Electrochemical water splitting is a promising technology for sustainable conversion, storage, and transport of hydrogen energy. Searching for earth-abundant hydrogen/oxygen evolution reaction (HER/OER) electrocatalysts with high activity and durability to replace noble-metal-based catalysts plays paramount importance in the scalable application of water electrolysis. A freestanding electrode architecture is highly attractive as compared to the conventional coated powdery form because of enhanced kinetics and stability. Herein, recent progress in developing transition-metal-based HER/OER electrocatalytic materials is reviewed with selected examples of chalcogenides, phosphides, carbides, nitrides, alloys, phosphates, oxides, hydroxides, and oxyhydroxides. Focusing on self-supported electrodes, the latest advances in their structural design, controllable synthesis, mechanistic understanding, and strategies for performance enhancement are presented. Remaining challenges and future perspectives for the further development of self-supported electrocatalysts are also discussed.

Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, it is described how researchers are working to improve the performance of TMS-based materials by manipulating their internal and external nanoarchitectures. A general introduction to the water-splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials are explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in the HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both the HER and the OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The aim herein is to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

https://espace.library.uq.edu.au/view/UQ:df1181f/UQdf1181f_OA.pdf

Nanoarchitectonics for Transition-Metal-Sulfide-Based Electrocatalysts for Water Splitting.

Transition metal dichalcogenides (TMDs) have attracted considerable attention in recent years due to their unique properties and promising applications in electrochemical energy storage and conversion. However, the limited number of active sites and blocked ion and mass transport severely impair the electrochemical performance of TMDs. Construction of three-dimensional (3D) architectures from TMD nanomaterials has been proven an effective strategy to solve the aforementioned problems due to their large specific surface area and short ion and mass transport distance. Here, we summarize the commonly used routes to build 3D TMD architectures and highlight their applications in electrochemical energy storage and conversion, including batteries, supercapacitors, and electrocatalytic hydrogen evolution. Moreover, the challenges and outlooks in this research area are also discussed.

Three-Dimensional Architectures Constructed from Transition-Metal Dichalcogenide Nanomaterials for Electrochemical Energy Storage and Conversion.

Recent years have witnessed an upsurge in the development of non-precious catalysts (NPCs) for alkaline water electrolysis (AWE), especially with the strides made in experimental and computational techniques. In this contribution, the most recent advances in NPCs for AWE were systematically reviewed, emphasizing the application of in situ/operando experimental methods and density functional theory (DFT) calculations in their understanding and development. First, we briefly introduced the fundamentals of the anode and cathode reaction for AWE, i.e., the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), respectively. Next, the most popular in situ/operando approaches for characterizing AWE catalysts, including hard and soft XAS, ambient-pressure XPS, liquid and identical location TEM, electrochemical mass spectrometry, and Raman spectroscopy were thoroughly summarized. Subsequently, we carefully discussed the principles, computational methods, applications, and combinations of DFT with machine learning for modeling NPCs and predicting the alkaline OER and HER. With the improved understanding of the structure-property-performance relationship of NPCs for AWE, we proceeded to overview their current development, summarising state-of-the-art design strategies to boost their activity. In addition, advances in various extensively investigated NPCs for AWE were evaluated. By conveying these methods, progress, insights, and perspectives, this review will contribute to a better understanding and rational development of non-precious AWE electrocatalysts for hydrogen production.

Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances

Development of nonnoble metal catalysts for hydrogen evolution reaction (HER) is critical to enable an efficient production of hydrogen at low cost and large scale. In this work, a novel bimetallic carbide nanostructure consisting of Mo2C and WC is synthesized. Based on a highly conductive WC backbone, nanosized Mo2C particles are integrated onto WC, forming a well-defined and highly robust nanowire structure. More importantly, it is found that electrochemical activation can partially remove surface carbon and activate the catalyst by changing its surface hydrophilicity. As a result, the residual carbon contributes positively to the activity, besides its role of protecting carbide from oxidation. Benefiting from the structure, the catalyst achieves high activity, stable electrolysis towards HER.

Novel molybdenum carbide-tungsten carbide composite nanowires and their electrochemical activation for efficient and stable hydrogen evolution

The hydrogen evolution reaction in an alkaline environment using a non-precious catalyst with much greater efficiency represents a critical challenge in research. Here, a robust and highly active system for hydrogen evolution reaction in alkaline solution is reported by developing MoS2 nanosheet arrays vertically aligned on graphene-mediated 3D Ni networks. The catalytic activity of the 3D MoS2 nanostructures is found to increase by 2 orders of magnitude as compared to the Ni networks without MoS2. The MoS2 nanosheets vertically grow on the surface of graphene by employing tetrakis(diethylaminodithiocarbomato)molybdate(IV) as the molybdenum and sulfur source in a chemical vapor deposition process. The few-layer MoS2 nanosheets on 3D graphene/nickel structure can maximize the exposure of their edge sites at the atomic scale and present a superior catalysis activity for hydrogen production. In addition, the backbone structure facilitates as an excellent electrode for charge transport. This precious-metal-free and highly efficient active system enables prospective opportunities for using alkaline solution in industrial applications.

