A Study Of Non-Pt Catalysts For Electrochemical Hydrogen-Water Interconversion

In the future energy system using renewable energy (e.g., solar energy, wind energy, tidal energy) for electricity generation, highly-efficient conversion and high-density storage of electricity is an enabling technology for energy distribution. For instance, the electricity can be converted to chemical energy for convenient storage and transportation, and the chemical energy can be converted back to electricity when necessary. A clean and efficient implementation of these energy conversions is the electrochemical hydrogen-water interconversion (H2+O2(?) H2O). Specifically, the electricity can be converted to chemical energy stored in H2 and O2 through water electrolysis, and these two molecules can be re-combined into H2O to output electricity through fuel cells. Apparently, the development of efficient, low-cost water electrolysis and fuel cell technology is of great significance for such a renewable and clean energy system.The state-of-the-art technology of fuel cells and water electrolysis is based on proton-exchange-membrane electrolytes. Albeit compact in size and high in power density, these devices rely severely on catalysts made of Pt, a highly scarce resource, which has been preventing them from widespread applications. Exploring non-Pt catalysts for both polymer electrolyte fuel cells and polymer electrolyte water electrolysis is the key to addressing this issue, also is the research subject of this thesis. In fuel cell related studies, I focus on Pd-based catalysts, aiming at designing high-performance Pd catalysts through morphology tailoring and electronic structure manipulating. In the study of water electrolysis, I work on developing a new technology based on alkaline polymer electrolyte, and have eventually realized a prototype of this new device. The main progress of this research includes the following four aspects:1. The morphology-activity relationship of Pd catalysts for fuel-cell cathode. In the study of electrochemical deposition of Pd, I found that the morphology of Pd can be changed from nanoparticles to nanorods by controlling the deposition rate. More importantly, such a morphology change had lead to a ten-fold increase in the catalytic activity (CA) of Pd toward the oxygen reduction reaction (ORR), a performance comparable to that of Pt catalysts. According to the electrochemical fingerprint obtained from CO-stripping experiments, it was proved that the morphology of Pd nanorods features the exposure of{110} facets. Density functional theory (DFT) calculations showed that the adsorption energy of oxygen atoms on Pd(110) is exceptionally small, violating the popular theory of d-band center. Such a weak binding to oxygen species renders the Pd(110) a high CA toward the ORR, which may explain the observed peculiarity of Pd nanorods. This study reveals an unknown nature of Pd nano-catalysts, and provides a new guideline for the rational design of Pd catalysts through morphology tailoring.2. Manipulation of the surface reactivity of Pd alloy catalysts for fuel cell anode. In order to manipulate the surface reactivity (SR) of Pd through the ligand effects of alloying,I synthesized a series of Pd-Cu and Pd-Au alloys with controlled ratios through electrochemical co-depositions. The study showed that, by alloying with Cu or Au, the SR of Pd can be linearly raised or reduced, respectively. I chosed the formic acid oxidation reaction (FAOR) and the hydrogen oxidation reaction (HOR), two typical anodic reactions of fuel cells, to investigate how the change of SR affected the CA. For the FAOR, although Pd is the best catalyst, its CA can be further enhanced by alloying with Cu, indicating that the rate-determine step of FAOR is the dissociative adsorption of formic acid molecules. Therefore, enhancing the SR is a design principle for Pd catalysts toward the FAOR. For the HOR, alloying with Au, on the contrary, can increase the CA of Pd, suggesting that the oxidative desorption of Hads is the rate-determine step of HOR. Thus the corresponding design principle is to reduce the SR of Pd.3. Study of catalysts for alkaline polymer electrolyte water electrolysis. As the electrochemical co-deposition of Ni-Fe anode catalysts is of low efficiency and energy consuming, and difficult for large-scale manufacture, I invented a new method, solid-state electrochemical reduction, to prepare Ni-Fe catalysts by directly electrochemical reduction of the solid precursors cast on the electrode substrate, and optimized many technical parameters including the electrode substrate, the Ni/Fe ratio, metal loadings, and reduction conditions. The Ni-Fe anode catalyst thus obtained delivers a current density of 370 mA/cm2 with an anodic polarization of 300 mV at room temperature, the highest performance record in the literature. As for the Ni-Mo cathode catalyst, in order to meet the requirements of membrane-electrode assembly, I invented a new method involving the co-precipitation of Ni-Mo precursors and a subsequent hydrogen reduction step. This method can readily produce high Ni-Mo loading cathodes. The resulting Ni-Mo cathode with a metal loading of 40 mg/cm2 gives a hydrogen evolution current density of 350 mA/cm2 at 100 mV polarization at room temperature, which is the best performance reported in the patents.4. Development of prototype devices of alkaline polymer electrolyte water electrolysis. Different from the traditional alkaline water electrolysis, alkaline polymer electrolyte water electrolysis requires special technology for the alkaline membrane-electrode assembly. I have investigated the preparation of diffusion layers for the anode and the cathode, the preparation of catalyst layers with polymer electrolyte impregnated, the hot pressing of membrane-electrode composite, and the operating conditions for this new device. By using a nickel-foam supported Ni-Fe anode, a stainless-steel-fiber-felt supported Ni-Mo cathode, and an alkaline polymer electrolyte (quaternary ammonium polysulfone) developed in our group, I have successfully assembled a prototype device of alkaline polymer electrolyte water electrolysis which works at 70℃with an electrolytic current density of 600 mA/cm2 at a cell voltage of 1.88 V. Thus far, this device can only work stably at 50℃because cross-linked polymer electrolytes have not been applied and membrane-electrode assembly technology is still not optimized.