Anne Hauch

Highly efficient high temperature electrolysis

High temperature electrolysis of water and steam may provide an efficient, cost effective and environmentally friendly production of H2 using electricity produced from sustainable, non-fossil energy sources. To achieve cost competitive electrolysis cells that are both high performing i.e. minimum internal resistance of the cell, and long-term stable, it is critical to develop electrode materials that are optimal for steam electrolysis. In this article electrolysis cells for electrolysis of water or steam at temperatures above 200 °C for production of H2 are reviewed. High temperature electrolysis is favourable from a thermodynamic point of view, because a part of the required energy can be supplied as thermal heat, and the activation barrier is lowered increasing the H2 production rate. Only two types of cells operating at high temperature (above 200 °C) have been described in the literature, namely alkaline electrolysis cells (AEC) and solid oxide electrolysis cells (SOEC). In the present review emphasis is on state-of-the art electrode materials and development of new materials for SOECs. Based on the state-of-the-art performance for SOECs H2 production by high temperature steam electrolysis using SOECs is competitive to H2 production from fossil fuels at electricity prices below 0.02–0.03 € per kWh. Though promising SOEC results on H2 production have been reported a substantial R&D is still required to obtain inexpensive, high performing and long-term stable electrolysis cells.

Solid Oxide Electrolysis Cells: Microstructure and Degradation of the Ni/Yttria-Stabilized Zirconia Electrode

Solid oxide fuel cells produced at Riso DTU have been tested as solid oxide electrolysis cells for steam electrolysis by applying an external voltage. Varying the sealing on the hydrogen electrode side of the setup verifies that the previously reported passivation over the first few hundred hours of electrolysis testing was an effect of the applied glass sealing. Degradation of the cells during long-term galvanostatic electrolysis testing [850°C, -1/2 A/cm 2 , p(H 2 O)/p(H 2 ) = 0.5/0.5] was analyzed by impedance spectroscopy and the degradation was found mainly to be caused by increasing polarization resistance associated with the hydrogen electrode. A cell voltage degradation of 2%/1000 h was obtained. Postmortem analysis of cells tested at these conditions showed that the electrode microstructure could withstand at least 1300 h of electrolysis testing, however, impurities were found in the hydrogen electrode of tested solid oxide electrolysis cells. Electrolysis testing at high current density, high temperature, and a high partial pressure of steam [-2 A/cm 2 , 950°C, p(H 2 O) = 0.9 atm] was observed to lead to significant microstructural changes at the hydrogen electrode-electrolyte interface.

Solid Oxide Electrolysis Cells: Degradation at High Current Densities

The degradation of Ni/yttria-stabilized zirconia (YSZ)-based solid oxide electrolysis cells operated at high current densities was studied. The degradation was examined at 850 degrees C, at current densities of -1.0, -1.5, and -2.0 A/cm(2), with a 50:50 (H(2)O:H(2)) gas supplied to the Ni/YSZ hydrogen electrode and oxygen supplied to the lanthanum, strontium manganite (LSM)/YSZ oxygen electrode. Electrode polarization resistance degradation is not directly related to the applied current density but rather a consequence of adsorbed impurities in the Ni/YSZ hydrogen electrode. However, the ohmic resistance degradation increases with applied current density. The ohmic resistance degradation is attributed to oxygen formation in the YSZ electrolyte grain boundaries near the oxygen electrode/electrolyte interface

Performance and Durability of Solid Oxide Electrolysis Cells

Solid oxide fuel cells produced at Riso National Laboratory have been tested as electrolysis cells by applying an external voltage. Results on initial performance and durability of such reversible solid oxide cells at temperatures from 750 to 950°C and current densities from -0.25 A/cm 2 to - 0.50 A/cm 2 are reported. The full cells have an initial area specific resistance as low as 0.27 Ωcm 2 for electrolysis operation at 850°C. During galvanostatic long-term electrolysis tests, the cells were observed to passivate mainly during the first ∼ 100 h of electrolysis. Cells that have been passivated during electrolysis tests can be partly activated again by operation in fuel cell mode or even at constant electrolysis conditions after several hundred hours of testing.

