The Role of Fe Species on NiOOH in Oxygen Evolution Reactions

Effect of Iron Doping in Ordered Nickel Oxide Thin Film Catalyst for the Oxygen Evolution Reaction

Water splitting has emerged as a promising route for generating hydrogen as an alternative to conventional production methods. Finding affordable and scalable catalysts for the anodic half-reaction, the oxygen evolution reaction (OER), could help with its industrial widespread implementation. Iron-containing Ni-based catalysts have a competitive performance for the use in commercial alkaline electrolyzers. Due to the complexity of studying the catalysts at working conditions, the active phase and the role that iron exerts in conjunction with Ni are still a matter of investigation. Here, we study this topic with NiO(001) and Ni0.75Fe0.25O x (001) thin film model electrocatalysts employing surface-sensitive techniques. We show that iron constrains the growth of the oxyhydroxide phase formed on top of the Ni or NiFe oxide, which is considered the active phase for the OER. Besides, operando Raman and grazing incidence X-ray absorption spectroscopy experiments reveal that the presence of iron affects both, the disorder level of the active phase and the oxidative charge around Ni during OER. The observed compositional, structural, and electronic properties of each system have been correlated with their electrochemical performance.

Ni and Co Active Site Transition and Competition in Fluorine-Doped NiCo(OH)2 LDH Electrocatalysts for Oxygen Evolution Reaction

The oxygen evolution reaction (OER) performance of NiCo LDH electrocatalysts can be improved through fluorine doping. The roles of Ni and Co active sites in such catalysts remain ambiguous and controversial. In addressing the issue, this study draws upon the molecular orbital theory and proposes the active center competitive mechanism between Ni and Co. The doped F-atoms can directly impact the valence state of metal atoms or exert an indirect influence through the dehydrogenation, thereby modulating the active center. As the F-atoms are progressively aggregate, the eg orbitals of Ni and Co transition from e2 g to e1 g, and subsequently to e0 g. The corresponding valence state elevates from +2 to +3, and then to +4, signifying an initial increase followed by a subsequent decrease in the electrocatalytic performance. Furthermore, a series of F-NiCo LDH catalysts are synthesized to verify the eg orbital occupancy analysis, and the catalytic OER overpotentials are 303, 243, 240, and 246 mV at the current density of 10 mA cm-2, respectively, which coincides well with the theoretical prediction. This investigation not only provides novel mechanistic insights into the transition and competition of Ni and Co in F-NiCo LDH catalysts but also establishes a foundation for the design of high-performance catalysts.

Designing a Phenalenyl-Based Dinuclear Ni(II) Complex: An Electrocatalyst with Two Single Ni Sites for the Oxygen Evolution Reaction (OER)

A new dinuclear Ni(II) complex 1, [Ni2II(dtbh-PLY)2], is synthesized from 9-(2-(3,6-di-tert-butyl-2-hydroxybenzylidene)hydrazineyl)-1H-phenalen-1-one, dtbh-PLYH2 ligand, and structurally characterized by various analytical tools including the single-crystal X-ray diffraction (SCXRD) technique. In the solid state, both Ni(II) metal centers in complex 1 exist in a distorted square planar geometry and display the presence of rare Ni···H-C anagostic interactions to form a one-dimensional (1-D) linear motif in the supramolecular array. Complex 1 is further stabilized in the solid state by π-π-stacking interactions between the highly delocalized phenalenyl rings. The redox features of complex 1 have been analyzed by the cyclic voltammetry (CV) technique in solution as well as in the solid state, revealing the crucial involvement of both the Ni(II) metal centers for undergoing quasi-reversible oxidation reactions on the application of an anodic sweep. A complex 1-modified glassy carbon electrode, GC-1, is employed as an electrocatalyst for oxygen evolution reaction (OER) in 1.0 M KOH, giving an OER onset at 1.45 V, and very low OER overpotential, 300 mV vs the reversible hydrogen electrode (RHE) to reach 10 mA cm-2 current density. Furthermore, GC-1 displayed fast OER kinetics with a Tafel slope of 40 mV dec-1, a significantly lower Tafel slope value than those of previously reported molecular Ni(II) catalysts. In situ electrochemical experiments and postoperational UV-vis, Fourier transform infrared (FT-IR), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) studies were performed to analyze the stability of the molecular nature of complex 1 and to gain reasonable insights into the true OER catalyst.

