General π‐Electron‐Assisted Strategy for Ir,Pt, Ru,Pd, Fe,Ni Single‐Atom Electrocatalysts with Bifunctional Active Sites for Highly Efficient Water Splitting

Both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are crucial to water splitting, but require alternative active sites. Now, a general π‐electron‐assisted strategy to anchor single‐atom sites (M=Ir, Pt, Ru, Pd, Fe, Ni) on a heterogeneous support is reported. The M atoms can simultaneously anchor on two distinct domains of the hybrid support, four‐fold N/C atoms (M@NC), and centers of Co octahedra (M@Co), which are expected to serve as bifunctional electrocatalysts towards the HER and the OER. The Ir catalyst exhibits the best water‐splitting performance, showing a low applied potential of 1.603 V to achieve 10 mA cm^?2 in 1.0 m KOH solution with cycling over 5 h. DFT calculations indicate that the Ir@Co (Ir) sites can accelerate the OER, while the Ir@NC₃ sites are responsible for the enhanced HER, clarifying the unprecedented performance of this bifunctional catalyst towards full water splitting.     

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.     

NixCo3‐xO4 Nanoneedle Arrays Grown on Ni Foam as an Efficient Bifunctional Electrocatalyst for Full Water Splitting

Developing highly active, stable and robust electrocatalysts based on earth‐abundant elements for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is important for many renewable energy conversion processes. Herein, Ni_xCo_3‐xO₄ nanoneedle arrays grown on 3D porous nickel foam (NF) was synthesized as a bifunctional electrocatalyst with OER and HER activity for full water splitting. Benefiting from the advantageous structure, the composite exhibits superior OER activity with an overpotential of 320 mV achieving the current density of 10 mA cm^?2. An exceptional HER activity is also acquired with an overpotential of 170 mV at the current density of 10 mA cm^?2. Furthermore, the catalyst also shows the superior activity and stability for 20 h when used in the overall water splitting cell. Thus, the hierarchical 3D structure composed of the 1D nanoneedle structure in Ni_xCo_3‐xO₄/NF represents an avenue to design and develop highly active and bifunctional electrocatalysts for promising energy conversion.     

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.     

Electrodeposited Cobalt‐Phosphorous‐Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting

One of the challenges to realize large‐scale water splitting is the lack of active and low‐cost electrocatalysts for its two half reactions: H₂ and O₂ evolution reactions (HER and OER). Herein, we report that cobalt‐phosphorous‐derived films (Co‐P) can act as bifunctional catalysts for overall water splitting. The as‐prepared Co‐P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mA cm^?2 at overpotentials of ?94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100 % Faradaic efficiency, rivalling the integrated performance of Pt and IrO₂. The major composition of the as‐prepared and post‐HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER.     

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.     

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.     

Single-atom rhodium anchored on S-doped black phosphorene as a promising bifunctional electrocatalyst for overall water splitting

Superior bifunctional electrocatalysts with ultra-high stability and excellent efficiency are crucial to boost the oxygen evolution reaction (OER) and the hydrogen evolution reduction (HER) in the overall water splitting (OWS) for the sustainable production of clean fuels. Herein, comprehensive density functional theory (DFT) computations were performed to explore the potential of several single transition metal (TM) atoms anchored on various S-doped black phosphorenes (TM/S_nx-BP) for bifunctional OWS electrocatalysis. The results revealed that these candidates display good stability, excellent electrical conductivity, and diverse spin moments. Furthermore, the Rh/S₁₂-BP catalyst was identified as an eligible bifunctional catalyst for OWS process due to the low overpotentials for OER (0.43 V) and HER (0.02 V), in which Rh and its adjacent P atoms were identified as the active sites. Based on the computed Gibbs free energies of OH*, O*, OOH* and H*, the corresponding volcano plots for OER and HER were established. Interestingly, the spin moments and the charge distribution of the active sites determine the catalytic trends of OER and HER. Our findings not only propose a promising bifunctional catalyst for OWS, but also widen the potential application of BP in electrocatalysis.     

Channel‐Rich RuCu Nanosheets for pH‐Universal Overall Water Splitting Electrocatalysis

Channel‐rich RuCu snowflake‐like nanosheets (NSs) composed of crystallized Ru and amorphous Cu were used as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting in pH‐universal electrolytes. The optimized RuCu NSs/C‐350 °C and RuCu NSs/C‐250 °C show attractive activities of OER and HER with low overpotentials and small Tafel slopes, respectively. When applied to overall water splitting, the optimized RuCu NSs/C can reach 10 mA cm^?2 at cell voltages of only 1.49, 1.55, 1.49 and 1.50 V in 1 m KOH, 0.1 m KOH, 0.5 m H₂SO₄ and 0.05 m H₂SO₄, respectively, much lower than those of commercial Ir/C∥Pt/C. The optimized electrolyzer exhibits superior durability with small potential change after up to 45 h in 1 m KOH, showing a class of efficient functional electrocatalysts for overall water splitting.     

