Electrocatalytic Reactions In Fuel Cells Study Of Mechanism And Low-Pt Catalysts

To address the problems of energy shortage and environmental pollution caused by our reliance on fossil fuels, many governments has committed to develop sustainable clean energy technology in the past decade. Hydrogen powered polymer electrolyte membrane fuel cell (H-PEMFC) is considered as an ideal alternative power device to traditional internal combustion engine for automobile application, due to its high electric power density, high energy conversion efficiency, low environmental impact as well as low temperature for start-up and operation. At present, the H-PEMFC requires a high loadings of precious metal platinum (Pt) as the electrocatalysts to maintain its high energy output; however, the high cost and low annual production of Pt greatly hinder the commercialization of this technology. Therefore, it is imperative to develop electrocatalytic materials with high performance and low Pt usage.In this dissertation, based on the applied electrochemistry, nanomaterials science, chemical thermodynamic and kinetic principles, and considering the key scientific problems associated with the application of H-PEMFC technology, we firstly carried out theoretical study on finding the key physical descriptor for electrocatalytic reactions related to fuel cells. Then, with the guidance of the above findings, novel electrocatalytic nanomaterial with high activity and low cost were designed and subsequently synthesized. Finally, we proposed a facile synthetic route for low-Pt catalysts in large scale. This study provided both theoretical and experimental basis for understanding the fundamental electrochemistry, and pushed forwards the application of low-Pt catalysts into practical fuel cells. The detail contents are as follows:1. For the mechanic study of anodic hydrogen reaction in fuel cell, we have proposed that the hydrogen binding energy (HBE) is not the sole reaction descriptor for hydrogen oxidation/evolution reaction (HOR/HER), and these reaction may also be governed by oxygen binding energy (OBE). Using HER as the probe reaction, we have studied the electrolyte pH effect of HER on three common weakly binding metal (Au, Cu and Ag, the metal located on the weakly binding branch of volcano plots) polycrystalline electrodes for the first time. When the electrolyte changed from acid to base, the HER activities of all three electrodes decreased. The largest variation was observed in the case of Au electrode, with 1.5 orders of magnitude decrease in exchange current density (i0). Through cyclic voltammetry (CV) and CO stripping analysis, we observed that the OBE of these weakly binding metals strongly depended on the media pH. Our results indicated the Sabatier principle could hardly explain the pH effect of HER on the weakly binding metals, and highlighted the role of working environmental in the design of electrocatalytic materials.2. For the mechanic study of cathodic oxygen reduction reaction (ORR), we have confirmed the importance of OBE in describing and controlling the ORR electrocatalytic behaviors. From a kinetic view, we systematically studied the role of OBE in bridging the ORR behaviors on two Au electrodes (polycrystalline Au electrode and nanoparticle Au electrode) and media pH for the first time. Electrochemical characterizations revealed the hydroxyl coverage, OBE, ORR activity and electron transfer number (n) all increased with the media pH. Besides, strong size effect of ORR on Au electrode was observed, again verifying the OBE theory. This work has provided theoretical guidance in structural design for promising catalytic materials.3. Based on the theoretical approach of ORR, we have designed and then constructed a trimetallic core-shell structured catalysts, denoted as Modified CuPd@Pt/C, via a combined method. A Pd interlayer was introduced to inhibit the dissolution of Cu and to enhance the electronic modification to the Pt shell. Compositional and structural characterizations indicated the core-shell structure and interaction of the metals in the synthesized catalyst. The mass activity, evaluated by rotating disk electrode (RDE) in liquid half-cell, was very close to the 2017 target value set by the U.S. Department of Energy (DOE,0.44 A/mgpt). We ascribed the enhanced ORR activity of Modified CuPd@Pt/C catalyst to the optimization of the electrochemical surface area, electronic and micro-geometric structure.4. In order to push forward the low-Pt catalyst strategy in the practical process of fuel cell, we attempted to develop the large-scale synthetic route for the Pt-based dealloyed catalyst, based on the consideration of catalytic activity, stability and cost. We proposed a microwave assisted polyol method to prepare Pt-based alloy precursor catalysts and a chemical dealloy process via acid leaching. With the synthesis of dealloyed CuxPty/C catalysts (denoted as Cux@Pty/C) as the probe experiment, we have synthesized gram-level dealloyed catalyst in laboratory by the above proposed route. Moreover, the Cux@Pty/C showed 3 times higher in mass activity than the benchmark Pt/C catalyst. This synthetic route has the advantage of short preparation period, high yield, energy saving and good repeatability, and can also be utilized for the preparation of other metal components of bimetallic or trimetallic dealloyed catalyst.