Active MoS_(2)-based electrode for green ammonia synthesis

Nitrogen electro-reduction under mild conditions is one promising alternative approach of the energyconsuming Haber-Bosch process for the artificial ammonia synthesis.One critical aspect to unlocking this technology is to discover the catalysts with high selectivity and efficiency.In this work,the N_(2)-to-NH_(3)conversion on the functional MoS_(2)is fully investigated by density functional theory calculations since the layered MoS_(2)provides the ideal platform for the elaborating copies of the nitrogenase found in nature,wherein the functionalization is achieved via basal-adsorption,basal-substitution or edge-substitution of transition metal elements.Our results reveal that the edge-functionalization is a feasible strategy for the activity promotion;however,the basal-adsorption and basal-substitution separately suffer from the electrochemical instability and the NRR inefficiency.Specifically,MoS_(2)functionalized via edge W-substitution exhibits an exceptional activity.The energetically favored reaction pathway is through the distal pathway and a limiting potential is less than 0.20 V.Overall,this work escalates the rational design of the high-effective catalysts for nitrogen fixation and provides the explanation why the predicated catalyst have a good performance,paving the guidance for the experiments.

Spatial-five coordination promotes the high efficiency of CoN_(4) moiety in graphene-based bilayer for oxygen reduction electrocatalysis: A density functional theory study

The searching of highly efficient catalysts for oxygen reduction reaction(ORR) has attracted particular attention. In this work, we construct the graphene-based bilayers BG/X that consists by the CoN_(4) embedded graphene as the upper layer and the X modified graphene as the bottom layer(X = Si, P, S). The interfacial bonding between CoN_(4) site and the X dopant is spontaneously formed due to the strong pd hybridization, which changes the Co ligand from the planar-four N_(4) coordination into spatial-five N_(4)+X one. The additive glue atom weakens too strong adsorptions of the ORR intermediates on CoN_(4) site and thereby improves the ORR activities in comparison with the monolayer counterpart. From the free energy profiles, the overpotentials η are 0.47, 0.49 and 0.45 V for BG/Si_(a), BG/P_(a) and BG/S_(a), respectively,being comparable to that of state-of-the-art Pt material. Besides, the kinetic barriers for the bilayers are less than 0.75 eV, an indicative of the room temperature activity. Furthermore, the combination of thermodynamic and kinetic analysis ensures the preference of 4e^(-)-OOH associative mechanism over 2e^(-)-H_(2)O_(2) mechanism, being beneficial for membrane stability against the H_(2)O_(2) corrosion. Therefore,the graphene-based bilayers deliver the high efficiencies for oxygen reduction electrocatalysis.Therefore, the interfacial bonding in the graphene-based bilayers provides an interesting strategy to suppress the poisoning phenomenon for the material design from atom scale.

Exploring single atom catalysts of transition-metal doped phosphorus carbide monolayer for HER:A first-principles study

Hydrogen has been identified as one of the most promising sustainable and clean energy. Developing hydrogen evolution reaction(HER) catalyst with high activity is essential for satisfying the future requirements. Considering novel advantages of two-dimensional materials and high catalytic activity of atomic transition metal, in this study, using density functional theory calculation, the HER on single transitionmetal(23 different TM atoms) doped phosphorus carbide monolayer(α-PC) has been investigated. The Volmer–Tafel and Volmer–Heyrovsky reaction mechanisms, and the stability of the most promising HER catalyst are also included. The results show that Ir-αPC with high physical and thermal stability has the most optimal value of Gibbs free adsorption energy for H atom. The relationship of d band center and the HER activity shows a volcano-like curve. The calculation of reaction energy barrier indicates that the Volmer-Heyrovsky step is more favorable than the Volmer-Tafel step.

Significantly improved hydrogen storage behaviors in MgH_(2) with Nb nanocatalyst

The study explores the excellent modification effect of Nb nanocatalyst prepared via surfactant assisted ball milling technique(SABM)on the hydrogen storage properties of MgH_(2).Optimal catalyst doping concentration was determined by comparing onset decomposition temperature,released hydrogen capacity,and reaction rate for different MgH_(2)-ywt%Nb(y=0,3,5,7,9)composites.The MgH_(2)-5wt%Nb composite started releasing hydrogen at 186.7℃ and a total of 7.0wt%hydrogen was released in the dehydrogenation process.In addition,5wt%Nb doped MgH_(2) also managed to release 4.2wt%H_(2) within 14 min at 250℃ and had the ability to absorb 4.0wt%hydrogen in 30 min at 100℃.Cycling tests revealed that MgH_(2)-5wt%Nb could retain 6.3wt%H_(2) storage capacity(89.2%)after 20 cycles.Dehydrogenation and hydrogenation activation energy values were decreased from 140.51±4.74 and 70.67±2.07 kJ·mol^(−1) to 90.04±2.83 and 53.46±3.33 kJ·mol^(−1) after doping MgH_(2) with Nb,respectively.Microstructure analysis proved that homogeneously distributed NbH acted as active catalytic unit for improving the hydrogen storage performance of MgH_(2).These results indicate SABM can be considered as an option to develop other nanocatalysts for energy related areas.

