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Oxygen evolution reaction (OER) plays a crucial role in developing energy conversion and adjusting electronic structure of the electrocatalysts can effectively improve the catalytic activity and stability. However, it is a challenge to adjust the electronic structure on two-dimensional iridium dioxide nanosheets (IrO2 NS), which have the advantages of high atom utilization. Here, we regulate the surface properties of IrO2 NS through sulfonated carbon dots (SCDs) to promote the OER catalytic process. The catalyst IrO2 NS/SCDs-2 exhibited excellent catalytic activity with a lower overpotential of 180 mV than IrO2 NS (230 mV) at the current density of 10 mA·cm−2 in a 0.5 M H2SO4 solution. And after 160 h of stability testing, the overpotential of IrO2 NS/SCDs-2 only decreased by 4 mV. Moreover, transient potential scanning test can visually demonstrate that the addition of SCDs improves the conductivity of the catalyst and increases the electron transfer rate. [ABSTRACT FROM AUTHOR]

Carbon dots modified iridium nanowires (Ir NWs/CDs) show high catalytic activity and stability for acidic hydrogen evolution/oxidation reactions, which are attributed to the large compression stress generated in Ir NWs and the fast proton/electron transfer ability and good conductivity of CDs. [Display omitted] • One-dimensional Ir nanowires modified with carbon dots are employed as catalysts for HER/HOR. • The Ir nanowires show a high compressive strain intensity of −0.563%. • CDs act as a bridge connecting hydrogen proton and electron with Ir active sites. • Ir NWs/CDs-15 requires 18 mV to obtain a current density of −10 mA cm−2 for HER. • Ir NWs/CDs-15 shows anti-CO ability which is twice the level of the PtRu/C. The effective harnessing of hydrogen energy relies on the development of bifunctional electrocatalysts that facilitate hydrogen evolution/oxidation reactions (HER/HOR) with high catalytic activity. The design of such electrocatalysts requires the consideration of not only the volcano relationship with hydrogen binding energy (HBE) or hydrogen adsorption Gibbs free energy (ΔG H) but also the regulation of catalytic kinetics such as interfacial proton/electron transfer. In this work, unique one-dimensional iridium nanowires with compressive stress are successfully prepared and combined with carbon dots (Ir NWs/CDs). Acting as an electrocatalyst for HER in 0.5 M H 2 SO 4 , the optimal Ir NWs/CDs only requires an 18 mV overpotential to achieve a current density of −10 mA cm−2. Furthermore, the optimal Ir NWs/CDs shows high HOR performance with a mass activity (@ 50 mV versus RHE) 1.5 times that of 20% Pt/C and excellent anti-CO toxicity ability which is twice the level of the PtRu/C catalyst. Ir NWs/CDs exhibit enhanced HER/HOR activity due to (1) the appropriate modulation of the binding energy to hydrogen intermediate facilitated by the compressive stress applied to the Ir structure and (2) the improved proton/electron transfer kinetics by optimizing the electronic properties and surface structures through tailored CDs. This study delivers a new strategy for designing and synthesizing efficient acidic HER/HOR bifunctional catalysts. [ABSTRACT FROM AUTHOR]

The increasing popularity of proton exchange membrane electrolysis technology for hydrogen production has brought attention to the electrolytic water reaction. However, the slow kinetics of the oxygen evolution reaction (OER) at the anode have great influence on the overall efficiency of the reaction. While iridium oxide shows excellent stability under acidic conditions, its OER activity still needs to be improved. Here, we synthesized two‐dimensional amorphous iridium oxide (Am−IrO2) nanosheets with the thickness of only 6 nm by a mixed molten salt method. Such nanosheets show an ultralow overpotential of only 230 mV at 10 mA cm−2 in 0.5 M H2SO4. The overpotential increases only 40 mV after 90 hours of the stability test at this current density. Am−IrO2 can maintain the current density of ~400 mA cm−2 after 120 hours of test at 1.8 V in the PEM device, demonstrating good industrial prospects. Density functional theoretical calculations show that the oxygen vacancies, together with the upshift of the O 2p band center, are responsible for the improvement of OER in Am−IrO2. [ABSTRACT FROM AUTHOR]

