Construction of Polarized Carbon-Nickel Catalytic Surfaces for Potent, Durable, and Economic Hydrogen Evolution Reactions

Constructing Asymmetric Charge Polarized NiCo Prussian Blue Analogue for Promoted Electrocatalytic Methanol to Formate Conversion

The highly selective electrochemical conversion of methanol to formate is of great significance for various clean energy devices, but understanding the structure-to-property relationship remains unclear. Here, the asymmetric charge polarized NiCo prussian blue analogue (NiCo PBA-100) is reported to exhibit remarkable catalytic performance with high current density (210 mA cm-2 @1.65 V vs RHE) and Faraday efficiency (over 90%). Meanwhile, the hybrid water splitting and Zinc-methanol-battery assembled by NiCo PBA-100 display the promoted performance with decent stability. X-ray absorption spectroscopy (XAS) and operando Raman spectroscopy indicate that the asymmetric charge polarization in NiCo PBA leads to more unoccupied states of Ni and occupied states of Co, thereby facilitating the rapid transformation of the high-active catalytic centers. Density functional theory calculations combining operando Fourier transform infrared spectroscopy demonstrate that the final reconstructed catalyst derived by NiCo PBA-100 exhibits rearranged d band properties along with a lowered energy barrier of the rate-determining step and favors the desired formate production.

A self-supported porous NiMo electrocatalyst to boost the catalytic activity in the hydrogen evolution reaction

To develop hydrogen energy production and address the issues of global warming, inexpensive, effective, and long-lasting transition metal-based electrocatalysts for the synthesis of hydrogen are crucial. Herein, a porous electrocatalyst NiMo/Ni/NF was successfully constructed by a two-step electrodeposition process, and was used in the hydrogen evolution reaction (HER) of electrocatalytic water decomposition. NiMo nanoparticles were coated on porous Ni/NF grown on nickel foam (NF), leading to a resilient porous structure with enhanced conductivity for efficient charge transfer, as well as distinctive three-dimensional channels for quick electrolyte diffusion and gas release. Notably, the low overpotential (42 mV) and fast kinetics (Tafel slope of 44 mV dec-1) at a current density of 10 mA cm-2 in 1.0 M KOH solution demonstrate the excellent HER activity of the electrode, which was superior to that of recently reported non-noble metal-based catalysts. Additionally, NiMo/Ni/NF showed extraordinary catalytic durability in stability tests at a current density of 10 mA cm-2 for 70 h. The porous structure catalyst and the electrodeposition-electrocatalysis technique examined in this study offer new approaches for the advancement of the electrocatalysis field because of these benefits.

Interfacial π-p Electron Coupling Prompts Hydrogen Evolution Reaction Activity in Acidic Electrolyte

The thermodynamically stable 2H-phase MoS2 is a brilliant material toward hydrogen evolution reaction (HER) owing to its excellent Gibbs free energy of hydrogen adsorption. Nevertheless, the poor intrinsic properties of 2H-MoS2 limit its electrocatalytic performances toward HER. In this work, graphitic carbon nitride covalently bridging 2H-MoS2 (MoS2/GCN) is proposed to construct robust HER electrocatalysts. The strong π-p electron coupling between the delocalized π electrons of GCN and the localized p electrons of S atoms sufficiently expose active sites and accelerate the reaction kinetics. To be specific, MoS2/GCN exhibits remarkable HER activity (160 mV at 10 mA·cm-2) and long-term durability. Importantly, MoS2/GCN also provides great potential for industrial application. Density functional theory (DFT) calculations disclose that the π-p electron coupling at the MoS2/GCN interface regulates the electronic structure of S atoms, consequently providing enhanced HER performance. This work presents a feasible pathway to develop advanced electrocatalysts for energy conversions.

Climbing the Hydrogen Evolution Volcano with a NiTi Shape Memory Alloy

Alkaline water electrolysis is a sustainable way to produce green hydrogen using renewable electricity. Even though the rates of the cathodic hydrogen evolution reaction (HER) are 2-3 orders of magnitude less under alkaline conditions than under acidic conditions, the possibility of using non-precious metal catalysts makes alkaline HER appealing. We identify a novel and facile route for substantially improving HER performance via the use of commercially available NiTi shape memory alloys, which upon heating undergo a phase transformation from the monoclinic martensite to the cubic austenite structure. While the room-temperature performance is modest, austenitic NiTi outperforms Pt (which is the state-of-the-art HER electrocatalyst) in terms of current density by ≤50% at 80 °C. Surface ensembles presented by the austenite phase are computed with density functional theory to bind hydrogen more weakly than either metallic Ni or Ti and to have binding energies ideally suited for HER.

