Computational high-throughput screening of electrocatalytic materials for hydrogen evolution

Conducting MoS₂ nanosheets as catalysts for hydrogen evolution reaction

We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS2 nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS2, we found that the activity of 2H MoS2 is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS2 remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS2.

Unifying the 2e(-) and 4e(-) Reduction of Oxygen on Metal Surfaces

Understanding trends in selectivity is of paramount importance for multi-electron electrochemical reactions. The goal of this work is to address the issue of 2e(-) versus 4e(-) reduction of oxygen on metal surfaces. Using a detailed thermodynamic analysis based on density functional theory calculations, we show that to a first approximation an activity descriptor, ΔGOH*, the free energy of adsorbed OH*, can be used to describe trends for the 2e(-) and 4e(-) reduction of oxygen. While the weak binding of OOH* on Au(111) makes it an unsuitable catalyst for the 4e(-) reduction, this weak binding is optimal for the 2e(-) reduction to H2O2. We find quite a remarkable agreement between the predictions of the model and experimental results spanning nearly 30 years.

Pd/C synthesized with citric acid: an efficient catalyst for hydrogen generation from formic acid/sodium formate

A highly efficient hydrogen generation from formic acid/sodium formate aqueous solution catalyzed by in situ synthesized Pd/C with citric acid has been successfully achieved at room temperature. Interestingly, the presence of citric acid during the formation and growth of the Pd nanoparticles on carbon can drastically enhance the catalytic property of the resulted Pd/C, on which the conversion and turnover frequency for decomposition of formic acid/sodium formate system can reach the highest values ever reported of 85% within 160 min and 64 mol H₂ mol⁻¹ catalyst h⁻¹, respectively, at room temperature. The present simple, low cost, but highly efficient CO-free hydrogen generation system at room temperature is believed to greatly promote the practical application of formic acid system on fuel cells.

Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts

Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water-alkali electrolysers. Here, using 3d-M hydr(oxy)oxides, with distinct stoichiometries and morphologies in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) regions, we establish the overall catalytic activities for these reaction as a function of a more fundamental property, a descriptor, OH-M(2+δ) bond strength (0 ≤ δ ≤ 1.5). This relationship exhibits trends in reactivity (Mn < Fe < Co < Ni), which is governed by the strength of the OH-M(2+δ) energetic (Ni < Co < Fe < Mn). These trends are found to be independent of the source of the OH, either the supporting electrolyte (for the OER) or the water dissociation product (for the HER). The successful identification of these electrocatalytic trends provides the foundation for rational design of 'active sites' for practical alkaline HER and OER electrocatalysts.

Metal atomic contact under electrochemical potential control

Electric conductance of the metal atomic contacts of Au and Pd was investigated using a scanning tunneling microscope (STM) in solution under electrochemical potential control. At the hydrogen evolution potential, a fractional conductance peak appeared around 0.5 G(0) (G(0) = 2e(2)/h) in the conductance histograms of Au contacts. For Pd contacts, peaks appeared around 1.0 G(0) in the conductance histograms at the hydrogen evolution potential. The conductance behavior and atomic configuration of the metal atomic contacts at hydrogen evolution potential were discussed based on previously reported experimental results and theoretical calculation results. We have proposed the formation of hydrogen adsorbed metal Au and Pd contacts in solution at the hydrogen evolution potential.

Substrate-mediated enhanced activity of Ru nanoparticles in catalytic hydrogenation of benzene

The impact of carbon substrate-Ru nanoparticle interactions on benzene and hydrogen adsorption that is directly related to the performance in catalytic hydrogenation of benzene has been investigated by first-principles based calculations. The stability of Ru(13) nanoparticles is enhanced by the defective graphene substrate due to the hybridization between the dsp states of the Ru(13) particle with the sp(2) dangling bonds at the defect sites. The local curvature formed at the interface will also raise the Ru atomic diffusion barrier, and prohibit the particle sintering. The strong interfacial interaction results in the shift of averaged d-band center of the deposited Ru nanoparticle, from -1.41 eV for a freestanding Ru(13) particle, to -1.17 eV for the Ru/Graphene composites, and to -1.54 eV on mesocellular foam carbon. Accordingly, the adsorption energies of benzene are increased from -2.53 eV for the Ru/mesocellular foam carbon composites, to -2.62 eV on freestanding Ru(13) particles, to -2.74 eV on Ru/graphene composites. A similar change in hydrogen adsorption is also observed, and all these can be correlated to the shift of the d-band center of the nanoparticle. Thus, Ru nanoparticles graphene composites are expected to exhibit both high stability and superior catalytic performance in hydrogenation of arenes.