A Method to Separate Process Contributions in Impedance Spectra by Variation of Test Conditions

Many processes contribute to the overall impedance of an electrochemical cell, and these may be difficult to separate in the impedance spectrum. Here, we present an investigation of a solid oxide fuel cell based on differences in impedance spectra due to a change of operating parameters and present the result as the derivative of the impedance with respect to ln(f). The method is used to separate the anode and cathode contributions and to identify various types of processes.

Silica Segregation in the Ni ∕ YSZ Electrode

Solid oxide fuel cells were tested as solid oxide electrolysis cells used for high-temperature steam electrolysis. The cells weretested at a variety of operation temperatures, current densities, and gas flows to the electrodes. The cell voltages monitored duringthe electrolysis operation increased significantly during the first few days of testing. Impedance spectroscopy obtained duringelectrolysis shows that it is the Ni/yttria-stabilized zirconia YSZ electrode that passivates. Reference cells and tested cells wereexamined in a scanning electron microscope after testing. These postmortem analyses reveal the reason for the observed passi-vation, because results from energy-dispersive spectroscopy clearly show evidence that silica-containing impurities have segre-gated to the hydrogen electrode/electrolyte interface during electrolysis testing. Examples of different microstructures and amountsof Si-containing impurities in the electrolyte/hydrogen electrode interface are presented and related to the electrolysis test condi-tions and the passivation histories of the electrolysis cells.© 2007 The Electrochemical Society. DOI: 10.1149/1.2733861 All rights reserved.Manuscript submitted October 24, 2006; revised manuscript received February 27, 2007. Available electronically May 4, 2007.

Poisoning of Solid Oxide Electrolysis Cells by Impurities

Electrolysis of H 2 0, CO 2 , and co-electrolysis of H 2 O and CO 2 was studied in Ni/yttria-stabilized zirconia (YSZ) electrode supported solid oxide electrolysis cells (SOECs) consisting of a Ni/YSZ support, a Ni/YSZ electrode layer, a YSZ electrolyte, and an lanthanum strontium manganite (LSM)/YSZ oxygen electrode When applying the gases as received, the cells degraded significantly at the Ni/YSZ electrode, whereas only minor (and initial) degradation was observed for either the Ni/YSZ or LSM/YSZ electrode. Application of clean gases to the Ni/YSZ electrode resulted in operation without any long-term degradation, in fact some cells activated slightly. This shows that the durability of these SOECs is heavily influenced by impurities in the inlet gases. Cleaning the inlet gases to the Ni/YSZ electrode may be a solution for operating these Ni/YSZ-based SOECs without long-term degradation.

Nanoscale Chemical Analysis and Imaging of Solid Oxide Cells

The performance of solid oxide cells (SOCs) is highly dependent on triple phase boundaries (TPBs). Therefore, detailed TPB characterization is crucial for their further development. We demonstrate that it is possible to prepare a ∼50 nm thick transmission electron microscopy (TEM) lamella of the interface between the dense ceramic electrolyte and the porous metallic/ceramic hydrogen electrode of an SOC using focused ion beam milling. We show combined TEM/scanning TEM/energy-dispersive spectroscopy investigations of the nanostructure at the TPBs in a high-performance SOC. The chemical composition of nanoscale impurity phases at the TPBs has been obtained with a few nanometers lateral resolution.

Advanced Test Method of Solid Oxide Cells in a Plug-Flow Setup

This paper describes a case study of two electrolysis tests of solid oxide cells [Ni/yttria-stabilized zirconia (YSZ)―YSZ-lanthanum strontium manganite (LSM)/YSZ] tested in a plug-flow setup. An extensively instrumented cell test setup was used, and the tests involved measurements of the cell impedance at open-circuit voltage and under current load, the cell voltage, and the in-plane voltage in the electrodes. From the cell-voltage measurements it was evident that a significant passivation of the cells occurred over the first ∼ 10 days. Thereafter, the cells reactivated at constant electrolysis conditions. From measurements of the in-plane voltages in the electrodes and impedance spectra obtained during the electrolysis operation, we derive information about the resistance distributions in the Ni electrodes and describe how these distributions evolve over time. Impedance spectra at open-circuit voltage before and after electrolysis testing at various gas compositions were used to show that the Ni electrode was affected by the electrolysis operation, whereas the LSM electrode was not.