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.     

A review of heteroatomic doped two-dimensional materials as electrocatalysts for hydrogen evolution reaction

Emerging two-dimensional (2D) materials, such as graphene, transition metal disulfide compounds (TMDCs), MXenes, layer double hydroxides (LDHs), black phosphorus (BP) and hexagonal boron nitride (h-BN), play an important role in speeding up hydrogen evolution reaction (HER) due to its large specific surface area as well as function of loading and efficient support. However, as an electrocatalyst, pure 2D materials cannot meet HER needs caused by their monotonous performance. Therefore, some nanoparticles are used to load and tune the 2D materials to develop efficient and inexpensive catalysts. Herein, we conduct a thorough analysis for materials based on heteroatoms, especially transition metal atoms and non-metal atoms (N, P, S, etc.) doped with graphene, TMDCs, MXenes, LDHs, BP and h-BN. It can be found that doping or coupling between 2D materials will affect the electronic structure, energy band, active area, conductivity and stability of the catalyst, which will induct a huge change in the catalytic performance. This review reveals the relationship between active centers, H₂O adsorption and chemical reaction processes. It also analyzes and summarizes the design principles and performance improvement mechanisms of hybrid catalysts. These discussions can provide references for other researchers to develop derivatives of related catalysts.     

The enhancement of hydrogen storage capacity in Li,Na and Mg-decorated BC3 graphene by CLICH and RICH algorithms

New hydrogen adsorption states on Li, Na, and Mg-decorated graphene-type BC₃ sheet have been investigated by first-principles calculations. The structural, electronic and binding properties, metal binding and nH₂ (n = 1–10) adsorption states of these systems are studied in detail with the Mulliken analysis, charge density differences, and partial density of states. To enhance the number of the adsorbed H₂ molecules per metal atom, and also to generate the better initial coordinates for reducing the simulation time, we present two masthematical algorithms (CLICH and RICH). The tested results on BC₃ sheet and boron-doped graphene (C₃₀B₂) show that these algorithms can increase the number of adsorbed hydrogen molecules by minimizing the computational time. It is seen that nH₂ (n = 1–10) adsorbed Li,/Na and/Mg-decorated BC₃ single- and double-sided systems are industrial materials for hydrogen storage technology, their adsorption energies fall into the acceptable adsorption energy range (0.20–0.60 eV/H₂). It is concluded from the optimized geometries and charge density differences for the higher number of H₂ adsorbed systems that not only decorated metal atom but also the sheet plays an important role in hydrogen storage process, because the boron atoms in the sheet expand the induced electric field between the adatoms and BC₃ sheet. From Mulliken analysis, there is a charge transfer mechanism between H₂ molecules and metal atoms. Moreover, the resonant peaks for the sheet, metal atoms and H₂ molecules in partial density of states curves indicate the possible hybridizations. Additionally, these adsorption processes are supported by charge density difference plots.     

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 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.     

3d transitional-metal single atom catalysis toward hydrogen evolution reaction on MXenes supports

Single atom catalysis involving atomically dispersed metal active sites on the appropriate supports is the effective way to magnify the catalytic efficiency and reduce the cost. By performing the first-principles calculations, we studied the anchoring of 3d transitional-metal single atoms M (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) on the surfaces of MXenes Cr₂CO₂ and Mo₂CO₂ and the catalytic activity of the single atom sites for hydrogen evolution reaction (HER). Sixteen single atom sites, M-Cr₂CO₂ (M = Sc, Ti, V, Cr, Mn, Fe, Co, Cu and Zn) and M-Mo₂CO₂ (M = Sc, Ti, V, Cr, Mn, Fe and Zn) have been chosen via examining the energetical and thermal stability of the isolated M atoms on the substrates. More importantly, we have calculated the Gibbs free energy change (ΔG_H) of H adsorption on the surface of M anchored Cr₂CO₂ and Mo₂CO₂ and find that Cr, Fe, Zn on Cr₂CO₂ and Sc, V on Mo₂CO₂ are the promising single atom active sites toward HER. Additionally, our results show that M atoms adsorbing turns the nearby sites to be active for catalyzing HER. MXenes Cr₂CO₂ and Mo₂CO₂, in terms of the supporting not only stabilize but also works together with the anchored single atom M as active catalyst toward HER.     