Three-Dimensional Structures of MoS2 Nanosheets with Ultrahigh Hydrogen Evolution Reaction in Water Reduction

In this study, robust nanoscale superamphiphobic (SAP) copper (Cu) sheet surfaces are obtained with an exceptionally simple and environmentally friendly technique. Copper oxide nanostructures are imparted onto the Cu sheet surface by a facile treatment of the surfaces with hot de-ionized water (≈80 °C) for different time periods. A morphological transformation starting from cube-like nanostructures associated with Cu2O to leaf-like nanostructures of CuO with the progress in treatment time is observed. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and laser scanning microscopy (LSM) analysis confirm the formation of copper oxide nanostructures. Consequently, the wetting properties of the treated Cu sheet surfaces toward water and organic liquids is varied after the surface energy reduction of the nanostructures with long chain fluorocarbon molecules of 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (PFDCS). The leaf-like nanostructured CuO surfaces demonstrate SAP properties with a water contact angle (CA) as high as 160° and organic liquid CAs of around 150°. The robustness of the obtained SAP Cu sheet surface is confirmed after being exposed to a stream of both water and organic liquids, annealed under various temperatures in ambient environment, and ultra-sonicated in acetone for various time periods.

Robust Superamphiphobic Nanoscale Copper Sheet Surfaces Produced by a Simple and Environmentally Friendly Technique

Microstructures offer enhancements in boiling heat transfer by increasing bubble departure frequency, active nucleation site density, critical cavity size, and surface area. Integration of microstructures to surfaces alters significant surface parameters such as porosity of the microstructured plates, contact angle, and configuration of microstructures on the surface, which all affect boiling heat transfer. The goal of this study is to investigate boiling heat transfer on different microstructured plates and the effect of various microscale surface morphologies on boiling heat transfer. The microstructured surfaces were formed on aluminum alloy 2024 sheets with the use of a simple and environmentally friendly technique of random mechanical sanding (grits of #36, #60, #400, and #1,000). Distilled water was pumped using a micro gear pump to the rectangular minichannel test section at flow rates of 100, 180, and 290 ml/min, which correspond to mass fluxes of 5.46, 10.58, and 16.15 kg/m(2).s, respectively. It was observed that surfaces with low grit (grit #36) showed no considerable enhancement, whereas the use of higher grit counts considerably enhanced boiling heat transfer up to a critical grit count. The results were supported by the images from the performed visualization of flow boiling.

Subcooled Flow Boiling Over Microstructured Plates In Rectangular Minichannels

In this study, the authors fabricated high performance core–shell nanostructured flexible photodetectors on a polyimide substrate of Kapton. For this purpose, p-type copper indium gallium selenide (CIGS) nanorod arrays (core) were coated with aluminum doped zinc oxide (AZO) films (shell) at relatively high Ar gas pressures. CIGS nanorods were prepared by glancing angle deposition (GLAD) technique using radio frequency (RF) magnetron sputtering unit at room temperature. AZO films were deposited by RF sputtering at Ar pressures of 1.0×10−2 mbar (high pressure sputtering) for the shell and at 3.0×10−3 mbar (low pressure sputtering) to create a top contact. As a comparison, the authors also fabricated conventional planar thin film devices incorporating CIGS film of similar material loading to that of CIGS nanorods. The morphological characterization was carried out by field-emission scanning electron microscope. The photocurrent measurement was conducted under 1.5 AM sun at zero electrical biasing, where CIGS...

High performance flexible copper indium gallium selenide core–shell nanorod array photodetectors

Vertically aligned core/shell nanorod array photodetectors were fabricated by high pressure sputter (HIPS) deposition of copper indium sulfide (CIS) films on glancing angle deposited (GLAD) indium sulfide (In2S3) nanorods. For comparison, we also studied nanorod photodetectors with conventional low pressure sputtered (LPS) CIS film coatings and counterpart thin film devices incorporating HIPS or LPS-CIS on In2S3 films. HIPS-GLAD core/shell photodetectors have shown a superior photocurrent density response along with lowest dark current density. Photoresponsivity defined with the photocurrent density/dark current density ratio γ = |J ph/J dark| was about ~1820 for HIPS-GLAD nanorod devices, which is several orders of magnitude higher compared to those of LPS-CIS thin film (γ ~ 2) and HIPS-CIS thin film (γ ~ 9) devices, and also about four-fold higher than LPS-CIS nanorod devices (γ ~ 490). Enhanced photoresponsivity is attributed to the porous microstructure and improved conformality of HIPS-CIS film around the In2S3 nanorods confirmed by SEM and EDS measurements. Due to randomization of the sputtered flux at higher working gas pressures, HIPS can provide a more conformal while at the same time a voidy low-density film around nanostructured surfaces. Reduced interelectrode distance and improved p–n junction interface due to the more uniform conformality of HIPS-CIS result in a higher photocurrent in our HIPS-GLAD devices. In addition, the voids in HIPS-CIS film as a result of its porous nature can behave as highly resistive spots that lower the dark current. Therefore, we have demonstrated that by utilizing a simple and low-temperature HIPS-GLAD method, high-photocurrent and low-dark-current photodetectors can be achieved by controlling the conformality and microstructure of a shell layer around nanorod arrays. HIPS shell coating method can be extended to almost any type of nanostructured substrate.

https://iopscience.iop.org/article/10.1088/2053-1591/3/10/105028/pdf

Matthew Brozak

Papers

Three-Dimensional Structures of MoS2 Nanosheets with Ultrahigh Hydrogen Evolution Reaction in Water Reduction

Robust Superamphiphobic Nanoscale Copper Sheet Surfaces Produced by a Simple and Environmentally Friendly Technique

Subcooled Flow Boiling Over Microstructured Plates In Rectangular Minichannels

High performance flexible copper indium gallium selenide core–shell nanorod array photodetectors

HIPS-GLAD core shell nanorod array photodetectors with enhanced photocurrent and reduced dark current