In-situ surface decoration of RuO_(2) nanoparticles by laser ablation for improved oxygen evolution reaction activity in both acid and alkali solutions

Improving the OER activity of noble metal-based materials is of profound importance to minimize the usage of noble metals and lower the cost.Here,we report considerable improvement on the catalytic activity of RuO_(2) particles for OER in both alkali and acid environments.The RuO_(2) nanoparticles were treated with a method of pulse laser ablation.Numerous Ru and RuO_(2) clusters were generated at the surface of RuO_(2) nanoparticles after the laser ablation,forming a lychee-shaped morphology.The larger pulse energy RuO_(2) nanoparticles are treated with,the better the OER activity can be.DFT calculations shows that the surface tension induced by the lychee-shaped morphology benefits the OER performance.Our best sample gives an overpotential of 172 mV(at 10 mA cm^(-2))and a Tafel slope of 53.5 mV dec^(-1) in KOH,while an overpotential of 219 mV and a Tafel slope of 44.9 mV dec^(-1) in H_(2)SO_(4),which are of topclass results.This work may inspire a new way to develop high-performance electrocatalysts for OER.

Ultralow-temperature assisted synthesis of single platinum atoms anchored on carbon nanotubes for efficiently electrocatalytic acidic hydrogen evolution

Although platinum(Pt) is highly active for hydrogen evolution reaction(HER)[1], it is crucial to explore the effective approach for minimizing the Pt loading amount in the practical application. Herein, one ultralow-temperature solution reduction approach is developed to anchor atomically dispersed Pt atoms on carbon nanotubes(Pt-CNTs), which decelerates the diffusion rate of Pt Cl2-6 ion reached onto the carbon nanotubes and lowers the free energy of Pt atoms in the solution to reduce the probability of the Pt aggregation. The obtained Pt-CNTs exhibits a low overpotential of 41 mV@10 mA cm^(-2) for HER in acidic media. The calculation results revealed that the improvement of the electrocatalytic activity is contributed by the interaction between CNTs and Pt atoms, which descreases the the Pt d band cneter referred to the Fermi level and lowers the Gibbs free energy of H*adsorption. This work may provide one easy and convenient strategy for the large-scale use of Pt catalysts in practical applications.

The effect of different Co phase structure(FCC/HCP)on the catalytic action towards the hydrogen storage performance of MgH_(2)

High hydrogen desorption temperature and sluggish reaction kinetics are the major limitations for the practical application of MgH_(2).In this study,Co particles with a face centered cubic(FCC)structure and a hexagonal close packed(HCP)structure were prepared facilely and proved to be good catalysts for magnesium hydride.Co particles with FCC structure presented better catalytic effect on MgH_(2)than that with HCP structure.Both 7%(mass)Co FCC and HCP particle modified MgH_(2)decreased the initial dehydrogenation temperature from 301.3℃ to approximately 195.0℃,but 7%(mass)Co with FCC structure modified MgH_(2)has a faster desorption rate,and around 6.5%(mass)H_(2)was desorbed in 10 min at325℃.Hydrogen uptake was detected at 70℃ under 3.25 MPa hydrogen pressure and 6.0%(mass)H_(2)was recharged in 40 min at 150℃.The hydrogen desorption and absorption activation energy for 7%(mass)FCC Co modified MgH_(2)was significantly decreased to(76.6±8.3)kJ·mol^(-1) and(68.3±6.0)kJ·mol^(-1),respectively.Thermodynamic property was also studied,the plateau pressures of MgH_(2)+7%(mass)FCC Co were determined to be 0.14,0.28,0.53 and 0.98 MPa for 300℃,325℃,350℃ and375℃.The decomposition enthalpy of hydrogen(ΔH)for MgH_(2)+7%(mass)FCC Co was(80.6±0.1)kJ·mol^(-1),5.8 kJ·mol^(-1)lower than that of as-prepared MgH_(2).Moreover,cycling performance for the first20 cycles revealed that the reaction kinetics and capacity of MgH_(2)-FCC Co composite remained almost unchanged.The result of density functional theory calculation demonstrated that cobalt could extract the Mg AH bond and reduced the decompose energy of magnesium hydride.Our paper can be presented as a reference for searching highly effective catalysts for hydrogen storage and other energy-related research fields.

