5f Covalency Synergistically Boosting Oxygen Evolution of UCoO4 Catalyst

Electrochemically Induced Ru/CoOOH Synergistic Catalyst as Bifunctional Electrode Materials for Alkaline Overall Water Splitting

Efficient and affordable price bifunctional electrocatalysts based on transition metal oxides for oxygen and hydrogen evolution reactions have a balanced efficiency, but it remains a significant challenge to control their activity and durability. Herein, a trace Ru (0.74 wt.%) decorated ultrathin CoOOH nanosheets (≈4 nm) supported on the surface of nickel foam (Ru/CoOOH@NF) is rationally designed via an electrochemically induced strategy to effectively drive the electrolysis of alkaline overall water splitting. The as-synthesized Ru/CoOOH@NF electrocatalysts integrate the advantages of a large number of different HER (Ru nanoclusters) and OER (CoOOH nanosheets) active sites as well as strong in-suit structure stability, thereby exhibiting exceptional catalytic activity. In particular, the ultra-low overpotential of the HER (36 mV) and the OER (264 mV) are implemented to achieve 10 mA cm-2. Experimental and theoretical calculations also reveal that Ru/CoOOH@NF possesses high intrinsic conductivity, which facilitates electron release from H2O and H-OH bond breakage and accelerates electron/mass transfer by regulating the charge distribution. This work provides a new avenue for the rational design of low-cost and high-activity bifunctional electrocatalysts for large-scale water-splitting technology and expects to help contribute to the creation of various hybrid electrocatalysts.

Balance between FeIV-NiIV synergy and Lattice Oxygen Contribution for Accelerating Water Oxidation

Hydrogen obtained from electrochemical water splitting is the most promising clean energy carrier, which is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Thus, the development of an efficient OER electrocatalyst using nonprecious 3d transition elements is desirable. Multielement synergistic effect and lattice oxygen oxidation are two well-known mechanisms to enhance the OER activity of catalysts. The latter is generally related to the high valence state of 3d transition elements leading to structural destabilization under the OER condition. We have found that Al doping in nanosheet Ni-Fe hydroxide exhibits 2-fold advantage: (1) a strong enhanced OER activity from 277 mV to 238 mV at 10 mA cm-2 as the Ni valence state increases from Ni3.58+ to Ni3.79+ observed from in situ X-ray absorption spectra. (2) Operational stability is strengthened, while weakness is expected since the increased NiIV content with 3d8L2 (L denotes O 2p hole) would lead to structural instability. This contradiction is attributed to a reduced lattice oxygen contribution to the OER upon Al doping, as verified through in situ Raman spectroscopy, while the enhanced OER activity is interpreted as an enormous gain in exchange energy of FeIV-NiIV, facilitated by their intersite hopping. This study reveals a mechanism of Fe-Ni synergy effect to enhance OER activity and simultaneously to strengthen operational stability by suppressing the contribution of lattice oxygen.

Anion Structure Regulation of Cobalt Silicate Hydroxide Endowing Boosted Oxygen Evolution Reaction

Transition metal silicates (TMSs) are attempted for the electrocatalyst of oxygen evolution reaction (OER) due to their special layered structure in recent years. However, defects such as low theoretical activity and conductivity limit their application. Researchers always prefer to composite TMSs with other functional materials to make up for their deficiency, but rarely focus on the effect of intrinsic structure adjustment on their catalytic activity, especially anion structure regulation. Herein, applying the method of interference hydrolysis and vacancy reserve, new silicate vacancies (anionic regulation) are introduced in cobalt silicate hydroxide (CoSi), named SV-CoSi, to enlarge the number and enhance the activity of catalytic sites. The overpotential of SV-CoSi declines to 301 mV at 10 mA cm-2 compared to 438 mV of CoSi. Source of such improvement is verified to be not only the increase of active sites, but also the positive effect on the intrinsic activity due to the enhancement of cobalt-oxygen covalence with the variation of anion structure by density functional theory (DFT) method. This work demonstrates that the feasible intrinsic anion structure regulation can improve OER performance of TMSs and provides an effective idea for the development of non-noble metal catalyst for OER.

