Cr‐Doped Ni 3 S 2 Nanosheet Arrays as Efficient and Stable Electrocatalysts for Alkaline Hydrogen Evolution Reaction

With increasing global demand for energy, rapid depletion of fossil fuels and intensification of environmental concerns, exploring clean and sustainable energy carriers to replace fossil fuel is becoming critical. Among the various alternatives, hydrogen has been intensively regarded as a promising energy carrier to fulfill the increasing energy demand due to its large energy density per unit mass and eco-friendly production possibilities. However, hydrogen does not exist in molecular structure in nature, and it is essential to obtain efficient and sustainable H2 production technologies. Alkaline water electrolysis is an effective, clean and sustainable process to produce high-quality hydrogen. In this process, highly active electrocatalysts for the hydrogen evolution reaction (HER) are required to accelerate the sluggish kinetics and lower the overpotentials (η) for efficient hydrogen evolution. To date, a noble metal, platinum (Pt), is the state-of-art electrocatalyst for HER. However, exploration of alternative electrocatalysts with low cost and excellent electrocatalytic activity is of vital importance to realize large-scale hydrogen production through water electrolysis. Generally, an electrochemically active catalyst should have an optimal hydrogen adsorption free energy to allow efficient catalytic hydrogen adsorption/desorption. In alkaline solution, dissociation of water onto the electrocatalyst determines the overall HER efficiency. This thesis focuses on rational design and synthesis of different earth-abundant electrocatalysts for electrocatalytic HER in alkaline media. Through facile anion or cation doping strategies, electrocatalysts with abundant accessible active sites, enhanced electronic conductivity and accelerated HER kinetics have been systematically fabricated, characterized and evaluated. First, an efficient HER electrocatalyst in alkaline media was fabricated by incorporating sulfur atoms into a cobalt (hydro)oxide crystal structure. The resultant catalyst exhibits a remarkably enhanced HER activity with a low-overpotential of 119 mV at 10 mA/cm2 and an excellent durability. The results suggest that cobalt hydroxide benefits water adsorption and cleavage, while the negatively charged sulfur ligands facilitate hydrogen adsorption and desorption on the surface of electrocatalysts, leading to significantly promoted Volmer and Heyrovsky steps for HER in alkaline media. Second, exploring bifunctional electrocatalysts which can simultaneously accelerate the HER and oxygen evolution reaction (OER) activities plays a key role in alkaline water splitting. Here, sulfur atoms were incorporated into the mixed transition metal hydroxide with high OER performance to render excellent HER activity. The enhanced catalytic activity towards HER was confirmed by a synergistic effect between the retained metal hydroxide host and the incorporated sulfur atoms. In addition, the full water splitting electrolyzer equipped with fabricated bifunctional electrocatalysts as anode and cathode materials exhibited remarkable overall water splitting performance comparable to that with benchmark Pt and RuO2 electrocatalysts. The S/Se co-doped Co3O4 nanosheets on carbon cloth were fabricated by a facile room temperature chalcogen atom incorporation methodology and were applied as the electrocatalyst for HER in alkaline media. The sulfur and selenium atoms were homogeneously distributed on the surface by forming Co-S or Co-Se bonds which play a key role in the structural change in electrochemical activation. The obtained electrocatalysts demonstrated remarkably improved HER activity compared to that of the original Co3O4. Finally, molybdenum doped cobalt hydroxide was fabricated with significantly accelerated HER kinetics. The introduced Mo sites not only effectively facilitate water dissociation process and desorption of the OHads intermediates, but also simultaneously optimize the hydrogen adsorption free energy. Therefore, the in situ-generated Mo-doped amorphous cobalt hydroxide exhibited a remarkable HER performance in alkaline media with an overpotential of only -80 mV at a current density of 10 mA/cm2. This thesis innovatively explores strategies to improve the catalytic activity towards HER of metal (hydro)oxide in alkaline media. The surface foreign atom doping was demonstrated to manipulate the surface structure of catalysts, thus not only improving the water dissociation processes, but also facilitating the hydrogen adsorption/desorption on the catalysts. The demonstrated facile and effective strategies could be adopted for the fabrication of cost-effective and highly active catalysts for other important chemical reactions for energy conversion applications.

Phase-mediated cobalt phosphide with unique core-shell architecture serving as efficient and bifunctional electrocatalyst for hydrogen evolution and oxygen reduction reaction

Electronic structure engineering and interface chemistry are critical for optimizing hydrogen evolution reaction (HER) electrocatalysts. Here, we report a solution-based synthesis of Cr-doped MnTe nanowires and a Cr2Te3/MnTe/Cr2Te3 metal/semiconductor/metal heterojunction that achieves outstanding HER performance. The epitaxial interface, defined by coherent lattice alignment between MnTe and Cr2Te3, enables directional electron transfer from the metallic Cr2Te3 caps to the MnTe semiconductor, as supported by XPS and Bader charge analyses. DFT calculations reveal that Cr doping enhances conductivity and introduces active electronic states, while the heterojunction simultaneously optimizes hydrogen adsorption free energy (ΔGH*) and reduces HER barriers. As a result, the catalyst achieves a low overpotential of -0.354 V at 10 mA cm-2 and a small Tafel slope of 53.9 mV dec-1, surpassing its individual components. This work offers strategic guidance for designing highly efficient non-noble metal HER electrocatalysts via interface engineering and electronic structure tuning.

