Understanding the Oxygen Evolution Reaction on a Two-Dimensional NiO2 Catalyst

Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to β-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals β-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate β-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared β-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Efficient synergism of NiO-NiSe2 nanosheet-based heterostructures shelled titanium nitride array for robust overall water splitting

Water splitting via the use of an efficient catalyst is a clean and cost-effective approach to produce green hydrogen. In this study, we successfully developed a novel hybrid coming from thin NiO-NiSe₂ nanosheet-based heterostructure shelled high-conductive titanium nitride nanoarrays (TiN@NiO-NiSe₂) supported on carbon cloth (CC) via an optimized in-situ synthesis strategy. The hybrid possesses unique physicochemical properties due to the combination of merits from individual components and their synergistic effects, thereby boosting number and type of electroactive sites, reasonably adjusting Gibbs free adsorption energy, and promoting charge/mass transfers. As a potential bifunctional electrocatalyst, the hybrid requires low overpotentials of 115 and 240 mV to reach a current response of 10 mA cm^-2 towards hydrogen evolution reaction and oxygen evolution reaction in 1.0 M KOH, respectively. Therefore, an electrolyzer of the TiN@NiO-NiSe₂ on CC exhibits a low operation voltage of 1.57 V at 10 mA cm^-2 together with a prospective durability, which exceed behaviors of Pt/C//RuO₂ as well as recently reported bifunctional electrocatalysts. The results suggest a promising approach for developing cost-effective catalyst towards green hydrogen production via water splitting.

Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides

Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non-intrinsic activity metrics, thus hampering the construction of consistent structure-activity-relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline α-M_A (II)M_B (III) LDH and β-M_A (OH)₂ electrocatalysts (M_A =Ni, Co, and M_B =Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe-free Co-containing catalysts > Fe-Co-free Ni-based catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual-metal-site nature of the reaction centers, which lead to composition-dependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance.

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.

Electronic Modulation of Hierarchical Spongy Nanosheets toward Efficient and Stable Water Electrolysis

The energy conversion efficiency of water electrolysis is determined by the activity of selected catalysts. Ideal catalysts should possess not only porous architecture for high-density assembly of active sites but also a subtle electronic configuration for the optimized activity at each site. In this context, the development of stable porous hosting materials that allow the incorporation of various metal elements is highly desirable for both experimental optimization and theoretical comparison/prediction. Herein, MOF-derived spongy nanosheet arrays constructed by assembly of carbon encapsulated hetero-metal doped Ni₂ P nanoparticles is presented as a superior bifunctional electrocatalyst for water splitting. This hierarchical structure can be stably retained when secondary metal dopants are introduced, providing a flexible platform for electronic modulation. The catalytic origin of activity enhancement via metal (Fe, Cr, and Mn) doping is deciphered through experimental and theoretical investigations. Combining the advantages in both morphological and electronic structures, the optimized catalyst NiMn-P exhibits remarkable activity in both hydrogen and oxygen evolution in the alkaline media, with an ultrasmall cell voltage of 1.49 V (at 10 mA cm^-2 ) and high durability for at least 240 h.

In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution

NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

Fast prediction of oxygen reduction reaction activity on carbon nanotubes with a localized geometric descriptor

By using the pyramidalization angle as a localized geometric descriptor for oxygen reduction reaction (ORR) activity of carbon nanotubes (CNTs), we show the ORR activity of these systems can be readily predicted with mere structural optimization of CNTs.

