , which are close to commercial Pt/C (16 and 102.6 mV, respectively).

Risk factors for death in hospitalized COVID-19 patients in Africa: A systematic review and meta-analysis

BACKGROUND

The coronavirus disease 2019 has quickly spread worldwide since it first appeared in Wuhan, China, in late 2019. The most affected country in Africa was South Africa. This study aimed to identify the risk factors for death in hospitalized COVID-19 patients in Africa.

METHODS

We conducted a systematic review following the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines. We searched articles from the following database: PubMed, Embase, Cochrane Library, Medline, and COVID-19 Research Database. We used Google Scholar for gray literature. The language used in this article was English. The last search was conducted on January 15, 2023. Pooled HRs, or ORs, and 95% confidence intervals, were calculated separately to identify the risk factors for death in hospitalized COVID-19 patients. Heterogeneity was assessed by Cochran's Q statistic and the I2 test. The Egger test was used to assess publication bias. Subgroup analysis was performed to determine the source of heterogeneity. Data analysis was performed using Stata version 17. A P value < .05 was considered significant.

RESULTS

A total of 16,600 articles were obtained from the database search; finally, 16 articles met the inclusion criteria and were eligible for data extraction. The analysis revealed that the pooled prevalence of mortality in hospitalized COVID-19 patients was 13.9%. Advanced age was a significant risk factor for death in hospitalized COVID-19 patients, with the pooled coronavirus mortality HR and OR being 3.73 (95% CI: 2.27-5.19) and 1.04 (95% CI: 1.02-1.06), respectively. In addition, male gender (pOR 1.23; 95% CI: 1.07-1.40), patients with diabetes mellitus (DM) (pOR 1.26; 95% CI: 1.01-1.51), hypertension (HTN) (pOR 1.56; 95% CI: 1.27-1.85), chronic kidney disease (CKD) (pHR 5.43; 95% CI: 0.18-10.67), severe or critical conditions (pOR 9.04; 95% CI: 3.14-14.94) had a significantly increased risk of coronavirus-related mortality. The main limitations of the present study stem from the predominant use of published studies, which could introduce publication bias.

CONCLUSION

According to this study, advanced age, male gender, hypertension, diabetes mellitus, chronic kidney disease, and severe or critical condition were clinical risk factors associated with death outcomes in hospitalized COVID-19 patients in Africa.

Facile synthesis of efficient Co3O4 nanostructures using the milky sap of Calotropis procera for oxygen evolution reactions and supercapacitor applications

The preparation of Co3O4 nanostructures by a green method has been rapidly increasing owing to its promising aspects, such as facileness, atom economy, low cost, scale-up synthesis, environmental friendliness, and minimal use of hazardous chemicals. In this study, we report on the synthesis of Co3O4 nanostructures using the milky sap of Calotropis procera (CP) by a low-temperature aqueous chemical growth method. The milky sap of CP-mediated Co3O4 nanostructures were investigated for oxygen evolution reactions (OERs) and supercapacitor applications. The structure and shape characterizations were done by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) techniques. The prepared Co3O4 nanostructures showed a heterogeneous morphology consisting of nanoparticles and large micro clusters. A typical cubic phase and a spinel structure of Co3O4 nanostructures were also observed. The OER result was obtained at a low overpotential of 250 mV at 10 mA cm-2 and a low Tafel slope of 53 mV dec-1. In addition, the durability of 45 hours was also found at 20 mA cm-2. The newly prepared Co3O4 nanostructures using the milky sap of CP were also used to demonstrate a high specific capacitance of 700 F g-1 at a current density of 0.8 A g-1 and a power density of 30 W h kg-1. The enhanced electrochemical performance of Co3O4 nanostructures prepared using the milky sap of CP could be attributed to the surface oxygen vacancies, a relatively high amount of Co2+, the reduction in the optical band gap and the fast charge transfer rate. These surface, structural, and optical properties were induced by reducing, capping, and stabilizing agents from the milky sap of CP. The obtained results of OERs and supercapacitor applications strongly recommend the use of the milky sap of CP for the synthesis of diverse efficient nanostructured materials in a specific application, particularly in energy conversion and storage devices.

