Anja Bieberle-Hütter

Modelling Study of Surface Reactions, Diffusion, and Spillover at a Ni/YSZ Patterned Anode

This paper presents a modelling study of the electrochemical hydrogen oxidation reaction at nickel/yttria-stabilized zirconia (Ni/YSZ) patterned anodes. An elementary kinetic reaction-diffusion model accounts for coupled heterogeneous chemistry and transport on the Ni and YSZ surfaces. Charge transfer is modeled as a spillover of adsorbates between the Ni and YSZ surfaces at the three-phase boundary (TPB). No a priori assumptions on rate-determining processes are made. Thermodynamic, kinetic, and transport parameters are compiled from various literature sources serving as a base for quantitative simulations. Seven different spillover reaction pathways of the hydrogen oxidation reaction are compared to experimental patterned anode data obtained previously by Bieberle et al. (J. Electrochem. Soc., 148, A646 2001) under a range of operating conditions. Only one reaction pathway, based on two hydrogen spillover reactions, is able to describe consistently the complete experimental data set. A sensitivity analysis for this case allows identification of rate-determining processes. Surface concentrations close to the TPB are predicted to differ from the concentration derived from thermodynamical equilibrium by up to 2 orders of magnitude. The simulation results and the validity of the model are critically discussed. Directions for future theoretical and experimental studies for elucidating the mechanistic details of Ni/YSZ anodes are given.

Modeling and Simulations in Photoelectrochemical Water Oxidation: From Single Level to Multiscale Modeling.

This review summarizes recent developments, challenges, and strategies in the field of modeling and simulations of photoelectrochemical (PEC) water oxidation. We focus on water splitting by metal-oxide semiconductors and discuss topics such as theoretical calculations of light absorption, band gap/band edge, charge transport, and electrochemical reactions at the electrode-electrolyte interface. In particular, we review the mechanisms of the oxygen evolution reaction, strategies to lower overpotential, and computational methods applied to PEC systems with particular focus on multiscale modeling. The current challenges in modeling PEC interfaces and their processes are summarized. At the end, we propose a new multiscale modeling approach to simulate the PEC interface under conditions most similar to those of experiments. This approach will contribute to identifying the limitations at PEC interfaces. Its generic nature allows its application to a number of electrochemical systems.

Fabrication of Micro Solid Oxide Fuel Cell by Thin Film Processing Hybridization: I. Multilayer Structure of Sputtered YSZ Thin Film Electrolyte and Ni-Based Anodes deposited by Spray Pyrolysis

Physical properties of sputtered YSZ thin film electrolytes on anode thin film by spray pyrolisis has been investigated to realize the porous electrode and dense electrolyte multilayer structure for micro solid oxide fuel cells. It is shown that for better crystallinity and density, YSZ need to be deposited at an elevated temperature. However, if pure NiO anode was used for high temperature deposition, massive defects such as spalling and delamination were induced due to high thermal expansion mismatch. By changing anode to NiO-CGO composite, defects were significantly reduced even at high deposition temperature. Further research on realization of full cells by processing hybridization and cell performance characterization will be performed in near future.

Electrical conductivity and crystallization of amorphous bismuth ruthenate thin films deposited by spray pyrolysis

Amorphous oxide thin films with tailored functionality will be crucial for the next generation of micro-electro-mechanical-systems (MEMS). Due to potentially favorable electronic and catalytic properties, amorphous bismuth ruthenate thin films might be applied in this regard. We report on the deposition of amorphous bismuth ruthenate thin films by spray pyrolysis, their crystallization behavior and electrical conductivity. At room temperature the 200 nm thin amorphous films exhibit a high electrical conductivity of 7.7 × 10(4) S m(-1), which was found to be slightly thermally activated (E(a) = 4.1 × 10(-3) eV). It follows that a long-range order of the RuO(6) octahedra is no precondition for the electrical conductivity of Bi(3)Ru(3)O(11). Upon heating to the temperature range between 490 °C and 580 °C the initially amorphous films crystallize rapidly. Simultaneously, a transition from a dense and continuous film to isolated Bi(3)Ru(3)O(11) particles on the substrate takes place. Solid-state agglomeration is proposed as the mechanism responsible for disintegration. The area specific resistance of Bi(3)Ru(3)O(11) particles contacted by Pt paste on gadolinia doped ceria electrolyte pellets was found to be 7 Ω cm(2) at 607 °C in air. Amorphous bismuth ruthenate thin films are proposed for application in electrochemical devices operating at low temperatures, where a high electrical conductivity is required.

Miniaturized Low-Temperature Solid Oxide Fuel Cells with an Yttria-Stabilized-Zirconia Foil Electrolyte

Micro-solid oxide fuel cells (SOFC) are promising power sources for portable electronic devices. This study presents the fabrication and electrochemical characterization of a new SOFC design which consists of prefabricated parts such as a nickel-mesh anode, a 3 µm thick yttria-stabilized-zirconia (YSZ) foil or a 140 µm thick YSZ-tape electrolyte and a thin-film cathode that are assembled on a Foturan® glass ceramic support structure. Pull-tests revealed a bonding strength of 100 - 200 g between the Ni-mesh anode and the YSZ electrolyte. An open-circuit voltage of 1.0 V and a maximum power density of 0.84 mW/cm2 were measured at 550{degree sign}C for SOFCs comprising a Ni-mesh anode, an YSZ-tape electrolyte and a Pt-paste cathode.