Strongly facet-dependent activity of iron-doped β-nickel oxyhydroxide for the oxygen evolution reaction

Iron (Fe)-doped β-nickel oxyhydroxide (β-NiOOH) is a highly active, noble-metal-free electrocatalyst for the oxygen evolution reaction (OER), with the latter being the bottleneck in electrochemical water splitting for sustainable hydrogen production. The mechanisms underlying how the Fe dopant modulates this host material's water electro-oxidation activity are still not entirely clear. Here, we combine hybrid density functional theory (DFT) and Hubbard-corrected DFT to investigate the OER activity of the most thermodynamically favorable (and therefore, expected to be the majority) crystallographic facets of β-NiOOH, namely (0001) and (101̄0). By considering active sites involving both oxidation and reduction of the transition-metal active center during the redox cycle on these two different facets, we show that six-fold-lattice-coordinated Fe in β-NiOOH is redox inactive towards both oxidation and reduction while five-fold-lattice-coordinated Fe in β-NiOOH does exhibit redox activity. However, the determined redox activity of Fe (or lack of it) is not indicative of good (or bad) performance as a dopant on these two facets. Three of the four active sites investigated (oxo and hydroxo sites on (0001) and a hydrated site on (101̄0)) exhibit only a marginal (<0.1 V) decrease or increase in the thermodynamic overpotential upon doping with Fe. Only one of the redox-active sites investigated, the hydroxo site on (101̄0), exhibits a large attenuation in the thermodynamic overpotential upon doping (to ∼0.52 V from 0.86 V), although the doped overpotential is larger than that observed experimentally for Fe-doped NiOOH. Thus, although pure β-NiOOH facets containing four-, five-, or six-fold lattice-coordinated Ni sites have roughly equal OER activities, yielding similar OER onset potentials (shown in A. Govind Rajan, J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2020, 142, 3600-3612), only those facets containing four-fold lattice-coordinated Fe (e.g., as shown in J. M. P. Martirez and E. A. Carter, J. Am. Chem. Soc., 2019, 141, 693-705) would be active under analogous conditions for the Fe-doped material. It follows that, while undoped β-NiOOH demonstrates a roughly facet-independent oxygen evolution activity, the activity of Fe-doped β-NiOOH strongly depends on the crystallographic facet. Our study further motivates the investigation of strategies for the selective growth of facets with low iron coordination number to enhance the water splitting activity of Fe-doped β-NiOOH.

Hierarchical Porous Nonprecious High-entropy Alloys for Ultralow Overpotential in Hydrogen Evolution Reaction

Water electrolysis is considered the cleanest method for hydrogen production. However, the widespread popularization of water splitting is limited by the high cost and scarce resources of efficient platinum group metals. Hence, it is imperative to develop an economical and high-performance electrocatalyst to improve the efficiency of hydrogen evolution reaction (HER). In this study, a hierarchical porous sandwich structure is fabricated through dealloying FeCoNiCuAl2 Mn high-entropy alloy (HEA). This free-standing electrocatalyst shows outstanding HER performance with a very small overpotential of 9.7 mV at 10 mA cm-2 and a low Tafel slope of 56.9 mV dec-1 in 1 M KOH solution, outperforming commercial Pt/C. Furthermore, this electrocatalytic system recorded excellent reaction stability over 100 h with a constant current density of 100 mA cm-2 . The enhanced electrochemical activity in high-entropy alloys results from the cocktail effect, which is detected by density functional theory (DFT) calculation. Additionally, micron- and nano-sized pores formed during etching boost mass transfer, ensuring sustained electrocatalyst performance even at high current densities. This work provides a new insight for development in the commercial electrocatalysts for water splitting.