Rational Design of Cu-Doped Tetrahedron of Spinel Oxide for High-Performance Nitric Oxide Electrochemical Sensor

The real-time detection of nitric oxide (NO) in living cells is essential to reveal its physiological processes. However, the popular electrochemical detection strategy is limited to the utilization of noble metals. The development of new detection candidates without noble metal species still maintaining excellent catalytic performance has become a big challenge. Herein, we propose a spinel oxide doped with heteroatom-Cu-doped Co₃O₄ (Cu-Co₃O₄) for the sensitive and selective detection of NO release from the living cells. The material is strategically designed with Cu occupying the tetrahedral (T_d) center of Co₃O₄ through the formation of a Cu-O bond. The introduced Cu regulates the local coordination environment and optimizes the electronic structure of Co₃O₄, hybridizing with the N 2p orbital to enhance charge transfer. The Cu_Td site can well inhibit the current response to nitrite (NO₂^-), resulting in a high improvement in the electrochemical oxidation of NO. The selectivity of Cu-Co₃O₄ can be markedly improved by the pore size of the molecular sieve and the negative charge on the surface. The rapid transmission of electrons is due to the fact that Cu-Co₃O₄ can be uniformly and densely in situ grown on Ti foil. The rationally designed Cu-Co₃O₄ sensor displays excellent catalytic activity toward NO oxidation with a low limit of detection of 2.0 nM (S/N = 3) and high sensitivity of 1.9 μA nM^-1 cm^-2 in cell culture medium. The Cu-Co₃O₄ sensor also shows good biocompatibility to monitor the real-time NO release from living cells (human umbilical vein endothelial cells: HUVECs; macrophage: RAW 264.7 cells). It was found that a remarkable response to NO was obtained in different living cells when stimulated by l-arginine (l-Arg). Moreover, the developed biosensor could be used for real-time monitoring of NO released from macrophages polarized to a M1/M2 phenotype. This cheap and convenient doping strategy shows universality and can be used for sensor design of other Cu-doped transition metal materials. The Cu-Co₃O₄ sensor presents an excellent example through the design of proper materials to implement unique sensing requirements and sheds light on the promising strategy for electrochemical sensor fabrication.

Unveiling Hierarchical Dendritic Co3O4-SnO2 Heterostructure for Efficient Water Purification

The construction of a desirable, environmentally friendly, and cost-effective nanoheterostructure photoanode to treat refractory organics is critical and challenging. Herein, we unveiled a hierarchical dendritic Co₃O₄-SnO₂ heterostructure via a sequential hydrothermal process. The time of the secondary hydrothermal process can control the size of the ultrathin SnO₂ nanosheets on the basis of the Ostwald solidification mass conservation principle. Ti/Co₃O₄-SnO₂-168h with critical growth size demonstrated a photoelectrocatalysis degradation rate of ∼93.3% for a high dye concentrate of 90 mg/L with acceptable long-term cyclability and durability over reported Co₃O₄-based electrodes because of the large electrochemically active area, low charge transfer resistance, and high photocurrent intensity. To gain insight into the photoelectric synergy, we proposed a type-II heterojunction between Co₃O₄ and SnO₂, which prevents photogenerated carriers' recombination and improves the generation of dominant active species •O₂^-, ¹O₂, and h⁺. This work uncovered the Ti/Co₃O₄-SnO₂-168 as a promising catalyst and provided a simple and inexpensive assembly strategy to obtain binary integrated nanohybrids with targeted functionalities.

Synergistic Effect of N-NiMoO4 /Ni Heterogeneous Interface with Oxygen Vacancies in N-NiMoO4 /Ni/CNTs for Superior Overall Water Splitting

The exploring of economical, high-efficiency, and stable bifunctional catalysts for hydrogen evolution and oxygen evolution reactions (HER/OER) is highly imperative for the development of electrolytic water. Herein, a 3D cross-linked carbon nanotube supported oxygen vacancy (V_o )-rich N-NiMoO₄ /Ni heterostructure bifunctional water splitting catalyst (N-NiMoO₄ /Ni/CNTs) is synthesized by hydrothermal-H₂ calcination method. Physical characterization confirms that V_o -rich N-NiMoO₄ /Ni nanoparticles with an average size of ≈19 nm are secondary aggregated on CNTs that form a hierarchical porous structure. The formation of Ni and NiMoO₄ heterojunctions modify the electronic structure of N-NiMoO₄ /Ni/CNTs. Benefiting from these properties, N-NiMoO₄ /Ni/CNTs drives an impressive HER overpotential of only 46 mV and OER overpotential of 330 mV at 10 mA cm^-2 , which also shows exceptional cycling stability, respectively. Furthermore, the as-assembled N-NiMoO₄ /Ni/CNTs||N-NiMoO₄ /Ni/CNTs electrolyzer reaches a cell voltage of 1.64 V at 10 mA cm^-2 in alkaline solution. Operando Raman analysis reveals that surface reconstruction is essential for the improved catalytic activity. Density functional theory (DFT) calculations further demonstrate that the enhanced HER/OER performance should be attributed to the synergistic effect of V_o and heteostructure that improve the conductivity of N-NiMoO₄ /Ni/CNTs and facilitatethe desorption of reaction intermediates.

