Co-Doped Ni3N Nanosheets with Electron Redistribution as Bifunctional Electrocatalysts for Efficient Water Splitting

Enhancing the electronic structure of Ni-based electrocatalysts through N element substitution for the hydrogen evolution reaction

The weak orbital coupling between Ni3N and H2O, caused by its interstitial structure and attenuated Ni-N interaction, is attributed to the high unoccupied d orbital energy of Ni3N. Consequently, the kinetics for water dissociation in the HER are slow. In this study, we effectively lowered the energy state of vacant d orbitals in Ni3N, which resulted in an exceptionally efficient HER. The as-synthesized Ni3N catalyst demonstrates an overpotential of 135 mV when subjected to a current density of 10 mA cm-2. The refined structural characterization suggests that the introduction of oxygen results in a reduction in electron densities surrounding the Ni sites. Furthermore, DFT calculations provide additional evidence that the electrocatalyst of Ni3N generates a greater number of lowest unoccupied orbitals (LUMOs) and improved alignment, thereby enhancing the adsorption and splitting of water. The notion of orbital-regulated electronic levels on Ni sites introduces a distinctive methodology for the systematic development of catalysts used in hydrogen evolution and other applications.

MXene boosted MOF-derived cobalt sulfide/carbon nanocomposites as efficient bifunctional electrocatalysts for OER and HER

The development of effective bifunctional electrocatalysts that can realize water splitting to produce oxygen and hydrogen through oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is still a great challenge to be addressed. Herein, we report a simple and versatile approach to fabricate bifunctional OER and HER electrocatalysts derived from ZIF67/MXene hybrids via sulfurization of the precursors in hydrogen sulfide gas atmosphere at high temperatures. The as-prepared CoS@C/MXene nanocomposites were characterized using a series of technologies including X-ray diffraction, gas sorption, scanning electronic microscopy, transmission electronic microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy. The synthesized CoS@C/MXene composites are electrocatalytically active in both HER and OER, and the CSMX-800 composite displayed the highest electrocatalytic performance towards OER and HER among all the produced samples. CSMX-800 exhibited overpotentials of 257 mV at 10 mA cm-2 for OER and 270 mV at 10 mA cm-2 for HER. Moreover, it also possesses small Tafel slope values of 93 mV dec-1 and 103 mV dec-1 for OER and HER, respectively. The enhanced electrocatalytic performance of the MXene-containing composites is due to their high surface area, enhanced conductivity, and faster charge transfer. This work demonstrated that CoS@C/MXene based electrocatalyst has great potential in electrochemical water splitting for hydrogen production, thus reducing carbon emissions and protecting the environment.

Electronic redistribution through the interface of MnCo2O4-Ni3N nano-urchins prompts rapid In situ phase transformation for enhanced oxygen evolution reaction

One of the most coveted objectives in the realm of energy conversion technologies is the development of highly efficient and economically viable electrocatalysts for the oxygen evolution reaction. The commercialization of such techniques has thus far been impeded by their slow response kinetics. One of the many ways to develop highly effective electrocatalysts is to judiciously choose a coupling interface that maximizes catalyst performance. In this study, the in situ electrochemical phase transformation of MnCo2O4-Ni3N into MnCo2O4-NiOOH is described. The catalyst has an exceptional overpotential of 224 mV to drive a current density of 10 mA cm-2. Strong interfacial contact is seen in the MnCo2O4-Ni3N catalyst, leading to a considerable electronic redistribution between the MnCo2O4 and Ni3N phases. This causes an increase in the valence state of Ni, which makes it an active site for the adsorption of *OH, O*, and *OOH (intermediates). This charge transfer facilitates the rapid phase transformation to form NiOOH from Ni3N. At a higher current density of 300 mA cm-2, the catalyst remained stable for a period of 140 h. DFT studies also revealed that the in situ-formed NiOOH on the MnCo2O4 surface results in superior OER kinetics compared to that of NiOOH alone.

