Catalytic activity and underlying atomic rearrangement in monolayer CoOOH towards HER and OER

For efficient hydrogen and oxygen production, design and synthesis of cost-effective, stable and active materials are inevitable. In this work, the catalytic activity of 2D CoOOH towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been investigated using first principles calculations based on density functional theory. The adatom induced structural rearrangement have been investigated from structural parameters as well as charge redistribution in 2D CoOOH. The preferred site for hydrogen and oxygen adsorption were found to be the top site of oxygen atom of 2D CoOOH. The catalytic activity of HER and OER towards 2D CoOOH was studied by calculating the Gibbs free energy. Our study revealed that the 2D CoOOH serve better as a catalyst for HER than OER with adsorption energy of −0.45 and −3.68 eV respectively suggesting its efficient use for hydrogen production. We further investigated the changes in electronic properties of 2D CoOOH on adsorption of hydrogen and oxygen atom.     

Catalytic hydrogen evolution reaction on surfaces of metal-nanoparticle-coated zinc-based oxides by first-principles calculations

Using first-principles calculations, we investigate the hydrogen evolution reaction (HER) on the noble-metal-nanoparticle-coated zinc or zinc-tin oxide surfaces. Among the six catalytic structures, the adsorption free energy of Au/ZnO structure is closest to zero. The relative energy diagrams reveal that Volmer-Heyrovsky mechanism on the NP (oxide) reactive site dominants the HER process on NP/ZTO (Pt/ZnO and Au/ZnO) structures. However, comparing the adsorption free energy and primary energy barrier of the prefer path of each structure, the Pt/ZnO structure shows the best performance for the HER process.     

Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer

Many transition metal di-selenides such as MoSe₂ and WSe₂ show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the number of electrocatalytic active sites and in turn to improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, we have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe₂ for HER and OER catalyst. The state-of-art density functional theory (DFT) based computations are performed for estimating the catalytic activity of the pristine and doped SnSe₂ by means of evaluating the adsorption and Gibbs free energies subjected to hydrogen and oxygen adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, we have included the dispersion correction term to exchange-correlation functional. Results show that the pristine SnSe₂ is not a good HER catalyst when hydrogen is adsorbed on its basal plane. However, edge-sites show the good hydrogen adsorption and indicates that the edges of SnSe₂ are the most preferential site for hydrogen adsorption. As far as the catalytic activity of SnSe₂ with dopants is concerned, the Na-doped SnSe₂ among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe₂ show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe₂ can be improved by selective doping of alkali metals.     

Theoretical study the component and facet dependence of HER performance on nickel phosphides surfaces

Energy depletion and environmental pollution are still serious challenges for human beings. The application of hydrogen energy should be a promising strategy to address this issue. However, the hydrogen production should be one shortcoming for hydrogen energy. The hydrogen evolution reaction (HER) based on electrocatalysis is an effective way to enhance the hydrogen generation with small energy consumption under ambient conditions. Many works have been devoted to develop high performance catalysts to satisfy the HER processes. Nevertheless, the mechanism about facet-dependence and composition-dependence influence is still need to deeply study. Hereon, based on density functional theory calculations, the [100], [110], and [111] facets of Ni_xP_y (Ni₃P, Ni₂P, NiP, NiP₂, NiP₃) systems were created and their HER catalytic activity were used to reveal the underline mechanism. By analyzing the variation of Gibbs free energy, it was found that the structural composition has a greater effect on HER than the facet. Significantly, the Ni₂P(111) surface with Ni/P-termination has the best HER performance for all samples in present work. Through exploring the electron transfer of H with surrounding atoms during the HER process, the H adsorption mechanism as well as its reaction mechanism has been revealed. The deep insights in this work provide an important fundamental that the contents of non-metal for compounds catalysts can heavily influence the performance of HER, which should give more guidance for designing new catalysts.     

