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[Display omitted] Pentlandite (Fe,Ni) 9 S 8 is a promising transition-metal catalyst for the hydrogen evolution reaction. However, little is explained about the long activation process that has been observed in experiments, and its facet-dependent hydrogen evolution activity is still theoretically unrevealed. To explain some experimental phenomena and to guide subsequent studies, density functional theory calculations are used to study the main synthetic surfaces: (1 1 1) and (3 1 1) in this work. The results show that the small metal cube plays an important role in the surface stability, and it is suggested that such cubes remain intact during catalysis. The linking sites serve as a bridge across the metal cubes and are the main catalytic active sites for hydrogen evolution. This is because the metal cubes can tune the electronic structures of the linking sites, and then the free energy of the linking sites is optimized. The (3 1 1) surface is a composite surface that consists of (1 0 0) and (1 1 1) facets and has the profile of a step. A surface conversion between the (3 1 1) and (1 1 1) facets may occur when the cube layer length increases. Therefore, the active sites can be feasibly engineered by the surface structures, and this could be helpful in further applications of (Fe,Ni) 9 S 8. [ABSTRACT FROM AUTHOR]

[Display omitted] Pentlandite is reported to exhibit good catalytic activity in hydrogen evolution reaction (HER). Many studies have paid attention to metal catalysis of pentlandite. However, the nonmetal catalysis is not considered for HER. Here, we unravel one probable catalytic mechanism of pentlandite toward HER using density functional theory. In our study models, (0 0 1) and (1 0 0) surfaces are created because there are three types of S-bridged M–M groups on them. Our study reveals that (Fe-Ni)-S center has a moderate value of Gibbs free energy while the corresponding value for (Fe-Fe)-S or (Ni-Ni)-S center is largely positive or negative. In (Fe-Ni)-S group, Fe and Ni can regulate the antibonding state of S, and then balance adsorption and desorption of proton. In addition, an intrinsic electronic potential difference exists between Fe and Ni in (Fe-Ni)-S group, which may boost the charge transfer. Particularly, (Fe-Ni)-S groups are perpendicular to the surface, and four of them make up one closed loop in the surface. It is suggested that the behaviors of such configuration composed of reaction centers resemble edge sites along the layers of MoS 2 toward HER. This study provides a deep insight into the synergistic effect of S-bridged Fe-Ni groups and enables the modulation of electrocatalytic reaction of pentlandite toward HER. [ABSTRACT FROM AUTHOR]

• Distribution of metal atoms at octahedral sites has little effect on stability. • Heterogeneous distribution at tetrahedral sites is beneficial to stability. • Fe atom tends to increase occupying ratio at octahedral sites. • Foreign atom can be easily introduced into octahedral sites. Pentlandite (Fe,Ni) 9 S 8 has been recently reported in experiments to show a good performance in hydrogen evolution reaction (HER) whose atomic scale mechanism necessarily requires explicit metal distribution. However, the knowledge of its explicit Fe/Ni distribution is lacking, impeding further HER research. In this work, we employed first-principles method to study metal distribution in pentlandite. The results demonstrated that the metal distribution at tetrahedral sites plays a major role in structural stability, where Fe Ni contact can significantly increase it. Additionally, it is importantly revealed that the metal atoms at octahedral sites can be tailored easily compared with those at tetrahedral sites. [ABSTRACT FROM AUTHOR]

