Sune Dalgaard Ebbesen

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

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.

PRODUCTION OF SYNTHETIC FUELS BY CO-ELECTROLYSIS OF STEAM AND CARBON DIOXIDE

Co-electrolysis of H2O and CO2 was studied in solid oxide cells (SOCs) supported by nickel-/yittria-stabilized zirconia (Ni/YSZ) electrode. Polarization characterization indicates that electrochemical reduction of both CO2 and H2O occurs during co-electrolysis. In parallel with the electrochemical reactions, the equilibrium of the water–gas shift reaction is reached, and moreover, CO is produced via the water–gas shift reaction. The degradation observed when performing co-electrolysis in these SOCs occurs mainly at the Ni/YSZ cathode and may be a consequence of impurities in the gas stream, adsorbing on active sites in the SOC. The low degradation is most likely acceptable for long-time operation.

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.

Exceptional Durability of Solid Oxide Cells

Extensive efforts to resolve the degradation normally associated with solid oxide electrolysis cells SOECs have been conducted during the past decade. To date, the degradation is assumed to be caused by adsorption of impurities in the cathode, although no firm evidence for this degradation mechanism has been presented. In this article, we demonstrate that the rapid degradation of these SOECs is indeed caused by impurities, and that operation without degradation is possible when removing these impurities from the inlet gases. Cleaning the inlet gases may be a solution for operating SOECs without long-term degradation. Production of synthetic hydrocarbon fuels from renewable energy is a solution to reduce oil consumption and carbon dioxide emissions without the need for modifications of existing infrastructure, e.g., in the production of methane synthetic natural gas or petrol/diesel, the infrastructure already exists in many countries. The raw material for synthetic hydrocarbon fuels is synthesis gas H2 +C O, which is traditionally produced via coal gasification or steam reforming of natural gas. Both processes consume fossil fuels and emit greenhouse gases. Coelectrolysis of H2O and CO2 H2O +C O2 → H2 cathode +C Ocathode +O 2 anode using renewable energy sources may be an alternative route for producing synthesis gas without consumption of fossil fuels and without emitting greenhouse gases. CO2 captured from air and/or recycling or reusing of CO2 from energy systems combined with coelectrolysis of H2O and CO2 seems to be an attractive method to provide CO2 neutral synthetic hydrocarbon fuels. Solid oxide electrolysis cells SOECs have the potential for cost-competitive production of hydrogen 1-4 and carbon monoxide, 1

Carbon Nanotube Growth on Nanozirconia under Strong Cathodic Polarization in Steam and Carbon Dioxide

Growth of carbon nanotubes (CNTs) catalyzed by zirconia nanoparticles was observed in the Ni–yttria doped zirconia (YSZ) composite cathode of a solid oxide electrolysis cell (SOEC) at approximately 875 °C during co‐electrolysis of CO2 and H2O to produce CO and H2. CNT was observed to grow under large cathodic polarizations specifically at the first 1 to 2 μm Ni–YSZ active cathode layer next to the YSZ electrolyte. High resolution transmission electron microscopy (HRTEM) shows that the CNTs are multi‐walled with diameters of approximately 20 nm and the catalyst particles have diameters in the range of 5 to 25 nm. The results of HRTEM and energy dispersive X‐ray spectroscopy (EDS) analysis confirm that the catalyst particles attached to the CNT are cubic zirconia. Most of the zirconia particles are located at one end of the CNTs, but particles embedded in the walls or inside the CNTs are also observed. Apart from the CNTs, graphitic layers covering zirconia nanoparticles are also widely observed. This work describes nano‐zirconia acting as a catalyst for the growth of CNT during electrochemical conversion of CO2 and H2O in a Ni‐YSZ cermet under strong cathodic polarization. An electrocatalytic mechanism is proposed for the CNT growth in SOECs. These findings provide further understanding not only on the mechanism of the catalytic growth of CNTs, but also on the local electrochemical properties of a highly polarized Ni–YSZ cathode at the micro and nano level.

Origin of electrolyte-dopant dependent sulfur poisoning of SOFC anodes

The mechanisms governing the sulfur poisoning of the triple phase boundary (TPB) of Ni-XSZ (X2O3 stabilized zirconia) anodes have been investigated using density functional theory. The calculated sulfur adsorption energies reveal a clear correlation between the size of the cation dopant X(3+) and the sulfur tolerance of the Ni-XSZ anode; the smaller the ionic radius, the higher the sulfur tolerance. The mechanistic study shows that the size of X(3+) strongly influences XSZs surface energy, which in turn determines the adhesion of Ni to XSZ. The Ni-XSZ interaction has a direct impact on the Ni-S interaction and on the relative stability of reconstructed and pristine Ni(100) facets at the TPB. Together, these two effects control the sulfur adsorption on the Ni atoms at the TPB. The established relationships explain experimentally observed dopant-dependent anode performances and provide a blueprint for the future search for and preparation of highly sulfur tolerant anodes.

Origin of Polarization Losses in Solid Oxide Electrolysis Cells under High Current Density

Under high current density, polarization losses in a solid oxide cell are higher in electrolysis mode than in fuel cell mode. Although part of these differences under high current density can be attributed to gas diffusion differences between steam and hydrogen. Diffusion coefficient differences can only partially explain the higher polarization losses during electrolysis operation.