Tailoring of Pd-Pt bimetallic clusters with high stability for oxygen reduction reaction

The composition-dependent equilibrium structure and thermal stability of Pd-Pt clusters with the size of 55 atoms, and CO, O, OH, and O(2) adsorption on these clusters have been studied using molecular simulation based on the Gupta empirical potential and density functional theory (DFT) calculations. It is found that Pd(43)Pt(12) with a three-shell onionlike structure (TS-cluster) exhibits the highest relative stability in both DFT and Gupta levels and also the highest melting point at the Gupta level among these Pd-Pt clusters. In addition, the Pd(43)Pt(12) TS-cluster possesses the weakest CO, O, OH, and O(2) adsorption strength, compared to the Pt(55), Pd(55), and Pd(13)Pt(42) clusters, indicating good catalytic activities toward the oxygen reduction reaction (ORR) among these Pd-Pt clusters considered. We expect that this kind of DFT-guided strategy by controlling the composition could provide a simple way for possibly searching new electrocatalysts.

Interplay between subsurface ordering, surface segregation, and adsorption on Pt-Ti(111) near-surface alloys

Using the first-principles cluster expansion (CE) method, we studied the subsurface ordering of Pt/Pt-Ti(111) surface alloys and the effect of this ordering on segregation and adsorption behavior. The clusters included in the CE are optimized by a genetic algorithm to better describe the interactions between Pt and Ti atoms in the subsurface layer. Similar to bulk Pt-Ti alloys, Pt-Ti(111) subsurface alloys show a strong ordering tendency. A series of stable ordered Pt-Ti subsurface structures are identified from the two-dimensional (2D) CE. As an indication of the connection between the 2D and the bulk ordering, the CE predicts a ground-state Pt(8)Ti structure in the (111) subsurface layer, which is the same ordering as the close-packed plane of the bulk Pt(8)Ti compound. We carried out Monte Carlo simulations (MC) using the CE Hamiltonian to study the finite temperature stability of the Pt-Ti subsurface structures. The MC results show that subsurface structures in the Pt-rich range have higher order-disorder transition temperatures than their Ti-rich subsurface counterparts. We calculate the binding energy of different adsorbates (O, S, H, and NO) on Pt-terminated and Ti-segregated surfaces of ordered PtTi and Pt(8)Ti subsurface alloys. The binding of these adsorbates is generally stronger on Ti-segregated surfaces than Pt-terminated surfaces. The adsorption-induced Ti surface segregation is determined by two factors: (i) the unfavorable energy penalty for the Ti atom to segregate to the clean surface and (ii) the favorable energy decrease from stronger adsorbate binding on the Ti-segregated surface. The two factors introduce similar magnitude in energy change for the S and NO adsorption on Ti-segregated surfaces of PtTi subsurface alloys. We predict an adsorption-induced Ti surface segregation that is dependent on the atomic configurations of the Ti-segregated surfaces resulting from the competition of the two factors.

Nanofaceted C/Re(1121): fabrication, structure, and template for synthesizing nanostructured model Pt electrocatalyst for hydrogen evolution reaction

We report the first observation of carbon-induced nanofaceting of a Re single crystal and its application in synthesizing a nanostructured model Pt electrocatalyst investigated using multiple surface science techniques, including low-energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy ion scattering, and scanning tunneling microscopy, combined with electrochemical reaction measurements. Upon annealing in acetylene at 700 K followed by annealing in vacuum at 1100 K, an initially planar Re(112̅1) surface becomes completely faceted and covered with three-sided nanopyramids exposing (011̅1), (101̅1), and (112̅0) faces. Using the faceted C/Re(112̅1) surface as a template, we have successfully fabricated a nanostructured Pt monolayer (ML) electrocatalyst. The Pt ML supported on the C/Re nanotemplate exhibits higher activity for the hydrogen evolution reaction than Pt(111). This is the first application of faceted metal surfaces as templates for synthesis of nanoscale model electrocatalyst with well-defined (facet) surface structure and controlled (facet) size on the nanometer scale, illustrating the potential for future studies of nanostructured bimetallic systems relevant to electrocatalytic reactions.