Designing Efficient Non-Precious Metal Electrocatalysts for High-Performance Hydrogen Production: A Comprehensive Evaluation Strategy

Developing abundant Earth-element and high-efficient electrocatalysts for hydrogen production is crucial in effectively reducing the cost of green hydrogen production. Herein, a strategy by comprehensively considering the computational chemical indicators for H* adsorption/desorption and dehydrogenation kinetics to evaluate the hydrogen evolution performance of electrocatalysts is proposed. Guided by the proposed strategy, a series of catalysts are constructed through a dual transition metal doping strategy. Density Functional Theory (DFT) calculations and experimental chemistry demonstrate that cobalt-vanadium co-doped Ni3N is an exceptionally ideal catalyst for hydrogen production from electrolyzed alkaline water. Specifically, Co,V-Ni3N requires only 10 and 41 mV in alkaline electrolytes and alkaline seawater, respectively, to achieve a hydrogen evolution current density of 10 mA cm-2. Moreover, it can operate steadily at a large industrial current density of 500 mA cm-2 for extended periods. Importantly, this evaluation strategy is extended to single-metal-doped Ni3N and found that it still exhibits significant universality. This study not only presents an efficient non-precious metal-based electrocatalyst for water/seawater electrolysis but also provides a significant strategy for the design of high-performance catalysts of electrolyzed water.

Surface Reconstruction on Metal Nitride during Photo-oxidation

The efficient conversion and storage of solar energy for chemical fuel production presents a challenge in sustainable energy technologies. Metal nitrides (MNs) possess unique structures that make them multi-functional catalysts for water splitting. However, the thermodynamic instability of MNs often results in the formation of surface oxide layers and ambiguous reaction mechanisms. Herein, we present on the photo-induced reconstruction of a Mo-rich@Co-rich bi-layer on ternary cobalt-molybdenum nitride (Co3 Mo3 N) surfaces, resulting in improved effectiveness for solar water splitting. During a photo-oxidation process, the uniform initial surface oxide layer is reconstructed into an amorphous Co-rich oxide surface layer and a subsurface Mo-N layer. The Co-rich outer layer provides active sites for photocatalytic oxygen evolution reaction (POER), while the Mo-rich sublayer promotes charge transfer and enhances the oxidation resistance of Co3 Mo3 N. Additionally, the surface reconstruction yields a shortened Co-Mo bond length, weakening the adsorption of hydrogen and resulting in improved performance for both photocatalytic hydrogen evolution reaction (PHER) and POER. This work provides insight into the surface structure-to-activity relationships of MNs in solar energy conversion, and is expected to have significant implications for the design of metal nitride-based catalysts in sustainable energy technologies.

Bi-cation incorporated Ni3N nanosheets boost water dissociation kinetics for enhanced alkaline hydrogen evolution activity

Nickel nitride (Ni3N) is a promising electrocatalyst for the hydrogen evolution reaction (HER) owing to its excellent metallic features and has been demonstrated to exhibit considerable activity for water oxidation. However, its undesirable characteristics as an HER electrocatalyst due to its poor unfavourable d-band energy level significantly limit its water dissociation kinetics. Herein, the HER electrocatalytic activity of Ni3N was prominently enhanced via the simultaneous incorporation of bi-cations (vanadium (V) and iron (Fe), denoted as V-Fe-Ni3N). The optimized V-Fe-Ni3N displays impressive performance with an overpotential of 69 mV at 10 mA cm-2 and good stability in 1.0 M KOH, which is remarkably better than pristine Ni3N, V-doped Ni3N, and Fe-doped Ni3N and considerably closer to a commercial Pt/C catalyst. Based on density functional theory (DFT) studies, V and Fe atoms not only serve as active sites for promoting water dissociation kinetics but also tune the electronic structure of Ni3N to achieve optimized hydrogen adsorption capabilities. This work presents an inclusive understanding of the rational designing of high-performance transition metal nitride-based electrocatalysts for hydrogen production. Its electrocatalytic performance can be significantly enhanced by doping transition metal cations.

Acidic media enables oxygen-tolerant electrosynthesis of multicarbon products from simulated flue gas

Renewable electricity powered electrochemical CO2 reduction (CO2R) offers a valuable method to close the carbon cycle and reduce our overreliance on fossil fuels. However, high purity CO2 is usually required as feedstock, which potentially decreases the feasibility and economic viability of the process. Direct conversion of flue gas is an attractive option but is challenging due to the low CO2 concentration and the presence of O2 impurities. As a result, up to 99% of the applied current can be lost towards the undesired oxygen reduction reaction (ORR). Here, we show that acidic electrolyte can significantly suppress ORR on Cu, enabling generation of multicarbon products from simulated flue gas. Using a composite Cu and carbon supported single-atom Ni tandem electrocatalyst, we achieved a multicarbon Faradaic efficiency of 46.5% at 200 mA cm-2, which is ~20 times higher than bare Cu under alkaline conditions. We also demonstrate stable performance for 24 h with a multicarbon product full-cell energy efficiency of 14.6%. Strikingly, this result is comparable to previously reported acidic CO2R systems using pure CO2. Our findings demonstrate a potential pathway towards designing efficient electrolyzers for direct conversion of flue gas to value-added chemicals and fuels.