Fe-doping and oxygen vacancy achieved by electrochemical activation and precipitation/dissolution equilibrium in NiOOH for oxygen evolution reaction

The poor conductivities and instabilities of accessible nickel oxyhydroxides hinder their use as oxygen evolution reaction (OER) electrocatalysts. Herein, we constructed Fe-NiOOH-OV-600, an Fe-doped nickel oxide hydroxide with abundant oxygen vacancies supported on nickel foam (NF), using a hydrothermal method and an electrochemical activation strategy involving 600 cycles of cyclic voltammetry, assisted by the precipitation/dissolution equilibrium of ferrous sulfide (FeS) in the electrolyte. This two-step method endows the catalyst with abundant Fe-containing active sites while maintaining the ordered structure of nickel oxide hydroxide (NiOOH). Characterization and density functional theory (DFT) calculations revealed that synergy between trace amounts of the Fe dopant and the oxygen vacancies not only promotes the generation of reconstructed active layers but also optimizes the electronic structure and adsorption capacity of the active sites. Consequently, the as-prepared Fe-NiOOH-OV-600 delivered large current densities of 100 and 1000 mA cm-2 for the OER at overpotentials of only 253 and 333 mV in 1 mol/L KOH. Moreover, the catalyst is stable for at least 100 h at 500 mA cm-2. This work provides insight into the design of efficient transition-metal-based electrocatalysts for the OER.

Cooperative Fe sites on transition metal (oxy)hydroxides drive high oxygen evolution activity in base

Fe-containing transition-metal (oxy)hydroxides are highly active oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form across many materials systems. The complexity and dynamics of the Fe sites within the (oxy)hydroxide have slowed understanding of how and where the Fe-based active sites form-information critical for designing catalysts and electrolytes with higher activity and stability. We show that where/how Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and structural properties of the host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni2+/3+ redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to impressively high per-Fe OER turn-over frequency (TOFFe) of ~40 s-1 at 350 mV overpotential which we attribute to under-coordinated "surface" Fe. By systematically controlling the concentration of surface Fe, we find TOFFe increases linearly with the Fe concentration. This suggests a changing OER mechanism with increased Fe concentration, consistent with a mechanism involving cooperative Fe sites in FeOx clusters.

Laser Promoting Oxygen Vacancies Generation in Alloy via Mo for HMF Electrochemical Oxidation

It is well known that nickel-based catalysts have high electrocatalytic activity for the 5-hydroxymethylfurfural oxidation reaction (HMFOR), and NiOOH is the main active component. However, the price of nickel and the catalyst's lifetime still need to be solved. In this work, NiOOH containing oxygen vacancies is formed on the surface of Ni alloy by UV laser (1J85-laser). X-ray absorption fine structure (XAFS) analyses indicate an interaction between Mo and Ni, which affects the coordination environment of Ni with oxygen. The chemical valence of Ni is between 0 and 2, indicating the generation of oxygen vacancies. Density functional theory (DFT) suggests that Mo can increase the defect energy and form more oxygen vacancies. In situ Raman electrochemical spectroscopy shows that Mo can promote the formation of NiOOH, thus enhancing the HMFOR activity. The 1J85-laser electrode shows a longer electrocatalytic lifetime than Ni-laser. After 15 cycles, the conversion of HMF is 95.92%.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 45.4 mV dec^-1, which outperforms commercial RuO₂ and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Facile formation of Zn-incorporated NiFe layered double hydroxide as highly-efficient oxygen evolution catalyst

Electrochemical water splitting is the primary method to produce green hydrogen, which is considered an efficient alternative to fossil fuels for achieving carbon neutrality. For meeting the increasing market demand for green hydrogen, high-efficiency, low-cost, and large-scale electrocatalysts are crucial. In this study, we report a simple spontaneous corrosion and cyclic voltammetry (CV) activation method to fabricate Zn-incorporated NiFe layered double hydroxide (LDH) on commercial NiFe foam, which shows excellent oxygen evolution reaction (OER) performance. The electrocatalyst achieves an overpotential of 565 mV and outstanding stability of up to 112 h at 400 mA cm^-2. The active layer for OER is shown to be β-NiFeOOH according to the results of in-situ Raman. Our findings suggest that the NiFe foam treated by simple spontaneous corrosion has promising industrial applications as a highly efficient OER catalyst.

