Tuning the Electronic Structures of Multimetal Oxide Nanoplates to Realize Favorable Adsorption Energies of Oxygenated Intermediates.

Highly active oxygen evolution reaction (OER) electrocatalysts are important to effectively transform renewable electricity to fuel and chemicals. In this work, we construct a series of multimetal oxide nanoplate OER electrocatalysts through successive cation exchange followed by electrochemical oxidation, whose electronic structure and diversified metal active sites can be engineered via the mutual synergy among multiple metal species. Among the examined multimetal oxide nanoplates, CoCeNiFeZnCuOx nanoplates exhibit the optimal adsorption energy of OER intermediates. Together with the high electrochemical active surface area, the CoCeNiFeZnCuOx nanoplates manage to deliver a small overpotential of 211 mV at an OER current density of 10 mA cm-2 (η10) with a Tafel slope as low as 21 mV dec-1 in 1 M KOH solution, superior to commercial IrO2 (339 mV at η10, Tafel slope of 55 mV dec-1), which can be stably operated at 10 mA cm-2 (at an overpotential of 211 mV) and 100 mA cm-2 (at an overpotential of 307 mV) for 100 h.

Architecting a Stable High-Energy Aqueous Al-Ion Battery.

Aqueous Al-ion batteries (AAIBs) are the subject of great interest due to the inherent safety and high theoretical capacity of aluminum. The high abundancy and easy accessibility of aluminum raw materials further make AAIBs appealing for grid-scale energy storage. However, the passivating oxide film formation and hydrogen side reactions at the aluminum anode as well as limited availability of the cathode lead to low discharge voltage and poor cycling stability. Here, we proposed a new AAIB system consisting of an AlxMnO2 cathode, a zinc substrate-supported Zn-Al alloy anode, and an Al(OTF)3 aqueous electrolyte. Through the in situ electrochemical activation of MnO, the cathode was synthesized to incorporate a two-electron reaction, thus enabling its high theoretical capacity. The anode was realized by a simple deposition process of Al3+ onto Zn foil substrate. The featured alloy interface layer can effectively alleviate the passivation and suppress the dendrite growth, ensuring ultralong-term stable aluminum stripping/plating. The architected cell delivers a record-high discharge voltage plateau near 1.6 V and specific capacity of 460 mAh g-1 for over 80 cycles. This work provides new opportunities for the development of high-performance and low-cost AAIBs for practical applications.

Layered Oxide Cathode for Potassium-Ion Battery: Recent Progress and Prospective.

Rechargeable potassium-ion batteries (KIBs) have demonstrated great potential as alternative technologies to the currently used lithium-ion batteries on account of the competitive price and low redox potential of potassium which is advantageous to applications in the smart grid. As the critical component determining the energy density, appropriate cathode materials are of vital need for the realization of KIBs. Layered oxide cathodes are promising candidates for KIBs due to high reversible capacity, appropriate operating potential, and most importantly, facile and easily scalable synthesis. In light of this trend, the recent advancements and progress in layered oxides research for KIBs cathodes, covering material design, structural evolution, and electrochemical performance are comprehensively reviewed. The structure-performance correlation and some effective optimization strategies are also discussed. Furthermore, challenges and prospects of these layered cathodes are included, with the purpose of providing fresh impetus for future development of these materials for advanced energy storage systems.

Electronic Modulation of Nickel Disulfide toward Efficient Water Electrolysis.

Developing highly efficient earth-abundant nickel-based compounds is an important step to realize hydrogen generation from water. Herein, the electronic modulation of the semiconducting NiS2 by cation doping for advanced water electrolysis is reported. Both theoretical calculations and temperature-dependent resistivity measurements indicate the semiconductor-to-conductor transition of NiS2 after Cu incorporation. Further calculations also suggest the advantages of Cu dopant to cathodic water electrolysis by bringing Gibbs free energy of H adsorption at both Ni sites and S sites much closer to zero. It is noteworthy that water dissociation on Cu-doped NiS2 (Cu-NiS2 ) surface is even more favorable than those on NiS2 and Pt(111). Thus, the prepared Cu-NiS2 shows noticeably improved performance toward alkaline hydrogen and oxygen evolution reactions (HER and OER). Specifically, it requires merely 232 mV OER overpotential to drive 10 mA cm-2 ; in parallel with Tafel slopes of 46 mV dec-1 . Regarding HER, an onset overpotential of only 68 mV is achieved. When integrated as both electrodes for water electrolysis, Cu-NiS2 needs only 1.64 V to drive 10 mA cm-2 , surpassing the state-of-the-art Ir/C-Pt/C couple (1.71 V). This work opens up an avenue to engineer low-cost and earth-abundant catalysts performing on par with the noble-metal-based one for water splitting.