Emerging materials and technologies for electrocatalytic seawater splitting

The limited availability of freshwater in renewable energy-rich areas has led to the exploration of seawater electrolysis for green hydrogen production. However, the complex composition of seawater presents substantial challenges such as electrode corrosion and electrolyzer failure, calling into question the technological and economic feasibility of direct seawater splitting. Despite many efforts, a comprehensive overview and analysis of seawater electrolysis, including electrochemical fundamentals, materials, and technologies of recent breakthroughs, is still lacking. In this review, we systematically examine recent advances in electrocatalytic seawater splitting and critically evaluate the obstacles to optimizing water supply, materials, and devices for stable hydrogen production from seawater. We demonstrate that robust materials and innovative technologies, especially selective catalysts and high-performance devices, are critical for efficient seawater electrolysis. We then outline and discuss future directions that could advance the techno-economic feasibility of this emerging field, providing a roadmap toward the design and commercialization of materials that can enable efficient, cost-effective, and sustainable seawater electrolysis.

Emerging Trends of Carbon-Based Quantum Dots: Nanoarchitectonics and Applications

Carbon-based quantum dots (QDs) have emerged as a fascinating class of advanced materials with a unique combination of optoelectronic, biocompatible, and catalytic characteristics, apt for a plethora of applications ranging from electronic to photoelectrochemical devices. Recent research works have established carbon-based QDs for those frontline applications through improvements in materials design, processing, and device stability. This review broadly presents the recent progress in the synthesis of carbon-based QDs, including carbon QDs, graphene QDs, graphitic carbon nitride QDs and their heterostructures, as well as their salient applications. The synthesis methods of carbon-based QDs are first introduced, followed by an extensive discussion of the dependence of the device performance on the intrinsic properties and nanostructures of carbon-based QDs, aiming to present the general strategies for device designing with optimal performance. Furthermore, diverse applications of carbon-based QDs are presented, with an emphasis on the relationship between band alignment, charge transfer, and performance improvement. Among the applications discussed in this review, much focus is given to photo and electrocatalytic, energy storage and conversion, and bioapplications, which pose a grand challenge for rational materials and device designs. Finally, a summary is presented, and existing challenges and future directions are elaborated.

A combined experimental and theoretical study of RuO2/TiO2 heterostructures as a photoelectrocatalyst for hydrogen evolution

We report a joint experimental and theoretical study of RuO₂/TiO₂ heterostructures. In the experimental section, mesoporous RuO₂/TiO₂ heterostructures were prepared by impregnation of mesoporous TiO₂ nanoparticles which were synthesized from a new precursor, Na₂[Ti(C₂O₄)₃], in an aqueous solution of ruthenium(III) chloride followed by calcination at 300 °C. Using various techniques, the prepared TiO₂ and RuO₂/TiO₂ heterostructures were extensively characterized. The photoelectocatalytic application of the as-prepared heterostructures was then investigated toward the hydrogen evolution reaction (HER). The results illustrated that RuO₂ is dispersed uniformly on the TiO₂ surface. The loading of RuO₂ on TiO₂ decreases the band gap energy and extends the absorption edge to the visible light region. This wide absorption extends the photoelectrocatalytic activity of RuO₂/TiO₂ heterostructures. To obtain a deeper understanding of the increase of the photoelectrocatalytic activity of RuO₂/TiO₂ heterostructures compared to pure TiO₂, theoretical calculations at the density functional theory (DFT) level were performed on some model clusters of pure TiO₂ and the RuO₂/TiO₂ heterostructure. The theoretical results elucidated that the recombination ratio of electron-hole pairs decreases effectively for RuO₂/TiO₂ compared to pure TiO₂.

