Atomically-thin molybdenum nitride nanosheets with exposed active surface sites for efficient hydrogen evolution

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Probing competent electrocatalysts for hydrogen evolution reaction (HER) of water splitting is one of the most hopeful approaches to confront the energy and environmental crisis. Herein, we highlight ultrathin N-doped Mo2C nanosheets (N-Mo2C NSs) in the role of greatly efficient platinum-free-based electrocatalysts for the HER. The transformation of crystal phase and structure between MoO2 nanosheets with a thickness of ∼1.1 nm and N-Mo2C NSs with a thickness of ∼1.0 nm is studied in detail. Structural analyses make clear that the surfaces of the N-Mo2C NSs are absolutely encompassed by apical Mo atoms, hence affording an ideal catalyst prototype to expose the role of Mo atoms for the duration of HER catalysis. Theoretical calculations demonstrate that the nanosheet structure, N doping, and particular crystalline phase of Mo2C produce more exposed Mo active sites, including Mo atoms on the C plane and doped N atoms. Through detailed electrochemical investigations, N-Mo2C NSs possess HER activity with an onset potential of -48.3 mV vs RHE, Tafel slope of 44.5 mV dec-1, and overpotential of 99 mV vs RHE at the cathodic current density of 10 mA cm-2 with excellent long-term stability. Lastly, the calcination temperature and dicyandiamide amount can obviously affect the phase transformation and surface structure of molybdenum carbide, resulting in an adjustable HER activity. This synthesis mechanism will facilitate the understanding and optimization of Mo-based electrocatalysts in the energy conversion field.

Enhanced hydrogen evolution via in situ generated 2D black phosphorous nanocomposites at the liquid/liquid interfaces

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.

An electron-deficient nickel porphyrin complex undergoes facile ring reduction to form a nickel isobacteriochlorin complex under hydrogen evolution conditions. Spectroscopic experiments indicate that the reduced nickel porphyrin undergoes subsequent reduction and protonation to form a phlorin anion rather than a metal hydride, demonstrating that the key initial proton-coupled electron transfer step is directed toward the ligand versus the metal. The phlorin anion facilely converts to the isobacteriochlorin in the presence of two-electron and three-proton equivalents. Cyclic voltammetry (CV) and spectroscopic experiments reveal that the four-electron, four-proton electrochemical reduction of nickel porphyrin to isobacteriochlorin occurs promptly in the presence of the strong proton donor tosic acid, followed by hydrogen evolution reaction (HER) catalysis at slightly more negative potentials. CVs of independently synthesized Ni isobacteriochlorin show a catalytic HER at the same potentials as those observed for the HER in CVs of the Ni porphyrin. We find that, under strongly acidic conditions, the HER catalysis arises from conversion of the Ni isobacteriochlorin into a nickel-containing, catalytically active electrode-adsorbed species. These results show that Ni porphyrin converts to Ni isobacteriochlorin under HER catalysis conditions via a ligand-based PCET process and that it is the isobacteriochlorin complex which gives rise to an active HER catalysis.

It is presently imperative to find efficient and practical catalysts for the hydrogen evolution reaction (HER) for hydrogen production with high efficiency. In this work, the catalytic HER activity of a class of two-dimensional (2D) metal–organic frameworks (MOFs), i.e., M3(C6O3S3)2 with M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Ru, Ta, W, and Re, is studied by using first-principles calculations. It is found that nonmetal atoms of the first coordination sphere to the central metal atom could be the active site for HER, and V3(C6O3S3)2 (ΔGH* = 0.02 eV, on V), Cr3(C6O3S3)2 (ΔGH* = −0.02 eV, on S, and ΔGH* = −0.05 eV, on O), and Cu3(C6O3S3)2 (ΔGH* = −0.03 eV, on S) are screened out as promising HER catalysts. To complete the picture of how the metal–ligand matching affects the activity, a different coordination microenvironment is considered by regulating the first coordination shell to, for example, the central V atom. It is of interest that the newly constructed moieties present high catalytic HER activity with ΔGH* almost zero, proving the significance of metal–ligand interaction. In view of these results, we propose a descriptor Δε↑↓ that correlates the local electronic structure and the catalytic HER activity. In short, our results not only identify a series of efficient HER electrocatalysts but also unravel the underneath factors that affect the activity and thus provide new insights into the rational design of catalysts for other reactions.