Maize-like CoP nanorod arrays as an efficient and robust electrocatalyst for superior hydrogen generation

High-performance, low-cost and robust electrocatalysts for the hydrogen evolution reaction (HER) play a critical role in large-scale hydrogen production via water splitting. Herein, we proposed a synthesis strategy for the self-assembly of maize-like CoP nanorod arrays with abundant active sites via a combination of conventional hydrothermal reaction and low-temperature phosphorization. This unique architecture exhibited remarkable catalytic performance for the HER, with a low overpotential of 130 mV at a current density of 10 mA cm^?2 and a small Tafel slope of 59 mV dec^?1 in 1.0 M KOH electrolyte, as well as good stability as verified by chronoamperometry measurement for 10 h. Density functional theory calculations further revealed that these maize-like CoP nanorod arrays with dense active sites and a high phosphorization degree could boost the HER performance in terms of low adsorption energy and free energy. This work provided a facile strategy towards manipulating morphology engineering to enhance the HER activity of CoP-based catalysts.     

Reaction coordinate mapping of hydrogen evolution mechanism on Mg3N2 monolayer

In this work, we have envisaged the hydrogen evolution reaction (HER) mechanism on Mg₃N₂ monolayer based on electronic structure calculations within the framework of density functional theory (DFT) formalism. The semiconducting nature of Mg₃N₂ monolayer motivates us to investigate the HER mechanism on this sheet. We have constructed the reaction coordinate associated with HER mechanism after determining the hydrogen adsorption energy on Mg₃N₂ monolayer, while investigating all possible adsorption sites. After obtaining the adsorption energy, we subsequently obtain the adsorption free energy while adding zero point energy difference (ΔZPE) and entropic contribution (TΔS). We have not only confined our investigations to a single hydrogen, but have thoroughly observed the adsorption phenomena for increasing number of hydrogen atoms on the surface. We have determined the projected density of states (DOS) in order to find the elemental contribution in the valence band and conduction band regime for all the considered cases. We have also compared the work function value among all the cases, which quantifies the amount of energy required for taking an electron out of the surface. The charge transfer mechanism is also being investigated in order to correlate with the HER mechanism with amount of charge transfer. This is the first attempt on this material to the best of our knowledge, where theoretical investigation has been done to mapping the reaction coordinate of HER mechanism with the associated charge transfer process and the work function values, not only for single hydrogen adsorption, but also for increasing number of adsorbed hydrogen.     

Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study

In the ever growing demand of future energy resources, hydrogen production reaction has attracted much attention among the scientific community. In this work, we have investigated the hydrogen evolution reaction (HER) activity on an open-shell polyaromatic hydrocarbon (PAH), graphene quantum dot “triangulene” using first principles based density functional theory (DFT) by means of adsorption mechanism and electronic density of states calculations. The free energy calculated from the adsorption energy for graphene quantum dot (GQD) later guides us to foresee the best suitable catalyst among quantum dots. Triangulene provides better HER with hydrogen placed at top site with the adsorption energy as −0.264 eV. Further, we have studied platinum decorated triangulene for hydrogen storage. Three different sites on triangulene were considered for platinum atom adsorption namely top site of carbon (C) atom, hollow site of the hexagon carbon ring near triangulene's unpaired electron and bridge site over C–C bond. It is found that the platinum atom is more stable on the hollow site than top and bridge site. We have calculated the density of states (DOS), highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and HOMO-LUMO gap of hydrogen molecule adsorbed platinum decorated triangulene. Our results show that the hydrogen molecule (H₂) dissociates instinctively on all three considered sites of platinum decorated triangulene resulting in D-mode. The fundamental understanding of adsorption mechanism along with analyses of electronic properties will be important for further spillover mechanism and synthesis of high-performance GQD for H₂ storage applications.     