DFT with corrections for an efficient and accurate description of strong electron correlations in NiO

An efficient and accurate description of the electronic structure of a strongly correlated metal-oxide semiconductor like NiO has been notoriously difficult. Here, we study the capabilities and limitations of two frequently employed correction schemes, a DFT+Uon-site correction and a DFT+1/2 self-energy correction. While both methods individually are unable to provide satisfactory results, in combination they provide a very good description of all relevant physical quantities. Since both methods cope with different shortcomings of common density-functional theory (DFT) methods (using local-density or generalized-gradient approximations), their combination is not mutually dependent and remains broadly applicable. The combined approach retains the computational efficiency of DFT calculations while providing significantly improved predictive power.

Ensemble Effect of Ruthenium Single-Atom and Nanoparticle Catalysts for Efficient Hydrogen Evolution in Neutral Media

Hydrogen evolution reaction (HER) plays a key role in electrochemical water splitting, which is a sustainable way for hydrogen production. The kinetics of HER is sluggish in neutral media that requires noble metal catalysts to alleviate energy consumption during the HER process. Here, we present a catalyst comprising a ruthenium single atom (Ru₁) and nanoparticle (Ru_n) loaded on the nitrogen-doped carbon substrate (Ru₁-Ru_n/CN), which exhibits excellent activity and superior durability for neutral HER. Benefiting from the synergistic effect between single atoms and nanoparticles in the Ru₁-Ru_n/CN, the catalyst exhibits a very low overpotential down to 32 mV at a current density of 10 mA cm^-2 while maintaining excellent stability up to 700 h at a current density of 20 mA cm^-2 during the long-term test. Computational calculations reveal that, in the Ru₁-Ru_n/CN catalyst, the existence of Ru nanoparticles affects the interactions between Ru single-atom sites and reactants and thus improves the catalytic activity of HER. This work highlights the ensemble effect of electrocatalysts for HER and could shed light on the rational design of efficient catalysts for other multistep electrochemical reactions.

Modulation of Lewis and Brønsted Acidic Sites to Enhance the Redox Ability of Nb2O5 Photoanodes for Efficient Photoelectrochemical Performance

Accelerated surface redox reaction and regulated carrier separation are the crux to the development of highly reactive oxide semiconductors for efficient photoelectrochemical water splitting. Here, we have selected Nb₂O₅ materials that combine unique surface acidity and semiconductor properties, and first used surface phosphorylation to change its surface acidic sites (Lewis and Brønsted acidic sites) to achieve efficient photoelectrochemical water splitting. The resulting photoanode born from this strategy exhibits a high photocurrent density of 0.348 mA/cm² at 1.23 V_RHE, which is about 2-fold higher than that of the bare Nb₂O₅, and a cathodic shift of 60 mV. Detailed experimental results show that the large increase in the Lewis acidic site can effectively modulate the electronic structure of the active sites involved in catalysis in [NbO₅] polyhedra and promote the activation of lattice oxygen. As a result, higher redox properties and the ability to inhibit carrier recombination are exhibited. In addition, the weakening of the Brønsted acidic site drives the reduction of protons in the oxygen evolution reaction (OER) and accelerates the reaction kinetics. This work advances the development of efficient photoelectrochemical water splitting on photoanodes driven by the effective use of surface acidity and provides a strategy for enhancing redox capacity to achieve highly active photoanodes.