Abstract The hydrogen adsorptions on 2H-phased Mo X 2 (X = S, Se) monolayers as a function of biaxial strain or external vertical electric field were investigated on the basis of density-functional theory. The results show the intrinsic bandgap and electron transfer effects on the hydrogen adsorption of Mo X 2 monolayer, which reveal the relationship between the electronic structures of Mo X 2 and corresponding hydrogen adsorption behaviors. Both biaxial strain and external vertical electric field have the ability to change bandgap of Mo X 2 monolayer. Bandgap narrowing can lead to hydrogen adsorption strengthening. The hydrogen adsorption on Mo X 2 monolayer can also be finely tuned by an electron transfer between H adatom and substrate by means of vertical electric field. Therefore, we suggest that these two factors can possibly be regulated and synergistically tune the hydrogen adsorption on Mo X 2 monolayer. Highlights • The mechanism of H adsorption on MoS 2 (MoSe 2) monolayer versus biaxial strain/electric field is explored. • The relationship between intrinsic electronic structures and H adsorption behaviors is investigated systematically. • The behavior of H adsorption tightly related to HER activity can be tuned by biaxial strain/electric field. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effect of structure evolution under working condition of MNC electrocatalysts on the OER activity and stability has been studied with the DFT method. • This work conducted a comprehensive evaluation method for MNC electrocatalysts by taking ΔG *OH as the screening criterion for OER activity and ΔE b of electrocatalysts under working conditions as the stability descriptor. • The stability of MNC electrocatalysts under working conditions, including high temperature, dynamic condition, local electric field and strong specific adsorption has been studied in detail. Developing low-cost, high activity and stability oxygen evolution reaction (OER) catalysts is significantly important but still challenging for water electrolyzers. In this work, we calculated the OER activity and stability of Metal-Nitrogen-Carbon (MNC, M = Co, Ru, Rh, Pd, Ir) based electrocatalyst with different structures (MN 4 C 8 , MN 4 C 10 , MN 4 C 12) using density functional theory (DFT) method. These electrocatalysts were divided into three groups based on the value of ΔG *OH , that is ΔG *OH > 1.53 eV (PdN 4 C 8 , PdN 4 C 10 , PdN 4 C 12), ΔG *OH < 1.23 eV (RuN 4 C 8 , RuN 4 C 10 , RuN 4 C 12 , CoN 4 C 8 , CoN 4 C 10) and 1.23 eV < ΔG *OH < 1.53 eV (RhN 4 C 8 , RhN 4 C 10 , RhN 4 C 12 , IrN 4 C 8 , IrN 4 C 10 , IrN 4 C 12 , CoN 4 C 12), and ΔG *OH determine whether the structure evolution will appear. The results proved that MNC (M = Rh, Ir) with 1.23 eV < ΔG *OH < 1.53 eV shows higher OER activity due to moderate binding energy between reaction intermediates and MNC. Furthermore, these catalysts could maintain M N C structure without further oxidation and structural evolution under working conditions (high temperature, dynamic condition, local electric field and strong specific adsorption), therefore show excellent stability. However, MNC electrocatalyst with ΔG *OH > 1.53 eV or ΔG *OH < 1.23 eV revealed less stability under working conditions, due to their low intrinsic stability or structural evolution under working conditions, respectively. In conclusion, we proposed a comprehensive evaluation method for MNC electrocatalysts by taking ΔG *OH as the screening criterion for OER activity and stability, as well as ΔE b under working condition as descriptor of stability. This is of great significance for the design and screening of ORR, OER and HER electrocatalysts under working conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Study of crystal plane, strain and ligand effects on the intrinsic stability of Pt alloy electrocatalysts. • Study of specific adsorption and local electric field on the stability of Pt alloy electrocatalysts. • Highlighting the importance of intrinsic stability and working environment on the durability of Pt alloy electrocatalysts. Electrocatalyst stability is a key parameter for commercializing high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Here, we used density functional theory (DFT) to investigate the stability of Pt-based electrocatalysts by calculating the binding energy (ΔE b) of surface Pt atoms under working conditions. Our results show that Pt(1 1 1) is more stable than Pt(1 0 0) and Pt(1 1 0) under vacuum conditions. Stress hurts the stability of Pt(1 1 1), regardless of compressive or tensile stress. Most of the transition metals alloyed with Pt improve the stability of Pt(1 1 1), especially PtV, PtY, and PtTa, which increase the ΔE b by 0.60–0.70 eV (ΔΔE b). However, the stability of Pt is significantly destroyed