Approach to Evaluation of Electrocatalytic Water Splitting Parameters, Reflecting Intrinsic Activity: Toward the Right Pathway

The development of transition metal-based non-precious-metal electrocatalysts for energy storage and conversion systems has received a lot of interest recently. To further this subject in the proper way given the development of electrocatalysts, a fair comparison of their respective performance is necessary. This Review investigates the parameters used for the comparison of electrocatalyst activity. Significant evaluation criteria employed in electrochemical water splitting studies are the overpotential at defined current density usually at 10 mA per geometric surface area, Tafel slope, exchange current density, mass activity, specific activity and turnover frequency (TOF). This Review will discuss how to identify the specific activity and TOF by electrochemical and non-electrochemical methods to represent intrinsic activity as well as the benefits and uncertainties of each technique, ensuring that each method is applied correctly when calculating intrinsic activity metrics.

Collaborative Interface Optimization Strategy Guided Ultrafine RuCo and MXene Heterostructure Electrocatalysts for Efficient Overall Water Splitting

Developing highly active and robust electrocatalysts for the hydrogen/oxygen evolution reaction (HER/OER) is crucial for the large-scale utilization of green hydrogen. In this study, a collaborative interface optimization guided strategy was employed to prepare a metal-organic framework (MOF) derived heterostructure electrocatalyst (MXene@RuCo NPs). The obtained electrocatalyst requires overpotentials of only 20 mV for the HER and 253 mV for the OER to deliver a current density of 10 mA/cm² in alkaline media, respectively, and it also exhibits great performance at high current density. Experiments and theoretical calculations reveal that the doped Ru introduces second active sites and decreases the diameter of nanoparticles, which greatly enhances the number of active sites. More importantly, the MXene/RuCo NPs heterogeneous interfaces in the catalysts exhibit great synergistic effects, decreasing the work function of the catalyst and improving the charge transfer rate, thus reducing the energy barrier of the catalytic reaction. This work represents a promising strategy for the development of MOF-derived highly active catalysts to achieve efficient energy conversion in industrial applications.

Oxygen Vacancies and Interface Engineering on Amorphous/Crystalline CrOx -Ni3 N Heterostructures toward High-Durability and Kinetically Accelerated Water Splitting

Manipulating catalytic active sites and reaction kinetics in alkaline media is crucial for rationally designing mighty water-splitting electrocatalysts with high efficiency. Herein, the coupling between oxygen vacancies and interface engineering is highlighted to fabricate a novel amorphous/crystalline CrO_x -Ni₃ N heterostructure grown on Ni foam for accelerating the alkaline hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory (DFT) calculations reveal that the electron transfer from amorphous CrO_x to Ni₃ N at the interfaces, and the optimized Gibbs free energies of H₂ O dissociation (ΔG_H-OH ) and H adsorption (ΔG_H ) in the amorphous/crystalline CrO_x -Ni₃ N heterostructure are conducive to the superior and stable HER activity. Experimental data confirm that numerous oxygen vacancies and amorphous/crystalline interfaces in the CrO_x -Ni₃ N catalysts are favorable for abundant accessible active sites and enhanced intrinsic activity, resulting in excellent catalytic performances for HER and OER. Additionally, the in situ reconstruction of CrO_x -Ni₃ N into highly active Ni₃ N/Ni(OH)₂ is responsible for the optimized OER performance in a long-term stability test. Eventually, an alkaline electrolyzer using CrO_x -Ni₃ N as both cathode and anode has a low cell voltage of 1.53 V at 10 mA cm^-2 , together with extraordinary durability for 500 h, revealing its potential in industrial applications.

Two-Dimensional Porous Structure of V-Doped NiO with Enhanced Electrochromic Properties

In this work, a two-dimensional porous structure of a V-doped NiO film with excellent electrochromic properties on an ITO substrate was synthesized by a hydrothermal method. The influence of V⁵⁺ ions on the NiO film was explored by adjusting the amount of V doping, including refining the crystal grains, increasing the specific surface area of the film, and accelerating the diffusion rate of OH^- in the film. Compared with the undoped NiO film, a 3 atom % V-doped NiO film comes out with superior electrochromic properties with large optical transmittance modulation (81.9% at 600 nm), fast response times (1.2 and 0.9 s), and excellent cycle stability (90.6%). This work creates innovation direction in the field of intelligent energy-saving window materials with high electrochromic properties.