Ir Nanoparticles Anchored on Metal-Organic Frameworks for Efficient Overall Water Splitting under pH-Universal Conditions

The construction of high-activity and low-cost electrocatalysts is critical for efficient hydrogen production by water electrolysis. Herein, we developed an advanced electrocatalyst by anchoring well-dispersed Ir nanoparticles on nickel metal-organic framework (MOF) Ni-NDC (NDC: 2,6-naphthalenedicarboxylic) nanosheets. Benefiting from the strong synergy between Ir and MOF through interfacial Ni−O−Ir bonds, the synthesized Ir@Ni-NDC showed exceptional electrocatalytic performance for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting in a wide pH range, superior to commercial benchmarks and most reported electrocatalysts. Theoretical calculations revealed that the charge redistribution of Ni−O−Ir bridge induced the optimization of H₂O, OH* and H* adsorption, thus leading to the accelerated electrochemical kinetics for HER and OER. This work provides a new clue to exploit bifunctional electrocatalysts for pH-universal overall water splitting.     

Fe/Co‐based Bimetallic MOF‐derived Co3Fe7@NCNTFs Bifunctional Electrocatalyst for High‐Efficiency Overall Water Splitting

Electrocatalytic water splitting to produce hydrogen and oxygen is regarded as one of the most promising methods to generate clean and sustainable energy for replacing fossil fuels. However, the design and development of an efficient bifunctional catalyst for simultaneous generation of hydrogen and oxygen remains extremely challenging yet is critical for the practical implementation of water electrolysis. Here, we report a facile method to fabricate novel N‐doped carbon nanotube frameworks (NCNTFs) by the pyrolysis of a bimetallic metal organic framework (MIL‐88‐Fe/Co). The resultant electrocatalyst, Co₃Fe₇@NCNTFs, exhibits excellent oxygen evolution reaction (OER) activity, achieving 10 mA/cm² at a low overpotential of just 264 mV in 1 M KOH solution, and 197 mV for the hydrogen evolution reaction. The high electrocatalytic activity arises from the synergistic effect between the chemistry of the Co₃Fe₇ and the NCNTs coupled to the novel framework structure. The remarkable electrocatalytic performance of our bifunctional electrocatalyst provides a promising pathway to high‐performance overall water splitting and electrochemical energy devices.     

Nitrogen,Phosphorus, and Fluorine Tri‐doped Graphene as a Multifunctional Catalyst for Self‐Powered Electrochemical Water Splitting

Electrocatalysts are required for clean energy technologies (for example, water‐splitting and metal‐air batteries). The development of a multifunctional electrocatalyst composed of nitrogen, phosphorus, and fluorine tri‐doped graphene is reported, which was obtained by thermal activation of a mixture of polyaniline‐coated graphene oxide and ammonium hexafluorophosphate (AHF). It was found that thermal decomposition of AHF provides nitrogen, phosphorus, and fluorine sources for tri‐doping with N, P, and F, and simultaneously facilitates template‐free formation of porous structures as a result of thermal gas evolution. The resultant N, P, and F tri‐doped graphene exhibited excellent electrocatalytic activities for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The trifunctional metal‐free catalyst was further used as an OER–HER bifunctional catalyst for oxygen and hydrogen gas production in an electrochemical water‐splitting unit, which was powered by an integrated Zn–air battery based on an air electrode made from the same electrocatalyst for ORR. The integrated unit, fabricated from the newly developed N, P, and F tri‐doped graphene multifunctional metal‐free catalyst, can operate in ambient air with a high gas production rate of 0.496 and 0.254 μL s⁻¹ for hydrogen and oxygen gas, respectively, showing great potential for practical applications.     

From a Molecular 2Fe‐2Se Precursor to a Highly Efficient Iron Diselenide Electrocatalyst for Overall Water Splitting

A highly active FeSe₂ electrocatalyst for durable overall water splitting was prepared from a molecular 2Fe‐2Se precursor. The as‐synthesized FeSe₂ was electrophoretically deposited on nickel foam and applied to the oxygen and hydrogen evolution reactions (OER and HER, respectively) in alkaline media. When used as an oxygen‐evolution electrode, a low 245 mV overpotential was achieved at a current density of 10 mA cm⁻², representing outstanding catalytic activity and stability because of Fe(OH)₂/FeOOH active sites formed at the surface of FeSe₂. Remarkably, the system is also favorable for the HER. Moreover, an overall water‐splitting setup was fabricated using a two‐electrode cell, which displayed a low cell voltage and high stability. In summary, the first iron selenide material is reported that can be used as a bifunctional electrocatalyst for the OER and HER, as well as overall water splitting.     

