Interconnected Network of Core-Shell CoP@CoBiPi for Efficient Water Oxidation Electrocatalysis under Near Neutral Conditions

Human-Machine Collaboration for Accelerated Discovery of Promising Oxygen Evolution Electrocatalysts with On-Demand Elements

A drastically efficient method for identifying electrocatalysts with desirable functionality is a pressing necessity for making a breakthrough in advanced water-electrolyzers toward large-scale green hydrogen production and addressing the significant challenge of carbon neutrality. Despite extensive investigations over the last several centuries, it remains a time-consuming task to identify even one promising affordable electrocatalyst without platinum-group-metal (PGM) for one electrochemical reaction due to its great complexities, particularly for the key anode reaction in the water-electrolyzer of the oxygen evolution reaction (OER). In this study, we demonstrate that a human-machine collaboration based on stepwise-evolving artificial intelligence (se-AI) can significantly shorten the development period of PGM-free multimetal OER electrocatalysts with performance beyond a PGM of RuO2. We were able to reach optimized materials only after 2% experimental trials of the entire candidate pool. The best PGM-free electrocatalyst discovered exhibited excellent activity comparable to RuO2 and, surprisingly, also demonstrated superior stability with a high current density of up to 1000 mA/cm2 at even pH 9.2, which condition is a thermodynamically challenging for typical PGM-free materials. This work illustrates that human's material discovery can be significantly accelerated through collaboration with AI.

A Green Synthesis of CoFe2O4 Decorated ZIF-8 Composite for Electrochemical Oxygen Evolution

Low-cost, sustainable hydrogen production requires noble metal-free electrocatalysts for water splitting. In this study, we prepared zeolitic imidazolate frameworks (ZIF) decorated with CoFe₂O₄ spinel nanoparticles as active catalysts for oxygen evolution reaction (OER). The CoFe₂O₄ nanoparticles were synthesized by converting agricultural bio-waste (potato peel extract) into economically valuable electrode materials. The biogenic CoFe₂O₄ composite showed an overpotential of 370 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 283 mV dec^-1, whereas the ZIF@CoFe₂O₄ composite prepared using an in situ hydrothermal method showed an overpotential of 105 mV at 10 mA cm^-2 and a low Tafel slope of 43 mV dec^-1 in a 1 M KOH medium. The results demonstrated an exciting prospect of high-performance noble metal-free electrocatalysts for low-cost, high-efficiency, and sustainable hydrogen production.

Highly Efficient Conversion of Propargylic Alcohols and Propargylic Amines with CO2 Activated by Noble-Metal-Free Catalyst Cu2 O@ZIF-8

The cyclization reactions of propargylic alcohols and propargylic amines with CO₂ are important in industrial applications, but it was a great challenge that non-noble-metal catalysts catalyzed both reactions under mild conditions. Herein, the catalyst Cu₂ O@ZIF-8 was prepared by encapsulating Cu₂ O nanoparticles into robust ZIF-8, and it can effectively catalyze the cyclization of both propargylic alcohols and propargylic amines with CO₂ into valuable α-alkylidene cyclic carbonates and oxazolidinones with turnover numbers (TONs) of 12.1 and 19.6, which can be recycled at least five times. The mechanisms were further uncovered by NMR, FTIR, ¹³ C isotope-labeling experiments and DFT calculations, in which Cu₂ O and DBU can synergistically activate the C≡C bond and the hydroxy/amino group of substrates. Importantly, it is the first example of a noble-metal-free catalyst that can catalyze both propargylic alcohols and propargylic amines with CO₂ simultaneously.

Nickel supported on Nitrogen-doped biomass carbon fiber fabricated via in-situ template technology for pH-universal electrocatalytic hydrogen evolution