Modulating 3d Charge State via Halogen Ions in Neighboring Molecules of Metal-Organic Frameworks for Improving Water Oxidation

Modulating the coordination environment of the metal active center is an effective method to boost the catalytic performances of metal-organic frameworks (MOFs) for oxygen evolution reaction (OER). However, little attention has been paid to the halogen effects on the ligands engineering. Herein, a series of MOFs X─FeNi-MOFs (X = Br, Cl, and F) is constructed with different coordination microenvironments to optimize OER activity. Theoretical calculations reveal that with the increase in electronegativity of halogen ions in terephthalic acid molecular (TPA), the Bader charge of Ni atoms gets larger and the Ni-3d band center and O-2p bands move closer to the Fermi level. This indicates that an increase in ligand negativity of halogen ions in TPA can promote the adsorption ability of catalytic sites to oxygen-containing intermediates and reduce the activation barrier for OER. Experimental also demonstrates that F─FeNi-MOFs exhibit the highest catalytic activity with an ultralow overpotential of 218 mV at 10 mA cm-2, outperforming most otate-of-the-art Fe/Co/Ni-based MOFs catalysts, and the enhanced mass activity by seven times compared with that for the sample before ligands engineering. This work opens a new avenue for the realization of the modulation of NiFe─O bonding by halogen ion in TPA and improves the OER performance of MOFs.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Engineering High-Spin State Cobalt Cations in Sulfide Spinel for Enhancing Water Oxidation

The spin states of active sites have a significant impact on the adsorption/desorption ability of the reaction intermediates during the oxygen evolution reaction (OER). Sulfide spinel is not generally considered a highly efficient OER catalyst owing to the low spin state of Co3+ and the lack of unpaired electrons available for adsorption of reaction intermediates. Herein, it is proposed a novel Nd-evoked valence electronic adjustment strategy to engineer the spin state of Co ions. The unique f-p-d orbital electronic coupling effect stimulates the rearrangement of Co d orbital electrons and increases the eg electron filling to achieve high-spin state Co ions, which promotes charge transport by propagating a spin channel and generates a high number of active sites for intermediate adsorption. The optimized CuCo1.75 Nd0.25 S4 catalyst exhibits outstanding electrocatalytic properties with a low overpotential of 320 mV at 500 mA cm-2 and a 48 h stability at 300 mA cm-2 . In situ synchrotron radiation infrared spectra confirm the quick accumulation of key *OOH and *O intermediates. This work deepens the comprehensive understanding of the relationship between OER activity and spin configurations of Co ions and offers a new design strategy for spinel compound catalysts.

Facilitating alkaline hydrogen evolution reaction on the hetero-interfaced Ru/RuO2 through Pt single atoms doping

Exploring an active and cost-effective electrocatalyst alternative to carbon-supported platinum nanoparticles for alkaline hydrogen evolution reaction (HER) have remained elusive to date. Here, we report a catalyst based on platinum single atoms (SAs) doped into the hetero-interfaced Ru/RuO2 support (referred to as Pt-Ru/RuO2), which features a low HER overpotential, an excellent stability and a distinctly enhanced cost-based activity compared to commercial Pt/C and Ru/C in 1 M KOH. Advanced physico-chemical characterizations disclose that the sluggish water dissociation is accelerated by RuO2 while Pt SAs and the metallic Ru facilitate the subsequent H* combination. Theoretical calculations correlate with the experimental findings. Furthermore, Pt-Ru/RuO2 only requires 1.90 V to reach 1 A cm-2 and delivers a high price activity in the anion exchange membrane water electrolyzer, outperforming the benchmark Pt/C. This research offers a feasible guidance for developing the noble metal-based catalysts with high performance and low cost toward practical H2 production.

4f-2p-3d orbital overlap in a metal-organic framework-derived CeO2/CeCo-LDH heterostructure promotes water oxidation

Herein, we have demonstrated a facile method for the synthesis of CeO2/Ce-Co-LDH heterostructures using zeolitic imidazolate framework-67 as the precursor. The Ce-incorporation in Co-LDH results in 4f-2p-3d orbital overlap to tune the electronic structure whereas the oxygen-deficient CeO2 controls the interface charge transfer. This results in excellent water oxidation activity to attain 500 mA cm-2 current density at 320 overpotential.