Electronic redistribution over the active sites of NiWO4-NiO induces collegial enhancement in hydrogen evolution reaction in alkaline medium

The activity-enhancement of a new-generation catalyst focuses on the collegial approach among specific solids which exploit the mutual coactions of these materials for HER applications. Strategic manipulation of these solid interfaces typically reveals unique electronic states different from their pure phases, thus, providing a potential passage to create catalysts with excellent activity and stability. Herein, the formation of the NiWO₄-NiO interface has been designed and synthesized via a three-step method. This strategy enhances the chance of the formation of abundant heterointerfaces due to the fine distribution of NiWO₄ nanoparticles over Ni(OH)₂ sheets. NiWO₄-NiO has superior HER activity in an alkaline (1 M KOH) electrolyte with modest overpotentials of 68 mV at 10 mA cm^-2 current density. The catalyst is highly stable in an alkaline medium and negligible change was observed in the current density even after 100 h of continuous operation. This study explores a unique method for high-performance hydrogen generation by constructing transition metal-oxides heterojunction. The XPS studies reveal an electronic redistribution driven by charge transfer through the NiWO₄-NiO interface. The density functional theory (DFT) calculations show that the NiWO₄-NiO exhibits a Pt-like activity with the hydrogen Gibbs free energy (ΔG_H*) value of 0.06 eV compared to the Pt(ΔG_H* = -0.02 eV).

In Situ Reconstruction NiO Octahedral Active Sites for Promoting Electrocatalytic Oxygen Evolution of Nickel Phosphate

Electrochemical activation strategy is very effective to improve the intrinsic catalytic activity of metal phosphate toward the sluggish oxygen evolution reaction (OER) for water electrolysis. However, it is still challenging to operando trace the activated reconstruction and corresponding electrocatalytic dynamic mechanisms. Herein, a constant voltage activation strategy is adopted to in situ activate Ni₂ P₄ O₁₂ , in which the break of NiONi bond and dissolution of PO₄ ^3- groups could optimize the lattice oxygen, thus reconstructing an irreversible amorphous Ni(OH)₂ layer with a thickness of 1.5-3.5 nm on the surface of Ni₂ P₄ O₁₂ . The heterostructure electrocatalyst can afford an excellent OER activity in alkaline media with an overpotential of 216.5 mV at 27.0 mA cm^-2 . Operando X-ray absorption fine structure spectroscopy analysis and density functional theory simulations indicate that the heterostructure follows a nonconcerted proton-electron transfer mechanism for OER. This activation strategy demonstrates universality and can be used to the surface reconstruction of other metal phosphates.

Hydrothermally Grown Dual-Phase Heterogeneous Electrocatalysts for Highly Efficient Rechargeable Metal-Air Batteries with Long-Term Stability

Metal-air batteries as alternatives to the existing lithium-ion battery are becoming increasingly attractive sources of power due to their high energy-cost competitiveness and inherent safety; however, their low oxygen evolution and reduction reaction (OER/ORR) performance and poor operational stability must be overcome prior to commercialization. Herein, it is demonstrated that a novel class of hydrothermally grown dual-phase heterogeneous electrocatalysts, in which silver-manganese (AgMn) heterometal nanoparticles are anchored on top of 2D nanosheet-like nickel vanadium oxide (NiV2 O6 ), allows an enlarged surface area and efficient charge transfer/redistribution, resulting in a bifunctional OER/ORR superior to those of conventional Pt/C or RuO2 . The dual-phase NiV2 O6 /AgMn catalysts on the air cathode of a zinc-air battery lead to a stable discharge-charge voltage gap of 0.83 V at 50 mA cm-2 , with a specific capacity of 660 mAh g-1 and life cycle stabilities of more than 146 h at 10 mA cm-2 and 11 h at 50 mA cm-2 . The proposed new class of dual-phase NiV2 O6 /AgMn catalysts are successfully applied as pouch-type zinc-air batteries with long-term stability over 33.9 h at 10 mA cm-2 .