under the working conditions of oxygen reduction reaction (ORR). The specific adsorption of ORR intermediates and electrolyte ions decreases the ΔE b of Pt(1 1 1) by 0.35–0.95 eV, and the effect of PO 4 3-* is more significant. Furthermore, when the electric field of the electrochemical double layer is coupled with PO 4 3-* specific adsorption, ΔΔE b further increases to 1.12 eV. Our results highlight the importance of the intrinsic ΔE b and the working conditions for the stability of Pt-based electrocatalysts. This work provides important guidelines for the design of stable ORR electrocatalysts for HT-PEMFCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Design of the prior PA-resistance electrocatalysts through theoretical calculation. • Discovery of different scaling relationship of the adsorption energy difference of O 2 and H x PO(3-x)- 4 on Pt 3 M(1 1 1) alloy surface. • Pt X Cu as the object for the research of internal mechanism of the potentially high PA resistance property. • Higher PA-resistance performance of Pt 2 Cu compared to the commercial PtC in half-cell and full-cell tests. In high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), dissociative H 3 PO 4 in cathode will block the active sites of Pt-based electrocatalyst, which thus failure to catalyze the oxygen reduction reaction (ORR). Therefore, the electrocatalysts for the ORR in HT-PEMFCs with high resistance to H 3 PO 4 poisoning are significantly important. Due to the similar bonding structure between H x PO 4 3-x/O 2 and Pt (Pt-O), the design of catalysts which could selectively reduce the adsorption energy of the H 3 PO 4 is urgently needed. In this work, we screen proper H 3 PO 4 resistance electrocatalysts through theoretical calculation based on the adsorption energy difference between H x PO 4 3-x and O 2 as an evaluation criterion. Then, we prepared a series of Pt x Cu catalysts based on the theoretical calculation results and further studied the structure-performance relationship of phosphoric acid-resistant catalysts through combining physical property characterization and electrochemical tests including half-cell and single-cell. The mass activity of Pt 2 Cu/C is 0.51 A/mg Pt , exceeding the DOE's 2020 target of 0.44 A/mg Pt. After add 0.1 M H 3 PO 4 into the HClO 4 electrolyte, the half-wave potential (E 1/2) of Pt 2 Cu/C is only negatively shifted by 47 mV, much lower than 90 mV of TKK Pt/C. Furthermore, the HT-PEMFC with Pt 2 Cu/C in the cathode shows a peak power density of 383.4 mW/cm2, which surpasses that of TKK Pt/C for 231.0 mW/cm2. Our strategy of screening catalysts based on the adsorption energy difference study and further verified by experimental methods provides a promising strategy for the design of practical electrocatalyst in the cathode of HT-PEMFCs. [ABSTRACT FROM AUTHOR]

Highlighting sentence: Fe-Ni heterostructure is synergistically beneficial to overall water splitting on the (1 1 0) fact of pentlandite. [Display omitted] • Bimetallic Fe-Ni can synergistically catalyze HER and OER, respectively. • The OER catalytic activity can be improved by anions-poisonous adsorption. • The mechanism of pentlandite for water splitting is uncovered at atomic level. Developing stable, low cost, and efficient HER/OER bifunctional electrocatalysts is highly desired in overall water splitting. The bimetallic Fe-Ni-based systems can exhibit a great potential for overall water splitting. However, its electrocatalytic mechanism remain elusive as a bifunctional catalyst. Herein, in acidic medium, HER and OER mechanisms on the (1 1 0) facet of pentlandite are systematically investigated by means of density functional theory. The results unravel that the bimetallic Fe-Ni heterostructure can delicately regulate the charge transfer to accommodate themselves separately to oxidation and reduction. The Fe site exhibits the excellent catalytic HER activity while the Ni site shows the high catalytic OER activity. Interestingly, during the OER process, the Fe site with *OH-poisonous adsorption can further optimize catalytic performance of its adjacent Ni site. These studies provide an in-depth understanding in the synergistic effect of Fe-Ni toward rational design and fabrication of efficient water splitting electrocatalysts. [ABSTRACT FROM AUTHOR]