One-step achievement of Fe-doped and interfacial Ru nanoclusters co-engineered Ni(OH)2 electrocatalyst on Ni foam for promoted oxygen evolution reaction

The creation of inexpensive, high-performance catalysts to reduce the overpotential of the oxygen evolution reaction (OER) process is critical for the electrolysis of water for hydrogen production. Therefore, we applied a one-step hydrothermal method using cation exchange reaction (CER) to prepare Fe-doped and interfacial Ru nanoclusters co-engineered Ni(OH)₂ nanosheets directly grafted on Ni foam (Ru@Fe-Ni(OH)₂/NF) for OER process. Results of electrochemical tests reveal that Ru@Fe-Ni(OH)₂/NF has excellent OER activity, and its overpotential (η) is only 266.4 mV when the current density is 50 mA cm^-2 in 1 M KOH solution, even lower than that of commercial OER catalyst RuO₂ (355 mV). The Tafel slope also decreases from 133.8 mV dec^-1 for pristine Ni(OH)₂/NF material to 24.1 mV dec^-1 for Ru@Fe-Ni(OH)₂/NF, indicating the higher charge transfer rates and fastest kinetics for water oxidation. At an overpotential of 300 mV the optimal turnover frequency (TOF) of 0.062 s^-1 for Ru@Fe-Ni(OH)₂/NF is achieved compared to that of Ni(OH)₂/NF (0.014 s^-1, NN), demonstrating the fast reaction kinetics of the as-prepared electrocatcalyst. After 24 h stability test, the catalytic activity of Ru@Fe-Ni(OH)₂/NF was only attenuated by 2 %, showing excellent OER stability and durability. Our results show that we have successfully developed an efficient OER catalyst for green and efficient electrocatalytic hydrolysis to produce H₂ and O₂, providing a promising method for clean H₂ production.

Activated FeS2 @NiS2 Core-Shell Structure Boosting Cascade Reaction for Superior Electrocatalytic Oxygen Evolution

Unlike single-step reactions, multi-step reactions can be greatly facilitated only if all the intermediate reactions can be catalyzed simultaneously and progressively. Herein, the theoretical analysis and experiments to illustrate the superiority of the cascade oxygen evolution reaction (OER) are conducted. As different OER intermediate reactions demand Fe_x Ni_1-x OOH with altered Fe/Ni ratios, gradient Fe-doped NiOOH can be an ideal electrocatalyst for the efficient cascade OER in line. Fine controlling of the nucleation sequence of iron and nickel sulfides leads to a FeS₂ @NiS₂ core-shell structure. The activated outward diffusion of Fe dopants results in the gradient Fe/Ni ratios in the Fe_x Ni_1-x OOH shell, where a cascade OER can happen. Electrochemical tests suggest that the FeS₂ @NiS₂ only needs an overpotential of 237 mV to reach the current density of 10 mA cm^-2 , with fast reaction kinetics and good stability.

Effect of Fe on Calcined Ni(OH)2 Anode in Alkaline Water Electrolysis

Ni (hydr)oxide is a promising and inexpensive material for oxygen evolution reaction (OER) catalysts and is known to dramatically increase the activity when used with Fe. Herein, we basified a Ni(II) solution and coated layered Ni(OH)2 on Ni coins to prepare a template with high stability and activity. To evaluate the stability and catalytic activity during high-current-density operation, we analyzed the electrochemical and physicochemical properties before and after constant current (CC) operation. The electrode with a Ni(OH)2 surface exhibited higher initial activity than that with a NiO surface; however, after the OER operation at a high-current density, degradation occurred owing to structural destruction. The activity of the electrodes with a NiO surface improved after the CC operation because of the changes on the electrode-surface caused by the CC operation and the subsequent Fe incorporation from the Fe impurity in the electrolyte. After confirming the improvement in activity due to Fe, we prepared NiFe-oxide electrodes with improved catalytic activity and optimized the Ni precursor and Fe loading solution concentrations. The Ni-Fe oxide electrode prepared under the optimal concentrations exhibited an overpotential of 287 mV at a current density of 10 mA/cm2, and a tafel slope of 37 mV dec−1, indicating an improvement in the OER activity.

The role of crystal facets and disorder on photo-electrosynthesis

Photoelectrochemistry has the potential to play a crucial role in the storage of solar energy and the realisation of a circular economy. From a chemical viewpoint, achieving high conversion efficiencies requires subtle control of the catalyst surface and its interaction with the electrolyte. Traditionally, such control has been hard to achieve in the complex multinary oxides used in PEC devices and consequently the mechanisms by which surface exposed facets influence light-driven catalysts are poorly understood. Yet, this understanding is critical to further improve conversion yields and fine-tune reaction selectivities. Here, we review the impact that crystal facets and disorder have on photoelectrochemical reactivity. In particular, we discuss how the crystal orientation influences the energetics of the surface, the existence of defects and the transport of reactive charges, ultimately dictating the PEC activity. Moreover, we evaluate how facet stability dictates the tendency of the solid to undergo reconstructions during catalytic processes and highlight the experimental and computational challenges that must be overcome to characterise the role of the exposed facets and disorder in catalytic performance.

