Nanosheet arrays derived from ZIF-67 grown on three-dimensional frameworks for the electrocatalytic oxygen evolution reaction

Hydrogen energy has become one of the most promising substitutes for conventional fuels because of its high calorific value and green and renewable advantages. Among various hydrogen production strategies, the water splitting hydrogen production strategy stands out. Therefore, it is very important to develop efficient and cheap oxygen evolution reaction (OER) electrocatalysts for hydrogen production by electrolysis of water. In this work, nickel selenide grown on nickel foam (NF) with good electrical conductivity and excellent catalytic performance, i.e. NiSex/NF, was selected as the three-dimensional conducting substrate, and the active material ZIF-67 was successfully compounded on the conductive substrate by using the in situ growth strategy. A series of self-supporting materials ZIF-67/NiSex/NF were obtained, which can be directly used as working electrodes for the electrocatalytic OER. The self-supporting material ZIF-67/NiSex/NF-1 can achieve a low overpotential of 353 mV at a current density of 100 mA cm-2 with a small Tafel slope of 107 mV dec-1, and excellent stability for 55 hours of continuous OER at a current density of 50 mA cm-2 in an alkaline medium. Benefiting from the unique layered structure and the synergy between Co and Se optimizing the electronic structure, ZIF-67/NiSex/NF-1 when used directly as an electrode shows exceptional OER catalytic performance at high current density.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Core-shell nanostructured electrocatalysts for water splitting

As the cornerstone of the hydrogen economy, water electrolysis consisting of the hydrogen and oxygen evolution reactions (HER and OER) greatly needs cost-efficient electrocatalysts that can decrease the dynamic overpotential and save on energy consumption. Over past years, observable progress has been made by constructing core-shell structures free from or with few noble-metals. They afford particular merits, e.g., a highly-exposed active surface, modulated electronic configurations, strain effects, interfacial synergy, or reinforced stability, to promote the kinetics and electrocatalytic performance of the HER, OER and overall water splitting. So far, a large variety of inorganics (carbon and transition-metal related components) have been introduced into core-shell electrocatalysts. Herein, representative efforts and progress are summarized with a clear classification of core and shell components, to access comprehensive insights into electrochemical processes that proceed on surfaces or interfaces. Finally, a perspective on the future development of core-shell electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials for energy conversion and storage.

A Superaerophobic Bimetallic Selenides Heterostructure for Efficient Industrial-Level Oxygen Evolution at Ultra-High Current Densities

Cost-effective and stable electrocatalysts with ultra-high current densities for electrochemical oxygen evolution reaction (OER) are critical to the energy crisis and environmental pollution. Herein, we report a superaerophobic three dimensional (3D) heterostructured nanowrinkles of bimetallic selenides consisting of crystalline NiSe2 and NiFe2Se4 grown on NiFe alloy (NiSe2/NiFe2Se4@NiFe) prepared by a thermal selenization procedure. In this unique 3D heterostructure, numerous nanowrinkles of NiSe2/NiFe2Se4 hybrid with a thickness of ~ 100 nm are grown on NiFe alloy in a uniform manner. Profiting by the large active surface area and high electronic conductivity, the superaerophobic NiSe2/NiFe2Se4@NiFe heterostructure exhibits excellent electrocatalytic activity and durability towards OER in alkaline media, outputting the low potentials of 1.53 and 1.54 V to achieve ultra-high current densities of 500 and 1000 mA cm-2, respectively, which is among the most active Ni/Fe-based selenides, and even superior to the benchmark Ir/C catalyst. The in-situ derived FeOOH and NiOOH species from NiSe2/NiFe2Se4@NiFe are deemed to be efficient active sites for OER.

