Theoretical Investigation of HER and OER Electrocatalysts Based on the 2D R-graphyne Completely Composed of Anti-Aromatic Carbon Rings

Based on the DFT calculations, two-dimensional (2D) R-graphyne has been demonstrated to have high stability and good conductivity, which can be conducive to the relevant electrocatalytic activity of the material. Different from the poor graphene, R-graphyne, which is completely composed of anti-aromatic structural units, can exhibit certain HER catalytic activity. In addition, doping the TM atoms in Group VIIIB can be considered an effective strategy to enhance the HER catalytic activity of R-graphyne. Particularly, Fe@R-graphyne, Os@R-graphyne, Rh@R-graphyne and Ir@R-graphyne can exhibit higher HER catalytic activities due to the formation of more active sites. Usually, the shorter the distance between the TM and C atoms is, the better the HER activity of the C-site is. Furthermore, doping Ni and Rh atoms of Group VIIIB can significantly improve the OER catalytic performance of R-graphyne. It can be found that ΔG_O* can be used as a good descriptor for the OER activities of TM@R-graphyne systems. Both Rh@R-graphyne and Ni@R-graphyne systems can exhibit bifunctional electrocatalytic activities for HER/OER. In addition, all the relevant catalytic mechanisms are analyzed in detail. This work not only provides nonprecious and highly efficient HER/OER electrocatalysts, but also provides new ideas for the design of carbon-based electrocatalysts.

Doping P atom with a lone pair: an effective strategy to realize high HER catalytic activity and avoid deactivation under wide H* coverage on 2D silicene and germanene by increasing the structural rigidity

In view of the weak aromatic characteristic resulting from the weak π-bonding ability (different from the analogous graphene), employing two-dimensional (2D) silicene and germanene monolayers could be one of the most promising ways to realize a new type of highly efficient and nonprecious catalyst for the hydrogen evolution reaction (HER). However, the HER activity of pristine silicene and germanene has to be improved, although both of them can exhibit a good change trend. Particularly, the hydrogen phenomenon can occur under moderate or high H* coverage on 2D silicene and germanene. To overcome these bottlenecks, in this study we identify the most effective strategy through doping P with a lone pair to significantly improve the HER catalytic activity under a high H* coverage, by screening a series of IIIA (i.e., B, Al, Ga, In and Tl) and VA (i.e., N, P, As, Sb and Bi) heteroatoms with different electronegativity under detailed DFT calculations. It is revealed that the doped P atoms and almost all the Si/Ge atoms can uniformly serve as highly active sites. Especially, in view of the existence of the lone pair, doping P effectively prevents hydrogenation (even under full H* coverage) by increasing the structural rigidity. Moreover, the P-doping concentration also plays a crucial role in obtaining high HER activity. The relevant mechanisms have been analyzed in detail. Clearly, all these fascinating findings are beneficial for realizing new HER electrocatalysts based on the excellent silicene or germanene nanomaterials, and even other Si/Ge-related materials in the near future.

Syngas Conversion to C2 Species over WC and M/WC (M = Cu or Rh) Catalysts: Identifying the Function of Surface Termination and Supported Metal Type

Improving the selectivity and activity of C₂ species from syngas is still a challenge. In this work, catalysts with monolayer Cu or Rh supported over WC with different surface terminations (M/WC (M = Cu or Rh)) are rationally designed to facilitate C₂ species generation. The complete reaction network is analyzed by DFT calculations. Microkinetics modeling is utilized to consider the experimental reaction temperature, pressure, and the coverage of the species. The thermal stabilities of the M/WC (M = Cu or Rh) catalysts are confirmed by AIMD simulations. The results show that the surface termination and supported metal types in the M/WC (M = Cu or Rh) catalysts can alter the existence form of abundant CH_x (x = 1-3) monomer, as well as the activity and selectivity of CH_x monomer and C₂ species. Among these, only the Cu/WC-C catalyst is screened out to achieve outstanding activity and selectivity for C₂H₂ generation, attributing to that the synergistic effect of the subsurface C atoms and the surface monolayer Cu atoms presents the noble-metal-like character to promote the generation of CH_x and C₂ species. This work demonstrates a new possibility for rational construction of other catalysts with the non-noble metal supported by the metal carbide, adjusting the surface termination of metal carbide and the supported metal types can present the noble-metal-like character to tune catalytic performance of C₂ species from syngas.