The oxygen evolution reactions in acid play an important role in multiple energy storage devices. The practical promising Ru-Ir based catalysts need both the stable high oxidation state of the Ru centers and the high stability of these Ru species. Here, we report stable and oxidative charged Ru in two-dimensional ruthenium-iridium oxide enhances the activity. The Ru0.5Ir0.5O2 catalyst shows high activity in acid with a low overpotential of 151 mV at 10 mA cm−2, a high turnover frequency of 6.84 s−1 at 1.44 V versus reversible hydrogen electrode and good stability (618.3 h operation). Ru0.5Ir0.5O2 catalysts can form more Ru active sites with high oxidation states at lower applied voltages after Ir incorporation, which is confirmed by the pulse voltage induced current method. Also, The X-ray absorption spectroscopy data shows that the Ru-O-Ir local structure in two-dimensional Ru0.5Ir0.5O2 solid solution improved the stability of these Ru centers. Stabilizing high oxidation state of Ru centers is important to achieve stable performance for acidic oxygen evolution reaction. Here the authors report two-dimensional ruthenium-iridium oxide for enhanced stability and activity for acidic water oxidation in proton exchange membrane electrolyzer. [ABSTRACT FROM AUTHOR]

Metastable metal oxides with ribbon morphologies have promising applications for energy conversion catalysis, however they are largely restricted by their limited synthesis methods. In this study, a monoclinic phase iridium oxide nanoribbon with a space group of C2/m is successfully obtained, which is distinct from rutile iridium oxide with a stable tetragonal phase (P42/mnm). A molten-alkali mechanochemical method provides a unique strategy for achieving this layered nanoribbon structure via a conversion from a monoclinic phase K0.25IrO2 (I2/m (12)) precursor. The formation mechanism of IrO2 nanoribbon is clearly revealed, with its further conversion to IrO2 nanosheet with a trigonal phase. When applied as an electrocatalyst for the oxygen evolution reaction in acidic condition, the intrinsic catalytic activity of IrO2 nanoribbon is higher than that of tetragonal phase IrO2 due to the low d band centre of Ir in this special monoclinic phase structure, as confirmed by density functional theory calculations. Well-defined metastable phase nanostructures are a core issue for catalyst design. Here, the authors report metastable monoclinic phase IrO2 nanoribbons obtained via a molten-alkali mechanochemical method, which exhibit intrinsic high performance towards the acidic oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

Lattice strain plays a critical role in regulating the electronic structure and improving the performance of catalysts. Here, Ir−IrO2/C catalysts with different iridium content are obtained by annealing Ir/C in air. The growth of IrO2 on Ir nanoparticles can form a core–shell structure and introduces strain at their interface. The local compressive strain on Ir nanoparticles is 7.2 % in the optimal Ir−IrO2/C sample with Ir content of 14.3 % (Ir−IrO2/C‐3), which regulates the adsorption energy of intermediates and improves the catalytical performance of oxygen evolution reaction (OER). The overpotential of Ir−IrO2/C‐3 at 10 mA ⋅ cm−2 is 264 mV in acidic electrolyte, 35 mV lower than that of Ir−IrO2/C with no lattice strain. Ir−IrO2/C‐3 also exhibits the highest mass activity of 1304 A ⋅ gIr−1 at 1.55 V vs. RHE, which is higher than those of commercial catalysts. Furthermore, the activity of Ir−IrO2/C‐3 can maintain 42 h with the potential increase of only 18 mV at 10 mA ⋅ cm−2, proving that the lattice strain can also effectively improve the electrochemical stability. [ABSTRACT FROM AUTHOR]

Exploring new materials is essential in the field of material science. Especially, searching for optimal materials with utmost atomic utilization, ideal activities and desirable stability for catalytic applications requires smart design of materials' structures. Herein, we report iridium metallene oxide: 1 T phase-iridium dioxide (IrO2) by a synthetic strategy combining mechanochemistry and thermal treatment in a strong alkaline medium. This material demonstrates high activity for oxygen evolution reaction with a low overpotential of 197 millivolt in acidic electrolyte at 10 milliamperes per geometric square centimeter (mA cmgeo−2). Together, it achieves high turnover frequencies of 4.2 sUPD−1 (3.0 sBET−1) at 1.50 V vs. reversible hydrogen electrode. Furthermore, 1T-IrO2 also shows little degradation after 126 hours chronopotentiometry measurement under the high current density of 250 mA cmgeo−2 in proton exchange membrane device. Theoretical calculations reveal that the active site of Ir in 1T-IrO2 provides an optimal free energy uphill in *OH formation, leading to the enhanced performance. The discovery of this 1T-metallene oxide material will provide new opportunities for catalysis and other applications. Identifying new, active material phases provides a promising avenue in the development of efficient catalysts. Here, authors demonstrate a metastable 1T-phase IrO2 metallene oxide as an oxygen evolution electrocatalyst in acidic electrolytes. [ABSTRACT FROM AUTHOR]