Developing alternatives to noble metal electrocatalysts for hydrogen production via water splitting is a challenging task. Herein, a novel electrocatalyst with Ni nanoparticles disperesed on N-doped biomass carbon fibers (NBCFs) was prepared through a simple in-situ growth process using Ni-ethanediamine complex (NiC) as the structure-directing agent. The in-situ template effect of the NiC facilitated the formation of Ni-N bonds between the Ni nanoparticles and NBCFs, which not only prevented the aggregation and corrosion of the Ni nanoparticles, but also accelerated the electron transfer in the electrochemical reaction, thus improving the hydrogen evolution reaction (HER) activity of the electrocatalyst. As expected, the optimal Ni/NBCF-1-H₂ electrocatalyst exhibited better HER activity over the entire pH range than the control Ni/NBCF-1-N₂ and Ni/NBCF-1-NaBH₄ samples. The HER overpotentials of the Ni/NBCF-1-H₂ electrocatalyst were as low as 47, 56, and 100 mV in alkaline (pH = 13.8), acidic (pH = 0.3), and neutral (pH = 7.3) electrolytes, respectively at the current density of 10 mA cm^-2. Meanwhile, the Ni/NBCF-1-H₂ sample could run continuously for 100 h, exhibiting outstanding stability. This work provides a feasible method for developing efficient and cheap electrocatalysts derived from biomass carbon materials using the in-situ template technology.

Strategies to Develop Earth-Abundant Heterogeneous Oxygen Evolution Reaction Catalysts for pH-Neutral or pH-Near-Neutral Electrolytes

The anodic oxygen evolution reaction (OER) is the bottleneck of water splitting to produce hydrogen due to its sluggish kinetics. In order to lower the energy cost, highly active and cost-efficient OER catalysts need to be used to overcome the OER reaction barrier, especially in neutral pH. Compared to alkaline or acidic electrolytes, pH-neutral or pH-near-neutral electrolytes are considered to be cheaper and safer, and water from rivers and the sea could be used directly under such conditions. However, OER under neutral pH is challenging compared to the OER catalysts for alkaline conditions. Therefore, OER catalysts for neutral or near-neutral pH have not been pursued significantly and, hence, there are limited advances in this area. Here, the progress made in the research and development of earth-abundant heterogeneous catalysts for OER in three pH-neutral or pH-near-neutral systems, namely, the phosphate system, the carbonate system, and the borate system, are systematically reviewed and summarized for the first time. Strategies to develop high-performance OER catalysts for neutral pH are reviewed and summarized. In addition, future challenges and opportunities in this field are discussed, which may shed some light on the future developments of earth-abundant heterogeneous catalysts for OER in neutral or near-neutral pH.

LiCl as Phase-Transfer Catalysts to Synthesize Thin Co2 P Nanosheets for Oxygen Evolution Reaction

Inorganic salts have been widely studied as templates for the synthesis of 2D layer structures. However, these salts normally can only serve as templates without any catalytic activity. Here, we report that LiCl used for the synthesis of ultrathin nanosheets not only serves as template for the synthesis of ultrathin Co₂ P nanosheets with a thickness of 0.7 nm but also acts as a catalyst that induces the phase-transfer from CoP to Co₂ P. The Co₂ P nanosheets have a high electrochemical performance for oxygen evolution reaction.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Template-Directed Growth of Bimetallic Prussian Blue-Analogue Nanosheet Arrays and Their Derived Porous Metal Oxides for Oxygen Evolution Reaction

Coordination polymers (CPs) are ideal precursors for synthesizing porous catalysts. However, the direct thermolysis of CPs is prone to generate agglomerates, greatly reducing the electrical conductivity and active sites of their derived catalysts. The construction of well-ordered CP nanostructures is a promising strategy for alleviating the above issue, but it remains challenging. Here, a facile chemical etching approach is developed for the fabrication of well-aligned three-dimensional (3D) bimetallic Prussian blue-analogue nanosheet arrays. Impressively, the derived porous metal oxide (Fe-NiO) acts as a remarkable oxygen evolution reaction (OER) catalyst, which merely requires overpotentials as low as 218 and 270 mV to achieve 10 and 100 mA cm^-2 in 1.0 m KOH aqueous solution, respectively. The excellent electrocatalytic performance of Fe-NiO is ascribed to the 3D porous nanosheet array architecture, which endows the bimetallic catalyst with abundant electrocatalytic active sites, enhanced surface permeability, and high electronic conductivity. It is expected that the proposed strategy can pave a new way for fabricating highly efficient electrocatalysts for energy storage and conversion.