Exploring the hidden catalyst from boron pnictide family for HER and OER

A systematic investigation of catalytic activity of boron phosphide nanowire (BP NW) towards over-all water-splitting reaction has been performed by evaluating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities. Intended to the mentioned aim, we have utilized Kohn-Sham formulated extensively popular ab initio method based on density functional theory (DFT). The structural and electronic properties of the BP NW are computed and compared with its bulk phase. We observe dramatic indirect to direct bandgap transition with pronounced energy gap after introducing two-dimensional confinement that is akin to the other reported III-V NWs. The calculated partial density of states with van Hove singularity also confirms the same. Owing to its moderate bandgap value, the applicability of the BP NW as an HER/OER catalyst is assessed by computing the site dependent HER/OER activities. Our computation on Gibbs free energy for the case of hydrogen adsorption with −1.19 eV magnitude gives better results; whereas in case of OER, the results with higher magnitude of Gibbs energy implicate over binding of oxygen with adsorbent thus revealing non-feasible desorption of oxygen from adsorbent. Significant perturbation in electronic states of NW under hydrogen adsorption confirms high sensitivity of BP NW for hydrogen adsorption. Further, the effect of substitutional doping on HER and OER activities suggests that the doped NW shows poor HER activity in contrast to the site-dependent better OER activity in case of Ga doped BP NW. The present BP NW shows potential as an HER catalyst owing to its lower adsorption and Gibbs free energies (−1.07 and −0.84 eV), as compared to previously conventionally utilized III-V NWs. Henceforth, we believe that the present study would serve as a blueprint for the researchers to design and develop toxic and/or metal-free catalyst that can be utilized for efficient water-reduction.     

Revealing the catalytic micro-mechanism of MoN,WN and WC on hydrogen evolution reaction

Catalytic performance of MoN, WN and WC on hydrogen evolution reaction (HER)were investigated by first-principles calculations, especially considering the effect of strain. From the calculation we can see the catalytic ability has an opposite trend with the adsorption capacity. H coverage was found to affect WN catalytic activity obviously, however has no effect on MoN and WC. Comparing with the experiments, we inferred that although the top site has the strongest potential catalytic ability, the actual catalytic site for HER is hollow1 site. Some specific transition metal doping (Ni > Fe > Mn > W for MoN, Fe > Co for WN, Ni > Co > Fe> Mn for WC) may indirectly improve the HER catalytic performance. It is worth noting that applying a certain strain (e.g. 2.5% tensile strain for MoN, 5% compressive strain for WN, WC) is helpful for improving the HER catalytic performance. Our work is instructive for HER catalyst development in terms of doping and strain.     

Activating basal plane of Janus VSSe for efficient hydrogen evolution reaction by non-noble metal element doping: A first-principal study

Searching electrocatalysts with excellent hydrogen evolution reaction (HER) performance is very important for developing clean hydrogen energy. Two-dimensional (2D) materials have been widely studied as HER electrocatalysts, however, the basal planes of 2D materials, which dominate the surface area, are usually with poor activity. In this work, we theoretically studied the HER activity of Janus 2H–VSSe with or without non-noble metal element doping. Density functional theory (DFT) calculations suggest that doping As and Si atoms in the S or Se sites of VSSe and the C and Ge atoms in the Se site of VSSe greatly promote the HER performance of the basal plane of VSSe, resulting in hydrogen adsorption free energy close to zero (i.e. ?0.022, ?0.040, 0.066, 0.065, ?0.030, 0.058 eV, respectively), which are better than the Pt catalyst (?0.09 eV). The doped atoms strengthen the interaction between their pz-orbital and the hydrogen s-orbital, resulting in a lower bonding state in energy and higher bind strength for the hydrogen atom. This work opens up a new way to design highly efficient and low-cost catalysts for HER.     

The strain and transition metal doping effects on monolayer Cr2O3 for hydrogen evolution reaction: The first principle calculations

Chromic oxide (Cr₂O₃) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr₂O₃ monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr₂O₃ monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔG_H1) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr₂O₃ monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr₂O₃ monolayer.     

Properties,mechanisms and advantages of metallic glass for electrocatalysis and HER in water splitting: A review

H₂ is one of the most promising and attractive alternative energy sources for fossil fuels to solve the current energy crisis and environmental problems. Hydrogen production from water electrocatalysis has become one of the most popular hydrogen production methods due to its high purity and environmental friendliness. The most commonly used catalyst for hydrogen evolution reaction (HER) is powdered Pt/C. However, the scarcity and complicated post-treatment processes severely limit its industrial application. Therefore, it is a key challenge to develop low-cost, high-performance self-supporting catalytic electrodes. Recently, metallic glasses (abbreviated as MGs) ribbons have attracted great interest in catalytic applications due to their unique short-range ordered and long-range disordered atomic structures. The metastable properties of MGs offer great potential for their application in HER. Here, this review introduces the preparation, application first. Secondly, we introduce the advantages and application progress of MGs as HER catalytic electrodes. Then, we summarize the factors affecting the catalytic performance of MGs in HER process. Finally, current challenges and future development prospects for realizing highly active and durable electrocatalysts are presented. This review aims to provide a guide for designing and developing MGs with high HER performance.     