Phosphorene-Based Electrocatalysts.

Phosphorene, generally defined as two-dimensional (2D) black phosphorus (BP) with monolayered or few-layered structure, has emerged as a promising member of the family of 2D materials. Since its discovery in 2014, extensive research has been focused on broadening its applications, covering the biological, photoelectric, and electrochemical fields, owing to the unique physicochemical and structural properties. As a single-elemental material, phosphorene has demonstrated its applicability for the preparation of efficient electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), nitrogen reduction reaction (NRR), and other electrocatalytic applications. In this Minireview, a summary of the very recent research progresses of phosphorene in electrocatalysis is offered, with a special focus on the effective synthetic strategies towards performance improvement. In the concluding section, challenges and perspectives are also discussed.

Ultrathin Amorphous Nickel Doped Cobalt Phosphates with Highly Ordered Mesoporous Structures as Efficient Electrocatalyst for Oxygen Evolution Reaction.

Herein, the facile preparation of ultrathin (≈3.8 nm in thickness) 2D cobalt phosphate (CoPi) nanoflakes through an oil-phase method is reported. The obtained nanoflakes are composed of highly ordered mesoporous (≈3.74 nm in diameter) structure and exhibit an amorphous nature. Attractively, when doped with nickel, such 2D mesoporous Ni-doped CoPi nanoflakes display decent electrocatalytic performances in terms of intrinsic activity, and low kinetic barrier toward the oxygen evolution reaction (OER). Particularly, the optimized 10 at% Ni-doped CoPi nanoflakes (denoted as Ni10-CoPi) deliver a low overpotential at 10 mA cm-2 (320 mV), small Tafel slope (44.5 mV dec-1 ), and high stability for OER in 1.0 m KOH solution, which is comparable to the state-of-the-art RuO2 tested in the same condition (overpotential: 327 mV at 10 mA cm-2 , Tafel slope: 73.7 mV dec-1 ). The robust framework coupled with good OER performance enables the 2D mesoporous Ni10-CoPi nanoflakes to be a promising material for energy conversion applications.

Cu- and Fe-Codoped Ni Porous Networks as an Active Electrocatalyst for Hydrogen Evolution in Alkaline Medium.

Highly active catalysts from the earth-abundant metals are essential to materialize the low-cost production of hydrogen through water splitting. Herein, nickel porous networks codoped with Cu and Fe prepared by thermal reduction of presynthesized Cu, Fe-codoped Ni(OH)2 nanowires are reported. The sample consists of nanoparticles of ∼80 nm, which form highly porous network clusters of ∼1 µm with a pore size of 10-100 nm. Among the various doped compositions, the NiCu0.05Fe0.025 porous network exhibits the best catalytic activity with a low overpotential of 60 mV for a hydrogen evolution reaction (HER) in 1 M KOH solution and a specific activity of 0.1 mA cm-2 at 117 mV overpotential calculated based on the electrochemical active surface area (ECSA). The density functional theory calculations reveal that codoping of Fe and Cu into the Ni lattice results in a shift of d-bands of nickel to lower energy levels and thus in the reduced hydrogen adsorption energy (ΔGH = -0.131 eV), which is close to ΔGH for Pt (-0.09 eV). When NiCu0.05Fe0.025(OH)2 nanowires is used as an oxygen evolution reaction (OER) catalyst and is coupled with NiCu0.05Fe0.025 porous networks for overall water splitting, the NiCu0.05Fe0.025∥NiCu0.05Fe0.025(OH)2 catalyst couple achieves a current density of 10 mA cm-2 at 1.491 V, similar to that of the Pt/C∥RuO2 couple and offers a negligible loss in the performance when operated at 20 mA cm-2 for 30 h.