Nickel-Vanadium-Manganese Trimetallic Nitride as Energy Saving, Efficient Bifunctional Electrocatalyst for Alkaline Water Splitting via Urea Electrocatalysis

Hydrogen production through pure water electrolysis is often found less economic as it requires high potential for water oxidation. The presence of urea in water involving effective urea oxidation can be considered as an effective strategy to produce hydrogen economically. Herein, we develop trimetallic nickel vanadium manganese nitride porous microspheres as an efficient bifunctional electrocatalyst for both urea oxidation reaction (UOR) as well as hydrogen evolution reaction (HER) mechanisms. The optimized NiVMn nitride exhibits eye-catching UOR activity along with HER activity that required only 1.36 and -0.253 V electrode potentials, respectively, to achieve a high current density of 100 mA cm^-2. Combining its bifunctional activity in UOR and HER in a two-electrode system, an energy saving by 0.26 V potential compared to water electrolysis through water oxidation can be acquired to reach 50 mA cm^-2 current density. The presence of manganese(II) has a significant influence in stabilizing high valence V(V) and Ni(II), offering large number of active sites, and during UOR, the effective electronic transitions are more between Mn → Ni rather than Mn → V, leading to excellent and stable UOR performance. Indeed, the electrocatalyst and the approach offering considerable energy saving phenomena are believed to make hydrogen production more economic and sustainable.

Biomimicry-inspired fish scale-like Ni3N/FeNi3N/NF superhydrophilic/superaerophobic nanoarrays displaying high electrocatalytic performance

The mass transfer efficiency and structural stability of the electrode are critical for industrialized water electrolysis operations. Herein, the biomimicry-inspired design of Ni₃N/FeNi₃N/NF nanoarrays with a fish scale-like structure, which endowed the Ni₃N/FeNi₃N/NF nanoarrays with rapid infiltration of aqueous solution within 60 ms and 169° bubble contact angle, is demonstrated. The optimal Ni₃N/FeNi₃N/NF sample displayed catalytic activity with hydrogen evolution reaction (HER) overpotentials of only 48 mV at 10 mA cm^-2 and 102 mV at 100 mA cm^-2. Similarly, the overpotential of the anodic-coupled urea oxidation reaction (UOR) was only 1.3 V at 10 mA cm^-2 and 1.35 V at 100 mA cm^-2. Besides, the small impact resulting from the rapid bubble extraction within the Ni₃N/FeNi₃N/NF nanoarrays ensured excellent HER cycling stability over 100 h at a current density of 50 mA cm^-2. The further scale-up experiment suggests the industrialization prospects of the prepared Ni₃N/FeNi₃N/NF electrocatalysts.

Chemical Energy-Driven Lithiation Preparation of Defect-Rich Transition Metal Nanostructures for Electrocatalytic Hydrogen Evolution

Transition metal nanostructures are widely regarded as important catalysts to replace the precious metal Pt for hydrogen evolution reaction (HER) in water splitting. However, it is difficult to obtain uniform-sized and ultrafine metal nanograins through general high-temperature reduction and sintering processes. Herein, a novel method of chemical energy-driven lithiation is introduced to synthesize transition metal nanostructures. By taking advantage of the slow crystallization kinetics at room temperature, more surface and boundary defects can be generated and remained, which reduce the atomic coordination number and tune the electronic structure and adsorption free energy of the metals. The obtained Ni nanostructures therein exhibit excellent HER performance. In addition, the bimetal of Co and Ni shows better electrocatalytic kinetics than individual Ni and Co nanostructures, reaching 100 mA cm^-2 at a low overpotential of 127 mV. The high HER performance originates from well-formed synergistic effect between Ni and Co by tuning the electronic structures. Density functional theory simulations confirm that the bimetallic NiCo possesses a low Gibbs free energy of hydrogen adsorption, which are conducive to enhance its intrinsic activity. This work provides a general strategy that enables simultaneous defect engineering and electronic modulation of transition metal catalysts to achieve an enhancement in HER performance.

