Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir–Ni Oxide Catalysts for Electrochemical Water Splitting (OER)

Electrochemical characterization of manganese oxides as a water oxidation catalyst in proton exchange membrane electrolysers

The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO₂/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.

Iridium-Based Catalysts for Solid Polymer Electrolyte Electrocatalytic Water Splitting

Chemical energy conversion/storage through water splitting for hydrogen production has been recognized as the ideal solution to the transient nature of renewable energy sources. Solid polymer electrolyte (SPE) water electrolysis is one of the most practical ways to produce pure H₂ . Electrocatalysts are key materials in the SPE water electrolysis. At the anode side, electrode materials catalyzing the oxygen evolution reaction (OER) require specific properties. Among the reported materials, only iridium presents high activity and is more stable. In this Minireview, an application overview of single iridium metal and its oxide catalysts-binary, ternary, and multicomponent catalysts of iridium oxides and supported composite catalysts-for the OER in SPE water electrolysis is presented. Two main strategies to improve the activity of an electrocatalyst system, namely, increasing the number of active sites and the intrinsic activity of each active site, are reviewed with detailed examples. The challenges and perspectives in this field are also discussed.

Transition-Metal-Doped RuIr Bifunctional Nanocrystals for Overall Water Splitting in Acidic Environments

The establishment of electrocatalysts with bifunctionality for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic environments is necessary for the development of proton exchange membrane (PEM) water electrolyzers for the production of clean hydrogen fuel. RuIr alloy is considered to be a promising electrocatalyst because of its favorable OER performance and potential for HER. Here, the design of a bifunctional electrocatalyst with greatly boosted water-splitting performance from doping RuIr alloy nanocrystals with transition metals that modify electronic structure and binding strength of reaction intermediates is reported. Significantly, Co-RuIr results in small overpotentials of 235 mV for OER and 14 mV for HER (@ 10 mA cm^-2 current density) in 0.1 m HClO₄ media. Therefore a cell voltage of just 1.52 V is needed for overall water splitting to produce hydrogen and oxygen. More importantly, for a series of M-RuIr (M = Co, Ni, Fe), the catalytic activity dependence at fundamental level on the chemical/valence states is used to establish a novel composition-activity relationship. This permits new design principles for bifunctional electrocatalysts.

Rational construction of self-supported triangle-like MOF-derived hollow (Ni,Co)Se2 arrays for electrocatalysis and supercapacitors

In this work, we have adopted a facile three-step method for constructing an intriguing bifunctional electrode of self-supported hollow (Ni,Co)Se2 arrays with a metal-organic framework (MOF) precursor. The triangle-like cobalt-based MOF arrays are first grown on a carbon cloth at room temperature, which is followed by an ion exchange/etching process with Ni(NO3)2 to form a critical hollow nanostructure with an incorporated hetero-metal element. The intermediate is then transformed into the final product through solvothermal selenization treatment. Taking advantages of the structural and compositional merits as well as the self-supporting nature, the resultant (Ni,Co)Se2 electrode exhibits excellent electrochemical activity and stability. When tested as an electrocatalyst for the oxygen evolution reaction (OER), the (Ni,Co)Se2 array electrode displayed a low onset overpotential of 226 mV and a small overpotential of 256 mV to afford a current density of 10 mA cm-2. The (Ni,Co)Se2 electrode is also utilized in a supercapacitor, which delivers a high specific capacitance of 2.85 F cm-2 at 2 mA cm-2 and exhibits excellent cycling stability with a capacitance retention of 80.8% after 2000 charge-discharge cycles at 20 mA cm-2. These results demonstrate the significance of the rational design of electrode materials and disclose the potential of our MOF-derived hollow (Ni,Co)Se2 array electrode for a variety of practical applications in energy conversion and storage.

Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Controllable synthesis of highly active micro/nanostructured metal electrocatalysts for oxygen evolution reaction (OER) is a particularly significant and challenging target. Herein, we report a 3D porous sponge-like Ni material, prepared by a facile hydrothermal method and consisting of cross-linked micro/nanofibers, as an integrated binder-free OER electrocatalyst. To further enhance the electrocatalytic performance, an N-doping strategy is applied to obtain N-doped sponge Ni (N-SN) for the first time, via NH₃ annealing. Due to the combination of the unique conductive sponge structure and N doping, the as-obtained N-SN material shows improved conductivity and a higher number of active sites, resulting in enhanced OER performance and excellent stability. Remarkably, N-SN exhibits a low overpotential of 365 mV at 100 mA cm^-2 and an extremely small Tafel slope of 33 mV dec^-1, as well as superior long-term stability, outperforming unmodified sponge Ni. Importantly, the combination of X-ray photoelectron spectroscopy and near-edge X-ray adsorption fine structure analyses shows that γ-NiOOH is the surface-active phase for OER. Therefore, the combination of conductive sponge structure and N-doping modification opens a new avenue for fabricating new types of high-performance electrodes with application in electrochemical energy conversion devices.

Electron Correlations Engineer Catalytic Activity of Pyrochlore Iridates for Acidic Water Oxidation

The development of highly efficient oxygen-evolving catalysts compatible with powerful proton-exchange-membrane-based electrolyzers in acid environments is of prime importance for sustainable hydrogen production. In this field, understanding the role of electronic structure of catalysts on catalytic activity is essential but still lacking. Herein, a family of pyrochlore oxides R₂ Ir₂ O₇ (R = rare earth ions) is reported as acidic oxygen-evolving catalysts with superior-specific activities. More importantly, it is found that the intrinsic activity of this material significantly increases with the R ionic radius. Electronic structure studies reveal that the increased R ionic radius weakens electron correlations in these iridate oxides. This weakening induces an insulator-metal transition and an enhancement of IrO bond covalency, both of which promote oxygen evolution kinetics. This work demonstrates the importance of engineering the electron correlations to rationalize the catalytic activity toward water oxidation in strongly correlated transition-metal oxides.

The chemically reduced CuO–Co3O4 composite as a highly efficient electrocatalyst for oxygen evolution reaction in alkaline media

The fabrication of efficient, alkaline-stable and nonprecious electrocatalysts for the oxygen evolution reaction is highly needed; however, it is a challenging task.

Topotactic Transformations in an Icosahedral Nanocrystal to Form Efficient Water-Splitting Catalysts

Designing high-performance, precious-metal-based, and economic electrocatalysts remains an important challenge in proton exchange membrane (PEM) electrolyzers. Here, a highly active and durable bifunctional electrocatalyst for PEM electrolyzers based on a rattle-like catalyst comprising a Ni/Ru-doped Pt core and a Pt/Ni-doped RuO₂ frame shell, which is topotactically transformed from an icosahedral Pt/Ni/Ru nanocrystal, is reported. The RuO₂ -based frame shell with its highly reactive surfaces leads to a very high activity for the oxygen evolution reaction (OER) in acidic media, reaching a current density of 10 mA cm^-2 at an overpotential of 239 mV, which surpasses those of previously reported catalysts. The Pt dopant in the RuO₂ shell enables a sustained OER activity even after a 2000 cycles of an accelerated durability test. The Pt-based core catalyzes the hydrogen evolution reaction with an excellent mass activity. A two-electrode cell employing Pt/RuO₂ as the electrode catalyst demonstrates very high activity and durability, outperforming the previously reported cell performances.

Efficient oxygen evolution on mesoporous IrOx nanosheets

Amorphous iridium oxide (IrO_x) is a promising catalyst that has high activity in the oxygen evolution reaction (OER) over a broad range of pH values.