Bonding state synergy of the NiF2/Ni2P hybrid with the co-existence of covalent and ionic bonds and the application of this hybrid as a robust catalyst for the energy-relevant electrooxidation of water and urea

Among the energy-relevant electrochemical reactions, the electrochemical water and urea oxidation reactions are very significant for solving the increasing energy crisis and environmental pollution. Herein, the NiF2/Ni2P hybrid catalyst, in which covalent and ionic bonds co-existed, was found to be a very active catalyst for these electrochemical reactions occuring during the electrolysis of water. The bonding states of the covalent and ionic bonds were verified by the crystal structure and surface chemical state revealed by spectral analysis. As a bifunctional catalyst for the electrooxidation of water and urea, the NiF2/Ni2P hybrid structure demonstrated higher catalytic activity, kinetics and stability in the catalytic reaction than the individual components NiF2 and Ni2P under the same conditions. Specifically, an overpotential as low as 283 mV could drive the benchmark current density of 10 mA cm-2 for the oxygen evolution reaction, significantly lower than the overpotential required for the NiF2 (393 mV) and Ni2P materials (342 mV); the maximum current density for urea electrooxidation could reach 157.35 mA cm-2 at 1.53 V, which was much higher than those of NiF2 (23.55 mA cm-2) and Ni2P (102.72 mA cm-2). The catalytic performance also outperformed those of the recently reported similar advanced catalysts, and the high performance could be attributed to the highly exposed active sites, rough surface area, excellent charge transfer ability, and especially, the synergistic effects between the covalent and ionic bonds in the catalyst system. Using a commercial Pt/C catalyst as a cathode, the cell potential for urea-assisted water electrolysis could be reduced to 1.5 V to obtain the current density of nearly 40 mA cm-2 in a two-electrode system (Pt/C||NiF2/Ni2P), about 300 mV less than that required for water electrolysis in the general alkaline electrolyte. The current study demonstrates the significance of bonding state synergy in an advanced catalyst for water electrolysis and sheds some light on catalyst development in energy chemistry.

Enhancing Electrocatalytic Water Splitting by Strain Engineering

Electrochemical water splitting driven by sustainable energy such as solar, wind, and tide is attracting ever-increasing attention for sustainable production of clean hydrogen fuel from water. Leveraging these advances requires efficient and earth-abundant electrocatalysts to accelerate the kinetically sluggish hydrogen and oxygen evolution reactions (HER and OER). A large number of advanced water-splitting electrocatalysts have been developed through recent understanding of the electrochemical nature and engineering approaches. Specifically, strain engineering offers a novel route to promote the electrocatalytic HER/OER performances for efficient water splitting. Herein, the recent theoretical and experimental progress on applying strain to enhance heterogeneous electrocatalysts for both HER and OER are reviewed and future opportunities are discussed. A brief introduction of the fundamentals of water-splitting reactions, and the rationalization for utilizing mechanical strain to tune an electrocatalyst is given, followed by a discussion of the recent advances on strain-promoted HER and OER, with special emphasis given to combined theoretical and experimental approaches for determining the optimal straining effect for water electrolysis, along with experimental approaches for creating and characterizing strain in nanocatalysts, particularly emerging 2D nanomaterials. Finally, a vision for a future sustainable hydrogen fuel community based on strain-promoted water electrolysis is proposed.

Radially Aligned Hierarchical Nickel/Nickel-Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting

Designing well-controlled hierarchical structures on micrometer and nanometer scales represents one of the most important approaches for upgrading the catalytic abilities of electrocatalysts. Although NiFe (oxy)hydroxide has been widely studied as a water oxidation catalyst due to its high catalytic capability and abundance, its structural manipulation has been greatly restricted due to its inherent crystallographic stacking feature. In this work, we report for the first time the construction of a nanotube structure of NiFe (oxy)hydroxide with an inner Ni-rich layer, which was radially aligned on a macroporous nickel foam. Such a hierarchically structured material realized several crucial factors that are essential for excellent catalytic behaviors, including abundant catalytic sites, a high surface area, efficient ionic and electronic transport, etc., and the designed catalyst exhibited competitive electrocatalytic activity for reaction of not only oxygen evolution but also hydrogen evolution, which is very rare. As a result, this novel material was well-suited for the use as a bifunctional catalyst in an integrated water-splitting electrolyzer, which could be even driven by a single AA battery or a 1.5 V solar cell, outperforming a benchmark catalyst of noble-metal ruthenium-platinum combinations and most state-of-the-art electrocatalysts. The work provided important suggestions for the rational modulation of catalysts with new structures targeted for high-performance electrodes used in electrochemical applications.