Søren Højgaard Jensen

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.

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.

Eliminating degradation in solid oxide electrochemical cells by reversible operation

One promising energy storage technology is the solid oxide electrochemical cell (SOC), which can both store electricity as chemical fuels (electrolysis mode) and convert fuels to electricity (fuel-cell mode). The widespread use of SOCs has been hindered by insufficient long-term stability, in particular at high current densities. Here we demonstrate that severe electrolysis-induced degradation, which was previously believed to be irreversible, can be completely eliminated by reversibly cycling between electrolysis and fuel-cell modes, similar to a rechargeable battery. Performing steam electrolysis continuously at high current density (1 A cm(-2)), initially at 1.33 V (97% energy efficiency), led to severe microstructure deterioration near the oxygen-electrode/electrolyte interface and a corresponding large increase in ohmic resistance. After 4,000 h of reversible cycling, however, no microstructural damage was observed and the ohmic resistance even slightly improved. The results demonstrate the viability of applying SOCs for renewable electricity storage at previously unattainable reaction rates, and have implications for our fundamental understanding of degradation mechanisms that are usually assumed to be irreversible.

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.

A flow reactor study of HNCO oxidation chemistry

An experimental and theoretical study of HNCO oxidation has been carried out. The experiments were performed in an isothermal quartz flow reactor, and the effects of temperature, CO concentration, and NO concentration were investigated at atmospheric pressure in the temperature range 1025–1425 K. The reaction mechanism for RAPRENOx proposed by Miller and Bowman (1991) has been updated based on the present result as well as recent advances in the understanding of important elementary steps. Model predictions with the revised mechanism are in good agreement with our experimental data as well as data from the literature. Oxidation of HNCO proceeds mainly through NCO, which subsequently is oxidized to NO or reacts with NO to form N2 and N2O. This sequence of reactions is chain terminating, and for reaction to occur, radicals must be generated either by alternative oxidation pathways or by the presence of other combustibles. A chain-branching oxidation route initiated by reaction of HNCO with O2 is proposed in order to explain the observed HNCO decay in the absence of inlet CO. Addition of CO enhances HNCO oxidation and the RAPRENOx chemistry, since CO oxidation acts to replenish the radical pool. The experimental results show that the mutual presence of HNCO and NO strongly inhibits CO oxidation at lower temperatures. In addition to the chain terminating HNCO/NCO reactions, a second inhibition mechanism involving NO is necessary to explain this behavior. This mechanism is presently believed to be NONO2 interconversion, but additional work is needed to confirm this. Further progress in the understanding of the HNCO chemistry is dependent on an accurate determination of the rate and/or mechanism of a number of key reactions, including HNCO + OH, HNCO + O2, NCO + NO and NO + O + M.

Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4

Correction for ‘Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH4’ by S. H. Jensen et al., Energy Environ. Sci., 2015, 8, 2471–2479.

SU-8 based piezoresistive mechanical sensor

We present the first SU-8 based piezoresistive mechanical sensor. Conventionally, silicon has been used as a piezoresistive material due to its high gauge factor and thereby high sensitivity to strain changes in a sensor. By using the fact that SU-8 is much softer than silicon and that a gold resistor is easily incorporated in SU-8, we have proven that a SU-8 based cantilever sensor is almost as sensitive to stress changes as the silicon piezoresistive cantilever. We demonstrate the chip fabrication, and characterization with respect to sensitivity, noise and device failure.

Characterization of the microloading effect in deep reactive ion etching of silicon

Knowledge of the magnitude and characteristic length scales of chip-scale process variations due to varying substrate pattern density is essential if compensation measures, such as incorporation of dummy structures, are to be taken during mask layout. Effects of variations in local pattern density on a deep reactive ion etch (DRIE) process have been investigated, and a decrease of the etch rate with increasing local pattern density within a characteristic radius of approximately 4.5 mm has been found. Analytical and numerical calculations confirm the existence of a similar depletion radius under the experimental conditions used.