A promising engineering strategy for water electro-oxidation iridate catalysts via coordination distortion

Distorted IrO₆ is more conducive to improving OER activity. Specifically, regulating the transformation of the IrO₆ octahedron from D_4h compression to D_4h elongation is very beneficial for reducing the energy of the rate determining step of the OER process.

Ultrafine and monodispersed iridium nanoparticles supported on nitrogen-functionalized carbon: an efficient oxidase mimic for glutathione colorimetric detection

Ultrafine and monodispersed Ir NPs supported on nitrogen-functionalized carbon served as an efficient oxidase mimic for glutathione colorimetric detection.

Cobalt iron phosphide nanoparticles embedded within a carbon matrix as highly efficient electrocatalysts for the oxygen evolution reaction

Co₃FeP_x/C nanocomposites were derived from one-step phosphorization of anthraquinone-2-sulfonate (AQS2) intercalated Co₃Fe layered double hydroxides (Co₃Fe LDHs).

Robust noble metal-based electrocatalysts for oxygen evolution reaction

The oxygen evolution reaction (OER) is a kinetically sluggish anodic reaction and requires a large overpotential to deliver appreciable current. Despite the fact that non-precious metal-based alkaline water electrocatalysts are receiving increased attention, noble metal-based electrocatalysts (NMEs) applied in proton exchange membrane water electrolyzers still have advantageous features of larger current and power densities with lower stack cost. Engineering NMEs for OER catalysis with high efficiency, durability and utilization rate is of vital importance in promoting the development of cost-effective renewable energy production and conversion devices. In this tutorial review, we covered the recent progress in the composition and structure optimization of NMEs for OER including Ir- and Ru-based oxides and alloys, and noble-metals beyond Ir and Ru with a variety of morphologies. To shed light on the fundamental science and mechanisms behind composition/structure-performance relationships and activity-stability relationships, integrated experimental and theoretical studies were pursued for illuminating the metal-support interaction, size effect, heteroatom doping effect, phase transformation, degradation processes and single-atom catalysis. Finally, the challenges and outlook are provided for guiding the rational engineering of OER electrocatalysts for applications in renewable energy-related devices.

Electrochemical On-line ICP-MS in Electrocatalysis Research

Electrocatalyst degradation due to dissolution is one of the major challenges in electrochemical energy conversion technologies such as fuel cells and electrolysers. While tendencies towards dissolution can be grasped considering available thermodynamic data, the kinetics of material's stability in real conditions is still difficult to predict and have to be measured experimentally, ideally in-situ and/or on-line. On-line inductively coupled plasma mass spectrometry (ICP-MS) is a technique developed recently to address exactly this issue. It allows time- and potential-resolved analysis of dissolution products in the electrolyte during the reaction under dynamic conditions. In this work, applications of on-line ICP-MS techniques in studies embracing dissolution of catalysts for oxygen reduction (ORR) and evolution (OER) as well as hydrogen oxidation (HOR) and evolution (HER) reactions are reviewed.

Ir-Ni Bimetallic OER Catalysts Prepared by Controlled Ni Electrodeposition on Irpoly and Ir(111)

The alteration of electrocatalytic surfaces with adatoms lead to structural and electronic modifications promoting adsorption, desorption, and reactive processes. This study explores the potentiostatic electrodeposition process of Ni onto polycrystalline Ir (Irpoly) and assesses the electrocatalytic properties of the resulting bimetallic surfaces. The electrodeposition resulted in bimetallic Ni overlayer (OL) structures and in combination with controlled thermal post-deposition annealing in bimetallic near-surface alloys (NSA). The catalytic oxygen evolution reaction (OER) activity of these two different Ni-modified catalysts is assessed and compared to a pristine, unmodified Irpoly. An overlayer of Ni on Irpoly showed superior performance in both acidic and alkaline milieu. The reductive annealing of the OL produced a NSA of Ni, which demonstrated enhanced stability in an acidic environment. The remarkable activity and stability improvement of Ir by Ni modification makes both systems efficient electrocatalysts for water oxidation. The roughness factor of Irpoly is also reported. With the amount of deposited Ni determined by inductively coupled plasma mass spectrometry (ICP-MS) and a degree of coverage (monolayer) in the dependence of deposition potential is established. The density functional theory (DFT) assisted evaluation of H adsorption on Irpoly enables determination of the preferred Ni deposition sites on the three low-index surfaces (111), (110), and (100).