Investigating hydrogen evolution reaction properties of a new honeycomb 2D AlC

The hydrogen due to its high mass energy density is a new renewable, economically viable and clean resource. The most eco-friendly and economical approaches for the generation of hydrogen through hydrogen evolution is electrochemical water splitting. The two-dimensional (2D) nanomaterials have been recently found as potential candidates as non-noble metal catalyst for hydrogen evolution. In this work, we have systematically studied the structural and electronic properties of the newly predicted hexagonal-aluminium carbide monolayer (h-AlC ML) under the framework of dispersion-corrected density functional theory (DFT) calculations. The calculated electronic total density of states (TDOS) of h-AlC ML predict its metallic nature in contrast to other polar honeycomb 2D materials which are either semiconducting or semimetallic. The metallic behavior of h-AlC monolayer which motivates us to investigate its HER activity results due to the presence of delocalized charge density near Fermi level. Thus, we have investigated the HER activity of h-AlC ML by calculating hydrogen (H) adsorption energy (ΔE_H) and Gibbs free energy (ΔG_H) at three different sites of the 3 × 3 and 4 × 4 supercells of h-AlC ML; top of carbon atom (E_H-C), top of aluminium atom (E_H-Al) and hollow site (E_H-Hollow). Our results show that the hollow site is most catalytically active site in both supercells of h-AlC ML. We believe that our results will inspire experimentalists to fabricate this new 2D material for achieving the desired range of HER activity.     

Phosphorus-doped nickel selenides nanosheet arrays as highly efficient electrocatalysts for alkaline hydrogen evolution

Earth-abundant transition-metal dichalcogenides are considered as promising electrocatalysts to accelerate the hydrogen evolution reaction （HER）. Among them, the pyrite nickel diselenide (NiSe₂) has been received special attention due to its low cost and high conductivity, but it suffers a poor HER performance in alkaline media possibly attributed to its inadequate hydrogen adsorption free energies. Here, we report a novel P-doped NiSe₂ nanosheet arrays anchored on the carbon cloth with an obviously optimized HER performance. The catalyst only needs a low overpotential of 86 mV at a current density of 10 mA cm^?2 and a Tafel slop of 61.3 mV dec^?1，as well as maintains a long-term durability for 55 h in 1.0 M KOH, which is superior to the pristine NiSe₂ (135 mV@10 mA cm^?2) and most recently reported non-noble metal electrocatalysts. The XRD, EDS, TEM and XPS results validated the successful doping of P element into NiSe₂ nanosheet, while the density functional theory (DFT) calculation demonstrated the P doping can optimize the electronic structures and the hydrogen adsorption free energy of NiSe₂. This work thus opens up new ways for rationally designing high-efficient HER electrocatalysts and beyond.     

Tuning the electronic structure of NiSe2 nanosheets by Mn dopant for hydrogen evolution reaction

Designing earth-abundant and highly active hydrogen evolution reactions (HER) electrocatalysts is pivotal for developing renewable energies. Here, we report that Mn-doped NiSe₂ nanosheets is successfully fabricated on carbon fiber cloth by solvothermal process and followed selenide reaction, showing excellent catalytic performance for electrochemical water splitting. Benefiting from the unique electronic property modified by Mn doping and nanosheets structures, the obtained electrodes only requires low overpotential of 86 mV to achieve the current density of 10 mA/cm² in acid solution. Moreover, the higher normalized exchange current densities manifest that Mn doping can improve the intrinsic activity of NiSe₂. Furthermore, first-principle calculation results manifest that Mn doping can optimize the electronic structures to reduce hydrogen adsorption energy and consequently improve the HER active. This novel method is expected to provide new chances for developing efficient catalysts for energy conversion applications.     

