Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution

Morphology-Guided Phase-Controlled Synthesis of Nickel Selenide Nanocrystals through Cation Exchange Reactions

The precise control of the crystal phase during the synthesis of nickel selenide (NixSey) nanocrystals is crucial, as crystal structure and composition significantly influence their reactivity, growth kinetics, and properties. The cation exchange (CE) method provides a versatile and robust approach for synthesizing nanomaterials, enabling precise control over phase, composition, and morphology. However, the application of this method for phase-controlled synthesis of NixSey nanocrystals has received limited research attention. Here, we present a morphology-guided CE method for the synthesis of spinel Ni3Se4 nanoparticles (NPs) and rhombic Ni3Se2 nanorods (NRs), wherein berzelianite Cu2-xSe NPs and NRs are employed as sacrificial templates for CE with Ni2+. This phase-controlled behavior, which is guided by morphology and dependent on the stacking length of the close-packed facets, relies on the rearrangement of the Se2- sublattice accompanied by CE, providing a unique and precise approach to controlling phase during nanocrystal synthesis. Additionally, the obtained Ni3Se4 NPs and Ni3Se2 NRs exhibit structure-dependent catalytic activities in the oxygen evolution reaction.

Durable Pulse-Electrodeposited Ni-Fe-S Nanosheets Supported on a Ni-S Three Dimensional Pattern as Robust Bifunctional Electrocatalysts for Hydrogen Evolution and Urea Oxidation Reactions

This study aims to establish easy-to-fabricate and novel structures for the synthesis of highly active and enduring electrocatalysts for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Gradient electrodeposition and four different time regimes were utilized to synthesize Ni-S 3D patterns with the optimization of electrodeposition time. Pulse electrodeposition was employed for the synthesis of Ni-Fe-S nanosheets at three different frequencies and duty cycles to optimize the pulse electrodeposition parameters. The sample synthesized at 13 min of gradient electrodeposition with a 1 Hz frequency and 0.7 duty cycle for pulse electrodeposition demonstrated the best electrocatalytic performance. The optimized electrode further showed remarkable performance for HER and UOR reactions, requiring only 54 mV and 1.25 V to deliver 10 mA cm-2 for HER and UOR, respectively. Moreover, the overall cell voltage of the two-electrode system in 1 M KOH and 0.5 M urea was measured at 1.313 V, delivering 10 mA cm-2. Constructing Ni-Fe-S nanosheets on 3D Ni-S significantly increased the electrochemical surface area from 51 to 278 for the Ni-S and Ni-Fe-S layers. Tafel slopes were measured as 138 and 182 mV dec-1 for the HER and UOR for the Ni-S coating layer and 97 mV dec-1 for the HER and 131 mV dec-1 for the UOR for the optimal Ni-Fe-S nanosheets on Ni-S. Minimal changes in the potential were observed at 100 mA cm-2 in 50 h regarding the HER and UOR, signifying exceptional electrocatalytic stability. This study provides economically viable, highly active, and long-lasting electrocatalysts suitable for HER and UOR applications.

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.

Electrochemical preparation of nano/micron structure transition metal-based catalysts for the oxygen evolution reaction

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the existing electrolytic cells lack a sufficient number of robust and highly active anodic electrodes for the oxygen evolution reaction (OER). Electrochemical synthesis technology provides a feasible route for the preparation of independent OER electrodes with high utilization of active sites, fast mass transfer, and a simple preparation process. A comprehensive review of the electrochemical synthesis of nano/microstructure transition metal-based OER materials is provided. First, some fundamentals of electrochemical synthesis are introduced, including electrochemical synthesis strategies, electrochemical synthesis substrates, the electrolyte used in electrochemical synthesis, and the combination of electrochemical synthesis and other synthesis methods. Second, the morphology and properties of electrochemical synthetic materials are summarized and introduced from the viewpoint of structural design. Then, the latest progress regarding the development of transition metal-based OER electrocatalysts is reviewed, including the classification of metals/alloys, oxides, hydroxides, sulfides, phosphides, selenides, and other transition metal compounds. In addition, the oxygen evolution mechanism and rate-determining steps of transition metal-based catalysts are also discussed. Finally, the advantages, challenges, and opportunities regarding the application of electrochemical techniques in the synthesis of transition metal-based OER electrocatalysts are summarized. This review can provide inspiration for researchers and promote the development of water splitting technology.

