Coupling Magnetic Single‐Crystal Co2Mo3O8 with Ultrathin Nitrogen‐Rich Carbon Layer for Oxygen Evolution Reaction

Transition‐metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC‐800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N‐rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm^?2 and 422 mV@40 mA cm^?2, and a full water‐splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm^?2 in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate‐determining step to form *OOH.     

Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical Overall‐Water‐Splitting Activity

To achieve sustainable production of H₂ fuel through water splitting, low‐cost electrocatalysts for the hydrogen‐evolution reaction (HER) and the oxygen‐evolution reaction (OER) are required to replace Pt and IrO₂ catalysts. Herein, for the first time, we present the interface engineering of novel MoS₂/Ni₃S₂ heterostructures, in which abundant interfaces are formed. For OER, such MoS₂/Ni₃S₂ heterostructures show an extremely low overpotential of ca. 218 mV at 10 mA cm^?2, which is superior to that of the state‐of‐the‐art OER electrocatalysts. Using MoS₂/Ni₃S₂ heterostructures as bifunctional electrocatalysts, an alkali electrolyzer delivers a current density of 10 mA cm^?2 at a very low cell voltage of ca. 1.56 V. In combination with DFT calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygen‐containing intermediates, thus accelerating the overall electrochemical water splitting.     

Non‐3d Metal Modulation of a Cobalt Imidazolate Framework for Excellent Electrocatalytic Oxygen Evolution in Neutral Media

Cobalt imidazolate frameworks are classical electrocatalysts for the oxygen evolution reaction (OER) but suffer from the relatively low activity. Here, a non‐3d metal modulation strategy is presented for enhancing the OER activity of cobalt imidazolate frameworks. Two isomorphous frameworks [Co₄(MO₄)(eim)₆] (M=Mo or W, Heim=2‐ethylimidazole) having Co(eim)₃(MO₄) units and high water stabilities were designed and synthesized. In different neutral media, the Mo‐modulated framework coated on a glassy carbon electrode shows the best OER performances (1 mA cm^?2 at an overpotential of 210 mV in CO₂‐saturated 0.5 m KHCO₃ electrolyte and 2/10/22 mA cm^?2 at overpotential of 388/490/570 mV in phosphate buffer solution) among non‐precious metal catalysts and even outperforms RuO₂. Spectroscopic measurements and computational simulations revealed that the non‐3d metals modulate the electronic structure of Co for optimum reactant/product adsorption and tailor the energy of rate‐determining step to a more moderate value.     

Amorphous Nanocages of Cu‐Ni‐Fe Hydr(oxy)oxide Prepared by Photocorrosion For Highly Efficient Oxygen Evolution

Electrochemical water splitting requires efficient, low‐cost water oxidation catalysts to accelerate the sluggish kinetics of the water oxidation reaction. A rapid photocorrosion method is now used to synthesize the homogeneous amorphous nanocages of Cu‐Ni‐Fe hydr(oxy)oxide as a highly efficient electrocatalyst for the oxygen evolution reaction (OER). The as‐fabricated product exhibits a low overpotential of 224 mV on a glassy carbon electrode at 10 mA cm^?2 (even lower down to 181 mV when supported on Ni foam) with a Tafel slope of 44 mV dec^?1 for OER in an alkaline solution. The obtained catalyst shows an extraordinarily large mass activity of 1464.5 A g^?1 at overpotential of 300 mV, which is the highest mass activity for OER. This synthetic strategy may open a brand new pathway to prepare copper‐based ternary amorphous nanocages for greatly enhanced oxygen evolution.     

SrNb0.1Co0.7Fe0.2O3−δ Perovskite as a Next‐Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution

The perovskite SrNb_0.1Co_0.7Fe_0.2O_3?δ (SNCF) is a promising OER electrocatalyst for the oxygen evolution reaction (OER), with remarkable activity and stability in alkaline solutions. This catalyst exhibits a higher intrinsic OER activity, a smaller Tafel slope and better stability than the state‐of‐the‐art precious‐metal IrO₂ catalyst and the well‐known BSCF perovskite. The mass activity and stability are further improved by ball milling. Several factors including the optimized e_g orbital filling, good ionic and charge transfer abilities, as well as high OH^? adsorption and O₂ desorption capabilities possibly contribute to the excellent OER activity.     