The SO 3 -CDs/NiFe LDH was synthesized through electrochemical and hydrothermal methods. The P-SO 3 -CDs/NiFe LDH catalyst work electrode was obtained via polarized by an electric field of 10,000 V/cm for 30 mins. After that, sulfonate functional groups selectively enrich at the surface of catalysts. Such sulfonate functional groups have a strong ability to capture proton, exhibit excellent OER performances as confirmed by DFT calculation and experimental results. [Display omitted] • NiFe LDH/CDs composite was employed as OER catalysts. • Electric field polarization is used to regulate the exposed functional groups. • H+ captured from *OH or *OOH species by the sulfonated groups on CDs. • The overpotential is 200 mV@10 mA cm−2 and Tafel slope is 55.2 mV dec−1. • Polarized catalysts reach a high current density than the unpolarized one. Interfacial design and regulation for electrocatalysts can effectively enhance the performance of oxygen evolution reaction (OER). It is still a challenge to search for a simple and easy way to controllably adjust the interface of catalysts. Here we report a new strategy, electric field polarization to regulate the charged functional groups of nickel–iron layered double hydroxide (NiFe LDH)/carbon dots (CDs) composite catalysts. The selectively exposed sulfonate functional groups on CDs under the electric field have regulated the interfacial environment of the composite catalyst. The density function theory (DFT) calculation also reveals that the high electrocatalytic activity can be attributed to H+ capture from *OH or *OOH species by the sulfonated groups on the surface of CDs. Thus, the synergistic effect between the catalytic activity of NiFe LDH and the functional group-modified CDs enhances the OER performance of the composite catalyst, showing low overpotential of 200 mV at 10 mA cm−2 and outstanding kinetics with Tafel slope of 55.2 mV dec−1. Moreover, the polarized composite catalysts can reach a high current density of 51.7 mA cm−2 at 1.46 V vs. reversible hydrogen electrode (RHE), which is 9.2 times higher than the unpolarized one (5.6 mA cm−2). [ABSTRACT FROM AUTHOR]

The construction of high-performance bifunctional self-supported electrodes is significant for promoting the activity and stability of water splitting. The self-supported electrode not only has a strong structural characterization but also avoids the negative effects of using binders. Herein, we fabricated a self-supported electrode (Ni 2 P/MoP@CNTs-CC), which is composed of carbon nanotubes (CNTs) directly grown on carbon cloth (CC) filled with transition metal-based phosphide composite nanoparticles. The optimized Ni 2 P/MoP@CNTs-CC electrode exhibits promising hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic performance. Ni 2 P/MoP@CNTs-CC shows a low overpotential of 87 mV at − 10 mA cm−2 and 360 mV at 40 mA cm−2 for HER and OER, respectively. Besides, the Ni 2 P/MoP@CNTs-CC electrode was also used as an anode and cathode, simultaneously, showing a cell voltage of 1.89 V at 50 mA cm−2 and excellent stability at 50 mA cm−2 for more than 100 h. This work provides a facile method for preparing a low-cost and robust self-supported electrode for water splitting. • Individual Ni 2 P/MoP nanoparticles were successfully encapsulated in the cavity of CNTs. • The dynamic bubble escape behavior on the surface of the self-supporting electrode was monitored and analyzed. • Ni 2 P/MoP@CNTs-CC shows considerable water splitting performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • The porous rhodium nanosheets with tunable compressive strain were successfully obtained by directly annealing metastable trigonal rhodium oxide. • The record HzOR performance. • A clear strain effect and catalytic mechanism for alkaline HzOR in single-metal-element systems. Electrocatalytic water decomposition is an essential pathway for large-scale preparation of high-purity hydrogen, while the conventional oxygen evolution reaction (OER) with a kinetically sluggish four-electron process becomes a huge challenge for its further application. Hydrazine oxidation reaction (HzOR) is considered an effective strategy to replace the sluggish OER process, attributed to its lower thermodynamic oxidation potential. Here, porous rhodium nanosheets (Rh-NSs) with tunable lattice strain are successfully obtained by directly annealing metastable trigonal rhodium oxide (Tri-RhO 2) in a hydrogen atmosphere. More importantly, ligand and synergetic effects can be effectively avoided in a single-metal-element system, providing an ideal model electrocatalyst for understanding the correlation between reactivity and lattice strain. The electrochemical results show that the optimal Rh-NSs-300 (the Tri-RhO 2 precursor is annealed at 300 °C) may deliver ultralow working potential (vs. RHE) of −103 mV at the current density of 10 mA cm−2 with a Tafel slope of 15.4 mV dec-1 for HzOR. In addition, the working potential (vs. RHE) of Rh-NSs-300 only drops 35 mV after the 72 h test by chronopotentiometry method at the constant current density of 10 mA cm−2. Density functional theory (DFT) calculations indicate that HzOR can more easily occur on Rh-NSs with tunable compressive strain than pure Rh. [ABSTRACT FROM AUTHOR]