Hierarchical NiMn/NiMn-LDH/ppy-C induced by a novel phase-transformation activation process for long-life supercapacitor

For micron-sized nickel-based hydroxides sheets, the reaction and migration of anions/water molecules in the inner region tends to lag behind those along the edge, which can cause structure mismatch and capacity degradation during cycles. Nanosizing and structure design is a feasible solution to shorten the ion/electron path and improve the reaction homogeneity. Herein, this study reports a novel three-stage strategy (self-assembly of NiMn-LDH/ppy-C - reduction to NiMn/ppy-C - in situ phase transformation into NiMn/NiMn-LDH/ppy-C) to reduce the sheet size of NiMn-LDH to nanometer. Triggered by electrochemical activation, NiMn-LDH nanosheets can hereby easily and orderly grow on the exposed active (111) crystal plane of Ni to establish NiMn-LDH/NiMn heterostructure around ppy-C. Importantly, nanosizing and hierarchical structure play a synergistic role to maintain structural integrity and to promote the electron/mass transfer kinetics. The NiMn/NiMn-LDH/ppy-C composite delivers superior cycling stability with almost no decay of capacity retention after 40,000 cycles at 5 A g^-1. Our hierarchical morphology modulation provides an ingenious, efficient way to boost the performance of Ni-based layered hydroxide materials.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.

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.

In Situ Electrochemical Reconstitution of CF-CuO/CeO2 for Efficient Active Species Generation

Achievement of the intrinsic activity by in situ electrochemical reconstruction has been becoming a great challenge for designing a catalyst. Herein, an effective electrochemical strategy is proposed to reconstruct the surface of the CF-CuO/CeO₂ precursor. Under the stimulation of oxidative/reductive potential, abundant active sites were successfully generated on the surface of CF-CuO/CeO₂. Remarkably, the implantation of oxygen vacancy-rich CeO₂ synergistically optimizes the chemical composition and electronic structure of CF-CuO/CeO₂, greatly promoting the generation of active species. Systematic electrochemical experiments indicate that the superior catalytic performance of reconstructed CF-CuO/CeO₂ could be attributed to CuOOH/CeO₂ and Cu₂O/Ce₂O₃ active species, respectively. The oxidative-/reductive-activated CF-CuO/CeO₂ was further employed in a paired cell for the synergistic catalysis of hydroxymethylfurfural oxidation with 4-nitrophenol hydrogenation. As a result, nearly 100% Faraday efficiency for furandicarboxylic acid/4-aminophenol production was achieved in the paired system (-0.9 V vs Ag/AgCl, 1.5 h). Therefore, the electrochemical reconstruction via oxidative/reductive activation has been confirmed as a feasible approach to significantly excite the intrinsic activity of a catalyst.

Realizing Two-Electron Transfer in Ni(OH)2 Nanosheets for Energy Storage

The theoretical capacity of a given electrode material is ultimately determined by the number of electrons transferred in each redox center. The design of multi-electron transfer processes could break through the limitation of one-electron transfer and multiply the total capacity but is difficult to achieve because multiple electron transfer processes are generally thermodynamically and kinetically more complex. Here, we report the discovery of two-electron transfer in monolayer Ni(OH)₂ nanosheets, which contrasts with the traditional one-electron transfer found in multilayer materials. First-principles calculations predict that the first oxidation process Ni²⁺ → Ni³⁺ occurs easily, whereas the second electron transfer in Ni³⁺ → Ni⁴⁺ is strongly hindered in multilayer materials by both the interlayer hydrogen bonds and the domain H structure induced by the Jahn-Teller distortion of the Ni³⁺ (t_2g⁶e_g¹)-centered octahedra. In contrast, the second electron transfer can easily occur in monolayers because all H atoms are fully exposed. Experimentally, the as-prepared monolayer is found to deliver an exceptional redox capacity of ∼576 mA h/g, nearly 2 times the theoretical capacity of one-electron processes. In situ experiments demonstrate that monolayer Ni(OH)₂ can transfer two electrons and most Ni ions transform into Ni⁴⁺ during the charging process, whereas bulk Ni(OH)₂ can only be transformed partially. Our work reveals a new redox reaction mechanism in atomically thin Ni(OH)₂ nanosheets and suggests a promising path toward tuning the electron transfer numbers to multiply the capacity of the relevant energy storage materials.