Synthesis of Tri‐functional Core‐shell CuO@carbon Quantum Dots@carbon Hollow Nanospheres Heterostructure for Non‐enzymatic H2O2 Sensing and Overall Water Splitting Applications

A core‐shell structure with CuO core and carbon quantum dots (CQDs) and carbon hollow nanospheres (CHNS) shell was prepared through facile in‐situ hydrothermal process. The composite was used for non‐enzymatic hydrogen peroxide sensing and electrochemical overall water splitting. The core‐shell structure was established from the transmission electron microscopy image analysis. Raman and UV‐Vis spectroscopy analysis confirmed the interaction between CuO and CQDs. The electrochemical studies showed the limit of detection and sensitivity of the prepared composite as 2.4 nM and 56.72 μA μM^?1 cm^?2, respectively. The core‐shell structure facilitated better charge transportation which in turn exhibited elevated electro‐catalysis towards hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting. The overpotential of 159 mV was required to achieve 10 mA cm^?2 current density for HER and an overpotential of 322 mV was required to achieve 10 mA cm^?2 current density for OER in 1.0 M KOH. A two‐electrode system was constructed for overall water splitting reaction, which showed 10 and 50 mA cm^?2 current density at 1.83 and 1.96 V, respectively. The prepared CuO@CQDs@CHNS catalyst demonstrated excellent robustness in HER and OER catalyzing condition along with overall water splitting reaction. Therefore, the CuO@CQDs@CHNS could be considered as promising electro‐catalyst for H₂O₂ sensing, HER, OER and overall water splitting.     

Imine Gels Based on Ferrocene and Porphyrin and Their Electrocatalytic Property

A series of porphyrin‐based imine gels have been synthesized via dynamic covalent gelation between 5,10,15,20‐tetra(4‐aminophenyl)‐21H,23H‐porphyrin (H₂TAPP) derivatives and various aldehyde compounds. The porphyrin‐ferrocene imine gels based on MTAPP (M=H₂, Ni²⁺, Co²⁺, Pd²⁺ and Zn²⁺) and ferrocene‐1,1′‐dicarbaldehyde (NA) display efficient HER, OER and ORR activities in alkaline media. Among the gels, CoTAPP‐NA shows an HER current density of 10 mA cm^?2 at low overpotential of 470 mV and small Tafel slope of 110 mV decade^?1 in alkaline media. CoTAPP‐NA also exhibits OER catalytic activity with low overpotential (416 mV for 10 mA cm^?2). CoTAPP‐NA shows ability in overall water splitting in alkaline media. In addition, CoTAPP‐NA exhibits onset potential (E_p) of 0.95 V and half‐wave potential (E_1/2) of 0.84 V in 1.0 mol L^?1 KOH solution for oxygen reduction. Moreover, the gel catalyst shows good stability.     

Fast Modulation of d-Band Holes Quantity in the Early Reaction Stages for Boosting Acidic Oxygen Evolution

Synthesis of highly active and durable oxygen evolution reaction (OER) catalysts applied in acidic water electrolysis remains a grand challenge. Here, we construct a type of high-loading iridium single atom catalysts with tunable d-band holes character (h-HL−Ir SACs, ∼17.2 wt % Ir) realized in the early OER operation stages. The in situ X-ray absorption spectroscopy reveals that the quantity of the d-band holes of Ir active sites can be fast increased by 0.56 unit from the open circuit to a low working potential of 1.35 V. More remarkably, in situ synchrotron infrared and Raman spectroscopies demonstrate the quick accumulation of *OOH and *OH intermediates over holes-modulated Ir sites in the early reaction voltages, achieving a rapid OER kinetics. As a result, this well-designed h-HL−Ir SACs exhibits superior performance for acidic OER with overpotentials of 216 mV @10 mA cm⁻² and 259 mV @100 mA cm⁻², corresponding to a small Tafel slope of 43 mV dec⁻¹. The activity of catalyst shows no evident attenuation after 60 h operation in acidic environment. This work provides some useful hints for the design of superior acidic OER catalysts.     

