Fengjiao Chen

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.

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.

Controllably Interfacing with Ferroelectric Layer: A Strategy for Enhancing Water Oxidation on Silicon by Surface Polarization

Silicon (Si) is an important material in photoelectrochemical (PEC) water splitting because of its good light-harvesting capability as well as excellent charge-transport properties. However, the shallow valence band edge of Si hinders its PEC performance for water oxidation. Generally, thanks to their deep valence band edge, metal oxides are incorporated with Si to improve the performance, but they also decrease the transportation of carriers in the electrode. Here, we integrated a ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] layer with Si to increase the photovoltage as well as the saturated current density. Because of the prominent ferroelectric property from P(VDF-TrFE), the Schottky barrier between Si and the electrolyte can be facially tuned by manipulating the poling direction of the ferroelectric domains. The photovoltage is improved from 460 to 540 mV with a forward-poled P(VDF-TrFE) layer, while the current density increased from 5.8 to 12.4 mA/cm(2) at 1.23 V bias versus reversible hydrogen electrode.

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.

Silicon/Organic Heterojunction for Photoelectrochemical Energy Conversion Photoanode with a Record Photovoltage

Silicon (Si) is a good photon absorption material for photoelectrochemical (PEC) conversion. Recently, the relatively low photovoltage of Si-based PEC anode is one of the most significant factors limiting its performance. To achieve a high photovoltage in PEC electrode, both a large barrier height and high-quality surface passivation of Si are indispensable. However, it is still challenging to induce a large band bending and passivate Si surface simultaneously in Si-based PEC photoanodes so far, which hinders their performance. Here, we develop a simple Si/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) heterojunction with large band banding and excellent surface passiviation for efficient PEC conversion. A chemically modified PEDOT:PSS film acts as both a surface passiviation layer and an effective catalyst simultaneously without sacrificing band bending level. A record photovoltage for Si-based PEC photoanodes as high as 657 mV is achieved via optimizing the PEDOT:PSS film fabrication process. The density of electron state (DOS) measurement is utilized to probe the passivation quality of the organic/inorganic heterojunction, and a low DOS is found in the Si/PEDOT:PSS heterojunction, which is in accordance with the photovoltage results. The low-temperature solution-processed Si/organic heterojunction photoanode provides a high photovoltage, exhibiting the potential to be the next-generation economical photoanode in PEC applications.

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.

Photoelectroreduction of Building-Block Chemicals

Conventional photoelectrochemical cells utilize solar energy to drive the chemical conversion of water or CO2 into useful chemical fuels. Such processes are confronted with general challenges, including the low intrinsic activities and inconvenient storage and transportation of their gaseous products. A photoelectrochemical approach is proposed to drive the reductive production of industrial building-block chemicals and demonstrate that succinic acid and glyoxylic acid can be readily synthesized on Si nanowire array photocathodes free of any cocatalyst and at room temperature. These photocathodes exhibit a positive onset potential, large saturation photocurrent density, high reaction selectivity, and excellent operation durability. They capitalize on the large photovoltage generated from the semiconductor/electrolyte junction to partially offset the required external bias, and thereby make this photoelectrosynthetic approach significantly more sustainable compared to traditional electrosynthesis.

Designing effective Si/Ag interface via controlled chemical etching for photoelectrochemical CO2 reduction

Photoelectrochemical reduction of CO2 to value-added chemicals represents a promising approach for artificial photosynthesis, but often suffers from limited selectivity and stability. Improving its performance would require proper design of semiconductor and co-catalyst materials, along with a strategy for effective coupling. Here, we report that controlled chemical etching of Si wafer by Ag+ ions yields effective semiconductor/co-catalyst interface for photoelectrochemical CO2 reduction. Resultant photocathodes exhibit large photocurrent density (∼10 mA cm−2 under 0.5 sun), great CO faradaic efficiency (90% at −0.5 V versus reversible hydrogen electrode), and impressive operational stability (little activity or selectivity loss within 8 h). Further enhancement (by ∼20%) of photocurrent density is achieved by combining photolithography patterning with chemical etching. Our study applies long-known chemistry as an unexpected solution and may provide a new strategy for high-performance photoelectrochemical CO2 reduction.