Wenwen Xu

High‐Performance Water Electrolysis System with Double Nanostructured Superaerophobic Electrodes

Catalysts screening and structural optimization are both essential for pursuing a high-efficient water electrolysis system (WES) with reduced energy supply. This study demonstrates an advanced WES with double superaerophobic electrodes, which are achieved by constructing a nanostructured NiMo alloy and NiFe layered double hydroxide (NiFe-LDH) films for hydrogen evolution and oxygen evolution reactions, respectively. The superaerophobic property gives rise to significantly reduced adhesion forces to gas bubbles and thereby accelerates the hydrogen and oxygen bubble releasing behaviors. Benefited from these metrics and the high intrinsic activities of catalysts, this WES affords an early onset potential (≈1.5 V) for water splitting and ultrafast catalytic current density increase (≈0.83 mA mV(-1) ), resulting in ≈2.69 times higher performance compared to the commercial Pt/C and IrO2 /C catalysts based counterpart under 1.9 V. Moreover, enhanced performance at high temperature as well as prominent stability further demonstrate the practical application of this WES.

Dehydrated layered double hydroxides: Alcohothermal synthesis and oxygen evolution activity

Layered double hydroxides (LDHs) are a class of two-dimensional (2D) layered materials with extensive applications and well-developed synthesizing methods in aqueous media. In this work, we introduce an alcohothermal synthesis method for fabricating NiFe-LDHs with dehydrated galleries. The proposed process involves incomplete hydrolysis of urea for the simultaneous precipitation of metal ions, with the resulting water-deficient ethanol environment leading to the formation of a dehydrated structure. The formation of a gallery-dehydrated layer structure was confirmed by X-ray diffraction (XRD), as well as by a subsequent rehydration process. The methodology introduced here is also applicable for fabricating Fe-based LDHs (NiFe-LDH and NiCoFe-LDH) nanoarrays, which cannot be produced under the same conditions in aqueous media because of the different precipitation processes involved. The LDH nanoarrays exhibit excellent electrocatalytic performance in the oxygen evolution reaction, as a result of their high intrinsic activity and unique structural features. In summary, this study not only introduces a new method for synthesizing LDH materials, but also provides a new route towards highly active and robust electrodes for electrocatalysis.

Superwetting Electrodes for Gas-Involving Electrocatalysis

Gas-involving electrochemical reactions, including gas evolution reactions and gas consumption reactions, are essential components of the energy conversion processes and gathering elevating attention from researchers. Besides the development of highly active catalysts, gas management during gas-involving electrochemical reactions is equally critical for industrial applications to achieve high reaction rates (hundreds of milliamperes per square centimeter) under practical operation voltages. Biomimetic surfaces, which generally show regular micro/nanostructures, offer new insights to address this issue because of their special wetting capabilities. Although a series of nanoarray-based structured electrodes have been constructed and demonstrated with excellent performances for gas-involving electrochemical reactions, understanding of bubble wetting behavior remains elusive. In this Account, our recent works including understanding the way to achieve the superwetting properties of solid electrode surfaces, and our advanced design and fabrication of superwetting electrodes for different types of electrochemical gas-involving electrochemical reactions are summarized. To begin, we first put forward several criteria of superwetting surfaces, including superaerophobic surfaces and superaerophilic surfaces. Then, we discuss how the nanoarray-based surface engineering technology can achieve the superwetting properties, in which high roughness of the nanoarray architecture is discovered to be a critical factor for constructing superaerophobic and superaerophilic surfaces. Finally, the feasibility of superwetting electrodes for enhancing the performances of gas-involving electrochemical reactions is also analyzed. Based on theoretical guidance, a series of superaerophobic and superaerophilic electrodes with various methods, such as hydrothermal reactions, electrodeposition technology and high-temperature vapor phase growth, have been built for practice. By comparing with the traditional planar electrodes fabricated by drop-casting method, the superaerophobic electrodes afford a low adhesion force to gas products and accelerate gas bubbles evolution, resulting in fast current increase and stable current for gas evolution reactions. This phenomenon is confirmed by operating different gas evolution reactions (hydrogen evolution, oxygen evolution and hydrazine oxidation) using superaerophobic electrodes with different catalysts (e.g., MoS2, Pt and Cu). On the other side, the superaerophilic electrodes can improve the catalytic performance of gas consumption reaction (e.g., oxygen reduction reaction) by facilitating gas diffusion and electron transport. Following theoretical analyses and experimental demonstrations, we assemble several energy conversion systems (e.g., electrochemical water splitting and direct hydrazine fuel cells) based on superwetting electrodes and test their performances. By virtue of the structural advantages of electrodes, these energy conversion systems show much higher energy efficiencies than their counterparts. In the last section, we put forward several future fields which are worthy for further exploration as rational extensions of the superwetting electrodes.

Boosting Oxygen Reaction Activity by Coupled Sulfides for High-Performance Rechargeable Metal-Air Battery

Sluggish oxygen electrochemistry including both oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) greatly restricts the performance of rechargeable metal–air battery. Herein, we couple NiFe sulfide (NiFeS2) with S-doped graphene oxide (S-GO) via a simultaneous sulfurization strategy to significantly improve the OER and ORR activities. The NiFeS2/S-GO on glassy carbon yields an OER current density of 10 mA cm−2 at 1.47 V and an ORR half-wave potential at 0.74 V (vs. RHE), giving an overvoltage difference as low as 0.73 V. When assembled in a rechargeable Zn–air battery, the battery with NiFeS2/S-GO air electrode exhibits a steady charging potential (1.98 V) with very little decay in discharging potential (from 1.20 to 1.17 V) for 180 charging–discharging cycles at 10 mA cm−2. Our study provides new insights for the design of efficient bifunctional oxygen electrocatalysts for high-performance energy conversion and storage devices.