Metal nitride-based nanostructures for electrochemical and photocatalytic hydrogen production

The over-dependence on fossil fuels is one of the critical issues to be addressed for combating greenhouse gas emissions. Hydrogen, one of the promising alternatives to fossil fuels, is renewable, carbon-free, and non-polluting gas. The complete utilization of hydrogen in every sector ranging from small to large scale could hugely benefit in mitigating climate change. One of the key aspects of the hydrogen sector is its production via cost-effective and safe ways. Electrolysis and photocatalysis are well-known processes for hydrogen production and their efficiency relies on electrocatalysts, which are generally noble metals. The usage of noble metals as catalysts makes these processes costly and their scarcity is also a limiting factor. Metal nitrides and their porous counterparts have drawn considerable attention from researchers due to their good promise for hydrogen production. Their properties such as active metal centres, nitrogen functionalities, and porous features such as surface area, pore-volume, and tunable pore size could play an important role in electrochemical and photocatalytic hydrogen production. This review focuses on the recent developments in metal nitrides from their synthesis methods point of view. Much attention is given to the emergence of new synthesis techniques, methods, and processes of synthesizing the metal nitride nanostructures. The applications of electrochemical and photocatalytic hydrogen production are summarized. Overall, this review will provide useful information to researchers working in the field of metal nitrides and their application for hydrogen production.

Controllable Electrodeposition Adjusts the Electrochromic Properties of Co and Mo Co-Modified WO3 Films

Metal ion modification is considered to be an effective way to construct metal oxides with specific physical and chemical properties. In this paper, we prepare a tungsten oxide (WO3) film co-modified by Co-ion and Mo-ion to serve as the electrochromic material through a one-step electrodeposition method. The effect of electrodeposition time on film thickness, surface morphology and electrochromic properties is systematically studied as well. The results show that, compared with pure WO3 film, the surface morphology of the tungsten oxide film modified by Co-ion and Mo-ion (WO3: Co, Mo) is significantly different. The Co and Mo co-modified film possesses a higher transmission modulation (58.5% at 600 nm) and rapid switching speed (coloring and bleaching time are 2.7 s and 5.6 s, respectively), low impedance value and excellent cycle stability. The performance enhancement is mainly attributed to the coral-like structure of the membrane, which provides a larger specific surface area, more ion adsorption sites and faster ion diffusion. Therefore, this work provides a fast and low-cost method to prepare tungsten oxide electrochromic films co-modified with cobalt and molybdenum ions. At the same time, it also provides an idea to obtain films with different electrochromic properties by adjusting the film thickness.

Hierarchically structured nickel/molybdenum nitride heterojunctions as superior bifunctional electrodes for overall water splitting

A 3D hierarchical heterostructure of intermetallic compound heterojunctions is first rationally designed and presented as a highly-active bifunctional electrode for water splitting.

Tuning the Electronic Structure of the CoP/Ni2P Nanostructure by Nitrogen Doping for an Efficient Hydrogen Evolution Reaction in Alkaline Media

As one of the most sustainable, efficient, and cleanest ways for hydrogen production, electrochemical water splitting relies heavily on cost-efficient and stable electrocatalysts. Herein, a self-supported and nitrogen-doped hybrid CoP/Ni₂P was synthesized through a simple two-step hydrothermal process followed by low-temperature phosphorization and nitridation (N-CoP/Ni₂P@NF). Both experimental and density functional theory calculation results suggest that nitrogen doping can tune the electrical structure of the CoP/Ni₂P heterostructure and thus optimize the free energy of adsorbed H on the surface of N-CoP/Ni₂P@NF and accelerate the electronic transport activity. The prepared N-CoP/Ni₂P@NF exhibits excellent electrocatalytic hydrogen evolution reaction (HER) performance, which merely requires an overpotential of -46 mV at -10 mA cm^-2 and shows a negligible decay after a long durability test for 72 h in alkaline (1.0 M KOH) media. Consequently, this work supplies a novel strategy with great potential for designing transition metal phosphate-based catalysts with high HER performance.

Unveiling the Mechanisms Ruling the Efficient Hydrogen Evolution Reaction with Mitrofanovite Pt3Te4

By means of electrocatalytic tests, surface-science techniques and density functional theory, we unveil the physicochemical mechanisms ruling the electrocatalytic activity of recently discovered mitrofanovite (Pt₃Te₄) mineral. Mitrofanovite represents a very promising electrocatalyst candidate for energy-related applications, with a reduction of costs by 47% compared to pure Pt and superior robustness to CO poisoning. We show that Pt₃Te₄ is a weak topological metal with the Z2 invariant, exhibiting electrical conductivity (∼4 × 10⁶ S/m) comparable with pure Pt. In hydrogen evolution reaction (HER), the electrode based on bulk Pt₃Te₄ shows a very small overpotential of 46 mV at 10 mA cm^-2 and a Tafel slope of 36-49 mV dec^-1 associated with the Volmer-Heyrovsky mechanism. The outstanding ambient stability of Pt₃Te₄ also provides durability of the electrode and long-term stability of its efficient catalytic performances.

