First row transition metal doped B12P12 and Al12P12 nanocages as excellent single atom catalysts for the hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a promising process to produce high purity hydrogen gas. However, the overpotential of this reaction hinders its practical applications. Single atom catalysts (SACs) are recently investigated by the scientific community to facilitate the HER. Herein, we studied the doping of late first-row transition metals on the B₁₂P₁₂ and Al₁₂P₁₂ nano-cages as SACs via density functional theory (DFT) calculations. Results show that all transition metals are chemisorbed on the support, with interaction energies ranging from −0.65 to −3.85 eV. The calculated Gibbs free energies of hydrogen evolution are −0.01, −0.06 and −0.20 eV for Ni@Al₁₂P₁₂, Ni@B₁₂P₁₂, and Co@B₁₂P₁₂, respectively, which are close to the optimum value of 0.00 eV, and comparable to the highly active Pt-based catalysts in literature. Our results indicate that the designed Ni@Al₁₂P₁₂, Ni@B₁₂P₁₂, and Co@B₁₂P₁₂ SACs are excellent candidates as noble metal-free, sufficiently stable, and highly efficient electrocatalysts for HER.     

Theoretical screening of highly efficient single-atom catalysts for nitrogen reduction based on a defective C3N monolayer

Conversion of N₂ to NH₃ through electrochemical technology is one of the most attractive and promising alternatives to the traditional Haber-Bosch method. However, exploring the promising electrocatalysts with high stability, activity and selectivity for nitrogen reduction reaction (NRR) is still an important and long-standing challenge to accelerate the green production of NH₃. Herein, through the first-principles high-throughput screening, we systematically investigated the potentiality of single transition metal (TM) anchored on defective C₃N monolayer as TM-V_CC candidates for N₂ fixation. We carried out a comprehensive screening and systematical evaluation for stability, catalytic activity and selectivity toward NRR on TM-V_CC candidates. Our results reveal that, among 26 candidates, Mn-V_CC can significantly suppress HER and exhibit the outstanding NRR activity, with the most favorable limiting potential of ?0.75 V through the distal pathway, which is better than the currently stepped catalyst Ru (0001). More impressively, such a satisfactory NH₃ conversion is primarily ascribed to the strong back-donation interactions between d-electrons of Mn atom and the anti-orbitals of N₂ molecule, as well as efficient charge transfer of electrochemical process. Our findings not only broaden the development prospect of SACs for N₂ reduction but also pave a way for rational design and rapid screening of highly active C₃N-based catalysts for NRR.     

Metal-nonmetal atom co-doping engineered transition metal disulfide for highly efficient hydrogen evolution reaction

The development of effective and non-precious electrocatalyts for hydrogen evolution reaction (HER) has attracted massive research interests. Herein, we report a density functional theory (DFT) investigation on the activation and optimization of Molybdenum disulfide (MoS₂) monolayer as efficient HER electrocatalysts by cobalt-nonmetal atom (X = B, C, N, P, Se) codoping. Our results show that three CoX-MoS₂ (X = C, N, and Se) catalysts display enhanced HER performance with |ΔG_H|s in the range of 0.12–0.23 eV. Careful electronic structure analysis manifests that the favorable H adsorption process on the MoS₂ basal plane is induced by suitable in-gap states upon codoping. Furthermore, appropriate biaxial strain can help optimize the HER performance of these co-doped systems, e.g, the ΔG_Hs of CoC@MoS₂, CoN@MoS₂, and CoSe@MoS₂ reaches 0.0 eV, ?0.04 eV, and ?0.01 eV at 1.86% tensile strain, 5% compressive strain, and 4% compressive strain, respectively. Our work offers a highly promising catalyst for HER and guides the atomic design of more efficient non-noble electrocatalysts.     