NiFe Hydroxide Lattice Tensile Strain: Enhancement of Adsorption of Oxygenated Intermediates for Efficient Water Oxidation Catalysis

The binding strength of reactive intermediates with catalytically active sites plays a crucial role in governing catalytic performance of electrocatalysts. NiFe hydroxide offers efficient oxygen evolution reaction (OER) catalysis in alkaline electrolyte, however weak binding of oxygenated intermediates on NiFe hydroxide still badly limits its catalytic activity. Now, a facile ball‐milling method was developed to enhance binding strength of NiFe hydroxide to oxygenated intermediates via generating tensile strain, which reduced the anti‐bonding filling states in the d orbital and thus facilitated oxygenated intermediates adsorption. The NiFe hydroxide with tensile strain increasing after ball‐milling exhibits an OER onset potential as low as 1.44 V (vs. reversible hydrogen electrode) and requires only a 270 mV overpotential to reach a water oxidation current density of 10 mA cm^?2.     

Multifunctional Active‐Center‐Transferable Platinum/Lithium Cobalt Oxide Heterostructured Electrocatalysts towards Superior Water Splitting

Designing cost‐effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active‐center‐transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO₂) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO₂ nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO₂ for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO₂ acts as the co‐catalyst for HER, whereas the active center transfers to LiCoO₂ and Pt turns into the co‐catalyst for OER. The unique architecture of Pt/LiCoO₂ heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO₂ heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm^?2 for HER and OER in alkaline medium, respectively.     

Heterostructured Inter‐Doped Ruthenium–Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting

The development of transition‐metal‐oxides (TMOs)‐based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter‐doped ruthenium–cobalt oxide [(Ru–Co)O_x] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru–Co)O_x nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm^?2 and a small Tafel slope of 23.5 mV dec^?1, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm^?2. As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm^?2 for alkaline overall water splitting.     

A Polyimide Nanolayer as a Metal‐Free and Durable Organic Electrode Toward Highly Efficient Oxygen Evolution

The exploitation of metal‐free organic polymers as electrodes for water splitting reactions is limited by their presumably low activity and poor stability, especially for the oxygen evolution reaction (OER) under more critical conditions. Now, the thickness of a cheap and robust polymer, poly(p‐phenylene pyromellitimide) (PPPI) was rationally engineered by an in situ polymerization method to make the metal‐free polymer available for the first time as flexible, tailorable, efficient, and ultra‐stable electrodes for water oxidation over a wide pH range. The PPPI electrode with an optimized thickness of about 200 nm provided a current density of 32.8 mA cm^?2 at an overpotential of 510 mV in 0.1 mol L^?1 KOH, which is even higher than that (31.5 mA cm^?2) of commercial IrO₂ OER catalyst. The PPPI electrodes are scalable and stable, maintaining 92 % of its activity after a 48‐h chronoamperometric stability test.     

Electrodeposited Amorphous Tungsten‐doped Cobalt Oxide as an Efficient Catalyst for the Oxygen Evolution Reaction

Thin film of amorphous tungsten‐doped cobalt oxide (W:CoO) was successfully grown on a conducting electrode via an electrochemical oxidation process employing a [Co(WS₄)₂]^2? deposition bath. The W:CoO catalyst displays an attractive performance for the oxygen evolution reaction in an alkaline solution. In an NaOH solution of pH 13, W:CoO operates with a moderate onset overpotential of 230 mV and requires 320 mV overpotential to generate a catalytic current density of 10 mA cm^?2. A low Tafel slope of 45 mV decade^?1 was determined, indicating a rapid O₂‐evolving kinetics. The as‐prepared W:CoO belongs to the best cobalt oxide‐based catalysts ever reported for the oxygen evolution (OER) reaction.     