Boosting CO2 Electroreduction to C2H4 via Unconventional Hybridization: High-Order Ce4+ 4f and O 2p Interaction in Ce-Cu2O for Stabilizing Cu. ACS NANO 2023. [PMID: 37410800 DOI: 10.1021/acsnano.3c03952]

Efficient conversion of carbon dioxide (CO₂) into value-added materials and feedstocks, powered by renewable electricity, presents a promising strategy to reduce greenhouse gas emissions and close the anthropogenic carbon loop. Recently, there has been intense interest in Cu₂O-based catalysts for the CO₂ reduction reaction (CO₂RR), owing to their capabilities in enhancing C-C coupling. However, the electrochemical instability of Cu⁺ in Cu₂O leads to its inevitable reduction to Cu⁰, resulting in poor selectivity for C₂₊ products. Herein, we propose an unconventional and feasible strategy for stabilizing Cu⁺ through the construction of a Ce⁴⁺ 4f-O 2p-Cu⁺ 3d network structure in Ce-Cu₂O. Experimental results and theoretical calculations confirm that the unconventional orbital hybridization near E_f based on the high-order Ce⁴⁺ 4f and 2p can more effectively inhibit the leaching of lattice oxygen, thereby stabilizing Cu⁺ in Ce-Cu₂O, compared with traditional d-p hybridization. Compared to pure Cu₂O, the Ce-Cu₂O catalyst increased the ratio of C₂H₄/CO by 1.69-fold during the CO₂RR at -1.3 V. Furthermore, in situ and ex situ spectroscopic techniques were utilized to track the oxidation valency of copper under CO₂RR conditions with time resolution, identifying the well-maintained Cu⁺ species in the Ce-Cu₂O catalyst. This work not only presents an avenue to CO₂RR catalyst design involving the high-order 4f and 2p orbital hybridization but also provides deep insights into the metal-oxidation-state-dependent selectivity of catalysts.

Unusual double ligand holes as catalytic active sites in LiNiO2

Designing efficient catalyst for the oxygen evolution reaction (OER) is of importance for energy conversion devices. The anionic redox allows formation of O-O bonds and offers higher OER activity than the conventional metal sites. Here, we successfully prepare LiNiO₂ with a dominant 3d⁸L configuration (L is a hole at O 2p) under high oxygen pressure, and achieve a double ligand holes 3d⁸L² under OER since one electron removal occurs at O 2p orbitals for Ni^III oxides. LiNiO₂ exhibits super-efficient OER activity among LiMO₂, RMO₃ (M = transition metal, R = rare earth) and other unary 3d catalysts. Multiple in situ/operando spectroscopies reveal Ni^III→Ni^IV transition together with Li-removal during OER. Our theory indicates that Ni^IV (3d⁸L²) leads to direct O-O coupling between lattice oxygen and *O intermediates accelerating the OER activity. These findings highlight a new way to design the lattice oxygen redox with enough ligand holes created in OER process.

Ni-Doped CuO Nanoarrays Activate Urea Adsorption and Stabilizes Reaction Intermediates to Achieve High-Performance Urea Oxidation Catalysts

Urea oxidation reaction (UOR) with a low equilibrium potential offers a promising route to replace the oxygen evolution reaction for energy-saving hydrogen generation. However, the overpotential of the UOR is still high due to the complicated 6e^- transfer process and adsorption/desorption of intermediate products. Herein, utilizing a cation exchange strategy, Ni-doped CuO nanoarrays grown on 3D Cu foam are synthesized. Notably, Ni-CuO NAs/CF requires a low potential of 1.366 V versus a reversible hydrogen electrode to drive a current density of 100 mA cm^-2 , outperforming various benchmark electrocatalysts and maintaining robust stability in alkaline media. Theoretical and experimental studies reveal that Ni as the driving force center can effectively enhance the urea adsorption and stabilize CO*/NH* intermediates toward the UOR. These findings suggest a new direction for constructing nanostructures and modulating electronic structures, ultimately developing promising Cu-based electrode catalysts.