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Enhanced photocatalytic hydrogen evolution and ammonia sensitivity of double-heterojunction g-C3N4/TiO2/CuO

The performance of semiconductor photocatalysts has been limited by rapid electron-hole recombination. One strategy to overcome this problem is to construct a heterojunction structure to improve the survival rate of electrons. In this context, a novel g-C₃N₄/TiO₂/CuO double-heterojunction photocatalyst was developed and characterized. Its photocatalytic activity for hydrogen production from water-methanol photocatalytic reforming was explored. Methanol is always used to eliminate semiconductor holes. The g-C₃N₄/TiO₂/CuO double-heterojunction photocatalyst with a narrow bandgap of ∼1.38 eV presented excellent photocatalytic activity for hydrogen evolution (97.48 μmol (g h)^-1) under visible light irradiation. Compared with g-C₃N₄/TiO₂ and CuO/TiO₂, the photocatalytic activity of g-C₃N₄/TiO₂/CuO for hydrogen production was increased approximately 7.6 times and 1.8 times, respectively. Below 240 °C, the sensitivity of g-C₃N₄/TiO₂/CuO to ammonia was approximately 90% and 46% higher than that of g-C₃N₄/TiO₂ and CuO/TiO₂, respectively. The enhancement of the photocatalytic activity and gas sensing properties of the g-C₃N₄/TiO₂/CuO composite resulted from the close interface contact established by the double heterostructure. The trajectory of electrons in the double heterojunction conformed to the S-scheme. UV-vis, PL, and transient photocurrent characterization showed that the double heterostructure effectively inhibited the recombination of e^-/h⁺ pairs and enhanced the migration of photogenerated electrons.

Rational design, synthesis, and applications of carbon-assisted dispersive Ni-based composites

Herein, we review recent developments in the rational design and engineering of various carbon-assisted dispersive nickel-based composites, and boosted properties for protein adsorption and nitroaromatics reduction.

Ultrasensitive dual-quenching electrochemiluminescence immunosensor for prostate specific antigen detection based on graphitic carbon nitride quantum dots as an emitter

Early monitoring of prostate-specific antigen (PSA) is crucial in diagnosis and proactive treatment of prostate disease. Herein, a dual-quenching ternary ECL immunosensor was designed for PSA detection based on graphitic carbon nitride quantum dots (g-CNQDs, as an emitter), potassium persulfate (K₂S₂O₈, as a coreactant), and silver nanoparticles doped multilayer Ti₃C₂ MXene hybrids (Ag@TCM, as a coreaction accelerator). First, Ag@TCM was immobilized on the surface of a glassy carbon electrode, then g-CNQDs was further adsorbed on Ag@TCM to acquire a higher initial ECL signal at a potential window from - 1.3 to 0.0 V (vs. Ag/AgCl). Ag@TCM not only acted as the coreaction accelerator, but also as a matrix to load enormous g-CNQDs and prostate-specific capture antibody via Ag-N bond. Meanwhile, prostate-specific detection antibody was marked by gold nanoparticles modified manganese dioxide as a dual-quenching probe (Ab2- Au@MnO₂). When Ab2-Au@MnO₂ was introduced into the ternary ECL system via sandwiched immuno-reaction, the high-sensitive detection of PSA was achieved by the dual-quenching effect, caused by the resonant energy transfer from g-CNQDs (energy donor) to Au@MnO₂ (energy acceptor). As a result, this ECL immunosensor showed a good dynamic concentration range from 10 fg·mL^-1 to 100 ng·mL^-1 with a detection limit of 6.9 fg·mL^-1 for PSA detection. The dual-quenching ECL strategy presented high stability and good specificity to open up a new pathway for ultrasensitive immunoassay.

Visible/near-infrared light absorbed nano-ferroelectric for efficient photo-piezocatalytic water splitting and pollutants degradation

The band structure of ferroelectrics can be modulated by mechanical stress induced piezoelectric polarization charges, and thus to promote the separation of photo-excited carriers, endowing photo-piezocatalysts with good performance in hydrogen production and pollutants degradation. However, the catalytic performance of these conventional photo-piezocatalysts is restricted since they mainly harvest UV light and generally have limited piezoelectricity. Here, in this study, by using self-propagation high-temperature synthesis process, highly piezoelectric gap-state-engineered nano relaxor ferroelectric at the morphotropic phase boundary, such as (Na_0.5Bi_0.5)TiO₃-Ba(Ti_0.5Ni_0.5)O₃ is synthesized for the first time and shows unprecedently light harvesting from UV to near-infrared (λ < 1300 nm). We demonstrate a significantly enhanced photo-piezocatalytic performance for this photo-piezocatalyst. A high hydrogen production rate of ~ 450 μmol g^-1 h^-1 is obtained and the decomposition of Rhodamine B dye is nearly completed after 20 min under irradiation and ultrasonic vibration. Moreover, an unprecedently efficient NIR-driven photocatalytic degradation of RhB is also demonstrated by using photo-piezocatalysts. This kind of novel multifunctional nano photo-piezocatalysts opens up new horizons to all-day available photo-piezocatalytic technology for a more efficient use of multisource energies from environment.

Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions

Of all the available resources given to mankind, the sunlight is perhaps the most abundant renewable energy resource, providing more than enough energy on earth to satisfy all the needs of humanity for several hundred years. Therefore, it is transient and sporadic that poses issues with how the energy can be harvested and processed when the sun does not shine. Scientists assume that electro/photoelectrochemical devices used for water splitting into hydrogen and oxygen may have one solution to solve this hindrance. Water electrolysis-generated hydrogen is an optimal energy carrier to store these forms of energy on scalable levels because the energy density is high, and no air pollution or toxic gas is released into the environment after combustion. However, in order to adopt these devices for readily use, they have to be low-cost for manufacturing and operation. It is thus crucial to develop electrocatalysts for water splitting based on low-cost and land-rich elements. In this review, I will summarize current advances in the synthesis of low-cost earth-abundant electrocatalysts for overall water splitting, with a particular focus on how to be linked with photoelectrocatalytic water splitting devices. The major obstacles that persist in designing these devices. The potential future developments in the production of efficient electrocatalysts for water electrolysis are also described.

Alloying Nickel with Molybdenum Significantly Accelerates Alkaline Hydrogen Electrocatalysis

Bifunctional hydrogen electrocatalysis (hydrogen-oxidation and hydrogen-evolution reactions) in alkaline solution is desirable but challenging. Among all available electrocatalysts, Ni-based materials are the only non-precious-metal-based candidates for alkaline hydrogen oxidation, but they generally suffer from low activity. Here, we demonstrate that properly alloying Ni with Mo could significantly promote its electrocatalytic performance. Ni₄ Mo alloy nanoparticles are prepared from the reduction of molybdate-intercalated Ni(OH)₂ nanosheets. The final product exhibits an apparent hydrogen-oxidation activity exceeding that of the Pt benchmark and a record-high mass-specific kinetic current of 79 A g^-1 at an overpotential of 50 mV. A superior hydrogen-evolution performance is also measured in alkaline solution. These experimental data are rationalized by our theoretical simulations, which show that alloying Ni with Mo significantly weakens its hydrogen adsorption, improves the hydroxyl adsorption and decreases the reaction barrier for water formation.

Interface engineering of heterostructured electrocatalysts towards efficient alkaline hydrogen electrocatalysis

Boosting the alkaline hydrogen evolution and oxidation reaction (HER/HOR) kinetics is vital to practicing the renewable hydrogen cycle in alkaline media. Recently, intensive research has demonstrated that interface engineering is of critical significance for improving the performance of heterostructured electrocatalysts particularly toward the electrochemical reactions involving multiple reaction intermediates like alkaline hydrogen electrocatalysis, and the research advances also bring substantial non-trivial fundamental insights accordingly. Herein, we review the current status of interface engineering with respect to developing efficient heterostructured electrocatalysts for alkaline HER and HOR. Two major subjects-how interface engineering promotes the reaction kinetics and what fundamental insights interface engineering has brought into alkaline HER and HOR-are discussed. Specifically, heterostructured electrocatalysts with abundant interfaces have shown substantially accelerated alkaline hydrogen electrocatalysis kinetics owing to the synergistic effect from different components, which could balance the adsorption/desorption behaviors of the intermediates at the interfaces. Meanwhile, interface engineering can effectively tune the electronic structures of the active sites via electronic interaction, interfacial bonding, and lattice strain, which would appropriately optimize the binding energy of targeted intermediates like hydrogen. Furthermore, the confinement effect is critical for delivering high durability by sustaining high density of active sites. At last, our own perspectives on the challenges and opportunities toward developing efficient heterostructured electrocatalysts for alkaline hydrogen electrocatalysis are provided.