Cobalt layered double hydroxides derived CoP/Co2P hybrids for electrocatalytic overall water splitting

Cobalt layered double hydroxides (Co-LDHs) obtained by an imidazole-assisted route were used as precursors to prepare dual-functional electrocatalysts CoP/Co₂P. CoP/Co₂P showed an enhanced electrocatalytic performance for HER and OER. The reason was attributed to porous sheets providing more catalytic sites and multi-components improving conductivity and substrate adsorption capability.

Metallic Iridium Thin-Films as Model Catalysts for the Electrochemical Oxygen Evolution Reaction (OER)—Morphology and Activity

Iridium (Ir) oxide is known to be one of the best electrocatalysts for the oxygen evolution reaction (OER) in acidic media. Ir oxide-based materials are thus of great scientific interest in current research on electrochemical energy conversion. In the present study, we applied Ir metal films as model systems for electrochemical water splitting, obtained by inductive heating in a custom-made setup using two different synthesis approaches. X-ray photoelectron spectroscopy (XPS) and selected area electron diffraction (SAED) confirmed that all films were consistently metallic. The effects of reductive heating time of calcined and uncalcined Ir acetate films on OER activity were investigated using a rotating disk electrode (RDE) setup. The morphology of all films was determined by scanning electron microscopy (SEM). The films directly reduced from the acetate precursor exhibited a strong variability of their morphology and electrochemical properties depending on heating time. The additional oxidation step prior to reductive heating accelerates the final structure formation.

Comparative study of catalytic activities among transition metal-doped IrO2 nanoparticles

Catalytic activities of transition metal-doped IrO₂ nanoparticles (TM-IrO₂ NPs; TM = Cr, Mn, Fe, Co, or Ni) are compared for various oxidation reactions such as electrochemical oxygen evolution reaction (OER), gas-phase photo-oxidation of thiol function group, and CO oxidative conversion. Here, we discovered a series of TM-IrO₂ catalysts have a common activity trend for these oxidation reactions, and their activities are closely related with modified electronic states of IrO₂, strongly affected by the types of the transition metal across the periodic table. For all oxidation reactions, Cr- and Mn-IrO₂ achieved the highest oxidation catalytic activity, and sequentially decreased activities were obtained with Fe, Co, and Ni doped IrO₂. For instance, the highest OER activity was achieved by Cr or Mn doping exhibiting the smallest overpotential η = 275~230 mV at 10 mA/cm², while Ni-IrO₂ showed rather larger overpotential (η = 347 mV) even compared with non-doped IrO₂ (η = 314 mV). Scanning transmission X-ray microscopy and high-resolution photoemission spectra of TM-IrO₂ indicated dopant metals modified the Ir-O interaction and thus increasing oxygen vacancy defects in IrO₂. Strongly positive correlation was observed between the catalytic activities and vacancy states. The amount of defect related signals was observed the most for Cr- or Mn-IrO₂, less so for Fe- or Co-IrO₂, and unnoted for Ni-IrO₂ compared with bare IrO₂. Based on these catalytic activities and surface spectroscopic analysis results, vacancy defects induced by doping in TM-IrO₂ NPs are proposed to contribute to enhance the oxidation activities.