Homogenous Cr and C Doped 3D Self-Supporting NiO Cellular Nanospheres for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is one promising technique to obtain high-purity hydrogen, therefore, exploiting inexpensive and high-efficiency HER electrocatalysts is a matter of cardinal significance under the background of achieving carbon neutrality. In this paper, a hydrothermal method was used to prepare the Cr-NiC₂O₄/NF (Ni foam) precursor. Then, the NiO-Cr-C/NF self-supporting HER catalyst was obtained by heating the precursor at 400 °C. The catalyst presents a 3D cellular nanospheres structure which was composed of 2D nanosheets. Microstructure characterization shows that Cr and C elements were successfully doped into NiO. The results of electrochemical measurements and density functional theory (DFT) calculations show that under the synergy of Cr and C, the conductivity of NiO was improved, and the Gibbs free energy of H* (∆GH*) value is optimized. As a result, in 1.0 M KOH solution the NiO-Cr-C/NF-3 (Ni:Cr = 7:3) HER catalyst exhibits an overpotential of 69 mV and a Tafel slope of 45 mV/dec when the current density is 10 mA·cm^-2. Besides, after 20 h of chronopotentiometry, the catalytic activity is basically unchanged. It is demonstrated that C and Cr co-doping on the lattice of NiO prepared by a simple hydrothermal method and subsequent heat treatment to improve the catalytic activity and stability of the non-precious metal HER catalysts in an alkaline medium is facile and efficient.

Positively Charged Pt-Based Nanoreactor for Efficient and Stable Hydrogen Evolution

Positively charged Pt can work as the active center for hydrogen evolution reaction (HER) but the corresponding design of state-of-the-art electrocatalysts at high current densities has never been realized. Here the application of positively charged Pt in an effective Fe-PtNiPO nanoreactor for highly efficient and stable HER is demonstrated. Synchrotron radiation X-ray absorption spectroscopy confirms the formation of internal positively charged Pt and the in situ experiments reveal the quick charge transfer in the nanoreactor. Ni-based materials around Pt are used to tune the electronic structure and promote the water dissociation to form locally enriched H⁺ , while a porous Fe shell can both prevent the loss of active material and allow the efficient material transport. All the beneficial compositions work together to form an effective nanoreactor for HER. As a result, the Fe-PtNiPO nanoreactor shows a low overpotential of 19 mV to achieve 10 mA cm^-2 and exhibits a high mass activity of 10.93 A mg_Pt ^-1 (at 100 mV). Most importantly, it only needs an ultra-low overpotential of 193 mV to achieve a high current density of 1000 mA cm^-2 with an excellent stability over 300 h, which represents one of the best electrocatalysts for alkaline HER and might be used for large-scale industrial application in the future.

Versatile Bifunctional and Supported IrNi Oxide Catalyst for Photoelectrochemical Water Splitting

Designing a high-performance electrocatalyst that operates with photon-level energy is of the utmost importance in order to address the world’s urgent energy concerns. Herein, we report IrNi nanoparticles uniformly distributed on cost-effective activated carbon support with a low mass loading of 3% by weight to drive the overall water splitting reaction under light illumination over a wide pH range. The prepared IrNi nanomaterials were extensively characterized by SEM/EDX, TEM, XRD, Raman, and UV-visible absorption spectroscopy. The experimental results demonstrate that when the Ir:Ni ratio is 4:1, the water splitting rate is high at 32 and 25 mA cm−2 for hydrogen (at −1.16 V) and oxygen evolution reactions (at 1.8 V) in alkaline electrolyte, respectively, upon the light irradiation (100 mW cm−2). The physical and electrochemical characterization of metal and alloy combinations show that the cumulative effect of relatively high crystallinity (among the materials used in this study), reduced charge recombination rate, and improved oxygen vacancies observed with the 4Ir1Ni@AC electrode is the reason for the superior activity obtained. A high level of durability for hydrogen and oxygen evolution under light illumination is seen in the chronoamperometric study over 15 h of operation. Overall water splitting examined in 0.1 M of NaOH medium at a 50 mV s−1 scan rate showed a cell voltage of 1.94 V at a 10 mA cm−2 current density.

ZIF-67-Derived (NiCo)S2@NC Nanosheet Arrays Hybrid for Efficient Overall Water Splitting