NiFe single atom catalysts anchored on carbon for oxygen evolution reaction

Single atom catalysts (SACs) can improve the efficiency of oxygen evolution reaction (OER). However, metal SACs anchored on carbon materials are relatively uncommon for the OER. In this paper, using carbon black as carrier, NiFe SACs are fabricated by one step pyrolysis method. When the weight ratio of Ni/Fe is lower than 5:3, NiFe@C exibits highly-dispersed SACs over carbon in addition to some Ni₃Fe alloy. The prepared NiFe@C 5:3 SACs showed excellent OER performance with an overpotential of 322 mV and 438 mV for current density of 10 mA/cm² and 100 mA/cm², respectively. The Tafel slope of NiFe@C 5:3 was as small as 87.6 mV/dec, indicating fast charge transfer of NiFe@C 5:3 during OER process, which was also confirmed by electrochemical impedance spectra with R_ct = 18.07 Ωcm². Meanwhile, NiFe@C 5:3 had the highest specific capacitance of 5 mF/cm² with good stability. This work provides a reference for designing electocatalyst material for efficient, stable and economical OER process.     

Molecular transition metal corrole as an efficient electrocatalyst for the heterogeneous CO2 electroreduction: A theory study

Carbon dioxide electrochemical reduction (eCO₂RR) has been regarded as an important solution for low-carbon economy. However, challenges remain for searching low-cost and high selectivity catalysts. Here, we investigated electrocatalytic activity of molecular catalysts containing transition metal single atom supported on corrole as the eCO₂RR catalysts (TM/SACC) by DFT. Various C₁ products can be produced on the 14 TM/SACC, including methane (CH₄), formic acid (HCOOH) and CO. We found CO and formic acid are major products on TM/SACC (TM = Ni, Pd, Zn, Cu, Au, Ag) at higher overpotentials, while methane are major eCO₂RR products on TM/SACC (TM = Mn, Cr, Nb, Mo, Zr, V, Ti, Cd) at lower overpotentials. Our studies indicate Mn/SACC gives high selectivity for methane formation. Due to the lowest overpotential value of 0.46 V, Mn/SACC can be a quite promising catalyst with excellent performance for reduction of CO₂ to methane along the most favorable pathway: CO₂ → COOH1 → CO1 → CHO1 → CH₂O1 → CH₂OH1 → CH₃OH1→CH₃1→CH₄1→CH₄(g), among which the hydrogenation of CHO1 to CH₂O1 and CH₃OH1 to CH₃1 and H₂O are the limiting-potential step and rate-determining step, respectively. The study shows corrole with different transition metal could adjust the catalytic performance of electrocatalysts, which offer a hopeful strategy for the design of molecular catalysts.     

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.     

Atomically dispersed Co atoms in nitrogen-doped carbon aerogel for efficient and durable oxygen reduction reaction

Developing non-noble-metal-based electrocatalysts as alternatives to replace Pt-based catalysts for oxygen reduction reaction (ORR) is crucial for large scale industrial application of fuel cells. Herein, we report a facile method to synthesize atomically dispersed Co atoms anchored on nitrogen-doped carbon aerogels with a 3D hierarchically porous network structure via F127-assisted pyrolysis of a phenolic resin/Co²⁺ composite and subsequent HCl etching treatment. HRTEM, AC-STEM, XRD, XPS, and Raman spectroscopy measurements demonstrate that Co atoms are homogeneously atomically dispersed on nitrogen-doped carbon aerogels within the porous structure by coordination with pyridinic-N. Among a series of samples, the Co-NCA@F127-1: 0.56 catalyst exhibits an enhanced ORR activity with onset potential (E_onset) of 0.935 V vs. RHE, the high diffusion limiting current density of 5.96 mA cm⁻² at 0.45 V, as well as an excellent resistance to methanol poisoning and good long-term stability in alkaline medium, comparable to the state-of-the-art Pt/C catalyst. This work may provide a novel and ingenious thought in the design and engineering of efficient and robust electrocatalysts based on single transition-metal atoms supported by nitrogen-doped carbon materials.     