Hierarchical MnCo2O4 nanowire@NiFe layered double hydroxide nanosheet heterostructures on Ni foam for overall water splitting

It is of great importance to construct non-precious metal bifunctional electrocatalysts with low cost and high efficiency for overall water splitting.

Three-dimensionally hierarchical NiCoP@PANI architecture for high-performance hydrogen evolution reaction

Ternary phosphides are attracting great attention in the field of electrocatalytic hydrogen evolution and they have been verified to be highly active for water splitting. Herein, we developed polyaniline (PANI) coated nickel-cobalt metal phosphides nanowire arrays grown on nickel foam (NiCoP@PANI) as hydrogen evolution reaction electrocatalysts. The appearance of PANI with excellent electric conductivity can accelerate H⁺ in electrolyte transfer into H₂ and offer a masking layer to restrain the damage of electrode structural. Besides, the 3D porous structural of nickel foam can act as a skeleton to avoid electrode structure collapse and a channel for electron transfer. The optimized NiCoP@PANI can drive a current density of 10 mA cm^-2 at low overpotential of 80.6 mV in 1 M KOH solution, and satisfactory electrochemical stability with unbroken structure and unchanged composition after electrochemical test.

Genuine Active Species Generated from Fe3 N Nanotube by Synergistic CoNi Doping for Boosted Oxygen Evolution Catalysis

The surface reconstruction of oxygen evolution reaction (OER) catalysts has been proven favorable for enhancing its catalytic activity. However, what is the active site and how to promote the active species generation remain unclear and are still under debate. Here, the in situ synthesis of CoNi incorporated Fe₃ N nanotubes (CoNi-Fe₃ N) on the iron foil through the anodization/electrodeposition/nitridation process for use of boosted OER catalysis is reported. The synergistic CoNi doping induces the lattice expansion and up shifts the d-band center of Fe₃ N, which enhances the adsorption of hydroxyl groups from electrolyte during the OER catalysis, facilitating the generation of active CoNi-FeOOH on the Fe₃ N nanotube surface. As a result of this OER-conditioned surface reconstruction, the optimized catalyst requires an overpotential of only 285 mV at a current density of 10 mA cm^-2 with a Tafel slope of 34 mV dec^-1 , outperforming commercial RuO₂ catalysts. Density functional theory (DFT) calculations further reveal that the Ni site in CoNi-FeOOH modulates the adsorption of OER intermediates and delivers a lower overpotential than those from Fe and Co sites, serving as the optimal active site for excellent OER performance.

Size-Dependent Nickel-Based Electrocatalysts for Selective CO2 Reduction

Closing the anthropogenic carbon cycle by converting CO₂ into reusable chemicals is an attractive solution to mitigate rising concentrations of CO₂ in the atmosphere. Herein, we prepared Ni metal catalysts ranging in size from single atoms to over 100 nm and distributed them across N-doped carbon substrates which were obtained from converted zeolitic imidazolate frameworks (ZIF). The results show variance in CO₂ reduction performance with variance in Ni metal size. Ni single atoms demonstrate a superior Faradaic efficiency (FE) for CO selectivity (ca. 97 % at -0.8 V vs. RHE), while results for 4.1 nm Ni nanoparticles are slightly lower (ca. 93 %). Further increase the Ni particle size to 37.2 nm allows the H₂ evolution reaction (HER) to compete with the CO₂ reduction reaction (CO₂ RR). The FE towards CO production decreases to under 30 % and HER efficiency increase to over 70 %. These results show a size-dependent CO₂ reduction for various sizes of Ni metal catalysts.

Modified Nano-TiO2 Based Composites for Environmental Photocatalytic Applications

TiO2 probably plays the most important role in photocatalysis due to its excellent chemical and physical properties. However, the band gap of TiO2 corresponds to the Ultraviolet (UV) region, which is inactive under visible irradiation. At present, TiO2 has become activated in the visible light region by metal and nonmetal doping and the fabrication of composites. Recently, nano-TiO2 has attracted much attention due to its characteristics of larger specific surface area and more exposed surface active sites. nano-TiO2 has been obtained in many morphologies such as ultrathin nanosheets, nanotubes, and hollow nanospheres. This work focuses on the application of nano-TiO2 in efficient environmental photocatalysis such as hydrogen production, dye degradation, CO2 degradation, and nitrogen fixation, and discusses the methods to improve the activity of nano-TiO2 in the future.