Electrocatalytic water splitting is considered a promising approach to obtain clean and sustainable hydrogen energy. The integration of optimal nanoarchitecture and multicomponent synergy has been a significant factor for designing a bifunctional electrocatalyst to promote the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). In particular, the charge migration, mass transfer, and gas release rate in the catalyzing process are closely correlated with the architecture of the catalyst. Here, ZIF-67-derived N-doped carbon nanofiber-supported (NiCo)S₂ nanosheet [(NiCo)S₂/NCNF] as a bifunctional electrocatalyst was synthesized using electrospinning, template etching, and subsequent gas sulfidation method. The hierarchical hybrid nanofiber with inner hollow cubes and outer nanosheets provides easy electron penetration, high charge/mass transportation efficiency, and robust structure stability. Furthermore, the MOF-derived carbon-encapsuled bimetal-sulfide and the synergistic effect of double active centers are conducive to an exceptional performance, showing low overpotentials of 177 and 203 mV to drive a current density of 10 mA cm^-2 and robust stability for the HER and OER, respectively. Meanwhile, the (NiCo)S₂/NCNF electrodes exhibit a small voltage of 1.61 V for overall water splitting activity with an electrolyzer cell at current densities of 10 mA cm^-2 over 12 h. This work presents novel insights into the bifunctional catalyst for promoting the overall water splitting via a MOF-derived nanoarchitecture and multicomponent synergy.

Optimizing electronic structure of porous Ni/MoO2 heterostructure to boost alkaline hydrogen evolution reaction

Heterostructure engineering is an efficient strategy to synergisticallyimprove electrocatalytic activity. In this work, Ni/MoO₂ heterojunction nanorods with porous structure self-supported on nickel foam (NF) are elaborately designed through a facile solution-evaporationmethod followed by a thermal reduction process. Prominently, the optimal electrocatalyst Ni/MoO₂@NF-E delivers an exceptionally low overpotential of 19 mV at the current density of 10 mA cm^-2 and a small Tafel slope of 52.3 mV dec^-1 toward the hydrogen evolution reaction (HER) in alkaline solution. Concurrently, Ni/MoO₂@NF-E also maintains excellent stability after 120 h of electrolysis or 5000 cyclic voltammetry cycles. The experimental and density functional theory (DFT) results indicate that the enhanced HER performance of Ni/MoO₂@NF-E should be ascribed to the porous structure in the Ni/MoO₂ nanorods providing more active catalytic site, the moderate Gibbs free energy of hydrogen adsorption (ΔG_H*), as well as strong synergistic effect between Ni and MoO₂. This work provides an efficient route for developing HER electrocatalysts in alkaline media.

Bixbyite-type Ln2O3 as promoters of metallic Ni for alkaline electrocatalytic hydrogen evolution

The active-site density, intrinsic activity, and durability of Ni-based catalysts are critical to their application in industrial alkaline water electrolysis. This work develops a kind of promoters, the bixbyite-type lanthanide metal sesquioxides (Ln₂O₃), which can be implanted into metallic Ni by selective high-temperature reduction to achieve highly efficient Ni/Ln₂O₃ hybrid electrocatalysts toward hydrogen evolution reaction. The screened Ni/Yb₂O₃ catalyst shows the low overpotential (20.0 mV at 10 mA cm⁻²), low Tafel slope (44.6 mV dec⁻¹), and excellent long-term durability (360 h at 500 mA cm⁻²), significantly outperforming the metallic Ni and benchmark Pt/C catalysts. The remarkable hydrogen evolution activity and stability of Ni/Yb₂O₃ are attributed to that the Yb₂O₃ promoter with high oxophilicity and thermodynamic stability can greatly enlarge the active-site density, reduce the energy barrier of water dissociation, optimize the free energy of hydrogen adsorption, and avoid the oxidation corrosion of Ni.

While renewable H₂ evolution will require inexpensive, abundant catalysts, non-noble metals typically show relatively low activities. Here, authors examine lanthanide metal sesquioxide doped metallic Ni and show efficient, stable performances for alkaline H₂ evolution electrocatalysis.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Synergistic coupling of heterostructured porous CoP nanosheets with P doped NiO for highly efficient overall alkaline water splitting