Nitrogen-Doped MoS2/Ti3C2TX Heterostructures as Ultra-Efficient Alkaline HER Electrocatalysts

Molybdenum disulfide (MoS₂) is intrinsically inert for the hydrogen evolution reaction (HER) in alkaline media due to its electronic structures. Herein, we tune the electronic structures of MoS₂ by a combined strategy of post-N doping coupled with the synergistic effect of Ti₃C₂T_X. The as-prepared N-doped MoS₂/Ti₃C₂T_X heterostructures show remarkable alkaline HER activity with an overpotential of 225 mV at 140 mA cm^-2, which ranks the N-doped MoS₂/Ti₃C₂T_X heterostructures among the best MoS₂/MXene-based electrocatalysts reported for alkaline HER. The first-principles calculations indicate that the N doping can enhance the activation of nearby S sites of MoS₂/Ti₃C₂T_X and thus promote the HER process. This strategy provides a promising way to develop high-efficiency MoS₂/MXene heterostructure catalysts for alkaline HER.

Theoretical Insights into the Hydrogen Evolution Reaction on the Ni3N Electrocatalyst

Ni-based catalysts are attractive alternatives to noble metal electrocatalysts for the hydrogen evolution reaction (HER). Herein, we present a dispersion-corrected density functional theory (DFT-D3) insight into HER activity on the (111), (110), (001), and (100) surfaces of metallic nickel nitride (Ni3N). A combination of water and hydrogen adsorption was used to model the electrode interactions within the water splitting cell. Surface energies were used to characterise the stabilities of the Ni3N surfaces, along with adsorption energies to determine preferable sites for adsorbate interactions. The surface stability order was found to be (111) < (100) < (001) < (110), with calculated surface energies of 2.10, 2.27, 2.37, and 2.38 Jm−2, respectively. Water adsorption was found to be exothermic at all surfaces, and most favourable on the (111) surface, with Eads = −0.79 eV, followed closely by the (100), (110), and (001) surfaces at −0.66, −0.65, and −0.56 eV, respectively. The water splitting reaction was investigated at each surface to determine the rate determining Volmer step and the activation energies (Ea) for alkaline HER, which has thus far not been studied in detail for Ni3N. The Ea values for water splitting on the Ni3N surfaces were predicted in the order (001) < (111) < (110) < (100), which were 0.17, 0.73, 1.11, and 1.60 eV, respectively, overall showing the (001) surface to be most active for the Volmer step of water dissociation. Active hydrogen adsorption sites are also presented for acidic HER, evaluated through the ΔGH descriptor. The (110) surface was shown to have an extremely active Ni–N bridging site with ΔGH = −0.05 eV.

Spontaneous Enhanced Visible-Light-Driven Photocatalytic Water Splitting on Novel Type-II GaSe/CN and Ga2SSe/CN vdW Heterostructures

With the aggravation of environmental pollution and the energy crisis, it is particularly important to develop and design environment-friendly and efficient spontaneous enhanced visible-light-driven photocatalysts for water splitting. Herein novel type-II van der Waals (vdW) GaSe/CN and Ga₂SSe/CN heterostructures are proposed through first-principles calculations. Their electronic properties and photocatalytic performance are theoretically analyzed. In particular, their appropriate band gap and band-edge position meet the requirements of the oxygen evolution reaction, and the reaction is thermodynamically feasible in most pH ranges. The unique band alignment of these heterostructured photocatalysts leads to high solar-to-hydrogen energy conversion efficiencies up to 15.11%, which has a good commercial application prospect. More excitingly, with the application of 2% biaxial strain, the smooth progress of the water-splitting reaction of the GaSe/CN and Ga₂SSe/CN heterostructures can still be maintained, and the carrier mobility and optical absorption characteristics can be effectively improved. Consequently, these findings suggest that the GaSe/CN and Ga₂SSe/CN vdW heterostructures have promising potentials as photocatalysts for water splitting. This work may provide a promising clue for the design of efficient and stable photocatalytic water-splitting catalysts under visible spectroscopy.

Nb-Doped nickel nitride-derived catalysts for electrochemical water splitting

An alkaline electrolyzer using Nb-doped nickel nitride-derived catalysts as electrodes for overall water splitting delivers 10 mA cm−2 at a low cell voltage of 1.61 V.