Anodically electrodeposited amorphous molybdenum sulfide (AE‐MoSx) has attracted significant attention as a non‐noble metal electrocatalyst for its high activity toward the hydrogen evolution reaction (HER). The [Mo3S13]2− polymer‐based structure confers a high density of exposed sulfur moieties, widely regarded as the HER active sites. However, their intrinsic complexity conceals full understanding of their exact role in HER catalysis, hampering their full potential for water splitting applications. In this report, a unifying approach is adopted accounting for modifications in the inherent electrochemistry (EC), HER mechanism, and surface species to maximize the AE‐MoSx electroactivity over a broad pH region (0–10). Dramatic enhancements in HER performance by selective electrochemical cycling within reductive (overpotential shift, ηHER ≈ −350 mV) and electro‐oxidative windows (ηHER ≈ −290 mV) are accompanied by highly stable performance in mildly acidic electrolytes. Joint analysis of X‐ray photoelectron spectroscopy, Raman, and EC experiments corroborate the key role of bridging and terminal S ligands as active site generators at low pH, and reveal molybdenum oxysulfides (Mo5+OxSy) to be the most active HER moiety in AE‐MoSx in mildly acidic‐to‐neutral environments. These findings will be extremely beneficial for future tailoring of MoSx materials and their implementation in commercial electrolyzer technologies.

The nature of active sites of molybdenum sulfide-based catalysts for hydrogen evolution reaction

TMOs are generally considered as catalytically inert for hydrogen evolution reaction (HER).[1] This unique property has promoted a wide variety of applications of TMOs, such as aqueous-based lithium ionic batteries, electrochromic smart windows, electrocatalysts and/or catalyst supports. However, hydrogen evolution as an undesired side reaction is commonly observed when they are used in aqueous-based supercapacitors and battery applications.[2] The unexpected HER mechanism for intrinsic TMOs has not been studied until very recently by McKone and Mpourmpakis groups.[3] With careful theoretical calculations, they clearly demonstrated that intrinsic WO3 is not HER active. Bulk hydrogen (H) intercalation into WO3 is a prerequisite for its catalytic activity toward HER. Applying electrochemical potential to WO3 primarily sets the bulk H stoichiometry of thus formed hydrogen bronzes HxWO3. The level of intercalation (x), precisely to say, the density of the intercalated hydrogen in the WO3, in turn determines the formation of W-H*, the bonding strength of W-H*, and their consumption to H2 (HER). Significant HER is only seen at potentials negative enough when x > 0.5. Beyond this intercalation level, HER is no longer dictated by potential-dependent activation energies, instead, is controlled by the density of the hydrogen intercalation on the surface of WO3 catalysts. In line with this extensive theoretical study, Augustyn group very recently experimentally demonstrated the dependence of HER activity of WO3 materials on their electrochemical insertion characteristics.[4] By including crystal water between the WO3 sheets, proton intercalation is greatly facilitated likely with a Grotthuss mechanism. Accordingly, the HER kinetics is largely improved with a large positive shift of the HER on-set potential. On the other hand, by inclusion of molecular pillars in the WO3 lattice to physically block proton intercalation, the HER activity of WO3 was largely inhibited. Recently, we developed a facile approach to cationically dope WO3 nanosheets with red phosphorus. The proton interaction and the associated HER behavior as a function of electrochemical potentials were carefully studied with multiple electrochemical techniques. Interestingly, we found that proton interaction is largely suppressed even without including molecular pillars in the P-doped WO3 lattice. At lower intercalation levels before the so-called “potential independent regime”, the HER kinetics is largely decreased, as expected, compared to the non-doped WO3 control sample. While in the “potential independent regime”, the HER kinetics is largely enhanced. Furthermore, including alkaline metal ions into the electrolyte, the HER kinetics is further suppressed as demonstrated by largely negatively shifted onset potentials and the increased Tafel slopes. Interestingly, in the “potential independent regime”, the HER kinetics is further drastically enhanced. Extensive structural characterization and theoretical calculations are undergoing to reveal the mechanism behind these interesting tunable potential dependent - proton interaction and HER behavior, which will be reported in this work. Understanding how heteroatomic doping could modulate proton interaction and HER behaviors of TMOs at atomic levels would provide practical guidance for designing more efficient and selective electrochemical catalysts for various catalytic reduction reactions, such as carbon dioxide reductions and nitrogen/nitrate reductions to valuable compounds.[1] Zhang, S.; Zhang, X.; Rui, Y.; Wang, R.; Li, X., Recent advances in non-precious metal electrocatalysts for pH-universal hydrogen evolution reaction. Green Energy & Environment 2021, 6 (4), 458-478.[2] Wang, R.; Mitchell, J. B.; Gao, Q.; Tsai, W.-Y.; Boyd, S.; Pharr, M.; Balke, N.; Augustyn, V., Operando Atomic Force Microscopy Reveals Mechanics of Structural Water Driven Battery-to-Pseudocapacitor Transition. ACS Nano 2018, 12 (6), 6032-6039.[3] Miu, E. V.; McKone, J. R.; Mpourmpakis, G., The Sensitivity of Metal Oxide Electrocatalysis to Bulk Hydrogen Intercalation: Hydrogen Evolution on Tungsten Oxide. Journal of the American Chemical Society 2022, 144 (14), 6420-6433.[4] Spencer, M. A.; Fortunato, J.; Augustyn, V., Electrochemical proton insertion modulates the hydrogen evolution reaction on tungsten oxides. The Journal of Chemical Physics 2022, 156 (6), 064704. Figure 1