FeCoSe2 Nanoparticles Embedded in g-C3N4: A Highly Active and Stable bifunctional electrocatalyst for overall water splitting

To investigate cost affordable and robust HER and OER catalysts with significant low overpotentials, we have successfully embedded FeCoSe2 spheres on smooth surfaces of graphitic carbon nitride that demonstrated high stability and electrocatalytic activity for H2 production. We systematically analyzed the composition and morphology of FexCo1-xSe2/g-C3N4 and attributed the remarkable electrochemical performance of the catalyst to its unique structure. Fe0.2Co0.8Se2/g-C3N4 showed a superior HER activity, with quite low overpotential value (83 mV at -20 mA cm-2 in 0.5 M H2SO4) and a current density of -3.24, -7.84, -14.80, -30.12 mA cm-2 at 0 V (vs RHE) in Dulbecco's Phosphate-Buffered Saline (DPBS), artificial sea water (ASW), 0.5 M H2SO4 and 1 M KOH, respectively. To the best of our knowledge, these are the highest reported current densities at this low potential value, showing intrinsic catalytic activity of the synthesized material. Also, the catalyst was found to deliver a high and stable current density of -1000 mA cm-2 at an overpotential of just 317 mV. Moreover, the synthesized catalyst delivered a constant current density of -30 mA cm-2 for 24 h without any noticeable change in potential at -0.2 V. These attributes confer our synthesized catalyst to be used for renewable fuel production and applications.

Boosting visible-light-driven catalytic hydrogen evolution via surface Ti3+ and bulk oxygen vacancies in urchin-like hollow black TiO2 decorated with RuO2 and Pt dual cocatalysts

A novel hollow urchin-like black RuO₂/TiO₂/Pt nanomaterial with surface Ti³⁺ and bulk single-electron oxygen vacancies (Vo·) was used for enhancing the hydrogen evolution performance under visible light.

A flower-like CoS2/MoS2 heteronanosheet array as an active and stable electrocatalyst toward the hydrogen evolution reaction in alkaline media

CoS₂/MoS₂ heteronanosheet arrays (HNSAs) with vertically aligned flower-like architectures are fabricated through in situ topotactic sulfurization of CoMoO₄ nanosheet array (NSA) precursors on conductive Ni foam. CoMoO₄ NSAs are prepared by a self-template hydrothermal method without using any hard template and surfactant. Benefiting from a 3D flower-like architecture constituted by ultrathin nanosheets with abundant exposed heterointerfaces as highly active sites and predesigned void spaces, the as-synthesized CoS₂/MoS₂ HNSAs exhibit an excellent hydrogen evolution reaction (HER) performance with a low overpotential of 50 mV at 10 mA cm⁻², and a small Tafel slope of 76 mV dec⁻¹ in 1.0 M KOH, which outperforms most previously reported CoS₂ and MoS₂ based electrocatalysts with compositional or morphological similarity. This work demonstrates the great potential in developing high-efficiency and earth-abundant electrocatalysts for alkaline HER through heterointerface engineering and morphological design by utilizing transition metal molybdate as a promising platform.

CoS₂/MoS₂ heteronanosheet arrays with vertically aligned flower-like architecture are fabricated through in situ topotactic sulfurization of CoMoO₄ nanosheet arrays.

Interfacial engineering of Mo2C-Mo3C2 heteronanowires for high performance hydrogen evolution reactions

Non-precious metal-based electrocatalysts with high activity and stability for efficient hydrogen evolution reactions are of critical importance for low-cost and large-scale water splitting. In this work, Mo₂C-Mo₃C₂ heteronanowires with significantly enhanced catalytic performance are constructed from an MoAn precursor via an accurate phase transition process. The structure disordering and surface carbon shell of Mo₂C-Mo₃C₂ heteronanowires can be precisely regulated, resulting in an enlarged surface area and a defect-rich catalytic surface. Density functional theory calculations are used to identify the effect of the defective sites and carbon shell on the free energy for hydrogen adsorption in hydrogen evolution. Meanwhile, the synergistic effect between different phases and the introduced lattice defects of Mo₂C-Mo₃C₂ are considered to enhance the HER catalytic performance. The designed catalyst exhibits optimal electrocatalytic activity in both acidic and alkaline media: low overpotentials of 134 and 116 mV at 10 mA cm^-2, a small Tafel slope of 64 mV dec^-1, and a long-term stability for 5000 cycles. This work will provide new insights into the design of high-efficiency HER catalysts via interfacial engineering at the nanoscale for commercial water splitting.