Hierarchical Cobalt Borate/MXenes Hybrid with Extraordinary Electrocatalytic Performance in Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is a key reaction for many renewable energy storage and conversion techniques. Developing efficient non-precious metal-based electrocatalysts for OER has attracted increasing attention. Herein is reported a strategy to fabricate hierarchical cobalt borate/Ti₃ C₂ T_x MXene (Co-B_i /Ti₃ C₂ T_x ) hybrid through fast chemical reactions at room temperature. This interesting hierarchical structure of Co-B_i /Ti₃ C₂ T_x hybrid is beneficial for exposing more active sites, improving mass diffusion, and charge-transfer pathways for electrochemical reaction. Moreover, a strong interaction between Co-B_i and Ti₃ C₂ T_x ensures efficient charge transfer and facilitates the electrostatic attraction of more anionic intermediates for a fast redox process. Consequently, the hierarchical Co-B_i /Ti₃ C₂ T_x hybrid shows extraordinary OER catalytic activity to deliver a current density of 10 mA cm^-2 at an overpotential of 250 mV, and a Tafel slope of about 53 mV dec^-1 .

Co3 O4 /Fe0.33 Co0.66 P Interface Nanowire for Enhancing Water Oxidation Catalysis at High Current Density

Designing well-defined nanointerfaces is of prime importance to enhance the activity of nanoelectrocatalysts for different catalytic reactions. However, studies on non-noble-metal-interface electrocatalysts with extremely high activity and superior stability at high current density still remains a great challenge. Herein, a class of Co₃ O₄ /Fe_0.33 Co_0.66 P interface nanowires is rationally designed for boosting oxygen evolution reaction (OER) catalysis at high current density by partial chemical etching of Co(CO₃ )_0.5 (OH)·0.11H₂ O (Co-CHH) nanowires with Fe(CN)₆ ^3- , followed by low-temperature phosphorization treatment. The resulting Co₃ O₄ /Fe_0.33 Co_0.66 P interface nanowires exhibit very high OER catalytic performance with an overpotential of only 215 mV at a current density of 50 mA cm^-2 and a Tafel slope of 59.8 mV dec^-1 in 1.0 m KOH. In particular, Co₃ O₄ /Fe_0.33 Co_0.66 P exhibits an obvious advantage in enhancing oxygen evolution at high current density by showing an overpotential of merely 291 mV at 800 mA cm^-2 , much lower than that of RuO₂ (446 mV). Co₃ O₄ /Fe_0.33 Co_0.66 P is remarkably stable for the OER with negligible current loss under overpotentials of 200 and 240 mV for 150 h. Theoretical calculations reveal that Co₃ O₄ /Fe_0.33 Co_0.66 P is more favorable for the OER since the electrochemical catalytic oxygen evolution barrier is optimally lowered by the active Co- and O-sites from the Co₃ O₄ /Fe_0.33 Co_0.66 P interface.

Engineering a stereo-film of FeNi3 nanosheet-covered FeOOH arrays for efficient oxygen evolution

Electrochemical oxygen evolution reaction (OER) can be accelerated by employing transition-metal-based catalysts to obtain the desired activity and durability. Considering the promoting effect of the electrode structure on catalyzing OER, a stereo-film on carbon cloth comprising FeNi3 nanosheet-covered FeOOH (F@FeNi3-CC) was engineered by the hydrothermal reaction and subsequent synchronous electrodeposition of Fe and Ni ions. In F@FeNi3-CC, the FeOOH array not only provides the Fe source of FeNi3 nanosheets during the cathodic electrodeposition, but also functions as the support for the ultrathin FeNi3 nanosheets. Such a stereo-film with good electrolyte-permeability also offers expedited electrolyte/reactant transmission paths. The tailored FeNi3 nanosheet offers abundant exposed catalytic sites by virtue of the in situ derived hydroxide layer during anodic oxidation and also acts as the current collector to accelerate charge transport. The F@FeNi3-CC electrode yields outstanding catalytic activity towards OER in alkaline media, particularly at low potential of 1.50 V and large current density of 100 mA cm-2, accompanied with the excellent long-term stability. These results are significant for the construction of stereo-film electrodes for various engineering applications.

A Co-MOF nanosheet array as a high-performance electrocatalyst for the oxygen evolution reaction in alkaline electrolytes

A Co-MOF nanosheet array on Ni foam (Co-MOF/NF) acts as a superior electrocatalyst for the oxygen evolution reaction, needing an overpotential of only 311 mV to drive a geometrical catalytic current density of 50 mA cm⁻² in 1.0 M KOH.

