Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution

Novel CoxSy/WS2 nanosheets supported on carbon cloth as efficient electrocatalyst for hydrogen evolution reaction

Electrocatalysts for hydrogen evolution reaction (HER) from water splitting have been widely studied as one of the efficient energy storage methods by producing hydrogen (H2), the important clean energy resource. The best-known electrocatalyst for HER is platinum (Pt) that is expensive and scarce on earth. Molybdenum disulfide (MoS2) has attracted great attentions as one of promising catalysts for replacing Pt not only because of the relatively low cost and earth abundancy but also great HER activity, selectivity and stability as a great catalyst. Since the edge site of MoS2 was proved as an active site for HER, a great number of efforts on maximally exposing edge site have contributed to the studies about MoS2 based catalysts for HER. The basal plane of MoS2 is catalytically inert for HER since the free energy of adsorbed hydrogen (delGH*) known to a great descriptor for activation energy is thermodynamically uphill (~1.92eV) compared with that of active edge site (~0.06eV). However, sulfur (S) vacancy in the basal plane was very recently investigated as a new active site for HER demonstrating the basal plane that composes the majority part of MoS2 can be utilized for HER. Diverse methods to generate S-vacancy to make a use of the basal plane of MoS2 for efficient HER catalysts have been investigated such as Ar plasma treatment, hydrogen annealing and electrochemical desulfurization. However, HER activity of desulfurized MoS2-x still needs to be improved to be competitive with the HER activities of platinum. Cobalt is one of the non-precious metal promotors for MoS2 based catalysts for hydrogen evolution reaction, water gas shift reaction and oxygen reduction reaction. In very recently, Co was doped in the basal plane of S-vacancy rich MoS2 and Co-SMoS2 achieved a great catalytic activity for the hydrodeoxygenation (HDO) reaction. Since S-vacancy site is coordinatively unsaturated and has a high surface free energy like edge site, it is expected to provide a great nucleation site for doping or anchoring transition metal atoms or clusters. Thus, Co promotor can be employed for enhancing the catalytic activity of the basal plane of MoS2 based catalysts. Herein we report the great HER activity enhancement of the catalytically inert basal plane of MoS2 by combining the catalytic effects of S-vacancy and Co cluster addition as a non-precious metal oxide promotor. We use both density functional theory (DFT) calculations and experiments to support the claim. The polycrystalline 2H phase MoS2 multilayer whose basal plane is dominantly exposed was synthesized on fluorine-doped tin oxide (FTO) and carbon foam electrodes through thermolysis in the tube furnace. S-vacancy was generated on the basal plane of MoS2 through the same electrochemical desulfurization method. On top of that, Co cluster was electrodeposited to further promote the HER activity of the basal plane under different desulfurization conditions. Consequently, we demonstrate S-vacancy plays a critical role for Co nucleation during electrodeposition and thus both electrochemically active surface area and the intrinsic catalytic activity increased. Especially, the overpotential at -10 mA/cm2 was reduced around 300mV making the inert basal plane of MoS2 have comparable overpotential about -0.21 V vs. RHE at -10 mA/cm2 with the state of the art non-precious MoS2 based catalysts.

Superdispersed NiCo2S4 nanoparticles anchored on reduced graphene oxide for efficient hydrogen evolution reaction in acidic and alkaline media

MoSx supported graphene oxides with different degree of oxidation as efficient electrocatalysts for hydrogen evolution

P and Se-codopants triggered basal plane active sites in NbS2 3D nanosheets toward electrocatalytic hydrogen evolution

Although few-layer bismuth oxyhalides (BiOX, X = Cl, Br, and I) have been shown to be appropriate for photocatalytic hydrogen production, the hydrogen evolution reaction (HER) activity of BiOX is unrevealed. Herein, the origins of catalytic activity on single-layer BiOX are investigated by using the density functional theory. The grand potential calculations show that the Bi- and BiO-terminations of single-layer BiOX are stable in O-poor and O-rich environments, respectively. The Bi- and BiO-terminations of single-layer BiOX are found to have obviously active sites for HER, whereas the (001) basal planes are inert. The Gibbs free energies for the adsorption of hydrogen atoms on the Bi- and BiO-terminations are close to the optimal value of 0 eV, indicating that single-layer BiOX possess favorable HER performances. The enhanced HER activities on the Bi- and BiO-terminations are attributed to the localized edge states around the Fermi level, which are caused by the Bi 6p-orbital density of the fringe bismuth atoms and O 2p-orbital density of the fringe oxygen atoms, respectively. The results of this work suggest that single-layer BiOX are a family of promising catalysts for water splitting.

The performance of the hydrogen evolution reaction (HER) at the cathode in alkaline electrolysis can be compromised due to oxidation caused by the oxygen reduction reaction (ORR), led by oxygen gas crossover through the porous separator. In this study, we introduce N:NiFeP@FeNC, a highly efficient electrocatalyst for HER and ORR in an alkaline environment, featuring independent active sites for each reaction. N:NiFeP@FeNC demonstrated an overpotential of 78 mV to achieve a current density of 10 mA cm −2 for HER and a half-wave potential of 0.88 V RHE for ORR. Notably, the independent HER and ORR active sites effectively prevented oxidation of the HER active site during ORR durability tests. Through density functional theory (DFT) calculations, the mechanisms underlying HER and ORR on N:NiFeP@FeNC were elucidated, identifying key factors that enhance catalytic performance. The low activity of the HER active site (NiFeP) was attributed to the high energy barrier for *H 2 O dissociation, while the low activity of the ORR active site (Fe–N–C) was related to delayed desorption due to excessively strong interactions between intermediates and the active metal centers. The incorporation of N atoms into the catalyst induced electronic structure reconfiguration in the Ni and Fe atoms, thereby facilitating electrochemical reactions. This study addresses a previously overlooked yet critical issue in alkaline electrolysis cathode research, providing a simple and effective strategy that highlights significance for future exploration.