Synergy of Nb Doping and Surface Alloy Enhanced on Water-Alkali Electrocatalytic Hydrogen Generation Performance in Ti-Based MXene.

Presented are the theoretical calculation and experimental studies of a Ti3C2T x MXene-based nanohybrid with simultaneous Nb doping and surface transition metal alloy modification. Guided by the density functional theory calculation, the Nb doping can move up the Fermi energy level to the conduction band, thus enhancing the electronic conductivity. Meanwhile, the surface modification by Ni/Co alloy can moderate the surface M-H affinity, which will further enhance the hydrogen evolution reaction (HER) activity. A series of Ni/Co alloy attached on Nb-doped Ti3C2T x MXene nanohybrids (denoted as NiCo@NTM) are successfully prepared. As expected, the Ni0.9Co0.1@ NTM nanohybrids present an extraordinary HER activity in alkaline solution, which only needs an overpotential (η) of 43.4 mV to reach the current density of 10 mA cm-2 in 1 m KOH solution and shows good stability. The performance of the Ni0.9Co0.1@ NTM nanohybrids is comparable to the commercial 10% Pt/C electrode (34.4 mV@10 mA cm-2) and is better than most state-of-the-art Pt-free HER catalysts. Inspired by the facile synthesis process and chemical versatility of both MXene and transition metal alloys, the nanohybrids reported here are promising non-noble metal electrocatalysts for water-alkali electrolysis.

Facile Synthesis of Amorphous Ternary Metal Borides-Reduced Graphene Oxide Hybrid with Superior Oxygen Evolution Activity.

Metal borides represent an emerging family of advanced electrocatalyst for oxygen evolution reaction (OER). Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on reduced graphene oxide (rGO) sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only 230 mV is required to afford a current density of 15 mA cm-2 with a small Tafel slope of 50 mV dec-1. DFT calculations lead to a suggestion that the in situ formation of MO xH y during electrochemical activation acts as active sites for water oxidation. The superior OER activity of the as-prepared catalyst is attributed to (i) its unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents, and (iii) strong coupling of active material and highly conductive rGO. This work not only provides new perspectives to design a highly effective material for OER but also opens a promising avenue to tailor the electrochemical properties of metal borides, which could be extended to other materials for energy storage and conversion technologies.

Performance-improved Li-O2 batteries by tailoring the phases of MoxC porous nanorods as an efficient cathode.

Novel nitrogen-doped porous molybdenum carbide (α-MoC1-x and ß-Mo2C) architectures were prepared using Mo-based metal-organic frameworks (MOFs) as the precursor. The synthesized molybdenum carbides consist of numerous nanocrystals organized into micro-sized rods with interpenetrating mesoporous-channels and macroporous-tunnels along the axial direction. When employed as the cathode catalyst for Li-O2 batteries, this dual pore configuration offers abundant active sites for the electrochemical reaction and many nucleation sites for the discharge product of Li2O2; hence, decent performances were obtained. Among the two synthesized molybdenum carbides, the α-MoC1-x electrode stands out as being better due to its lower charge transfer resistance (395.8 Ω compared to 627.9 Ω) and better O2 adsorption (binding energy of -1.87 eV of α-(111)-Mo compared to -0.72 eV of ß-(101)-Mo). It delivered a high full discharge of 20 212 mA h g-1 with a discharge voltage of 2.62 V at 200 mA g-1. A good cycling stability was also obtained: i.e. 100 stable cycles with a fixed capacity of 1000 mA h g-1 (at a current density of 200 mA g-1) with a charging voltage of 4.24 V and maintaining a respectable round-trip efficiency of ∼70%.

Constructing Multifunctional Heterostructure of Fe2 O3 @Ni3 Se4 Nanotubes.

