| Fuel cell is an environment friendly energy conversion device and its development can date back to the nineteenth century. The first references to fuel cells appeared in 1838, physicist William Grove used Platinum black as electrode catalyst material to invent a simple hydrogen fuel cell and he published his research at December 1838. In the latter two hundred years especially since the 1950s, fuel cells are widely used in military, space, power plants, motor vehicles, mobile equipment, household and other fields. Depending on the electrolyte, fuel cells can divide into AFC (Alkaline fuel cell), PAFC (Phosphoric acid fuel cell), MCFC (Molten Carbonate fuel cell), SOFC (Solid Oxide fuel cell) and PEMFC (Proton exchange membrane fuel cell). PEMFC is the most promising one owing to many advantages such as environment friendly, high energy conversion rate, safety, easy assembly and maintenance. In the archetypical hydrogen-oxide proton exchange membrane fuel cell design, a proton-conducting polymer membrane (the electrolyte) separates the anode and cathode sides. On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons; on the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water. The traditional electrode catalysts for hydrogen fuel cell are Pt and its alloy. According to previous research, Pt-based materials have lower overpotential for both anode (HOR/HER) and cathode (ORR). However, Pt itself has significant disadvantages such as increasingly high price and instability during electrode cycling. Therefore, reaching for high efficiency, safety and cheap electrode catalysts which can rival Pt become increasingly important.In the past years, scientists have made some progress in developing cathode and anode catalysts which can replace Pt. For the cathode ORR reaction, the most promising materials are non-precious metal Me/N/C. This kind of material is first discover by Jasinski in 1964, in the following half century its catalytic performance and synthetic method both been improved. Recently years, some research groups declare their newly synthetic catalysts performance approaching platinum in both activity and stability. However, the nature of Me/N/C catalyst is still controversial and we can only make some hypothesis from experimental phenomena. All the experiment results reveal a MeN4 center is necessary for ORR process. The center metal can be some Co, Fe, Cu, Ni and among all the transition metal, Fe is the most promising one. For the anode HOR/HER reaction, precious metals such as Pt Ru Pd Au are still the optimal catalysts and Pt makes the best performance. However, owing to the high price of Pt, people gradually their pay attention to another cheaper metal Pd. It has been reported on experiments about the HER activity of Pd and the catalytic property of Pd is similar to platinum. Apart from its catalytic activity, another notable feature for Pd is CO poisoning immunity which makes Pd the most expected HER catalyst.Although researchers make much process in finding electrode catalysts for both cathode and anode, the reaction mechanism on Fe/N4 and Pd(111) are still unknown. The lack of theoretical data from atomic level is the key factors limiting the development of experiment. Therefore, there is strong need for theory to complement experimental efforts and help provide a fundamental understanding of electrochemistry phenomena. In this article, we aim to predictive basis for the design and the optimization of electrocatalysts with the help of first-principles method and a new theoretical model for soild/solution interface. The solid/liquid interface was described by using the periodic continuum solvation model based on the modified Poisson-Boltzmann equation (CM-MPB), which can take into account the long-range electrostatic interaction due to the solvation of electrolyte. For the short-range electrostatic interaction we add explicit water molecule as the first solution shell. In our approach, the surface is explicitly polarized by adding/subtracting charges and the counter charge is distributed as point charge in 3D-grid according to the modified Poisson-Boltzmann equation. The absolute electro chemical potential of the system can be calculated by computing the work function in solution and then referring it to the experimental work function of the standard hydrogen electrode. To study an electrochemical reaction under a constant potential as that encountered in experiment, we did a series of calculations with different surface charges for both the IS and the TS. The reaction barrier can then be obtained for each fixed charge condition. Next, we need to link the computed surface charge with the electrochemical potential. TS researching method used here is constrained Broyden dimer method which is newly developed by our group. In the new method, the reaction path searching starts from an initial state without the need for preguessing the TS-like or final state structure. It also reduces the computational cost in calculating the Hessian.The nature of Fe/N/C catalysts is still on debate. Experiment related the current density of ORR to the amount of in-plane FeN4 centers using Mossbauer spectra and a linear correlation was identified. FeN4 species also been detected in the catalyst prepared using Fe acetate, Fe porphyrin and NH3 as the precursor by other technique. Here we design two kinds of activity site namely four-coordinated FeN4 center and five-coordinated Fe(X)N4 and study their ORR behavior respectively. Our calculation results indicated ORR on FeN4 prefer 4-electron pathway and the energy barrier is 0.55eV lower than 2-electron pathway. The rate-determining step for 4-electron pathway lie in H2O desorption, besides, O-OH dissociation also has a high barrier. We also comparing adsorption energy of CO and O2, the results show this kind of FeN4 center is CO immunizing. We choose CN, OH, NH2 as the representative of X group in Fe(X)CN to investigate their ORR process and find rate-determining step for 4-electron pathway is 0-0 bond breaking.2-electron pathway can take place on Fe(CN)N4 marginally, the H2O2 productivity is 0.1%. Finally, we discuss the potential dependent ORR kinetics on the Fe(CN)N4 centers. Combining with the Kinetic formula, we calculation Tafel slop at low overpotential is 60mV which is identical with experiment.We use a six-layer slabs with adsorbates on both sides of the Pd(111) surfaces to simulate HER process. First, H coverage is taken into account. H coverage is highly dependent on the applied electrochemical potential. At PH=0, H coverage is 1ML on Pd(111) around equilibrium potential and when elevate the potential H coverage increasing. At PH=13 alkaline environment, H coverage behavior is similar with acid solution. HER on Pd(111) has two possible reaction pathway namely Tafel and Heyrovsky. Our results indicated barrier of Tafel pathway is 0.8leV which is 0.04eV higher than Heyrovsky process at acid medium. Both of them lower than HER on Pt(111) surface (0.92eV for Tafel and Heyrovsky). The calculated α is 0.08,0.53 for Tafel and Heyrovsky which is according to general electrochemistry law. At High potential area, HER is dominated by Heyrovsky pathway. At alkaline medium, barrier of Tafel step is 0.88eV at equilibrium potential and a is 0.1 which is similar with acid medium. Comparing the results on Pd(111) with other precious metal such as Pt(111), Pt(100), Au(111), we find Pd(111) is an eligible catalytic material.With the aim to explain the electrochemistry behavior observed by experiment from atomic level, we investigate two important reactions for hydrogen fuel cell with the help of DFT method. The thermodynamics and kinetics of ORR and HER on FeN4 and Pd(111) are clarify here which make sense to design new cathode/anode catalysts in the future work. |
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