Electrodeposition: An efficient method to fabricate self-supported electrodes for electrochemical energy conversion systems

The development of electrocatalysts for energy conversion systems is essential for alleviating environmental problems and producing useful energy sources as alternatives to fossil fuels. Improving the catalytic performance and stability of electrocatalysts is a major challenge in the development of energy conversion systems. Moreover, understanding their electrode structure is important for enhancing the energy efficiency. Recently, binder-free self-supported electrodes have been investigated because the seamless contact between the electrocatalyst and substrate minimizes the contact resistance as well as facilitates fast charge transfer at the catalyst/substrate interface and high catalyst utilization. Electrodeposition is an effective and facile method for fabricating self-supported electrodes in aqueous solutions under mild conditions. Facile fabrication without a polymer binder and controlability of the compositional and morphological properties of the electrocatalyst make electrodeposition methods suitable for enhancing the performance of energy conversion systems. Herein, we summarize recent research on self-supported electrodes fabricated by electrodeposition for energy conversion reactions, particularly focusing on cathodic reactions of electrolyzer system such as hydrogen evolution, electrochemical CO2 reduction, and electrochemical N2 reduction reactions. The deposition conditions, morphological and compositional properties, and catalytic performance of the electrocatalyst are reviewed. Finally, the prospective directions of electrocatalyst development for energy conversion systems are discussed.

Towards Highly Efficient Chalcopyrite Photocathodes for Water Splitting: The Use of Cocatalysts beyond Pt

Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrites such as Cu(In, Ga)(Se,S)₂ (CIGS) and CuIn(Se,S)₂ (CIS) present excellent semiconductor characteristics for this purpose, but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demanding improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material, but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present Minireview highlighted the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.

Optimization of nickel selenide for hydrogen and oxygen evolution reactions by response surface methodology

In this study, the electrocatalytic activity of Ni-Se electrode synthesized on nickel foam by pulse electrodeposition was optimized through the design of experiments (DOE) approach using the response surface methodology (RSM) for both hydrogen and oxygen evolution reactions. The frequency (f), duty cycle (dc), current density (i), and electrodeposition time (sum of t_ons) were chosen as the parameters of the pulse electrodeposition method. The analyses of variance (ANOVA) were performed on the responses of the designed experiments that included the required overpotential at the current density of 10 mA/cm² for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) (η_10,HER and η_10,OER), active surface area (R_f) and intrinsic electrocatalytic activity (i/R_f). The results indicated that η_10,HER, η_10,OER, and R_f are mainly influenced by duty cycle and electrodeposition time, while i/R_f is affected by frequency and time. The optimized NiSe₂ electrode synthesized under optimal conditions of pulse electrodeposition (low duty cycle and prolonged electrodeposition time) showed the most desirable values for η_10,HER, η_10,OER, and R_f, equal to 44 mV (vs. RHE), 235 mV (vs. RHE) and 14700, respectively. The nanostructured NiSe₂ demonstrated the highest potential in the bifunctional application of OER and HER.

Pulse Electrodeposition of a Superhydrophilic and Binder-Free Ni-Fe-P Nanostructure as Highly Active and Durable Electrocatalyst for Both Hydrogen and Oxygen Evolution Reactions

Development and fabrication of electrodes with favorable electrocatalytic activity, low-cost, and excellent electrocatalytic durability are one of the most important issues in the hydrogen production area using the electrochemical water splitting process. We use the pulse electrodeposition method as a versatile and cost-effective approach to synthesize three-dimensional Ni-Fe-P electrocatalysts on nickel nanostructures under various applied frequencies and duration times, in which nanostructures exhibit excellent intrinsic electrocatalytic activity. Benefiting from the three-dimensional structure, as well as the simultaneous presence of the three elements nickel, iron, and phosphorus, the electrode fabricated at the optimal conditions has indicated outstanding electrocatalytic activity with a η₁₀ of 66 and 198 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in a 1.0 M KOH solution. Also, the water electrolysis cell constructed with this electrode and tested as a bifunctional electrode exhibited 1.508 V for 10 mA cm^-2 in overall water splitting. In addition, the lowest amount of potential change in 100 mA cm^-2 was observed for HER and OER, indicating excellent electrocatalytic stability. This study proposes a binder-free and economical technique for the synthesis of three-dimensional electrocatalysts.