Theoretical Calculation Studies On The Electrocatalytsis In Hydrogen-Oxygen Fuel Cell

Hydrogen-Oxygen fuel cell is an important component in hydrogen economy, in which noble metal such as Pt and its alloy has been generally used as catalysis. The key in the large-scale application of fuel cells is to improve the activity and stability of catalysis and to reduce the utilizing of Pt. The design of catalysts depends on the deep understanding of reaction mechanism. Despite of the decades of extensive and intensive research, there remain a number of fundamental problems in fuel cell electrocatalysis. First-principle based theoretical calculation is one of the most effective method in studying such problems.Since there is still no clear understanding of the catalysis surface under reaction condition, large deviation exists between the model of the theoretical calculation and the practical situation, leading to inconvincible results. In this thesis, we built the theoretical model aiming at finding the exact potential dependent adsorption-coadsorption structure on the catalysis surface. Based on this, the mechanism and kinetic of hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) has been carefully studied.1. The mechanism and kinetics of HER&HOR on Pt-based catalysis surface.H* adsorption isotherm (the relation between H* coverage and potential) among the whole potential region (underpotential or overpotential region) under different pH has been built. Meanwhile, the kinetic function associated with acid-base neutralization reaction, mass transport of H+ and OH- and surface reaction has been proposed. The combining of kinetic function and H* adsorption isotherm gives potential-current relation associated with Heyrovsky-Volmer and Tafel-Volmer mechanism. With the Density Functional Theory (DFT) calculation, the cyclic voltammetry (CV) results on underpotential region and the fitted HER&HOR polarization curve, the following conclusion is made:(1)HER&HOR follows Heyrovsky-Volmer mechanism; (2)Under different pH, Heyrovsky reaction have two reaction pathway, namely, H2O and H3O+ as H doner, respectively (We name them H2O pathway and H3O+ pathway for convenience). It is found that the standard reaction kinetic constant of H2O pathway is two order of magnitude lower than H3O+ pathway; (3) In strong base, the HER&HOR mainly follows H3O+ pathway, while in strong acid and weak base, HER&HOR follows H2O pathway. This is different from the traditional understanding that in neutral and alkaline reaction HER&HOR goes through H2O pathway.The implication of the above conclusion towards the design of catalysis has been analyzed, which explains the HER active improvement method in experimental such as PtRu alloy and modifying Pt(111) with Pt island. The mechanism is that on the potential region that HER is taken place in base, the H* adsorption on Pt is too strong that it is hard to desorb. Exchanging Pt with PtRu, the desorption will be easier which accelerates HER. Besides, modifying Pt(111) surface with some island will provide some sites where H* is more prone to desorb, which also accelerates HER.2. The co-adsorption structure of O species in Pt(111) surface.Combing Canonical Monte Carlo (MC) simulation and DFT calculation, gradual dissociation process from water bilayer towards half dissociated water bilayer on Pt(111) surface has been studied. The results turns to be in good match with the butterfly peaks on the Pt(111) oxygen region of CV; Through the analysis of the change of energy and configurational entropy, the water repulsion between water bilayer and the surface has been introduced into the explanation of the butterfly peak forming in CV curve.Through the mean field DFT calculation, we study about the continuing oxidation of half dissociated water bilayer and O* cluster phase. We found that the kinetic barrier of O* formation from half dissociated water bilayer is high. However, once an O* forms, the OH* surrounding will be quite unstable. Thus lessening the barrier of continuing forming 0* cluster. This properties lead to nearly no current response in the potential region of 0.85 to 0.95 V and a wing peak around 0.95 V. This picture can explain the nucleation and growth mechanism found in experiments.3. The ORR mechanism on Pt(111) surface.Under the conclusion of the second part. We found out there are two kinds of adsorption phase on Pt(111) surface under ORR conditions. Based on this, we study adsorption phase dependent reaction pathway through DFT. The conclusion has been give below:(1) O* cluster phase has great hinder effect towards ORR. Thus below 1.1 V, ORR mainly takes place through the region covered with OH*/H20* network; (2) Under the phase of OH*/H2O* network, the rate determine step of ORR is the proton coupled electron transfer reaction from O2 to OOH* with an OH* becoming H2O*; (3) The origin of the high overpotential of ORR on Pt is the formation of a totally covered O* cluster phase on Pt above 1.07 V, which compeletely poisons the catalyst abd hinders ORR; (4) The altering trend of Tafel slope is caused by the potential dependent O* coverage.4. The volcano relation of ORR under Pt-base catalysis.Combining with the results in section 2 and 3, the shortage of current volcano plot has been analysted. It is suggested that the formation of the top of volcano plot was not the altering of the rate determine step but the catalyst based co-effect between the activation energy of the rate determine step and the active site on the surface. Based on this, a new type of volcano plot based on the co-effect of co-adsorption structure and the rate of ORR on single site has been proposed.