Atomically dispersed low-cost transition metals catalyze efficient hydrogen evolution on two-dimensional SnO nanosheets

Two-dimensional (2D) electrocatalyst plays an important role in hydrogen production via water splitting. In this work, the first-principles calculation was used to investigate the hydrogen evolution reaction (HER) performance of transition metal (TM) single atom catalysts (SACs) on 2D SnO nanosheets. Among the TM considered (TM = V, Cr, Mn, Fe, Co, Ni, Cu and Pt), V, Fe, Co, Ni, Cu and Pt can effectively improve the catalytic activity of SnO. More importantly, the low-cost Co can exhibit promising HER performance with the Gibbs free energy as low as ~0.015 eV, which is competitive with the precious catalyst Pt. The theoretical exchange current densities of Co SACs can reach ~10⁻¹⁶ A/site. The exciting HER activity is mainly facilitated through the d-d hybridization between the TM and Sn atoms on the SnO surface, which introduces new electron states near the Fermi level. Our work highlights the complexity and diversity of the effect of TM SACs on SnO nanosheets and implies their potential applications as efficient HER electrocatalysts.     

Screening of transition metal single atom catalysts supported on B36 cluster for nitrogen fixation

Developing efficient catalysis for dinitrogen (N₂) fixation is a very important and challenging issue in chemistry. A large number of recent studies have shown that boron containing materials have good catalytic activity for nitrogen reduction reaction (NRR). Meanwhile, many theoretical and experimental studies also focus on the pure boron cluster system. The small size and extremely lack of electrons of the pure boron cluster system make its catalytic performance after being anchored with transition metals (TM) become very promising. Herein, we reported that the potential of a series of TM atoms (including Sc–Zn, Nb, Mo, Ru, Rh, Pd, Ta, W, Re, Os, Ir, Pt) anchored on B₃₆ cluster for NRR. It is found that the Re-anchored B₃₆ is the most promising candidate with a limiting potential of ?0.48 V via distal mechanism, and it also has good stability, catalytic activity and selectivity for NRR. It is worth noting that there are relatively rare researches on Re-based catalysts for N₂ fixation, our findings suggest that the Re-doped materials should receive attention for NRR.     

Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

DFT-based study of single transition metal atom doped g-C3N4 as alternative oxygen reduction reaction catalysts

In this paper, the stability and the oxygen reduction reaction (ORR) catalytic activity of single transition metal atom doped g-C₃N₄ catalysts, M-C₃N₄ (M = Mn, Fe, Co, Ni, Cu, Rh, Pd, Ag, Pt, Au), were investigated in detail by performing density functional theory (DFT) calculations. The results of binding energy reveal all M-C₃N₄ are thermodynamically stable. Further dynamic calculations demonstrate they are also dynamically stable except Au-C₃N₄. Then, through comparing the value of overpotentials, we found that most of M-C₃N₄ exhibit no ORR catalytic activity except for Ag-C₃N₄ and Pd-C₃N₄, both of which have somewhat catalytic properties but still inferior to Pt(111). It may be caused by the strong adsorption between ORR intermediates (OOH, O, OH) and M-C₃N₄. We further preformed DFT calculation for the high-valent metal complexes of g-C₃N₄ (M-OH-C₃N₄) and the significant enhancement of activity is obtained. Due to the additional OH group, the overall adsorption energies of ORR intermediates on M-OH-C₃N₄ have been decreased and become more close to those on Pt(111), and ORR mechanisms have also been changed. In addition, the overpotentials of ORR on Ni-OH-C₃N₄ and Cu-OH-C₃N₄ are much close to that on the Pt(111), indicating that they possess the catalytic activity comparable to precious Pt catalyst.     

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.     

Effect of transition metal (M: Fe,Co or Mn) for the oxygen reduction reaction with non-precious metal catalysts in acid medium

The effect of the metal for the oxygen reduction reaction (ORR) in acid medium with non-precious metal catalysts has been investigated. A series of non-precious metal catalysts with typical formulation M/N/C with M being Mn, Co or Fe have been prepared by incorporating N onto an active carbon matrix by means of thermal treatments under inert atmospheres. The N-containing active carbons were further treated with the M-containing precursors based upon Mn, Co or Fe phthalocyanines and thermally treated under inert atmosphere. The performance for the ORR in acid medium of all of the catalysts has been evaluated by means of electrochemical techniques. The activity, both in terms of onset potential for the ORR and maximum current density at representative potentials between 900 and 700 mV follows the trend Fe > Co > Mn. In addition, the performance of the Fe-based catalysts obtained during the different stages of the catalyst preparation has been also evaluated. The catalysts obtained after the pyrolysis step are the only ones showing measurable rates for the ORR. Although the amount of N and Fe incorporated onto the carbon matrix decreases the pyrolysis treatment, this treatment leads to the formation of the real active sites for the ORR irrespectively of the nature of the transition metal.     