Recent Progress on Nickel-Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

Developing clean and sustainable energies as alternatives to fossil fuels is in strong demand within modern society. The oxygen evolution reaction (OER) is the efficiency-limiting process in plenty of key renewable energy systems, such as electrochemical water splitting and rechargeable metal-air batteries. In this regard, ongoing efforts have been devoted to seeking high-performance electrocatalysts for enhanced energy conversion efficiency. Apart from traditional precious-metal-based catalysts, nickel-based compounds are the most promising earth-abundant OER catalysts, attracting ever-increasing interest due to high activity and stability. In this review, the recent progress on nickel-based oxide and (oxy)hydroxide composites for water oxidation catalysis in terms of materials design/synthesis and electrochemical performance is summarized. Some underlying mechanisms to profoundly understand the catalytic active sites are also highlighted. In addition, the future research trends and perspectives on the development of Ni-based OER electrocatalysts are discussed.

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.

Designing transition metal alloy nanoparticles embedded hierarchically porous carbon nanosheets as high-efficiency electrocatalysts toward full water splitting

Developing high-efficiency catalysis electrodes towards both HER (hydrogen evolution reaction) and OER (oxygen evolution reaction) is critical to the popularity and practical application of devices for conversion and storage of clean and renewable hydrogen energy by overall water splitting. In this paper, a series of transition metal alloy nanoparticles embedded hierarchically porous carbon nanosheets (denoted as Fe_xCo_y@PCNSs) have been successfully designed and synthesized. After optimizing the metallic contents in Fe_xCo_y alloy, Fe₃Co₇@PCNSs displays superior water electrolysis performances compared to other control samples and previously reported non-noble metal catalysts. This advance is mainly due to the synergistic effect of increased carbon edges exposure, abundant accessible active sites, and improved electron/mass transport capability of 3D hierarchically porous architecture. More importantly, when Fe₃Co₇@PCNSs is applied as both cathode and anode in a two-electrode cell for carrying out the overall water splitting process, it just needs the corresponding cell voltages of 1.667 and 1.707 V to attain the 10 and 20 mA cm^-2 current densities respectively. The synthesis of Fe₃Co₇@PCNSs confirms the importance of Fe and Co elements in designing catalyst structures. The electrochemical measurements further display the key roles of structural features and metal alloy@C active sites for boosting water splitting performances.

Boosted Performance of Ir Species by Employing TiN as the Support toward Oxygen Evolution Reaction

Reducing the noble-metal loading without sacrificing the catalytic performance of the oxygen evolution reaction (OER) catalysts is paramount yet highly challenging. Herein, IrO₂@Ir/TiN electrocatalysts employing TiN as the support have been developed and shown high efficiency toward OER. TiN is found not only to disperse the IrO₂@Ir nanoparticles effectively but also to exert the electronic modulation of Ir by downshifting its d-band center of 0.21 eV compared to pure IrO₂. Excitingly, TiN remarkably enhances the catalytic performance of Ir, where the overpotential to achieve the current density of 10 mA cm^-2 is only 265 mV for the IrO₂@Ir/TiN (60 wt %) catalyst. As a result, 71.7 wt % of the Ir metal can be saved to compare with the commercial Ir-black counterpart. Moreover, TiN can inhibit the aggregation and oxidative dissolution of Ir species, thereby enhancing the operational stability. The combined advantages of TiN open a new solution to reduce the anodic catalyst cost through boosting the catalytic activity and stability.

Fe-CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large-Current-Density Oxygen Evolution and Overall Water Splitting

Industrial application of overall water splitting requires developing readily available, highly efficient, and stable oxygen evolution electrocatalysts that can efficiently drive large current density. This study reports a facile and practical method to fabricate a non-noble metal catalyst by directly growing a Co-Fe Prussian blue analogue on a 3D porous conductive substrate, which is further phosphorized into a bifunctional Fe-doped CoP (Fe-CoP) electrocatalyst. The Fe-CoP/NF (nickel foam) catalyst shows efficient electrocatalytic activity for oxygen evolution reaction, requiring low overpotentials of 190, 295, and 428 mV to achieve 10, 500, and 1000 mA cm-2 current densities in 1.0 m KOH solution. In addition, the Fe-CoP/NF can also function as a highly active electrocatalyst for hydrogen evolution reaction with a low overpotential of 78 mV at 10 mA cm-2 current density in alkaline solution. Thus, the Fe-CoP/NF electrode with meso/macropores can act as both an anode and a cathode to fabricate an electrolyzer for overall water splitting, only requiring a cell voltage of 1.49 V to afford a 10 mA cm-2 current density with remarkable stability. This performance appears to be among the best reported values and is much better than that of the IrO2-Pt/C-based electrolyzer.