Herein, the carbon dots bridges nickel oxide and manganese trioxide through the oxygen-containing group of carbon dots (NiO-Mn 2 O 3 -CDs) is successfully constructed and as an advanced bifunctional oxygen electrocatalysts. The CDs covalently bridging Ni and Mn atoms can reprogram the electronic structure of active sites and enhance charge transfer. Consequently, the optimal NiO-Mn 2 O 3 -CDs yields outstanding bifunctional activities for ORR and OER. Importantly, the rechargeable zinc-air battery test with NiO-Mn 2 O 3 -CDs as the air cathode delivers the excellent performances, outperforming the commercial Pt/C + IrO 2. [Display omitted] • NiO-Mn 2 O 3 -CDs yield remarkable electrocatalytic activity for ORR and OER. • The introduction of CDs reprograms the electronic structure of active sites. • Rechargeable Zn-air battery with NiO-Mn 2 O 3 -CDs shows power density of 287 mW cm−2. The development of high-efficiency, low-cost and durable bifunctional oxygen electrocatalysts is of great significance for the large-scale application of rechargeable zinc-air batteries. Herein, the carbon dots bridging nickel oxide and manganese trioxide through the oxygen-containing group of carbon dots (NiO-Mn 2 O 3 -CDs) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is proposed. The optimal NiO-Mn 2 O 3 -CDs yields remarkable electrocatalytic activity with low overpotential (298 mV) to derive the 10 mA·cm−2 for OER and a high half wave potential (0.84 V vs. RHE) for ORR. The outstanding performances are attributed to the CDs covalently bridging Ni and Mn atoms, which reprogram the electronic structure of active sites and enhance charge transfer. Consequently, the rechargeable zinc-air battery test with NiO-Mn 2 O 3 -CDs as the air cathode deliver the excellent performances, including a high open voltage (1.51 V), a maximum power density (287 mW∙cm−2 @ 0.35 A∙cm−2) and robust cycling life, outperforming the commercial Pt/C + IrO 2. This work presents an efficient strategy to construct the highly active and stability bifunctional air cathodes for electrochemical energy devices. [ABSTRACT FROM AUTHOR]

The hydrazine oxidation reaction (HzOR) has been considered as a more energy-efficient alternative to the oxygen evolution reaction in water electrolysis. To design efficient electrocatalysts for HzOR, precise engineering of materials is required to increase active sites and build electronic structures. Herein, nitrogen contained face-centered cubic rhodium (N-fcc-Rh) nanosheets are prepared by directly annealing metastable trigonal rhodium oxide precursor in the ammonia atmosphere. Benefiting from the abundant active sites and unique electronic structure, the optimal N-fcc-Rh-300 electrocatalyst achieves an ultra-low working potential of − 81 mV (vs. RHE) at 10 mA cm−2 in 1.0 M KOH/0.5 M N 2 H 4. Density functional theory calculations suggest the N element in the N-fcc-Rh electrocatalyst enables to the reduction of the formation energy of the potential-determining step from *NH 2 NH 2 to *NHNH 2 for the HzOR process. This work reveals a new strategy to prepare advanced metal electrocatalysts for various electrochemical applications. [Display omitted] • Nitrogen contained face-centered cubic rhodium nanosheets. • Excellent HzOR activity with working potential of − 81 mV (vs. RHE) at 10 mA cm−2. • N elements into Rh leads to a shift of the d-band center away from the Fermi level. [ABSTRACT FROM AUTHOR]