Strong Electronic Orbit Coupling between Cobalt and Single-Atom Praseodymium for Boosted Nitrous Oxide Decomposition on Co3O4 Catalyst

Nitrous oxide (N₂O) has gained increasing attention as an important noncarbon dioxide greenhouse gas, and catalytic decomposition is an effective method of reducing its emissions. Here, Co₃O₄ was synthesized by the sol-gel method and single-atom Pr was confined in its matrix to improve the N₂O decomposition performance. It was observed that the reaction rate varied in a volcano-like pattern with the amount of doped Pr. A N₂O decomposition reaction rate 5-7.5 times greater than that of pure Co₃O₄ is achieved on the catalyst with a Pr/Co molar ratio of 0.06:1, and further Pr doping reduced the activity due to PrO_x cluster formation. Combined with X-ray photoelectron spectroscopy, X-ray absorption fine structure, density functional theory and in situ near-ambient pressure X-ray photoelectron spectroscopy, it was demonstrated that the single-atom doped Pr in Co₃O₄ generates the "Pr 4f-O 2p-Co 3d" network, which redistributes the electrons in Co₃O₄ lattice and increases the t_2g electrons at the tetracoordinated Co²⁺ sites. This coupling between the Pr 4f orbit and Co²⁺ 3d orbit triggers the formation of a 4f-3d electronic ladder, which accelerates the electron transfer from Co²⁺ to the 3π* antibonding orbital of N₂O, thus contributing to the N-O bond cleavage. Moreover, the energy barrier for each elementary reaction in the decomposition process of N₂O is reduced, especially for O₂ desorption. Our work provides a theoretical grounding and reference for designing atomically modified catalysts for N₂O decomposition.

Tremella-Like Ni-NiO with O-Vacancy Heterostructure Nanosheets Grown In Situ on MXenes for Highly Efficient Hydrogen and Oxygen Evolution

Electronic modulation via heterostructures or vacancies has been recently regarded as an effective strategy to improve electrocatalytic activity by optimizing the adsorption free energies of hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) active intermediates during the reaction. Herein, tremella-like Ni-NiO with O-vacancy heterostructure nanosheets grown in situ on Ti₃C₂T_x MXenes (Ni-NiO/Ti₃C₂T_x MXene) are fabricated via a facile strategy. Benefitting from the heterointerfaces between Ni and NiO, the synergetic coupling effects of MXenes and Ni-NiO heterostructures, the O-vacancies, and the unique architecture, the as-prepared Ni-NiO/Ti₃C₂T_x MXene showed superior activity toward the HER and OER in alkaline electrolyte, only requiring overpotentials of 72 mV for the HER and 248 mV for the OER to offer 10 mA cm^-2. Density functional theory (DFT) calculations revealed that Ni-NiO with O-vacancies can effectively increase the electron density around the Fermi level and modulate the Gibbs free energies of the intermediates during catalytic reactions, thus accelerating the reaction kinetics.

Implanting an Electron Donor to Enlarge the d-p Hybridization of High-Entropy (Oxy)hydroxide: A Novel Design to Boost Oxygen Evolution

High-entropy (HE) electrocatalysts are becoming a research hotspot due to their interesting "cocktail effect" and have great potential for tailored catalytic properties. However, it is still a great challenge to illustrate their inherent catalytic mechanism for the "cocktail effect", and there is also a paucity of quantitative descriptors to characterize the specific catalytic activity and give logical design strategies for HE systems. Herein, the unexpected activation of all metal sites in HE Cu-Co-Fe-Ag-Mo (oxy)hydroxides for the oxygen evolution reaction (OER) is reported, and it is found that metal-oxygen d-p hybridization, as an effective descriptor, can indicate the intrinsic activity of each metal site. According to the quantitative hybridization, introducing an electron donor (e.g., Ag) is raised and verified to reinforce the electrocatalytic activity of the HE system. Consequently, Ag-decorated Co-Cu-Fe-Ag-Mo (oxy)hydroxide (Ag@CoCuFeAgMoOOH) electrocatalysts are constructed by an electrochemical reconstruction method, and their OER performances are thoroughly characterized. The Ag@CoCuFeAgMoOOH is verified with a low overpotential (270 mV at 100 mA cm^-2 ) and a small Tafel slope (35.3 mV dec^-1 ), as well as good electrochemical stability. The favorable activity of the electron donor and underlying synergistic "cocktail effect" are demonstrated and disclosed. This work opens up a new strategy to guide the design/fabrication of advanced HE electrocatalysts.