Heterostructures have attracted increasing attention due to their amazing synergetic effects, which may improve the electrochemical properties, such as good electrical/ionic conductivity, electrochemical activity, and mechanical stability. Herein, novel hierarchical Fe2 O3 @Ni3 Se4 nanotubes are successfully fabricated by a multistep strategy. The nanotubes show length sizes of ≈250-500 nm, diameter sizes of ≈100-150 nm, and wall thicknesses of ≈10 nm. The as-prepared Fe2 O3 @Ni3 Se4 nanotubes with INi:Fe = 1:10 show excellent Li storage properties (897 mAh g-1 high reversible charge capacity at 0.1 A g-1 ), good rate performance (440 mAh g-1 at 5 A g-1 ), and outstanding long-term cycling performance (440 mAh g-1 at 5 A g-1 during the 300th cycle) as an anode material for lithium ion batteries. In addition, the Fe2 O3 @Ni3 Se4 nanotubes with INi:Fe = 1:10 (the atomic ratio between Ni and Fe) show superior electrocatalytic performance toward the oxygen evolution reaction with an overpotential of only 246 mV at 10 mA cm-2 and a low Tafel slope of 51 mV dec-1 in 1 m KOH solution.

Few-layer NiPS3 nanosheets as bifunctional materials for Li-ion storage and oxygen evolution reaction.

The construction of two-dimensional (2D) ultrathin nanosheets is considered as a promising strategy for enhancing electrochemical performance, owing to their large surface area and fast electron transport. In this study, ultrathin few-layer NiPS3 nanosheets are obtained and systematically investigated by high-yield liquid phase exfoliation from their bulk layered crystals, and are exploited as anodes for lithium ion batteries (LIBs) and electrocatalysts for oxygen evolution reaction (OER). When evaluated as an anode for LIBs, NiPS3 nanosheets show excellent electrochemical properties in terms of stable cycling performance and rate capabilities. A stable reversible capacity of 796.2 mA h g-1 is delivered after the 150th cycle at a current density of 100 mA g-1. As for the OER, the exfoliated few-layer NiPS3 nanosheets have demonstrated excellent electrocatalytic performance, such as a low overpotential of 301 mV at a current density of 10 mA cm-2, a small Tafel slope of 43 mV dec-1, and an outstanding long-term durability. This work is expected to make a contribution to develop next generation high-performance electrochemically active materials for catalysts and batteries.

Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting.

Herein, the hydrothermal synthesis of porous ultrathin ternary NiFeV layer double hydroxides (LDHs) nanosheets grown on Nickel foam (NF) substrate as a highly efficient electrode toward overall water splitting in alkaline media is reported. The lateral size of the nanosheets is about a few hundreds of nanometers with the thickness of ≈10 nm. Among all molar ratios investigated, the Ni0.75 Fe0.125 V0.125 -LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for the oxygen evolution reaction (OER) and 125 mV for the hydrogen evolution reaction (HER) in 1.0 m KOH electrolyte. Its exceptional activity is further shown in its small Tafel slope of 39.4 and 62.0 mV dec-1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni0.75 Fe0.125 V0.125 -LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm-2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area. This work explores a promising way to achieve the optimal inexpensive Ni-based hydroxide electrocatalyst for overall water splitting.

Scalable synthesis of SnS2/S-doped graphene composites for superior Li/Na-ion batteries.

Tin disulfide (SnS2) has emerged as a promising anode material for lithium/sodium ion batteries (LIBs/SIBs) due to its unique layered structure, outstanding electrochemical properties and low cost. However, its poor cycling life and time-consuming synthesis as well as low-yield production hinder the practical utilization of nanostructured SnS2. In this work, we demonstrate a simple and reliable dissolution-regeneration strategy to construct a flexible SnS2/sulfur-doped reduced graphene oxide (S-rGO) composite as anodes for LIBs and SIBs, highlighting its mass-production feature. In addition, the robust affinity between SnS2 and S-rGO without interstitial volume is very beneficial for preventing the SnS2 particles from breaking themselves away from the rGO nanosheets into free nanoparticles. As a result, the SnS2/S-rGO composite as anodes delivers high reversible capacities of 1078 mA h g-1 and 564 mA h g-1 (at 0.1 A g-1) for LIBs and SIBs, respectively, and excellent rate capabilities and cycling stability (e.g. 532 mA h g-1 during the 600 cycles at 5.0 A g-1 for LIBs). Our proposed strategy may also possess great potential for the practical application of other electrochemically active metal sulfide composites for energy devices.