A bifunctional nanoporous Ni-Co-Se electrocatalyst with a superaerophobic surface for water and hydrazine oxidation

The sluggish kinetics of the oxygen evolution reaction (OER) has severely hindered the energetic convenience of water splitting. Thus, developing a highly efficient catalyst for the OER and replacing the OER with hydrazine oxidation (HzOR) are effective strategies for water electrolysis to achieve sustainable hydrogen production. Herein, bifunctional nanosheet arrays Ni0.6Co0.4Se with a porous structure were fabricated on Ni foam (NF) by the bubble dynamic template method during electrodeposition. Compared with CoSe2 and NiSe2, Ni0.6Co0.4Se exhibits excellent electrocatalytic performance for both the OER and HzOR. A low overpotential of only 249 mV is required to drive 10 mA cm-2, and a retention rate of nearly 100% after 24 h at 10 mA cm-2 is observed for Ni0.6Co0.4Se towards the OER. By substituting the OER by HzOR, an extremely high current density of 300 mA cm-2 at 0.4 V vs. RHE and a retention rate of 86.8% at 200 mA cm-2 after 12 h can be achieved. Interestingly, the mechanistic reason for the enhanced catalytic ability of Ni0.6Co0.4Se was studied, which is associated with the synergistic effects of Ni and Co, large ECSA, high electrical conductivity and most importantly the superaerophobic nature induced by the porous structure of Ni0.6Co0.4Se. The non-noble metal bifunctional electrocatalyst demonstrates a promising potential for application in both the OER and HzOR.

Vertically stacked bilayer heterostructure CoFe2O4@Ni3S2 on a 3D nickel foam as a high-performance electrocatalyst for the oxygen evolution reaction

The as-obtained CoFe₂O₄@Ni₃S₂/NF can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions.

Efficient photocatalytic hydrogen production by Mn0.05Cd0.95S nanoparticles anchored on cubic NiSe2

In the field of catalysis, three critical factors for evaluating catalyst activity include charge separation efficiency, photoabsorption, and surface activity sites.

Triggering Cation Exchange Reactions by Doping

Cation exchange (CE) reactions have emerged as a technologically important route, complementary to the colloidal synthesis, to produce nanostructures of different geometries and compositions for a variety of applications. Here it is demonstrated with first-principles simulations that an interstitial impurity cation in CdSe nanocrystals weakens nearby bonds and reduces the CE barrier in the prototypical exchange of Cd²⁺ ions by Ag⁺ ions. A Wannier function-based tight binding model is employed to quantify microscopic mechanisms that influence this behavior. To support our model, we also tested our findings in a CE experiment: both CdSe and interstitially Ag-doped CdSe nanocrystals (containing 4% of Ag⁺ ions per nanocrystal on average) were exposed to Pb²⁺ ions at room temperature and it was observed that the exchange reaction proceeds further in doped nanocrystals. The findings suggest doping as a possible route to promote CE reactions that hardly undergo exchange otherwise, for example, those in III-V semiconductor nanocrystals.

Recent Progresses in Electrocatalysts for Water Electrolysis

Abstract

The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.

Graphical Abstract

Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals

Stoichiometric Cu₂Se nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb²⁺ ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic Cu₂Se nanocrystals to Pb²⁺ cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating Cu₂Se@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb²⁺ ions inside the host lattice and to the absence of preferred entry points in cubic Cu₂Se. On the other hand, in hexagonal Cu₂Se nanocrystals, the entrance of Pb²⁺ ions generated PbSe stripes "sandwiched" in between hexagonal Cu₂Se domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb²⁺ ions through specific (a, b) planes of the hexagonal Cu₂Se structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.

Synthesis and characterization of [Cu(salen)]2 coordination nano-assembly by a green and simple method

Hierarchical structured microplates consisting of nanoparticles were grown based on a self-assembly approach via a hydrothermal synthesis route without using any additive. Experimental parameters, such as reaction temperature, time and solvent play crucial roles in determining the morphology of final product. The nanoscale shape and macroscopically assembled architecture of [Cu(salen)]2 {salen = N,N′-bis(salicylidene)1,2-ethyelenediimine} crystals were totally controlled by preparation conditions for crystal growth. The synthesized [Cu(salen)]2 hierarchical architectures were characterized with UV-Vis, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric (TGA) analyses. The thickness of the fabricated plates changes from 25 nm to 3.5 μm. The as-prepared nanoplates are actually composed of nanoparticles with diameters under 100 nm. Furthermore, the formation mechanism of the microplates decorated with nanorods has also been discussed, based on the influence of the kinetics of nucleation and crystal growth.