Regulating surface wettability and electronic state of molybdenum carbide for improved hydrogen evolution reaction

Molybdenum carbide (Mo₂C) is a cost-effective transition metal carbides (TMCs) electrocatalyst for hydrogen evolution reaction (HER) due to its electronic structure similar to Pt-group noble metal. Herein, we report that an effective strategy of regulating the surface wettability and electronic state of Mo₂C by ammonia and hydrothermal co-treatment to enhance its HER activity. The electrochemical results demonstrate that Mo₂C undergoing ammonia and hydrothermal co-treatment (Mo₂C-wh) exhibits remarkably improved electrocatalytic HER activity as compared to the pristine Mo₂C (Mo₂C-p) catalyst. The activity-structure relationship studies manifest that ammonia and hydrothermal co-treatment increases the content of hydroxyl group and pyridinic-N, thus endowing Mo₂C-wh with lower charge transfer resistance, larger electrochemical active surface area and higher surface wettability. DFT calculations reveal that ammonia and hydrothermal co-treatment enhances the Mo 3d-band center and reduces the hydrogen adsorption free energy. These changes in electronic states of Mo sites and physical properties of Mo₂C positively contribute to the improvement of electrocatalytic HER activity.     

Electronic structure modulation of CoSe2 nanowire arrays by tin doping toward efficient hydrogen evolution

The development of highly active, durable and earth-abundant electrocatalysts toward hydrogen evolution reaction (HER) is of great significance for promoting hydrogen energy. As one of the most potential substitutes for Pt-based materials, pyrite cobalt selenide (CoSe₂) still has shortcomings in terms of HER performance possibly due to its unfavorable hydrogen adsorption characteristics. Metal cation doping has been considered as one of the most available methods to modulate the electronic structure of electrocatalysts. Herein, non-transition metal tin (Sn) doped CoSe₂ nanowire arrays grown on carbon cloth have been constructed and fabricated via a simple gas-phase selenization treatment of hydroxide precursor. The successful doping of Sn element into CoSe₂ nanowires was confirmed by many experimental results. The as-prepared catalyst shows an obviously enhanced HER performance in alkaline media. Compared with pristine CoSe₂, the overpotential of Sn doped catalyst with optimal doping content decreases from 189 mV to 117 mV at 10 mA cm^?2 and the Tafel slope declines from 94 mV dec^?1 to 86 mV dec^?1, as well as shows long-term durability for 100 h. Experimental results and further density functional theory (DFT) calculations show that Sn doping can improve the ability of charge transfer and increase the electrochemical surface area, as well as optimize the hydrogen adsorption energy, all of which are instrumental in HER performance improvement. This work not only provides atomic-level insight into regulating the electronic structure of transition metal selenides by main group metal doping, but also broadens the avenue of developing high-efficiency and stable non-precious metal catalysts.     

Enhanced hydrogen evolution reaction performance of MoS2 by dual metal atoms doping

Molybdenum disulfide (MoS₂) has been considered a promising high-efficiency, low-cost hydrogen evolution reaction (HER) catalyst in acidic and alkaline media. However, the lack of active sites in the basal plane become the most significant obstacle hindering the widespread application of MoS₂. Here, we systematically studied the HER performance of MoS₂ plane or edge by co-doping Co atom and other 3d transition metals (TM = Ti–Fe, Ni) by density functional theory calculation methods. Interestingly, the dual atoms doping in both the basal plane and edges of MoS₂ is a feasible fabrication with small or negative formation energies. Compared with the pristine MoS₂ electrocatalyst, the HER performance in these doped systems is largely enhanced in both basal plane and edges due to the effective charge regulation on the S site by dual atom doping. Remarkably, close to zero H adsorption free energy (ΔG_H = ?0.161–0.119 eV) is identified for the TM-Co co-doped MoS₂ basal, indicating that they are potential alternate HER electrocatalysts of Pt. Our study provides a new strategy to design highly efficient non-noble metal electrocatalysts accessibility for energy-related applications.     

Doping sp-hybridized B atoms in graphyne supported single cobalt atoms for hydrogen evolution electrocatalysis

Electrochemical water splitting to hydrogen is considered as a promising approach for clean H₂ production. However, developing highly active and inexpensive electrocatalysts is an important part of the hydrogen evolution reaction (HER). Herein, we present a multifaceted atom (sp²-and sp-hybridized boron) doping strategy to directly fine-modify the electronic structures of the active site and the HER performance by the density functional theory calculations. It is found that the binding strength between the Co atom and the B doped graphyne nanosheets can be enhanced by doping B atoms. Meanwhile, the Co@B₁-GY and Co@B₂-GY catalysts exhibit good thermodynamic stability and high HER catalytic activity. Interestingly, the Co@B₂-GY catalyst has an ideal HER performance with the ΔG_H* value of −0.004 eV. Moreover, the d-band center of the Co atoms is upshifted by the sp²-or sp-hybridized B dopants. The concentrations of the sp-hybridized B atoms have a positive effect on the electrons transformation of the Co atoms. The interaction between the H and Co atoms becomes strong with the increase of the concentrations of the sp-hybridized B atoms and thus the corresponding catalysts show sluggish HER kinetics. This investigation could provide useful guidance for the experimental groups to directly and continuously control the catalytic activity towards HER by precisely doping multifaceted atoms.     