Exploring non-noble metal materials as bifunctional catalysts for water electrolysis is of great significance for the development and utilization of hydrogen energy. Herein, a flower branch-leaf shaped phosphide/oxide heterogeneous electrocatalyst located on Ni foam (CoP/P-NiO/NF) was developed through hydrothermal and phosphorization strategy. Benefiting from the strong ability to dissociate H₂O molecules on P-NiO and the suitable adsorption of intermediate H species on CoP, the optimal CoP/P-NiO/NF exhibited outstanding performance with low overpotentials of 52 mV at current density of 10 mA cm^-2, smaller Tafel slopes of 73.6 mV dec^-1 for hydrogen evolution reaction (HER). Meanwhile, CoP/P-NiO/NF indicated 265 mV at 100 mA cm^-2 with Tafel slope of 101.8 mV dec^-1 for oxygen evolution reaction (OER) due to the optimal redistribution of electrons among Ni²⁺, Co²⁺ and Co³⁺ for favorable adsorption/desorption of oxygen-intermediates. Both HER and OER shown robust stability during 32 h without decline. The corresponding two-electrode system for overall alkaline water splitting required a low voltage of 1.6 V at 100 mA cm^-2 with long stability (20 h) which is far lower than that on RuO₂-Pt/C and many other reported non-noble metal electrocatalysts. This work demonstrates that the synergistic effect and morphology engineering play vital roles in the enhanced electrocatalytic performance.

Systematic Engineering on Ni-Based Nanocatalysts Effectively Promote Hydrogen Evolution Reaction

Designing a synthesis of ultra-small Ni-based nanomaterials with high intrinsic activity and stability in alkaline hydrogen evolution reaction (HER) is a major challenge. Herein, a series of noble metal doped ultra-small size (4 nm) M-Ni/NiO nanoparticles supported on CNT are rationally designed by a solvent-free microwave reduction method that is fast (60 s), simple, includes no surfactants, extensive (>1 g), and has high yield (82.7%). The Ir-Ni/NiO@CNT has superior performance with a low overpotential of 24.6 mV at 10 mA cm^-2 . In addition, the turnover frequency (TOF) value up to 2.51 s^-1 and the exchange current density reaches 4.34 mA cm^-2 , indicating that the catalyst has better intrinsic catalytic activity. It is further proved by density functional theory (DFT) that the NiO surface is conducive to the adsorption of OH* in the Volmer step while the Ni is inclined to adsorb H*, which synergistically promotes the water-splitting reaction, thereby increasing the catalytic rate of HER. It is believed that this work will provide valuable contributions and inspirations toward the large-scale production of high-performance Ni-based electrocatalysts for HER.

Electrocatalytic Water Splitting: From Harsh and Mild Conditions to Natural Seawater

Electrocatalytic water splitting is regarded as the most effective pathway to generate green energy-hydrogen-which is considered as one of the most promising clean energy solutions to the world's energy crisis and climate change mitigation. Although electrocatalytic water splitting has been proposed for decades, large-scale industrial hydrogen production is hindered by high electricity cost, capital investment, and electrolysis media. Harsh conditions (strong acid/alkaline) are widely used in electrocatalytic mechanism studies, and excellent catalytic activities and efficiencies have been achieved. However, the practical application of electrocatalytic water splitting in harsh conditions encounters several obstacles, such as corrosion issues, catalyst stability, and membrane technical difficulties. Thus, the research on water splitting in mild conditions (neutral/near neutral), even in natural seawater, has aroused increasing attention. However, the mechanism in mild conditions or natural seawater is not clear. Herein, different conditions in electrocatalytic water splitting are reviewed and the effects and proposed mechanisms in the three conditions are summarized. Then, a comparison of the reaction process and the effects of the ions in different electrolytes are presented. Finally, the challenges and opportunities associated with direct electrocatalytic natural seawater splitting and the perspective are presented to promote the progress of hydrogen production by water splitting.

Synthesis of Uranium Single Atom from Radioactive Wastewater for Enhanced Water Dissociation and Hydrogen Evolution

As a common nuclide in radioactive wastewater, uranium (U) is generally treated by landfill, which induces the massive abandonment of uranium resources. In this work, a pulse voltammetry method for the synthesis of U single atoms on MoS₂ (U/MoS₂ ) nanosheets from radioactive wastewater for the electrocatalytic alkaline hydrogen evolution reaction (HER) is reported. The mass loading of U single atoms is facilely controlled with high selectivity for coexisting ions in radioactive wastewater. In the electrolyte of 1 m of KOH, U/MoS₂ nanosheets with 5.2% of U single atoms exhibit relatively low overpotentials of 72 mV at 10 mA cm^-2 . The mechanistic study reveals that the HER on U/MoS₂ includes the water dissociation on U single atoms to form OH* and H transfer from OH* to adjacent S-edge atoms. This procedure exhibits decreased activation energy for transition state in water dissociation and optimized Gibbs free energy for H* adsorption.