Uniform W-NiCoP microneedles by molten salt decomposition as bifunctional electrocatalyst for alkaline water splitting

Keplerate polyoxomolybdate nanoball mediated controllable preparation of metal-doped molybdenum disulfide for electrocatalytic hydrogen evolution in acidic and alkaline media

Boron and phosphorus co-doped NiVFe LDHs@NF as a highly efficient self-supporting electrocatalyst for the hydrogen evolution reaction

Recent advances in iron-based sulfides electrocatalysts for oxygen and hydrogen evolution reaction

Modulation of Volmer step for efficient alkaline water splitting implemented by titanium oxide promoting surface reconstruction of cobalt carbonate hydroxide

Cheap, efficient, and stable hydrogen evolution reaction (HER) electrocatalysts have long been pursued, owing to their scientific and technological importance. Currently, platinum has been regarded as the benchmarked HER electrocatalyst. Unfortunately, the low abundance and high cost impede its industrial applications. Here, we synthesize bimetallic carbide Mo6Ni6C grown on nickel foam as a HER catalyst, delivering a low overpotential of −51 mV at −10 mA cm–2 in 0.5 M H2SO4 for more than 200 h, which is among the best reported benchmarked HER catalysts in acid to date. On the basis of experimental observations and theoretical modeling, we ascribe the good activity to the proper Gibbs free energy of adsorbed hydrogen (ΔG(H*)) for the carbon active sites and attribute the stability to the corrosion-stable Mo–Mo bonds in the crystal structure. This work demonstrates the possibility for Mo6Ni6C to be one of the best candidates for HER electrocatalysts in the large-scale electrolysis industry.

Incorporating ultra-small N-doped Mo2C nanoparticles onto 3D N-doped flower-like carbon nanospheres for robust electrocatalytic hydrogen evolution

Study on density functional theory of MFe2O4 (M=Co, Ni, Cu) for electrocatalytic hydrogen and oxygen evolution reaction

Reported herein is a highly active and durable hydrogen evolution reaction (HER) electrocatalyst, which is constructed following a tandem interface strategy and functional in alkaline and even neutral medium (pH ≈ 7). The ternary composite material, consisting of conductive nickel foam (NF) substrate, Ni3 S2 -MoS2 heterostructure, and TiO2 coating, is synthesized by the hydrothermal method and atomic layer deposition (ALD) technique. Representative results include: (1) versatile characterizations confirm the proposed composite structure and strong electronic interactions among comprised sulfide and oxide species; (2) the material outperforms commercial Pt/C by recording an overpotential of 115mV and a Tafel slope of 67mV dec-1 under neutral conditions. A long-term stability in alkaline electrolytes up to 200 h and impressive overall water splitting behavior (1.56V @ 10mA cm-2 ) are documented; (3) implementation of ALD oxide tandem layer is crucial to realize the design concept with superior HER performance by modulating a variety of heterointerface and intermediates electronic structure.

A large surface area of catalytic sites and low electric resistance are desirable properties for electrocatalysts that lower the overpotential required for electrochemical reactions, such as the hydrogen evolution reaction (HER), in this study. In the presence of polyoxometalate (POM) and triblock copolymer pluronic (P123) as a hybrid soft template, the hydrothermal sulfurization of nickel foam leads to the formation of a hollow microsphere, assembled from the Ni3S2 motif. Sonication for preparing POM+P123 hybrids, while adjusting the POM content, is an effective strategy for synthesizing a Pt-like electrocatalyst with excellent hydrogen evolution efficiency. The reason is that both sonication and optimal POM content can simultaneously enhance the electroactive surface area and electron transfer of the Ni3S2 electrocatalyst. Adopting the optimal conditions, the three-dimensional porous network, composed of Ni3S2 hollow microspheres on the nickel foam, shows that the HER in an alkaline medium requires only 77 mV overpotential for a current density of 10 mA cm-2, with a robust long-term stability during a 25 h test. The performance at a large current density outperforms the current benchmark electrocatalyst (Pt) for HER.

Anion-Modulated Platinum for High-Performance Multifunctional Electrocatalysis toward HER, HOR, and ORR.

Nanocrystalline transition metal tetraborides as efficient electrocatalysts for hydrogen evolution reaction at the large current density