First-principles investigation of the hydrogen evolution reaction of transition metal phosphides CrP, MnP, FeP, CoP, and NiP

We comprehensively investigated the hydrogen evolution reaction (HER) activity of a series of transition metal phosphides (MPs) (M = Cr, Mn, Fe, Co, and Ni) using first-principles calculations. The free energy difference was calculated for possible sites on the surface to pinpoint the reactive sites and the associated catalytic activities. We found that the chemical properties of these considered MPs are different from those of WP, including CrP which has the same electronic configuration as WP but was shown not to be a good electrocatalyst. Different reactive sites other than WP were predicted, and notably, unlike WP, phosphorus can participate/catalyze the HER on the considered MP. Among these MPs, there are more active sites on FeP, CoP, and NiP than CrP and MnP. Our electronic structure analysis suggests that the spin polarization is critical in determining the hydrogen adsorption and hence the HER performance. We further explored the HER of metal- or phosphorus-deficit MPs, as samples can be grown under different conditions. In particular, phosphorus-deficit FeP, CoP, and NiP were found to have enhanced HER performance, with either better catalytic activities or more active sites. Therefore, we proposed that controlling of these defects can be an effective approach to tune the HER catalytic ability of these MPs. It can serve as the design principle to synthesize new MP based electrocatalysts.

Computational Screening toward Hydrogen Evolution Reaction by the Introduction of Point Defects at the Edges of Group IVA Monochalcogenides: A First-Principles Study

Exploring materials with high hydrogen evolution reaction (HER) performance is of importance for the development of clean hydrogen energy, and the defects on the surfaces of catalysts are essential. In this work, we evaluate the HER performance among group IVA monochalcogenides MXs (M = Ge/Sn, X = S/Se) with M/X point defects on the edges. Compared with basal planes and bare edges, the GeS edge with Ge vacancy (ΔG_H* = 0.016 eV), GeSe edge with Se vacancy (ΔG_H* = 0.073 eV), and SnSe edge with Sn vacancy (ΔG_H* = -0.037 eV) hold the best HER performances, which are comparable to or even better than the value for Pt (-0.07 eV). Furthermore, the relationships between ΔG_H* and p-band centers of considered models are summarized. The stability of proposed electrocatalysts are analyzed by vacancy-formation energy and strain engineering. In summary, the HER performance of MXs is greatly improved by introduction of point defects at the edges, which is promising for their use as electrocatalysts for the conversion and storage of energy in the future.

Confinement Effect of Mesopores: In Situ Synthesis of Cationic Tungsten-Vacancies for a Highly Ordered Mesoporous Tungsten Phosphide Electrocatalyst

Engineering defects in crystalline electrocatalysts is an effective approach to tailor the electronic structure and number of active sites, which are essential for the intrinsic activity of the hydrogen evolution reaction (HER). Unlike previously reported methods, we demonstrate a confinement effect using a mesoporous template for in situ fabrication of cationic W vacancies in as-prepared ordered mesoporous tungsten phosphide (WP) nanostructures by adjusting the nonstoichiometric ratio of the precursor elements. With a plenty of W vacancies and ordered mesoporosity, the as-prepared catalyst WP-Mesop exhibits better catalytic performance than the catalysts without mesopores and/or vacancies. The WP-Mesop shows an ultralow overpotential of 175 mV in acid and 229 mV in alkaline at 100 mA cm^-2 and stability of 48 h without structural collapse in both acid and alkaline media. Meanwhile, density functional theory calculations further reveal that the activation barrier for HER can be lowered by introducing cationic W vacancies. This strategy can be extended to generate cationic defects in other transition metal phosphides to improve their HER activities.

Embedding tetrahedral 3d transition metal TM4 clusters into the cavity of two-dimensional graphdiyne to construct highly efficient and nonprecious electrocatalysts for hydrogen evolution reaction

Coupled with the high structural stability and good conductivity, all the new 2D composite nanostructures TM₄@GDY (TM = Sc, Ti, Mn, Fe, Co, Ni and Cu) can uniformly exhibit considerably high catalytic activity for hydrogen evolution reaction.

MOF-derived cobalt-nickel phosphide nanoboxes as electrocatalysts for the hydrogen evolution reaction

The development of high-efficiency nonprecious electrocatalysts based on inexpensive and Earth abundant elements is of great significance for renewable energy technologies. Group VIII transition metal phosphides (TMPs) gradually stand out due to their intriguing properties including low resistance and superior catalytic activity and stability. Herein, we adopt a unique MOF-derived strategy to synthesize transition metal phosphide nanoboxes which can be employed as electrocatalysts for the hydrogen evolution reaction. During this process, we converted a Co-MOF to a CoNi-MOF by ion exchange and low-temperature phosphating to achieve CoNiP nanoboxes. The CoNiP nanoboxes can reach a current density of 10 mA cm^-2 at a low overpotential of 138 mV with a small Tafel slope of 65 mV dec^-1.

Theoretical predication of the high hydrogen evolution catalytic activity for the cubic and tetragonal SnP systems

Both the cubic and tetragonal SnP systems, with a layered structure similar to phosphorene, can exhibit a considerably high HER catalytic activity over a wide range of hydrogen coverage.

Theoretical design of a series of 2D TM–C3N4 and TM–C3N4@graphene (TM = V, Nb and Ta) nanostructures with highly efficient catalytic activity for the hydrogen evolution reaction

Coupled with high structural stability and metallic conductivity, all of the new composite systems TM–C₃N₄ and TM–C₃N₄@graphene (TM = V, Nb and Ta) can possess considerably high catalytic activity for the hydrogen evolution reaction.