Iridium-Tungsten Alloy Nanodendrites as pH-Universal Water-Splitting Electrocatalysts

The development of highly efficient and durable electrocatalysts for high-performance overall water-splitting devices is crucial for clean energy conversion. However, the existing electrocatalysts still suffer from low catalytic efficiency, and need a large overpotential to drive the overall water-splitting reactions. Herein, we report an iridium-tungsten alloy with nanodendritic structure (IrW ND) as a new class of high-performance and pH-universal bifunctional electrocatalysts for hydrogen and oxygen evolution catalysis. The IrW ND catalyst presents a hydrogen generation rate ∼2 times higher than that of the commercial Pt/C catalyst in both acid and alkaline media, which is among the most active hydrogen evolution reaction (HER) catalysts yet reported. The density functional theory (DFT) calculations reveal that the high HER intrinsic catalytic activity results from the suitable hydrogen and hydroxyl binding energies, which can accelerate the rate-determining step of the HER in acid and alkaline media. Moreover, the IrW NDs show superb oxygen evolution reaction (OER) activity and much improved stability over Ir. The theoretical calculation demonstrates that alloying Ir metal with W can stabilize the formed active iridium oxide during the OER process and lower the binding energy of reaction intermediates, thus improving the Ir corrosion resistance and OER kinetics. Furthermore, the overall water-splitting devices driven by IrW NDs can work in a wide pH range and achieve a current density of 10 mA cm^-2 in acid electrolyte at a low potential of 1.48 V.

Mesoporous Metallic Iridium Nanosheets

Two-dimensional (2D) metals are an emerging class of nanostructures that have attracted enormous research interest due to their unusual electronic and thermal transport properties. Adding mesopores in the plane of ultrathin 2D metals is the next big step in manipulating these structures because increasing their surface area improves the utilization of the material and the availability of active sites. Here, we report a novel synthetic strategy to prepare an unprecedented type of 2D mesoporous metallic iridium (Ir) nanosheet. Mesoporous Ir nanosheets can be synthesized with close-packed assemblies of diblock copolymer (poly-(ethylene oxide)- b-polystyrene, PEO- b-PS) micelles aligned in the 2D plane of the nanosheets. This novel synthetic route opens a new dimension of control in the synthesis of 2D metals, enabling new kinds of mesoporous architectures with abundant catalytically active sites. Because of their unique structural features, the mesoporous metallic Ir nanosheets exhibit a high electrocatalytic activity toward the oxygen evolution reaction (OER) in acidic solution as compared to commercially available catalysts.

IrCo Nanodendrite as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting under Acidic Conditions

Investigation of high-efficiency electrocatalysts for acidic overall water splitting is of great significance toward fulfillment of proton exchange membrane (PEM) electrolyzers but still remains challenging. Herein, we report the colloidally synthesis of IrCo alloy nanodendrites with petal-like architecture (NDs). Benefiting from unique hierarchical architecture and strong electronic interaction arising from synergistic alloying effect of IrCo at the atomic level, the resultant IrCo_0.65 NDs display remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performances with overpotentials of 17 and 281 mV to achieve 10 mA cm^-2 in 0.1 M HClO₄, respectively. Moreover, when further used as bifunctional electrocatalyst toward acidic overall splitting, a low cell voltage of 1.593 V is achieved at 10 mA cm^-2.