Highly efficient oxygen evolution electrocatalysts prepared by using reduction-engraved ferrites on graphene oxide

A simple and efficient method was explored for synthesizing efficient ferrite-based OER electrocatalysts by using reduction-engraved ultrafine ferrite nanoparticles on a conductive GO support.

Oriented CuCo2S4 nanograss arrays/Ni foam as an electrode for a high-performance all-solid-state supercapacitor

An oriented nanograss array consisting of CuCo₂S₄ nanocrystallines was directly prepared on Ni foam by a hydrothermal method. The uniform arrays with a single blade diameter of ∼100 nm and length of ∼4 μm can grow tightly on Ni foam without any binders. The ordered one-dimensional structure facilitates the electron transport to Ni substrates along the axial direction. The electrochemical properties were presented in three- and two-electrode configurations, demonstrating an enhanced specific capacitance and a long-term cycling stability. As a practical all-solid-state device, it achieved a high energy density of 31.88 W h kg^-1 at a power density of 3.20 kW kg^-1. Even at a higher power density of 15.23 W h kg^-1, the device still had an energy density of 16.5 kW kg^-1. After 5000 cycles, the retention ratio was higher than 99% at high current density of 11.36 A g^-1.

Architecting a Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as an Efficient Oxygen Evolution Electrocatalyst

To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size, and structure of the materials. We report here a facile preparation of orthorhombic CoTe₂ nanocrystals embedded in an N-doped graphitic carbon matrix to form a 3D architecture with a size of ∼500 nm and abundant mesopores of ∼4 nm for the oxygen evolution reaction (OER). The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm^-2, which is much lower than that for pristine CoTe₂ powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe₂ are responsible for the enhancement of the electrocatalytic performance.

Anion-exchange synthesis of a nanoporous crystalline CoB2O4 nanowire array for high-performance water oxidation electrocatalysis in borate solution

Developing nanoporous nanoarray electrocatalysts for efficient water oxidation in environmentally benign media is highly desired but still remains a key challenge. In this communication, we report the fabrication of a nanoporous crystalline CoB₂O₄ nanowire array on Ti mesh (CoB₂O₄/TM) from a Co(OH)F nanowire array on Ti mesh (Co(OH)F/TM) via an anion-exchange reaction. As a three dimensional (3D) catalyst electrode for water oxidation, CoB₂O₄/TM exhibits superior catalytic activity and needs an overpotential of only 446 mV to drive a geometrical catalytic current density of 10 mA cm^-2 in 0.1 M potassium borate (pH = 9.2). Notably, this catalyst also shows strong long-term electrochemical durability with high turnover frequency values of 0.19 and 0.81 mol O₂ per s at overpotentials of 400 and 500 mV, respectively.

Bimetallic Nickel-Substituted Cobalt-Borate Nanowire Array: An Earth-Abundant Water Oxidation Electrocatalyst with Superior Activity and Durability at Near Neutral pH

There is an urgent demand to develop earth-abundant electrocatalysts for efficient and durable water oxidation under mild conditions. A nickel-substituted cobalt-borate nanowire array is developed on carbon cloth (Ni-Co-Bi/CC) via oxidative polarization of NiCo₂ S₄ nanoarray in potassium borate (K-Bi). As a bimetallic electrocatalyst for water oxidation, such Ni-Co-Bi/CC is superior in catalytic activity and durability in 0.1 m K-Bi (pH: 9.2), with a turnover frequency of 0.33 mol O₂ s^-1 at the overpotential of 500 mV and nearly 100% Faradaic efficiency. To drive a geometrical catalytic current density of 10 mA cm^-2 , it only needs overpotential of 388 mV, 34 mV less than that for Co-Bi/CC, outperforming reported non-noble-metal catalysts operating under benign conditions. Notably, its activity is maintained over 80 000 s. Density functional theory calculations suggest that the O* to OOH* conversion is the rate-determining step and Ni substitution decreases the free energy on Co-Bi from 2.092 to 1.986 eV.