Mechanism of transition metal cluster catalysts for hydrogen evolution reaction

Studying the hydrogen evolution reaction (HER) catalyst is important for the global energy crisis. Clusters have many special characteristics due to quantum size effect and super high specific surface area, including optical performance, catalytic performance, etc. In this work, the structures of transition metal cluster TM_n (TM = Co, Ni, Cu, Pd, Pt, n = 4–10) were searched and optimized by quantum chemistry methods. To search for non-precious metal catalysts, we calculated the Gibbs free energies for HER process on different clusters. Furthermore, the electronic structures of clusters before and after the reaction with H were analyzed, including the molecular surface electron distribution, the frontier molecular orbital, and the charge transfer properties, which dominated the HER processes. The results show that the Cu clusters have excellent HER catalytic properties due to its suitable surface electron distribution and HOMO/LUMO levels, especially Cu₄, Cu₇ and Cu₉, which even comparable to Pt catalysts. These results can help us better understand the mechanism of clusters catalyze HER process.     

ZIF-8 derived Zn,Mn, S and P co-loaded on N doped carbon as efficient electrocatalyst for hydrogen evolution reaction (HER)

Developing low-cost electrocatalysts with excellent catalystic activity and superior stability for hydrogen evolution reaction (HER) is still a big challenge. In this paper, we adopted zeolitic imidazolate zinc framework (ZIF-8) as precursor to fabricate Zn, Mn, S and P co-loaded on N doped carbon (ZnMnSP/NDC) via pyrolysis, adsorption, sulphurization and phosphorization processes and investigated its catalytic activity for HER. After being carefully researched, the loading of Mn, S and P can obviously improve the catalytic activity of the catalyst for HER. Mn and P elements are loaded on porous carbon homogeneously. Zn can react with S to form ZnS. More interestingly, the loading of S element can highly improve the graphitic degree of porous carbon. Furthermore, ZnMnSP/NDC exhibits excellent long-term stability for HER in the alkaline solution. The excellent electrocatalytic performance of ZnMnSP/NDC may be attributed to the high loading of Mn, S, P element and the unique nanoparticle-embedded porous carbon structure. This work provides a valuable way to fabricate non-precious elements co-loaded N doped carbon with excellent catalytic performance for HER.     

Probing active sites on MnPSe3 and FePSe3 tri-chalcogenides as a design strategy for better hydrogen evolution reaction catalysts

Electronic structure and catalytic activity in hydrogen evolution reactions (HER) of two ternary tri-chalcogenide nano-ribbons, MnPSe₃ and FePSe₃, have been investigated using first-principles electronic structure calculations. Specific edge sites have been identified as the catalytic centers in both these materials. HER catalytic activity has been predicted through determination of the hydrogen adsorption free energy following Nørskov's approach. This has been done both with and without considering effects of the aqueous solvent. Hydrogen coverage dependency of the catalytic activity have also been studied. Identification of the catalytically active edge sites on these materials bridges experimental observations and theoretical predictions on these materials.     

Activated HER performance of defected single layered TiO2 nanosheet via transition metal doping

Doping engineering has been considered as a viable strategy to obtain highly efficient photocatalysts for hydrogen evolution reaction (HER). Since transition metal doping usually introduces of oxygen vacancy defect in oxide compounds, the first-principles calculations were performed to investigate the HER activity of transition metal-doped lepidocrocite-type TiO₂ nanosheets with oxygen vacancies. Different hydrogen adsorption sites are taken into account here. When adsorbed hydrogen occupies the oxygen vacancy, an optimal HER activity, similar to ideal Pt metal, was obtained in Fe-doped nanosheets. The tunable HER performance was correlated with the d-band center level of dopants. On the other hand, O-p_z band center level is responsible for the hydrogen adsorption free energy in the case of hydrogen adsorbed with the surface oxygen atom. Unexpectedly, for Mn doping cases, the pre-adsorbed hydrogen could activate surface O atom with additional hydrogen adsorption free energy being close to zero. In addition, the strain engineering also could effectively adjust HER activity in defected nanosheets.