The development of active and durable bifunctional electrocatalysts for overall water splitting is mandatory for renewable energy conversion. This study reports a general method for controllable synthesis of a class of IrM (M = Co, Ni, CoNi) multimetallic porous hollow nanocrystals (PHNCs), through etching Ir-based, multimetallic, solid nanocrystals using Fe ions, as catalysts for boosting overall water splitting. The Ir-based multimetallic PHNCs show transition-metal-dependent bifunctional electrocatalytic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic electrolyte, with IrCo and IrCoNi PHNCs being the best for HER and OER, respectively. First-principles calculations reveal a ligand effect, induced by alloying Ir with 3d transition metals, can weaken the adsorption energy of oxygen intermediates, which is the key to realizing much-enhanced OER activity. The IrCoNi PHNCs are highly efficient in overall-water-splitting catalysis by showing a low cell voltage of only 1.56 V at a current density of 2 mA cm , and only 8 mV of polarization-curve shift after a 1000-cycle durability test in 0.5 m H SO solution. This work highlights a potentially powerful strategy toward the general synthesis of novel, multimetallic, PHNCs as highly active and durable bifunctional electrocatalysts for high-performance electrochemical overall-water-splitting devices.

Potassium-ion batteries (KIBs) are receiving increasing interest in grid-scale energy storage owing to the earth abundant and low cost of potassium resources. However, their development still stays at the infancy stage due to the lack of suitable electrode materials with reversible depotassiation/potassiation behavior, resulting in poor rate performance, low capacity, and cycling stability. Herein, the first example of synthesizing single-crystalline metallic graphene-like VSe nanosheets for greatly boosting the performance of KIBs in term of capacity, rate capability, and cycling stability is reported. Benefiting from the unique 2D nanostructure, high electron/K -ion conductivity, and outstanding pseudocapacitance effects, ultrathin VSe nanosheets show a very high reversible capacity of 366 mAh g at 100 mA g , a high rate capability of 169 mAh g at 2000 mA g , and a very low decay of 0.025% per cycle over 500 cycles, which are the best in all the reported anode materials in KIBs. The first-principles calculations reveal that VSe nanosheets have large adsorption energy and low diffusion barriers for the intercalation of K -ion. Ex situ X-ray diffraction analysis indicates that VSe nanosheets undertake a reversible phase evolution by initially proceeding with the K -ion insertion within VSe layers, followed by the conversion reaction mechanism.

Potassium-ion batteries (KIBs) have recently attracted intensive attention because of the abundant potassium resources and their low cost and high safety. However, the major challenge faced by KIBs lies in the lack of stable and high-capacity materials for the intercalation/deintercalation of large-size potassium ions. A unique pistachio-shuck-like MoSe /C core/shell nanostructure (PMC) is synthesized herein as an advanced anode for boosting the performance of KIBs. This PMC is featured with a few layers of molybdenum selenide as the core with an expanded interlayer spacing of ≈0.85 nm, facilitating the intercalation/deintercalation of K ions, and a thin amorphous carbon as the shell, which can confine the active molybdenum selenide nanosheets during cycling for maintaining the high structural stability. Most importantly, as a whole, the PMC has the advantages of reducing the surplus hollow interior space for improving its packing density and buffering the volume expansion during the K-ion intercalation for further enhancing the stability. As a consequence, the PMC shows a very high capacity of 322 mAh g at 0.2 A g over 100 cycles, and can still remain 226 mAh g at 1.0 A g for a long period of 1000 cycles, which is among the best-reported KIBs anodes.

Developing bifunctional efficient electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is in high demand for the development of overall water-splitting devices. In particular, the electrocatalytic performance can be largely improved by designing positive nanoscale-heterojunction with well-tuned interfaces. Herein, a novel top-down strategy is reported to construct the oxide/ sulfide heterostructures (N-NiMoO 4 /NiS 2 nanowires/nanosheets) as a multisite HER/OER catalyst. Starting with the NiMoO 4 nanowires, nitridation in a controlled manner enables activation of Ni sites in NiMoO 4 and then yields oxide/sulfide heterojunction by directly vulcanizing the highly composition-segregated N-NiMoO 4 nanowires. The abundant epitaxial heterogeneous interfaces at atomic-level facilitate the electron transfer from N-NiMoO 4 to NiS 2 , which further cooperate synergistically toward both the hydrogen and oxygen generation in alkali solution. Furthermore, with N-NiMoO 4 /NiS 2 grown carbon fiber cloth as the engineering electrode, the assembled N-NiMoO 4 /NiS 2 -N-NiMoO 4 /NiS 2 system can deliver a current density of 10 mA cm −2 with the cell voltage of 1.60 V in the water-splitting reaction. This current density is 3.39 times higher than that of the Pt-Ir set (2.95 mA cm −2 ). The excellent catalytic performance offered of N-NiMoO 4 /NiS 2 nanowires/nanosheets presents a great example to demonstrate the significance of interface engineering in the field of electrocatalysis.

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.