The Spin Modulation Stimulated Efficient Electrocatalytic Oxygen Evolution Reaction over LaCoO3 Perovskite

Perovskite is a promising non-noble catalyst and has been widely investigated for the electrochemical oxygen evolution reaction (OER). However, there is still serious lack of valid approaches to further enhance their catalytic performance. Herein, we propose a spin state modulation strategy to improve the OER electrocatalytic activity of typical perovskite material of LaCoO₃. Specifically, the electronic configuration transition was realized by a simple high temperature thermal reduction process. M-H hysteresis loop results reveal that the reduction treatment can produce more unpaired electrons in 3d orbit by promoting the electron transitions of Co from low spin state to high spin state, and thus lead to the increase of the spin polarization. Electrochemical measurements show that the catalytic performance of LaCoO₃ is strongly dependent on its electronic configuration. With the optimized reduction treatment, the overpotential for the OER process in 0.5 M KOH electrolyte solution at 10 mA cm⁻² current density was 396 mV, significantly lower than that of the original state. Furthermore, it can mediate efficient OER with an overpotential of 383 mV under an external magnetic field, which is attributed to the appropriate electron filling. Our results show that electron spin state regulation is a new way to boost the OER electrocatalytic activity.     

Coupling Magnetic Single-Crystal Co2Mo3O8 with Ultrathin Nitrogen-Rich Carbon Layer for Oxygen Evolution Reaction

Transition-metal oxides as electrocatalysts for the oxygen evolution reaction (OER) provide a promising route to face the energy and environmental crisis issues. Although palmeirite oxide A₂Mo₃O₈ as OER catalyst has been explored, the correlation between its active sites (tetrahedral or octahedral) and OER performance has been elusive. Now, magnetic Co₂Mo₃O₈@NC-800 composed of highly crystallized Co₂Mo₃O₈ nanosheets and ultrathin N-rich carbon layer is shown to be an efficient OER catalyst. The catalyst exhibits favorable performance with an overpotential of 331 mV@10 mA cm⁻² and 422 mV@40 mA cm⁻², and a full water-splitting electrolyzer with it as anode catalyst shows a cell voltage of 1.67 V@10 mA cm⁻² in alkaline. Combined HAADFSTEM, magnetic, and computational results show that factors influencing the OER performance can be attributed to the tetrahedral Co sites (high spin, t₂³e⁴), which improve the OER kinetics of rate-determining step to form *OOH.     

Sulfur‐Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x‐Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co?S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x‐Vo‐S to exhibit much superior OER activity. FeCoO_x‐Vo‐S exhibits a mass activity of 2440.0 A g^?1 at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec^?1, indicative of its excellent charge transfer rate. When FeCoO_x‐Vo‐S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm^?2 and 406.0 mA cm^?2 at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.     

Coupling of Bifunctional CoMn‐Layered Double Hydroxide@Graphitic C3N4 Nanohybrids towards Efficient Photoelectrochemical Overall Water Splitting