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H₂ is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.     

Facile synthesis of 3D hierarchical mesoporous Fe-C-N catalysts as efficient electrocatalysts for oxygen reduction reaction

A salt crystal-templating synthesis route is proposed to synthesize a Fe-N-C catalyst with well-controlled mesoporous structure. In the presence of glucose, NaCl-template can efficiently tune the porous structure of catalyst and help to improve the oxygen reduction reaction (ORR) activity. The optimized catalyst possesses a hierarchical mesopore size distribution, a high Brunauer-Emmett-Teller surface area (up to 911.56 m² g^?1) and homogeneous distribution of abundant active sites. As a result, the obtained catalyst shows a desirable ORR activity in alkaline medium (half-wave potential of 0.84 V and kinetic mass activity at 0.8 V of 24.95 A g^?1), high selectivity (electron transfer number >3.92), excellent long term durability (only 16 mV negative shift of half-wave potential after 5000 potential cycles in O₂-saturated 0.1 M KOH) and good tolerance to methanol. The enhanced electrochemical performance enables the proposed catalyst to be the promising electrocatalyst candidate to commercial Pt/C towards ORR.     

Mechanistic understanding of CO2 Reduction Reaction Towards C1 products by molecular transition metal porphyrin catalysts

Designing low-cost, highly efficient, and durable electrocatalysts for carbon dioxide reduction reaction (eCO₂RR) to value-added chemicals is an appealing approach to balance the global carbon emission. Heterogeneous molecular catalysts is emerging as an important area for CO₂ utilization. Herein, a series of atomically isolated transition metal-N₄ sites anchored on porphyrin framework (TM/PRF) molecular catalysts are employed as electrocatalysts for eCO₂RR because of their well-defined structure, the versatility and relatively low cost, and remarkable activity, which allow the establishment of precise structural model for a better understanding of the CO₂ reduction mechanism. Based on density functional theory calculations and the computational hydrogen electrode model on 27 candidates, we propose a number of TM/PRF (TMs = Ni, Co, Mn, Cu, Ag, Au, Zn, Pd, Ti, V, Cr, Ta, W), which show promise of highly active and selective catalysts for carbon monoxide, formic acid, methane and methanol production, while simultaneously suppress competing hydrogen evolution reaction. Studies have shown that the TM center plays a crucial role in determining the catalytic performance of the material. And the TM/PRF molecule tends to stabilize different radical intermediates at the TM site. Thus, when reacted with a greater number of electrons, highly reduced products are formed. Among the different TM/PRF materials, Ta/PRF and W/PRF have been shown to produce CH₃OH selectively at low overpotentials (0.39 and 0.58 V), whereas Ti/PRF has been found as highly selective toward CH₄ production with low overpotential of 0.58 V, outperforming most known electrodes. The calulation of activation energy and the d-band center reveal that Cr/PRF shows higher catalystic activity in comparison with other TM/PRF. The advantages of the outstanding selectivity of products, the reduced overpotentials and the higher activity of catalyst allow these new systems TM/PRF (TMs = Ta, W, Ti, Cr) to be promising catalysts, which will motivate more fundamental mechanism studies and technological applications for eCO₂RR.     