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.     

Platinum doped iron carbide for the hydrogen evolution reaction: The effects of charge transfer and magnetic moment by first-principles approach

In this work, the catalytic activity towards hydrogen evolution reaction (HER) was studied for hydrogen adsorption on Pt doped Fe₂C (001) surface configuration (Pt/Fe₂C) and compared with pure Pt (001). The adsorption of H on the pristine Fe₂C, Pt doped Fe₂C, and pure Pt in (001) slab was computed. The best and promising HER activity (ΔG_H1 = −0.02 eV) is obtained at the hollow site adsorption of Pt/Fe₂C (Fe₁₃Pt₃C₈) compared to the experimental value of pure Pt (ΔG_H1 = −0.09 eV) suggesting the possibility of the H₂ formation on the surface of Fe₁₃Pt₃C₈. The structural stabilities of Fe₂C and Pt/Fe₂C were investigated by the formation energy analysis. Also, it is observed that to enhance the HER mechanism, the modification of the d-electron structure of Pt atoms is essential which can be achieved by the increased Pt doping. The Bader charge analysis demonstrated the charge transfer between the substrate and the adsorbed H atoms. The density of states (DOS) of pure Fe₂C and optimal Pt/Fe₂C were calculated which revealed the magnetic and metallic nature of these materials. In addition, the adsorption and resulted activation of H₂ were facilitated by the elongation of H–H bond length in Fe₁₃Pt₃C_8. This work supports the HER over single atom catalysts (SACs) with lower Pt loading but with high catalytic activity and the maximum atom utilization of SACs.     

Yttrium-decorated C48B12 as hydrogen storage media: A DFT study

We report a density functional theory calculation dedicated to analyze the behavior of hydrogen adsorption on Yttrium-decorated C₄₈B₁₂. Electron deficient C₄₈B₁₂ is found to promote charge transfer between Y atom and substrate leading to an enhanced local electric field which can significantly improve the hydrogen adsorption. The analysis shows that Y atoms can be individually adsorbed on the pentagonal sites without clustering of the metal atoms, and each Y atom can bind up to six H₂. molecules with an average binding energy of −0.46 eV/H₂, which is suitable for ambient condition hydrogen storage. The Y atoms are found to trap H₂ molecules through well-known “Kubas-type” interaction. Our simulations not only clarify the mechanism of the reaction among C₄₈. B₁₂, Y atoms and H₂ molecules, but also predict a promising candidate for hydrogen storage application with high gravimetric density (7.51%).     

Could one non-noble metal surface with non-noble substrate be a good hydrogen evolution catalyst: Performance of transition metal A monolayer on B substrate in theory frame

Could materials only including non-noble metals be good HER catalysts? To deal with this puzzle, computational screening of 132 different non-noble transition metal A/B surfaces for HER (A monolayer on B), are carried out by first-principles calculations systematically. The formation energies and dissolution potentials are calculated to access stabilities of A/B in vacuum and in solution, respectively. The realistic catalytic surfaces with oxygen or hydroxyl (co)adsorption are confirmed by analyzing Surface Pourbaix diagrams. Finally, three A/B surfaces (Cu/Mo, Mo/W, and Ti/Nb) with high stabilities and high HER exchange current densities (8.51, 3.39, 2.83 mA/cm²) are screened out. Further electronic properties analysis reveals that the different interactions between H 1s and d orbital, s orbital of A atoms will determine the feasible application of d and s band centers to uncover water effect and substrate effect on hydrogen adsorption ability, and tune the HER activity of A/B in the end.     