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.

Molybdenum disulfide (MoS2) based catalysts for the electrochemical hydrogen evolution reaction (HER) have been widely studied as alternatives to platinum based catalysts due to the earth-abundance and great catalytic activity. Since the edge sites of 2H phase MoS2 were shown to be the active sites for HER, a great number of studies have focused on maximally exposing catalytically active edge-sites through diverse engineering process. Recently, we demonstrated that active sites could be created on the basal plane of 2H-MoS2 by generating sulfur (S)-vacancies. At the S-vacancy sites, the under coordinated Mo atoms introduce gap states that allow for favorable hydrogen binding, introducing the highest per site turnover frequency (TOF) reported for any MoS2-based catalyst for HER. However, the S-vacancies in the basal plane have so far only been generated using controlled argon (Ar) plasma exposure and H2 annealing. A more industrially viable alternative to the argon plasma desulfurization process is needed. To effectively utilize S-vacancies in MoS2 catalysts for industrial applications, a facile, general, and scalable route for generating S-vacancies in MoS2 of any morphology is needed. Electrochemical desulfurization is one of the possible methods for generating S-vacancies by removing sulfur atoms from the basal plane of MoS2. This method removes the sulfur atoms in the basal plane of 2H-MoS2 to form hydrogen sulfide (H2S) gas through a desulfurizing activation cycle. In this work, we show an electrochemical (EC) desulfurization method for generating S-vacancies in monolayer as well as polycrystalline multilayer MoS2 supported on diverse electrodes. Density functional theory (DFT) calculations show that S-vacancies are expected to be thermodynamically favorable relative to the pristine basal plane at a sufficient negative potential. The concentration of S-vacancies can be controlled by changing the applied desulfurization voltage. These theoretical predictions are experimentally verified with the continuously grown MoS2 monolayers on gold (Au) showing that electrochemically generated S-vacancies are comparable to the recent work about Ar-plasma treated ones. In addition, we demonstrate the generality of the electrochemical desulfurization approach by generating S-vacancies on MoS2 multilayers supported on flat carbon rods and porous carbon foams leading to a significant HER activity enhancement. Finally, we experimentally show that the HER activity is stable under extended desulfurization durations as well as operating durations and that the concentration of S-vacancies and activity can be varied using the applied potential in polycrystalline multilayer MoS2 on carbon foam electrode.

Insight into the superior activity of bridging sulfur-rich amorphous molybdenum sulfide for electrochemical hydrogen evolution reaction

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Possessing large specific surface areas and rich metal redox sites, layered double hydroxides (LDHs) are potentially suitable oxygen evolution reaction catalysts. It is a pity that they usually show poor hydrogen evolution reaction (HER) activity on account of the limited conductivity and deficient active sites. Taking NiFe LDH nanosheets as an example, we develop a “one stone three birds” plasma engraving strategy to enhance the HER activity of NiFe LDH. The “three birds,” including the reduction of Ni2+ to Ni nanoparticles (Ni NPs), generation of more oxygen vacancies (Ov), and exfoliation of nanosheets into much thinner ones, can obviously improve the conductivity and active sites of NiFe LDH. The plasma processing can also enhance water adsorption and accelerate the Volmer step during HER. As expected, the plasma-engraved NiFe LDH (PEH) exhibits enhanced HER activity with a low overpotential of 22 mV at 10 mA cm−2 and a small Tafel slope of 38 mV dec−1 in 1 M KOH, much better than NiFe LDH (202 mV, 145 mV dec−1). By combining optical emission spectroscopy diagnosis and structural/electrochemical characterizations, the relationship among the electron excitation temperature (Texc) in plasma, the amount of Ni NPs and Ov in PEH, and the HER activity of PEH is established. Excitingly, the PEH also displays splendid HER activity in both alkaline real seawater and overall water splitting.

Two‐dimensional (2D) molybdenum disulfide (MoS2) has been regarded as an attractive non‐precious‐metal electrocatalyst for the hydrogen evolution reaction (HER). Engineering the crystal phase of MoS2 to activate the basal planes/edges and simultaneously improve the electronic conductivity is currently an effective strategy for enhancing its HER activity. Herein, we report a facile and efficient hydrothermal route to prepare 1T/2H‐MoS2 catalysts using ionic liquid (N‐butyl pyridinium bromide, [BPy]Br) as a structure‐directing agent, where the large steric hindrance of [BPy]Br and the mutual π‐stacking interaction induce the phase transition of MoS2 from 2H to the 1T phase. By adding a suitable amount of [BPy]Br in the reaction system, the portion of the 1T phase in 1T/2H‐MoS2 was increased, which can expose more active sites on its basal planes/edges as well as facilitate charge transfer for the HER. Consequently, 1T/2H‐MoS2 with the 1T portion of 91.9 % exhibits a significantly enhanced HER activity compared to that of the MoS2 synthesized without the aid of [BPy]Br, in terms of a lower Tafel slope of 59 mV dec−1. This synthesis strategy provides valuable guidance for designing the phase structure of MoS2‐based electrocatalysts to achieve high HER efficiency.

Electronic structure regulation of nickel-iron layered double hydroxides by tuning ternary component for overall water splitting