Density Functional Theory Investigation Of Several Catalysts Towards Hydrogen Energy

Since the 1950s,the increase in population and advances in technology have resulted in an unprecedented increase in global energy consumption.Nowadays,fossil fuels such as coal,oil and natural gas account for more than 80%of global energy consumption.A series of en-vironmental pollution and energy crisis problems coursed by excessive consumption of fossil fuels are becoming more and more serious.In order to improve this situation,the development of clean and efficient renewable energy to replace fossil fuels has become the focus of current research.Hydrogen has attracted people's attention as a very promising renewable energy car-rier due to its high energy density and environmental friendliness.However,the large-scale use of hydrogen energy is currently restricted by catalysts,and a large amount of work is focused on the development of catalysts.In this context,we adopted density functional theory methods to design a series of hydrogen energy catalysts.The main contents are divided into two parts:1.Design of formic acid decomposition catalyst.Formic acid has low toxicity,is liquid at room temperature,has high mass and volume hydrogen storage capacity,and is a very prom-ising hydrogen storage material.Hydrogen production under mild conditions via formic acid decomposition is promising for use in fuel cell vehicles.In this situation,the hydrogen needs to be released and utilized on demand.In order to achieve high efficiency and high purity hydrogen production with formic acid,it is necessary to develop formic acid decomposition catalysts with high catalytic activity and high selectivity.We discussed the influencing factors of the turnover frequency,and proposed two design directions for formic acid decomposition catalysts:increasing the number of active sites and decreasing the rate control step.According to theoretical calculation results,the optimal par-ticle diameter of catalyst is confirmed as 2.4 nm.Combining experiments and our calculations,the initial turnover frequency of each catalyst is related to the surface energy to understand the catalytic activity trend and mechanism.It is found that regulating the surface energy of the catalyst can change the energy barriers of O-H bond cleavage and bi-HCOO*rearrangement,thereby improving the catalytic activity.The obtained design principle takes into account the structure and surface physicochemical properties,clarifies the potential source of activity of catalyst,and provides a guiding method for the design of high efficiency catalysts in other applications.In order to further improve the catalytic activity of Au Pd formic acid decompo-sition catalyst,we introduced a third element on the basis of the Au Pd to fine tune its perfor-mance.It is found that doping small amount of Pt atoms to the surface can change the initial adsorption site of HCOOH*,break the symmetry of bi-HCOO*adsorption configuration and weaken the Pd-O,leading to the simultaneous promotion of O-H bond cleavage and bi-HCOO*rearrangement,and reducing the energy barrier of rate controlling step.In addition,we found that the special arrangement of Pd atoms on the Au₃Pd₁ ordered phase surface can decompose formic acid via HCOO^-pathway.By achievement of O-H bond by formic acid ionization in the solution,the problem that promoting the rearrangement of bi-HCOO*will inhibit the O-H bond breakage is solved.The energy barrier of rate controlling step of dehy-drogenation and dehydration pathway on Au₃Pd₁ is 0.67 e V and 0.94 e V,respectively.The design method of using ordered phase to achieve high activity formic acid decomposition via HCOO^-pathway is expected to become a basis prototype for exploring the design of formic acid decomposition catalyst.2.Design of electrochemical water splitting catalyst.The electrochemical water splitting method is an attractive energy conversion technology,which can store and release renewable intermittent energy such as solar energy and wind energy on demand.At present,noble metal catalysts are the most effective water splitting catalysts,but the high cost of noble metals limits its large-scale application.The development of non-noble metal catalysts with high activity and stability to reduce costs has become a research hotspot in the field of water splitting cata-lysts.We found that Cr-doped NiFe oxyhydroxide has high catalytic activity for oxygen evo-lution reaction,and the theoretical overpotential is only 0.29 V.This improvement in catalytic performance comes from the fact that Cr doping weakens the Fe-O bond around the active site of Fe atoms,reduces the valence of Fe,promotes the bonding and electron transfer between Fe atoms and O*.Therefore,the adsorption of the intermediate O*is enhanced,and the reac-tion energy of the O*generation step is decreased.The Cr-doped NiFe oxyhydroxide exhibits semi-metallic and has good electrical conductivity,which is good for the electrons transfor-mation during the electrochemical process.In addition,we designed a three-functional catalyst composed of Co-doped Fe₃C and nitrogen doped graphite.The carbon atoms adjacent to pyr-idine nitrogen are used as active sites for oxygen reduction reactions and hydrogen evolution reaction,and the Fe-Co site in the Co-doped Fe oxyhydroxide generated at the oxidation po-tential is used as the active site for the oxygen evolution reaction.By combining with Co doped Fe₃C,the intra-layer?bonds of nitrogen doped graphite are weakened,resulting in strong intermediate adsorption and low overpotential of oxygen reduction reaction and hydrogen evo-lution reaction.The appearance Fe-Co sites derived from the Co doped improves the adsorp-tion of OOH*intermediate during the oxygen evolution reaction,which results in low reaction energy of potential determining step and good catalytic activity.This designed catalyst has oxygen reduction reaction,hydrogen evolution,and oxygen evolution catalytic activity with the theoretical overpotential of 0.42V,0.14 V,and 0.39V,respectively.Finally,we designed Cu₅Zr₁ intermetallic compound catalyst for hydrogen evolution reaction.This catalyst has a good structural stability.The influence of the real solution environment on the adsorption state of the catalyst surface is considered in our calculation,and it is found that the free energy of H atom adsorption of Cu₅Zr₁ is-0.05 e V,which is close to Pt.Our results indicate that Cu₅Zr₁is a good potential candidate for a low-cost hydrogen evolution reaction.