Core-Shell NiFe-LDH@NiFe-Bi Nanoarray: In Situ Electrochemical Surface Derivation Preparation toward Efficient Water Oxidation Electrocatalysis in near-Neutral Media

The corrosion issue with acidic and alkaline water electrolyzers can be avoided by developing water oxidation catalysts performing efficiently under benign conditions. In this Letter, we report that a NiFe-borate layer can be generated on a NiFe-layered double hydroxide nanosheet array hydrothermally grown on carbon cloth via an in situ electrochemical surface derivation process in potassium borate (K-B_i) solution. The resulting 3D NiFe-LDH@NiFe-B_i nanoarray (NiFe-LDH@NiFe-B_i/CC) demonstrates high activity for water oxidation, demanding overpotentials of 444 and 363 mV to achieve 10 mA cm^-2 in 0.1 and 0.5 M K-B_i (pH: 9.2), respectively, rivaling the performances of most reported non-noble-metal catalysts in near-neutral media. Notably, this electrode also shows strong electrochemical durability with a high turnover frequency of 0.54 mol O₂ s^-1 at overpotential of 600 mV. All these features promise its use as an efficient earth-abundant catalyst material for water oxidation under eco-friendly conditions.

Core-shell CoFe2O4@Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pH

The exploration of high-performance and earth-abundant water oxidation catalysts operating under mild conditions is highly attractive and challenging. In this communication, core-shell CoFe₂O₄@Co-Fe-Bi nanoarray on carbon cloth (CoFe₂O₄@Co-Fe-Bi/CC) was successfully fabricated by in situ surface amorphization of CoFe₂O₄ nanoarray on CC (CoFe₂O₄/CC). As a 3D water oxidation electrode, CoFe₂O₄@Co-Fe-Bi/CC shows outstanding activity with an overpotential of 460 mV to drive a geometrical catalytic current density of 10 mA cm^-2 in 0.1 M potassium borate (pH 9.2). Notably, it also demonstrates superior long-term durability for at least 20 h with 96% Faradic efficiency. Density functional theory calculations indicate that the conversion from OOH* to O₂ is the rate-limiting step and the high water oxidation activity of CoFe₂O₄@Co-Fe-Bi/CC is associated with the lower free energy of 1.84 eV on a Co-Fe-Bi shell.

Anodically Grown Binder-Free Nickel Hexacyanoferrate Film: Toward Efficient Water Reduction and Hexacyanoferrate Film Based Full Device for Overall Water Splitting

One of the challenges in obtaining hydrogen economically by electrochemical water splitting is to identify and substitute cost-effective earth-abundant materials for the traditionally used precious-metal-based water-splitting electrocatalysts. Herein, we report the electrochemical formation of a thin film of nickel-based Prussian blue analogue hexacyanoferrate (Ni-HCF) through the anodization of a nickel substrate in ferricyanide electrolyte. As compared to the traditionally used Nafion-binder-based bulk film, the anodically obtained binder-free Ni-HCF film demonstrates superior performance in the electrochemical hydrogen evolution reaction (HER), which is highly competitive with that shown by a Pt-plate electrode. The HER onset and the benchmark cathodic current density of 10 mA cm^-2 were achieved at small overpotentials of 15 mV and 0.2 V (not iR-corrected), respectively, in 1 M KOH electrolyte, together with the long-term electrochemical durability of the film. Further, a metal-HCF-electrode-based full water-splitting device consisting of the binder-free Ni-HCF film on a Ni plate and a one-dimensional Co-HCF film on carbon paper as the electrodes for the HER and the oxygen evolution reaction (OER), respectively, was designed and was found to demonstrate very promising performance for overall water splitting.

Supercritical fluid processing for the synthesis of NiS2 nanostructures as efficient electrocatalysts for electrochemical oxygen evolution reactions

A facile supercritical fluid process was demonstrated for the synthesis of cubic NiS₂ nanostructures for efficient electrochemical oxygen evolution reactions.

An amorphous FeMoS4 nanorod array toward efficient hydrogen evolution electrocatalysis under neutral conditions

As a durable catalyst, an FeMoS₄ nanorod array on carbon cloth shows high activity for hydrogen evolution in neutral media, achieving a geometrical catalytic current density of 10 mA cm⁻² at an overpotential of 204 mV.