Na Han

Highly active and durable methanol oxidation electrocatalyst based on the synergy of platinum-nickel hydroxide-graphene.

Active and durable electrocatalysts for methanol oxidation reaction are of critical importance to the commercial viability of direct methanol fuel cell technology. Unfortunately, current methanol oxidation electrocatalysts fall far short of expectations and suffer from rapid activity degradation. Here we report platinum–nickel hydroxide–graphene ternary hybrids as a possible solution to this long-standing issue. The incorporation of highly defective nickel hydroxide nanostructures is believed to play the decisive role in promoting the dissociative adsorption of water molecules and subsequent oxidative removal of carbonaceous poison on neighbouring platinum sites. As a result, the ternary hybrids exhibit exceptional activity and durability towards efficient methanol oxidation reaction. Under periodic reactivations, the hybrids can endure at least 500,000 s with negligible activity loss, which is, to the best of our knowledge, two to three orders of magnitude longer than all available electrocatalysts.

Ultrathin nickel–iron layered double hydroxide nanosheets intercalated with molybdate anions for electrocatalytic water oxidation

There have been growing efforts to search for active, robust and cost-effective electrocatalysts for the oxygen evolution reaction (OER). Among the different candidates, Ni–Fe layered double hydroxides (LDHs) hold great promise due to their high activity closely approaching or even outperforming that of the precious metal benchmark in alkaline media. Here, we show that their activity can be further promoted when forming ultrathin LDH nanosheets intercalated with molybdate ions via an exfoliation-free hydrothermal method. In 1 M KOH, these nanosheets exhibit about 3-fold higher OER current density than regular NiFe LDH nanosheets, which was believed to be mostly contributed by their higher available density of electrochemically active sites associated with the ultrathin thickness. The great activity is also accompanied by remarkable durability at different current density levels. Finally, we demonstrate that these ultrathin nanosheets can also be directly grown on Ni foam for achieving significant current densities.

Nanostructured CuP2/C composites as high-performance anode materials for sodium ion batteries

Research on sodium ion batteries has recently been revived. Attention is now placed on the development of high-capacity and stable electrode materials at low costs. Among them, compounds operating on the conversion mechanism represent a promising class of anode materials. Unfortunately, they are generally plagued by poor electrical conductivity and large volume changes during repeated cycling. In this study, we exploit a new type of composite material made of copper phosphide and Super P carbon black (CuP2/C) as a potential anode candidate. The final products consisted of crystalline CuP2 cores coated with carbon black nanoparticles on the surface. Electrochemical measurements and multiple ex situ studies demonstrate that CuP2/C composites are capable of fast and reversible sodiation and desodiation based on the conversion mechanism. They deliver a large capacity in excess of 500 mA h g−1, high rate capability and decent short-term cycling stability. Our study suggests that these transition metal phosphides with a suitable carbon coating may hold great opportunities as anode materials for sodium ion batteries for effective and economical energy storage.

Hierarchical VS2 Nanosheet Assemblies: A Universal Host Material for the Reversible Storage of Alkali Metal Ions

Reversible electrochemical storage of alkali metal ions is the basis of many secondary batteries. Over years, various electrode materials are developed and optimized for a specific type of alkali metal ions (Li+ , Na+ , or K+ ), yet there are very few (if not none) candidates that can serve as a universal host material for all of them. Herein, a facile solvothermal method is developed to prepare VS2 nanosheet assemblies. Individual nanosheets are featured with a few atomic layer thickness, and they are hierarchically arranged with minimized stacking. Electrochemical measurements show that VS2 nanosheet assemblies enable the rapid and durable storage of Li+ , Na+ , or K+ ions. Most remarkably, the large reversible specific capacity and great cycling stability observed for both Na+ and K+ are extraordinary and superior to most existing electrode materials. The experimental results of this study are further supported by density functional theory calculations showing that the layered structure of VS2 has large adsorption energy and low diffusion barriers for the intercalation of alkali metal ions.

Iron-based sodium-ion full batteries

Rechargeable sodium-ion batteries have been an active area of research over the past several years. While a great deal of attention is now focused on the development and evaluation of single electrode materials, much less is paid to their combined performance in full batteries. Most full batteries currently available suffer from rapid capacity fading under extended cycling. In this study, we prepare ultra-small, poorly crystalline FeOx nanoparticles supported on carbon nanotubes as the anode material for sodium-ion batteries. It exhibits excellent half-cell performances; and, when combined with a Prussian blue cathode material, it leads to iron-based full batteries. Our prototypes have a working voltage of ∼2 V, specific energy density of ∼136 W h kg−1 and, most impressively, outstanding cycling stability at both low and high current rates with negligible capacity loss. Owing to their low material and fabrication cost, long cycle life and high efficiency, we believe that these iron-based sodium-ion batteries would be highly appealing toward the stationary energy storage.

CuWO4 Nanoflake Array-Based Single-Junction and Heterojunction Photoanodes for Photoelectrochemical Water Oxidation

Over recent years, tremendous efforts have been invested in the search and development of active and durable semiconductor materials for photoelectrochemical (PEC) water splitting, particularly for photoanodes operating under a highly oxidizing environment. CuWO4 is an emerging candidate with suitable band gap and high chemical stability. Nevertheless, its overall solar-to-electricity remains low because of the inefficient charge separation process. In this work, we demonstrate that this problem can be partly alleviated through designing three-dimensional hierarchical nanostructures. CuWO4 nanoflake arrays on conducting glass are prepared from the chemical conversion of WO3 templates. Resulting electrode materials possess large surface areas, abundant porosity and small thickness. Under illumination, our CuWO4 nanoflake array photoanodes exhibit an anodic current density of ∼0.4 mA/cm(2) at the thermodynamic potential of water splitting in pH 9.5 potassium borate buffer--the largest value among all available CuWO4-based photoanodes. In addition, we demonstrate that their performance can be further boosted to >2 mA/cm(2) by coupling with a solution-cast BiVO4 film in a heterojunction configuration. Our study unveils the great potential of nanostructured CuWO4 as the photoanode material for PEC water oxidation.