The development of durable, low‐cost, and efficient photo‐/electrolysis for the oxygen and hydrogen evolution reactions (OER and HER) is important to fulfill increasing energy requirements. Herein, highly efficient and active photo‐/electrochemical catalysts, that is, CoMn‐LDH@g‐C₃N₄ hybrids, have been synthesized successfully through a facile in situ co‐precipitation method at room temperature. The CoMn‐LDH@g‐C₃N₄ composite exhibits an obvious OER electrocatalytic performance with a current density of 40 mA cm^?2 at an overpotential of 350 mV for water oxidation, which is 2.5 times higher than pure CoMn‐LDH nanosheets. For HER, CoMn‐LDH@g‐C₃N₄ (η₅₀=?448 mV) requires a potential close to Pt/C (η₅₀=?416 mV) to reach a current density of 50 mA cm². Furthermore, under visible‐light irradiation, the photocurrent density of the CoMn‐LDH@g‐C₃N₄ composite is 0.227 mA cm^?2, which is 2.1 and 3.8 time higher than pristine CoMn‐LDH (0.108 mA cm^?2) and g‐C₃N₄ (0.061 mA cm^?2), respectively. The CoMn‐LDH@g‐C₃N₄ composite delivers a current density of 10 mA cm^?2 at 1.56 V and 100 mA cm^?2 at 1.82 V for the overall water‐splitting reaction. Therefore, this work establishes the first example of pure CoMn‐LDH and CoMn‐LDH@g‐C₃N₄ hybrids as electrochemical and photoelectrochemical water‐splitting systems for both OER and HER, which may open a pathway to develop and explore other LDH and g‐C₃N₄ nanosheets as efficient catalysts for renewable energy applications.     

Strong‐Coupled Cobalt Borate Nanosheets/Graphene Hybrid as Electrocatalyst for Water Oxidation Under Both Alkaline and Neutral Conditions

Developing highly active catalysts for the oxygen evolution reaction (OER) is of paramount importance for designing various renewable energy storage and conversion devices. Herein, we report the synthesis of a category of Co‐Pi analogue, namely cobalt‐based borate (Co‐B_i) ultrathin nanosheets/graphene hybrid by a room‐temperature synthesis approach. Benefiting from the high surface active sites exposure yield, enhanced electron transfer capacity, and strong synergetic coupled effect, this Co‐B_i NS/G hybrid shows high catalytic activity with current density of 10 mA cm^?2 at overpotential of 290 mV and Tafel slope of 53 mV dec^?1 in alkaline medium. Moreover, Co‐B_i NS/G electrocatalysts also exhibit promising performance under neutral conditions, with a low onset potential of 235 mV and high current density of 14.4 mA cm^?2 at 1.8 V, which is the best OER performance among well‐developed Co‐based OER electrocatalysts to date. Our finding paves a way to develop highly active OER electrocatalysts.     

Advanced Bifunctional Oxygen Reduction and Evolution Electrocatalyst Derived from Surface‐Mounted Metal–Organic Frameworks

Metal–organic frameworks (MOFs) and their derivatives are considered as promising catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are important for many energy provision technologies, such as electrolyzers, fuel cells and some types of advanced batteries. In this work, a “strain modulation” approach has been applied through the use of surface‐mounted NiFe‐MOFs in order to design an advanced bifunctional ORR/OER electrocatalyst. The material exhibits an excellent OER activity in alkaline media, reaching an industrially relevant current density of 200 mA cm^?2 at an overpotential of only ≈210 mV. It demonstrates operational long‐term stability even at a high current density of 500 mA cm^?2 and exhibits the so far narrowest “overpotential window” ΔE_ORR‐OER of 0.69 V in 0.1 m KOH with a mass loading being two orders of magnitude lower than that of benchmark electrocatalysts.     

Fluorographdiyne: A Metal‐Free Catalyst for Applications in Water Reduction and Oxidation

A highly efficient bifunctional metal‐free catalyst was prepared by growth of three‐dimensional porous fluorographdiyne networks on carbon cloth (p‐FGDY/CC). Our experiments and density functional theory (DFT) calculations show the 3D p‐FGDY/CC network is highly active and it is a high potential metal‐free catalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as overall water splitting (OWS) under both acidic and alkaline conditions. The experimental and theoretical results show very good consistency; for example, in the HER process, p‐FGDY/CC exhibits small overpotentials of 82 and 92 mV to achieve 10 mA cm^?2 under alkaline and acidic conditions, respectively. This ensures an even higher selectivity for the adsorption/desorption of various O/H intermediate species. The essential key promotion accomplishes a bifunctional H₂O redox performance application under pH‐universal electrochemical conditions.