Manganese coordinated with nitrogen in aligned hierarchical porous carbon for efficient electrocatalytic oxygen reduction reaction in alkaline and acidic medium

A Mn coordinated with N atoms aligned hierarchical porous carbon catalyst is prepared through an inorganic metal salt sublimation doping strategy. Gelatin is served as a carbon source and N source, Ca²⁺ is acted as templates to establish aligned porous structure during carbonization. MnCl₂ sublimates into gas to serve as Mn source after reaching the melting point. This method can effectively avoid the agglomeration of Mn atoms, which is beneficial to form Mn-N_x active sites. The prepared optimal catalyst exhibits a large specific surface area with an aligned hierarchical porous structure. XAFs result demonstrates that Mn coordinates with N atoms to form Mn-N_x configuration in the carbon structure. Notably, it exhibits outstanding catalytic ORR performance with a positive half-wave potential (0.86 V vs. RHE) and excellent durability, superior to Pt/C (20 wt%) catalyst under alkaline medium. Meanwhile, enhanced catalytic ORR performance and stability in an acidic medium are also achieved.     

Metallic iron doped vitamin B12/C as efficient nonprecious metal catalysts for oxygen reduction reaction

The development of efficient nonprecious metal catalysts for oxygen reduction reaction (ORR) is crucial but challenging. Herein, one simple and effective strategy is developed to synthesize bimetallic nitrogen-doped carbon catalysts by pyrolyzing Fe-doped Vitamin B12 (VB12) supported carbon black (Fe-VB12/C). A typical Fe₂₀-VB12/C catalyst with a nominal iron content of 20 wt% pyrolyzed at 700 °C exhibits remarkably ORR activity in alkaline medium (half-wave potential of 0.88 V, 10 mV positive than that of commercial Pt/C), high selectivity (electron transfer number > 3.93), excellent stability (only 6 mV negative shift of half-wave potential after 5000 potential cycles) and good methanol-tolerance. The superior ORR activity of the composite is mainly attributed to the improved mesoporous structure and co-existence of abundant Fe-N_x and Co-N_x active sites. Meanwhile, the metallic Fe are necessary for the improved ORR activity by means of the interaction of metallic Fe with neighboring active sites.     

Hydrogen evolution reaction of metal di-chalcogenides: ZrS2, ZrSe2 and Janus ZrSSe

Transition metal di-chalcogenides with stoichiometry MX₂ (M: Mo, V, W, Pt and X: S, Se) are considered as one of the non-precious and effective catalysts for the production of clean hydrogen energy via water-splitting mechanism. The major drawback of these materials is their inactive basal plane as compared to their edge sites. Recently, Janus MoSSe-a novel sandwiched structure has been synthesized and predicted theoretically to obtain increased catalytic activity by applying strain, external electric field and by creating vacancy. In this work, we have used state-of-the-art density functional theory with dispersion correction (DFT-D3) to study the catalytic activity for hydrogen evolution reaction (HER) of ZrS₂, ZrSe₂ and Janus ZrSSe. From our calculations, we conclude that among these three systems, the Janus ZrSSe ($\Delta G=1.19eV;\Delta G:Gibb\text{s}\text{free}energy$) is a good catalyst and can be utilized for HER at edge site. Janus ZrSSe shows enhanced catalytic activity at S-edge as compared to its basal plane and Se-edge site; whereas ZrSe₂ shows good catalytic activity at Se-edge rather than at the basal plane. The ZrS₂ shows good catalytic activity at S-edge. Further, we have doped Nb, Pt and W atoms in ZrS₂, ZrSe₂ and Janus ZrSSe to see their effect on catalytic activity of pristine compounds and found that the Nb-doped ZrSe₂ shows good catalytic activity for HER and is best among all considered systems with $\Delta G=0.63eV$ followed by Pt-doped ZrS₂. This study provides a theoretical basis for future application of ZrS₂, ZrSe₂ and Janus ZrSSe based catalysts for HER.     

碱性水电解非贵金属析氧催化剂的研究进展

"绿氢"能源的大规模应用依赖于电解水技术。和析氢反应(HER)相比,析氧反应(OER)是水电解过程中的关键反应,其动力学反应缓慢。目前,非贵金属OER电催化剂的活性和稳定性不佳,深入OER过程机理的理解和催化剂活性方面的研究有助于OER速率的提升。文章综述了近年来在碱性体系中对水电解制氢非贵金属析氧催化剂的研究讨论、分析和总结,主要从材料掺杂类、形貌调控类、调节电子结构类和复合结构类4个方面来阐述。