Improved Sodium-Ion Storage Performance of Ultrasmall Iron Selenide Nanoparticles

Sodium-ion batteries are potential low-cost alternatives to current lithium-ion technology, yet their performances still fall short of expectation due to the lack of suitable electrode materials with large capacity, long-term cycling stability, and high-rate performance. In this work, we demonstrated that ultrasmall (∼5 nm) iron selenide (FeSe2) nanoparticles exhibited a remarkable activity for sodium-ion storage. They were prepared from a high-temperature solution method with a narrow size distribution and high yield and could be readily redispersed in nonpolar organic solvents. In ether-based electrolyte, FeSe2 nanoparticles exhibited a large specific capacity of ∼500 mAh/g (close to the theoretical limit), high rate capability with ∼250 mAh/g retained at 10 A/g, and excellent cycling stability at both low and high current rates by virtue of their advantageous nanosizing effect. Full sodium-ion batteries were also constructed from coupling FeSe2 with NASICON-type Na3V2(PO4)3 cathode and demonstrated impressive capacity and cycle ability.

Ultrathin bismuth nanosheets from in situ topotactic transformation for selective electrocatalytic CO 2 reduction to formate

Electrocatalytic carbon dioxide reduction to formate is desirable but challenging. Current attention is mostly focused on tin-based materials, which, unfortunately, often suffer from limited Faradaic efficiency. The potential of bismuth in carbon dioxide reduction has been suggested but remained understudied. Here, we report that ultrathin bismuth nanosheets are prepared from the in situ topotactic transformation of bismuth oxyiodide nanosheets. They process single crystallinity and enlarged surface areas. Such an advantageous nanostructure affords the material with excellent electrocatalytic performance for carbon dioxide reduction to formate. High selectivity (~100%) and large current density are measured over a broad potential, as well as excellent durability for >10 h. Its selectivity for formate is also understood by density functional theory calculations. In addition, bismuth nanosheets were coupled with an iridium-based oxygen evolution electrocatalyst to achieve efficient full-cell electrolysis. When powered by two AA-size alkaline batteries, the full cell exhibits impressive Faradaic efficiency and electricity-to-formate conversion efficiency.The electroreduction of carbon dioxide to liquid products provides an appealing method to convert atmospheric carbon into valuable fuels. Here, the authors perform a topotactic transformation of bismuth oxyiodide to bismuth nanosheets that act as highly selective CO2-to-formate electrocatalysts.

Promoting Effect of Ni(OH)2 on Palladium Nanocrystals Leads to Greatly Improved Operation Durability for Electrocatalytic Ethanol Oxidation in Alkaline Solution

Most electrocatalysts for the ethanol oxidation reaction suffer from extremely limited operational durability and poor selectivity toward the CC bond cleavage. In spite of tremendous efforts over the past several decades, little progress has been made in this regard. This study reports the remarkable promoting effect of Ni(OH)2 on Pd nanocrystals for electrocatalytic ethanol oxidation reaction in alkaline solution. A hybrid electrocatalyst consisting of intimately mixed nanosized Pd particles, defective Ni(OH)2 nanoflakes, and a graphene support is prepared via a two-step solution method. The optimal product exhibits a high mass-specific peak current of >1500 mA mg-1Pd , and excellent operational durability forms both cycling and chronoamperometric measurements in alkaline solution. Most impressively, this hybrid catalyst retains a mass-specific current of 440 mA mg-1 even after 20 000 s of chronoamperometric testing, and its original activity can be regenerated via simple cyclic voltammetry cycles in clean KOH. This great catalyst durability is understood based on both CO stripping and in situ attenuated total reflection infrared experiments suggesting that the presence of Ni(OH)2 alleviates the poisoning of Pd nanocrystals by carbonaceous intermediates. The incorporation of Ni(OH)2 also markedly shifts the reaction selectivity from the originally predominant C2 pathway toward the more desirable C1 pathway, even at room temperature.

Engineering SnS2 nanosheet assemblies for enhanced electrochemical lithium and sodium ion storage

The reversible electrochemical storage of Li+ and Na+ ions is the operating basis of secondary lithium-ion and sodium-ion batteries. In recent years, there has been rapid growth in the search for appropriate electrode materials. Nevertheless, the development of host materials for active and durable electrochemical storage of both Li+ and Na+ ions remain challenging. In this study, we report a facile solvothermal method to prepare hierarchical assemblies of thin SnS2 nanosheets in N-methyl-2-pyrrolidone. The as-prepared product has an expanded layered structure due to the presence of organic intercalates. Mild annealing restores the normal 2H-SnS2 phase with the hierarchical architecture preserved. When annealed SnS2 was evaluated as the anode material of lithium-ion batteries, it exhibited large capacity in excess of 1200 mA h g−1 and decent short-term cycling stability. It was further coated with a thin carbon layer as the physical and electrical reinforcement, which led to a much improved cycle life at both low and high current rates. Moreover, carbon coated SnS2 also demonstrated a large capacity (∼600 mA h g−1) and decent cycling stability as the anode material of sodium-ion batteries.