Nitrogen,Phosphorus, and Fluorine Tri‐doped Graphene as a Multifunctional Catalyst for Self‐Powered Electrochemical Water Splitting

Electrocatalysts are required for clean energy technologies (for example, water‐splitting and metal‐air batteries). The development of a multifunctional electrocatalyst composed of nitrogen, phosphorus, and fluorine tri‐doped graphene is reported, which was obtained by thermal activation of a mixture of polyaniline‐coated graphene oxide and ammonium hexafluorophosphate (AHF). It was found that thermal decomposition of AHF provides nitrogen, phosphorus, and fluorine sources for tri‐doping with N, P, and F, and simultaneously facilitates template‐free formation of porous structures as a result of thermal gas evolution. The resultant N, P, and F tri‐doped graphene exhibited excellent electrocatalytic activities for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The trifunctional metal‐free catalyst was further used as an OER–HER bifunctional catalyst for oxygen and hydrogen gas production in an electrochemical water‐splitting unit, which was powered by an integrated Zn–air battery based on an air electrode made from the same electrocatalyst for ORR. The integrated unit, fabricated from the newly developed N, P, and F tri‐doped graphene multifunctional metal‐free catalyst, can operate in ambient air with a high gas production rate of 0.496 and 0.254 μL s^?1 for hydrogen and oxygen gas, respectively, showing great potential for practical applications.     

Hierarchical NiCo2O4 Hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water‐Splitting

Bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte may improve the efficiency of overall water splitting. Nickel cobaltite (NiCo₂O₄) has been considered a promising electrode material for the OER. However, NiCo₂O₄ that can be used as an electrocatalyst in HER has not been studied yet. Herein, we report self‐assembled hierarchical NiCo₂O₄ hollow microcuboids for overall water splitting including both the HER and OER reactions. The NiCo₂O₄ electrode shows excellent activity toward overall water splitting, with 10 mA cm^?2 water‐splitting current reached by applying just 1.65 V and 20 mA cm^?2 by applying just 1.74 V across the two electrodes. The synthesis of NiCo₂O₄ microflowers confirms the importance of structural features for high‐performance overall water splitting.     

Molybdenum-induced tuning 3d-orbital electron filling degree of CoSe2 for alkaline hydrogen and oxygen evolution reactions

The development of high-performance non-precious metal-based robust bifunctional electrocatalyst for both hydrogen evolution reaction(HER) and oxygen evolution reactions(OER) in alkaline media is essential for the electrochemical overall water splitting technologies. Herein, we demonstrate that the HER/OER performance of Co Se₂ can be significantly enhanced by tuning the 3d-orbital electron filling degree through Mo doping. Both density functional theory(DFT) calculations and experime...     

Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

Folded nanosheet-like Co<Subscript>0.85</Subscript>Se array for overall water splitting

Active, stable, and earth-abundant bifunctional electrocatalyst for overall water splitting is pivotal to actualize large-scale water splitting via electrolysis. In this work, the hierarchical folded nanosheet-like Co_0.85Se array on Ni foam is constructed by liquid-phase chemical conversion with cobalt precursor nanorod array. It can serve as an efficient bifunctional electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte, with a current density of 10 mA cm^?2 at overpotential of 232 mV for OER and 129 mV for HER and Tafel slope of 78.9 mV dec^?1 for OER and 95.0 mV dec^?1 for HER, respectively. The two-electrode alkaline water electrolyzer utilizing this folded nanosheet-like Co_0.85Se array as both anode and cathode toward overall water splitting offered a current of 10 mA cm^?2 at a cell voltage of 1.60 V. This work explores an efficient and low-cost electrocatalyst for overall water splitting application in alkaline electrolytes.     

Defects-engineered tailoring of tri-doped interlinked metal-free bifunctional catalyst with lower gibbs free energy of OER/HER intermediates for overall water splitting

Controllable tailoring of metal-free/carbon-based nanostructures tends an encouraging way to enhance the bifunctional activity of electrodes, but a great challenge owing to the sluggish kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, a facile tempted-defects assisted fractionation strategy is presented to synthesize N, S, and O tri-doped metal-free catalyst (DE-TDAP). Due to this effective tempted-defects and heteroatoms interlinking in DE-TDAP, it delivers the lowest overpotential toward both the OER (346 mV) and HER (154 mV) at 10 mA cm^?2. Remarkably, the DE-TDAP-electrode carries only a cell voltage of 1.81 V at 10 mA cm^?2 for overall water splitting and long-term stability. Considerably, the density functional theory (DFT) calculation exposes that the tailored-defects in tri-doped interlinking could enhance bifunctional catalytic performance devising from lower Gibbs free energy of OER/HER intermediates on active sites. This struggle henceforth provides a perceptive understanding of the synergetic principles of heteroatom-interlinking-tailoring nanostructures in water splitting.     

Nanosized IrOx–Ir Catalyst with Relevant Activity for Anodes of Proton Exchange Membrane Electrolysis Produced by a Cost‐Effective Procedure

We have developed a highly active nanostructured iridium catalyst for anodes of proton exchange membrane (PEM) electrolysis. Clusters of nanosized crystallites are obtained by reducing surfactant‐stabilized IrCl₃ in water‐free conditions. The catalyst shows a five‐fold higher activity towards oxygen evolution reaction (OER) than commercial Ir‐black. The improved kinetics of the catalyst are reflected in the high performance of the PEM electrolyzer (1 mg_Ir cm^?2), showing an unparalleled low overpotential and negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area, but rather to the ligand effect and low coordinate sites resulting in a high turnover frequency (TOF). The catalyst developed herein sets a benchmark and a strategy for the development of ultra‐low loading catalyst layers for PEM electrolysis.