Topological Formation of a Mo-Ni-Based Hollow Structure as a Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction in Alkaline Solutions

A Mo-Ni alloy has been demonstrated to be a benchmark noble-metal-free catalyst for the hydrogen evolution reaction (HER) in alkaline solutions. Nevertheless, further improvement on its catalytic activity is desired to meet industrial requirements. In this study, Mo-Ni-based hollow structures (MoNi-HS), backboned by MoO_{3- x} nanosheets and decorated with metallic MoNi₄ nanoparticles, were obtained via a topological transformation process by annealing MoNi-oxide hollow precursors in a reducing atmosphere. This hollow structure allowed for a large proportion of catalytic surface exposed in the electrolyte, leading to highly efficient utilization of active sites in the catalyst. As a result, robust catalytic activity toward HER was recorded in 1 M KOH electrolyte: a low overpotential of 38 mV to deliver a current density of 10 mA/cm² and a very small Tafel slope of 31.4 mV per dec. Such a remarkable performance of MoNi-HS even outperformed the catalytic activity of the commercial Pt/C electrocatalyst, addressing an effective strategy to promote the catalytic performance of noble-metal-free catalysts.

Recent Progress on Engineering Highly Efficient Porous Semiconductor Photocatalysts Derived from Metal-Organic Frameworks

Porous structures offer highly accessible surfaces and rich pores, which facilitate the exposure of numerous active sites for photocatalytic reactions, leading to excellent performances. Recently, metal-organic frameworks (MOFs) have been considered ideal precursors for well-designed semiconductors with porous structures and/or heterostructures, which have shown enhanced photocatalytic activities. In this review, we summarize the recent development of porous structures, such as metal oxides and metal sulfides, and their heterostructures, derived from MOF-based materials as catalysts for various light-driven energy-/environment-related reactions, including water splitting, CO₂ reduction, organic redox reaction, and pollution degradation. A summary and outlook section is also included.

Strongly Coupled Nickel-Cobalt Nitrides/Carbon Hybrid Nanocages with Pt-Like Activity for Hydrogen Evolution Catalysis

Designing non-precious-metal catalysts with comparable mass activity to state-of-the-art noble-metal catalysts for the hydrogen evolution reaction (HER) in alkaline solution still remains a significant challenge. Herein a new strongly coupled nickel-cobalt nitrides/carbon complex nanocage (NiCoNzocage) is rationally designed via chemical etching of ZIF-67 nanocubes with Ni(NO₃ )₂ under sonication at room temperature, following nitridation. The as-prepared strongly coupled NiCoN/C nanocages exhibit a mass activity of 0.204 mA µg^-1 at an overpotential of 200 mV for the HER in alkaline solution, which is comparable to that of commercial Pt/C (0.451 mA µg^-1 ). The strongly coupled NiCoN/C nanocages also possess superior stability for the HER with negligible current loss under the overpotentials of 200 mV for 10 h. Density functional theory (DFT) calculations reveal that the excellent HER performance under alkaline condition arises from the robust Co²⁺ →Co⁰ transformation achieved by strong (Ni, Co)N-bonding-induced efficient d-p-d coupled electron transfer, which is a key for optimal initial water adsorption and splitting. The high degree of amorphization urges the C-sites to be an electron-pushing bath to promote the inter-layer/sites electron-transfer with loss of the orbital-selection-forbidden-rule, which uniformly boosts the surface catalytic activities up to a high level independent of the individual surface active sites.

Cobalt/titanium nitride@N-doped carbon hybrids for enhanced electrocatalytic hydrogen evolution and supercapacitance

Hybridizing engineering: the electrocatalytic HER and supercapacitor activities of TiN can be significantly enhanced by hybridizing the optimal content of metallic cobalt.