Developing highly efficient and durable hydrogen evolution reaction (HER) electrocatalysts is crucial for addressing the energy and environmental challenges. Among the 2D-layered chalcogenides, MoSe2 possesses superior features for HER catalysis. The van der Waals attractions and high surface energy, however, stack the MoSe2 layers, resulting in a loss of edge active catalytic sites. In addition, MoSe2 suffers from low intrinsic conductivity and weak electrical contact with active sites. To overcome the issues, this work presents a novel approach, wherein the in situ incorporated diethylene glycol solvent into the interlayers of MoSe2 during synthesis when treated thermally in an inert atmosphere at 600 °C transformed into graphene (Gr). This widened the interlayer spacing of MoSe2, thereby exposing more HER active edge sites with high conductivity offered by the incorporated Gr. The resulting MoSe2-Gr composite exhibited a significantly enhanced HER catalytic activity compared to the pristine MoSe2 in an acidic medium and demonstrated a superior HER catalytic activity compared to the state-of-the-art Pt/C catalyst, particularly at a high current density beyond ca. 55 mA cm-2. Additionally, the MoSe2-Gr catalyst demonstrated long-term electrochemical stability during HER. This work, thus, presents a facile and novel approach for obtaining an efficient MoSe2 electrocatalyst applicable in green hydrogen production.

Coupled molybdenum carbide and nitride on carbon nanosheets: An efficient and durable hydrogen evolution electrocatalyst in both acid and alkaline media

Nickel‐molybdenum (NiMo) alloys can be a possible alternative to platinum as hydrogen evolution reaction (HER) catalysts because of the superior HER activity. However, the superior HER activity and the pH‐dependent kinetics are not currently fully understood. Herein, we present a study of HER kinetics and mechanisms of NiMo in alkaline, near‐neutral and acidic media by combining voltammetry measurements with electrochemical impedance spectroscopy and a microkinetic model. The results indicate that, compared to Ni, NiMo has significantly higher active surface area and intrinsic HER activity. In the subsequent measurements, we demonstrated that different from the existing explanations to the HER mechanisms for NiMo, the HER process in acidic, near‐neutral, and alkaline media is controlled by the Heyrovsky step. Our results show that increasing pH increases the hydrogen coverage, which increases the Tafel‐slope at low overpotentials, eventually resulting in only a single Tafel slope, which would commonly be interpreted as a Volmer‐limited reaction. Furthermore, the studies of thickness effect on HER kinetics show that the HER kinetics of NiMo are thickness‐dependent. In phosphate buffer, the increase in thickness did not significantly increase the double‐layer capacitance, but simulations with the microkinetic model indicate that the active surface area still increased similarly to other electrolytes, which is likely related to the type of electrolyte used.

Nickel phosphides are an emerging class of earth-abundant catalysts for hydrogen generation through water electrolysis. However, the hydrogen evolution reaction (HER) activity of Ni2P is lower than that of benchmark Pt group catalysts. To address this limitation, an integrated theoretical and experimental study was performed to enhance the HER activity and stability of hexagonal Ni2P through doping with synergistic transition metals. Among the nine dopants computationally studied, zinc emerged as an ideal candidate due to its ability to modulate the hydrogen binding free energy (ΔG H) closer to a thermoneutral value. Consequently, phase pure hexagonal Ni2-x Zn x P nanocrystals (NCs) with a solid spherical morphology, variable compositions (x = 0-17.14%), and size in the range of 6.8 ± 1.1-9.1 ± 1.1 nm were colloidally synthesized to investigate the HER activity and stability in alkaline electrolytes. As predicted, the HER performance was observed to be composition-dependent with Zn compositions (x) of 0.03, 0.07, and 0.15 demonstrating superior activity with overpotentials (η-10) of 188.67, 170.01, and 135.35 mV, respectively at a current density of -10 mA/cm2, in comparison to Ni2P NCs (216.2 ± 4.4 mV). Conversely, Ni2-x Zn x P NCs with x = 0.01, 0.38, 0.44, and 0.50 compositions showed a notable decrease in HER activity, with corresponding η-10 of 225.3 ± 3.2, 269.9 ± 4.3, 276.4 ± 3.7 and 263.9 ± 4.9 mV, respectively. The highest HER active catalyst was determined to be Ni1.85Zn0.15P NCs, featuring a Zn concentration of 5.24%, consistent with composition-dependent ΔG H calculations. The highest performing Ni1.85Zn0.15P NCs displayed a Heyrovsky HER mechanism, enhanced kinetics and electrochemically active surface area (ECSA), and superior corrosion tolerance with a negligible increase of η-10 after 10 h of continuous HER. This study provides critical insights into enhancing the performance of metal phosphides through doping-induced electronic structure variation, paving the way for the design of high-efficiency and durable nanostructures for heterogeneous catalytic studies.

Amorphous Molybdenum Sulfide Deposited Graphene Liquid Crystalline Fiber for Hydrogen Evolution Reaction Catalysis

Unraveling the mechanism of hydrogen evolution reaction on cobalt compound electrocatalysts