Transition metal decorated bismuthene for ammonia synthesis:A density functional theory study

The electrochemical nitrogen reduction reaction(NRR)for the ammonia production under ambient conditions is regarded as a sustainable alternative to the industrial Haber-Bosch process.However,the electrocatalytic systems that efficiently catalyze nitrogen reduction remain elusive.In the work,the nitrogen reduction activity of the transition metal decorated bismuthene TM@Bis is fully investigated by means of density functional theory calculations.Our results demonstrate that W@Bis delivers the best efficiency,wherein the potential-determining step is located at the last protonation step of^(*)NH_(2)+H^(+)+e^(-)→*NH_(3)via the distal mechanism with the limiting potential ULof 0.26 V.Furthermore,the dopants of Re and Os are also promising candidates for experimental synthesis due to its good selectivity,in despite of the slightly higher ULof NRR with the value of 0.55 V.However,the candidates of Ti,V,Nb and Mo delivered the relative lower ULof 0.35,0.37,0.41 and 0.43 V might be suffered from the side hydrogen evolution reaction.More interestingly,a volcano curve is established between ULand valence electrons of metal elements wherein W with 4 electrons in d band located at the summit.Such phenomenon originates from the underlying acceptance-back donation mechanism.Therefore,our work provides a fundament understanding for the material design for nitrogen reduction electrocatalysis.

Dopant-vacancy activated tetragonal transition metal selenide for hydrogen evolution electrocatalysis

Hydrogen production from water electrolysis using renewable electricity is a highly promising route to solve the energy crisis of human society. The tetragonal 3d-transition metal selenide with metallic feature has been discovered to efficiently catalyze the hydrogen evolution electrocatalysis;however, its performance is still unsatisfactory and further improvement is necessary. Herein, the hydrogen evolution reaction of the functional tetragonal 3d-transition metal selenide with the heteroatom-dopant as well as cationic vacancy is fully investigated by means of density functional theory calculations. Our results identify 53 promising candidates endowed with good activity due to the absolute free energy of hydrogen adsorption |ΔGH| ≤ 0.30 eV wherein 15 candidates with |ΔG_H| ≤ 0.09 eV possess compelling performance in comparison with the benchmark Pt material. Interestingly, the functional CuSe systems account for 29out of 53 candidates, being high attractive for experimental synthesis. According to the analysis of electronic structure, the enhanced performance stems from the upshift of the sp orbitals, which benefits for the improved affinity toward hydrogen capture. This work provides new direction and guidance for the design of novel electrocatalysts.

Investigation and optimization of high-valent Ta-doped SrFeO_(3-δ)as air electrode for intermediate-temperature solid oxide fuel cells

To explore highly active and thermomechanical stable air electrodes for intermediate-temperature solid oxide fuel cells(ITSOFCs),10mol%Ta5+doped in the B site of strontium ferrite perovskite oxide(SrTa_(0.1)Fe_(0.9)O_(3-δ),STF)is investigated and optimized.The effects of Ta^(5+)doping on structure,transition metal reduction,oxygen nonstoichiometry,thermal expansion,and electrical performance are evaluated systematically.Via 10mol%Ta^(5+)doping,the thermal expansion coefficient(TEC)decreased from 34.1×10^(-6)(SrFeO_(3-δ))to 14.6×10^(-6) K^(-1)(STF),which is near the TEC of electrolyte(13.3×10^(-6) K^(-1) for Sm_(0.2)Ce_(0.8)O_(1.9),SDC),indicates excellent thermomechanical compatibility.At 550-750℃,STF shows superior oxygen vacancy concentrations(0.262 to 0.331),which is critical in the oxygen-reduction reaction(ORR).Oxygen temperature-programmed desorption(O_(2)-TPD)indicated the thermal reduction onset temperature of iron ion is around 420℃,which matched well with the inflection points on the thermos-gravimetric analysis and electrical conductivity curves.At 600℃,the STF electrode shows area-specific resistance(ASR)of 0.152Ω·cm^(2) and peak power density(PPD)of 749 mW·cm^(-2).ORR activity of STF was further improved by introducing 30wt%Sm_(0.2)Ce_(0.8)O_(1.9)(SDC)powder,STF+SDC composite cathode achieving outstanding ASR value of 0.115Ω·cm2 at 600℃,even comparable with benchmark cobalt-containing cathode,Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ)(BSCF).Distribution of relaxation time(DRT)analysis revealed that the oxygen surface exchange and bulk diffusion were improved by forming a composite cathode.At 650℃,STF+SDC composite cathode achieving an outstanding PPD of 1117 mW·cm^(-2).The excellent results suggest that STF and STF+SDC are promising air electrodes for IT-SOFCs.