Engineering the activity of CoNx-graphene for hydrogen evolution

The sustainable generation of H₂ from water electrolysis is a promising approach to alleviating current energy crisis and environmental pollution. However, the efficiency of such a system is limited by the conventional noble metal catalysts. Herein, using first-principle calculations, we demonstrate the potential of non-precious Co and N-codoped graphene systems (CoNx-N/O-gra, x = 1–4) in efficiently catalyzing the hydrogen evolution reaction (HER). The findings showed that the activity of the Co sites was influenced by the concentration and configuration of N dopants, as well as the coordination environment of Co atom. From the volcano curve, obtained by plotting the exchange current as a function of the adsorption free energy, optimal performance was achieved on CoN4-gra, which was attributed to the moderate adsorption of CoN4-gra towards H. The electronic structures analyses confirmed that the hybridization between Co sp orbital and H s orbital played key role in tuning the interactions between the catalysts and H, which could be regulated by the coordination environment of Co. The present findings highlight the potential of non-precious metal/non-metal-codoped graphene as HER catalysts and offers an effective approach to tune the HER activity by modifying the coordination environment of the metal atoms.     

Hydrogen storage capacities of alkali and alkaline-earth metal atoms on SiC monolayer: A first-principles study

A detailed theoretical Density-Functional-Theory-based investigation of hydrogen adsorption on silicon carbide monolayers (SiC-ML) decorated with alkali and alkaline-earth metal atoms is presented. The results show that the favourable position for all adsorbed metal atoms is above a Si atom. These metal atoms are chemisorbed to the SiC-ML, except for Mg which is physisorbed. The adsorbed atoms act in turn as adsorption sites for H₂ molecules. The single-sided K-functionalized SiC-ML can store up to six H₂ molecules. For double-side K-decorated SiC-ML, up to ten H₂ molecules can be captured. In all cases, the H₂ molecules are physisorbed. This is beneficial because the breaking of chemical bonds, which otherwise would be needed to make use of the stored H₂, is energetically expensive. These results find decorated SiC-ML as a promising material for hydrogen storage systems.     

High-throughput screening of transition metal single-atom catalyst anchored on Janus MoSSe basal plane for hydrogen evolution reaction

Considerable efforts have been made to enhance the hydrogen evolution reaction (HER) catalytic performance of Janus MoSSe monolayer, which have been considered to be a promising candidate due to the unique asymmetry structure. However, the activation effect remains non-optimal for the inert Janus MoSSe basal plane at present. Herein, a train of transition metal (TM) atoms were anchored on the S-/Se-/Mo-defective MoSSe basal plane to screen effective TM single-atom catalysts for HER through density functional theory (DFT) computations. Interestingly, the single Co atom anchored on Mo-defective MoSSe and the single Zn or Cd atom anchored on S-defective MoSSe were judged to possess excellent HER performance yielding a near-zero ΔG_H (ΔG_H = ?0.050, ?0.095, ?0.098 eV, respectively), which is comparable to the optimized Pt-SACs. The enhanced HER activity is attributed to the doping of TM atoms (Co, Zn and Cd) which improves the conductivity of the original MoSSe and offers unoccupied states near the Fermi level decreasing the energy barrier of electrons transfer between H and TMs@MoSSe surface. In addition, the change of unoccupied antibonding states of active atoms leads to appropriate interaction between the active sites and H. The hybridization between H-s orbital and the TMs@MoSSe systems around the Fermi level also suggests the formation of stable bonding-antibonding hydrogen adsorption states. This work reveals an effective way of activating MoSSe basal plane for HER.     

Computational exploration of magnesium-decorated carbon nitride (g-C3N4) monolayer as advanced energy storage materials

Density functional theory (DFT) computational studies were conducted to explore the hydrogen storage performance of a monolayer material that is built on the base of carbon nitride (g-C₃N₄, heptazine structure) with decoration by magnesium (Mg). We found that a 2 × 2 supercell can bind with four Mg atoms. The electronic charges of Mg atoms were transferred to the g-C₃N₄ monolayer, and thus a partial electropositivity on each adsorbed Mg atom was formed, indicating a potential improvement in conductivity. This subsequently causes the hydrogen molecules’ polarization, so that these hydrogen molecules can be efficiently adsorbed via both van der Waals and electrostatic interactions. To note, the configurations of the adsorbed hydrogen molecules were also elucidated, and we found that most adsorbed hydrogen molecules tend to be vertical to the sheet plane. Such a phenomenon is due to the electronic potential distribution. In average, each adsorbed Mg atom can adsorb 1–9 hydrogen molecules with adsorption energies that are ranged from ?0.25 eV to ?0.1 eV. Moreover, we realised that the nitrogen atom can also serve as an active site for hydrogen adsorption. The hydrogen storage capacity of this Mg-decorated g-C₃N₄ is close to 7.96 wt %, which is much higher than the target value of 5.5 wt % proposed by the U.S. department of energy (DOE) in 2020 [1]. The finding in this study indicates a promising carbon-based material for energy storage, and in the future, we hope to develop more advanced materials along this direction.