Modeling Operando Electrochemical CO2 Reduction

Since the seminal works on the application of density functional theory and the computational hydrogen electrode to electrochemical CO₂ reduction (eCO₂R) and hydrogen evolution (HER), the modeling of both reactions has quickly evolved for the last two decades. Formulation of thermodynamic and kinetic linear scaling relationships for key intermediates on crystalline materials have led to the definition of activity volcano plots, overpotential diagrams, and full exploitation of these theoretical outcomes at laboratory scale. However, recent studies hint at the role of morphological changes and short-lived intermediates in ruling the catalytic performance under operating conditions, further raising the bar for the modeling of electrocatalytic systems. Here, we highlight some novel methodological approaches employed to address eCO₂R and HER reactions. Moving from the atomic scale to the bulk electrolyte, we first show how ab initio and machine learning methodologies can partially reproduce surface reconstruction under operation, thus identifying active sites and reaction mechanisms if coupled with microkinetic modeling. Later, we introduce the potential of density functional theory and machine learning to interpret data from Operando spectroelectrochemical techniques, such as Raman spectroscopy and extended X-ray absorption fine structure characterization. Next, we review the role of electrolyte and mass transport effects. Finally, we suggest further challenges for computational modeling in the near future as well as our perspective on the directions to follow.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

High-valence chromium accelerated interface electron transfer for water oxidation

High-valence Cr ions at the interface between an alloy core and a reconstruction layer shell served as powerful electron acceptors, accelerating the electron transfer by means of the exchange effect of the Ni–O–Cr configuration.

Triple captured iron by defect abundant NiO for efficient water oxidation

Fe-doped NiO host in well-defined nanorod assembly (Fe-NiMoO4@NiO-30) with large surface area is designed to achieve the triple capture of Fe via adjustable surface reconstruction and impregnation to optimize OER...

Electrochemical fabrication of an efficient cobalt–iron oxide/graphene heterostructure by a three-electrode system for electrocatalytic oxygen evolution reaction

Electrosynthesized cobalt–iron oxide/graphene heterostructure by a three-electrode electrolysis system for efficient oxygen evolution reaction.

Rapid mass production of iron nickel oxalate nanorods for efficient oxygen evolution reaction catalysis

Using a coprecipitation method we synthesized an oxalate, which has a good catalytic performance for oxygen evolution in an alkaline electrolyte. This method can efficiently synthesize a large number of electrocatalysts in a short time.

Design principles of noble metal-free electrocatalysts for hydrogen production in alkaline media: combining theory and experiment

Water electrolysis is a promising solution to convert renewable energy sources to hydrogen as a high-energy-density energy carrier. Although alkaline conditions extend the scope of electrocatalysts beyond precious metal-based materials to earth-abundant materials, the sluggish kinetics of cathodic and anodic reactions (hydrogen and oxygen evolution reactions, respectively) impede the development of practical electrocatalysts that do not use precious metals. This review discusses the rational design of efficient electrocatalysts by exploiting the understanding of alkaline hydrogen evolution reaction and oxygen evolution reaction mechanisms and of the electron structure-activity relationship, as achieved by combining experimental and computational approaches. The enhancement of water splitting not only deals with intrinsic catalytic activity but also includes the aspect of electrical conductivity and stability. Future perspectives to increase the synergy between theory and experiment are also proposed.

Engineering Ultrafine NiFe-LDH into Self-Supporting Nanosheets: Separation-and-Reunion Strategy to Expose Additional Edge Sites for Oxygen Evolution

Here, a strategy is reported to prepare Ni-Fe layered double hydroxide (NiFe-LDH) with abundant exposed edge planes for enhanced oxygen evolution reaction (OER). The edge-to-edge assembly of ultrafine NiFe-LDH directed by graphite-like carbon is performed through a one-step hydrothermal process to form self-supporting nanosheet arrays (named NiFe-LDH/C), in which ascorbic acid is employed as the carbon precursor to control both the platelet size and the assembly mode of NiFe-LDH. Benefiting from the unique structural engineering, NiFe-LDH/C can not only achieve a fast surface reconstruction into the highly active γ-phase structure, but also exposes abundant active edge sites, thus leading to a superior OER performance with the overpotential as low as 234 mV at a current density of 50 mA cm^-2 . Furthermore, density functional theory (DFT) calculations reveal that the unsaturated Fe-sites and the bridge-sites connecting Ni and Fe atoms, which only exist on the edge planes of NiFe-LDH, are the main active centers responsible for promoting the intrinsic OER activity. This work provides a specific and valuable reference for the rational design of high-quality electrocatalysts through structural engineering for renewable energy applications.

Push-Pull Electronic Effects in Surface-Active Sites Enhance Electrocatalytic Oxygen Evolution on Transition Metal Oxides

Sustainable electrocatalysis of the oxygen evolution reaction (OER) constitutes a major challenge for the realization of green fuels. Oxides based on Ni and Fe in alkaline media have been proposed to avoid using critical raw materials. However, their ill-defined structures under OER conditions make the identification of key descriptors difficult. Here, we have studied Fe-Ni-Zn spinel oxides, with a well-defined crystal structure, as a platform to obtain general understanding on the key contributions. The OER reaches maximum performance when: (i) Zn is present in the Spinel structure, (ii) very dense, equimolar 1 : 1 : 1 stoichiometry sites appear on the surface as they allow the formation of oxygen vacancies where Zn favors pushing the electronic density that is pulled by the octahedral Fe and tetrahedral Ni redox pair lowering the overpotential. Our work proves cooperative electronic effects on surface active sites as key to design optimum OER electrocatalysts.

Solid-state redox couple mediated water splitting

The solid-state redox couple is a vital charge transfer medium for electrochemical water splitting. In this Frontiers article, we summarize the versatile application of redox couples in promoting OER kinetics, in decoupling the HER and OER, and in combined electrochemical-thermochemical water splitting. These new ideas unlock vast potential for applying redox-couple-mediated water splitting to the storage of the intermittent and fluctuating energy derived from renewable sources.

High-performance NiOOH/FeOOH electrode for OER catalysis

The outstanding performance of NiOOH/FeOOH-based oxygen evolution reaction (OER) catalysts is rationalized in terms of a bifunctional mechanism involving two distinct active sites. In this mechanism, the OOH_ads reaction intermediate, which unfavorably affects the overall OER activity due to the linear scaling relationship, is replaced by O₂ adsorbed at the active site on FeOOH and H_ads adsorbed at the NiOOH substrate. Here, we use the computational hydrogen electrode method to assess promising models of both the FeOOH catalyst and the NiOOH hydrogen acceptor. These two materials are interfaced in various ways to evaluate their performance as bifunctional OER catalysts. In some cases, overpotentials as low as 0.16 V are found, supporting the bifunctional mechanism as a means to overcome the limitations imposed by linear scaling relationships.

Defects as catalytic sites for the oxygen evolution reaction in Earth-abundant MOF-74 revealed by DFT

The oxygen evolution reaction (OER) is the bottleneck of hydrogen production via water splitting and understanding electrocatalysts at atomic level becomes paramount to enhance the efficiency of this process.

Tunable photocatalytic water splitting and solar-to-hydrogen efficiency in β-PdSe2 monolayer

Direct production of hydrogen from photocatalytic water splitting is a potential solution to overcome global energy crisis.

The Effect of Iron Impurities on Transition Metal Catalysts for the Oxygen Evolution Reaction in Alkaline Environment: Activity Mediators or Active Sites?

There is an ongoing debate on elucidating the actual role of Fe impurities in alkaline water electrolysis, acting either as reactivity mediators or as co-catalysts through synergistic interaction with the main catalyst material. This perspective summarizes the most prominent oxygen evolution reaction (OER) mechanisms mostly for Ni-based oxides as model transition metal catalysts and highlights the effect of Fe incorporation on the catalyst surface in the form of impurities originating from the electrolyte or co-precipitated in the catalyst lattice, in modulating the OER reaction kinetics, mechanism and stability.

Graphic Abstract