Ni/YSZ electrodes structures optimized for increased electrolysis performance and durability

Abstract

Cermet Ni/YSZ electrodes are the most commonly applied fuel electrode for solid oxide cells (SOC) both when targeting solid oxide fuel cell (SOFC) applications and when used as solid oxide electrolysis cell (SOEC). In this work we report on the correlation between initial Ni/YSZ microstructure and the resulting electrochemical performance both initially and during long-term electrolysis testing at high current density and high p(H2O) inlet. Especially, this work focuses on microstructure optimization to hinder Ni mobility and migration during long-term operation and illustrates the key-role of electrode over-potential on the degradation of the Ni/YSZ electrodes in SOEC. We find that for long-term stability for electrolysis at high current densities and high p(H2O) the as-produced NiO/YSZ precursor electrode should be: 1) As dense as possible, 2) as fine particle and pore sized as possible and 3) the three phases (Ni, YSZ and pore phase) shall be size-matched and welldispersed. Applying such microstructure optimized Ni/YSZ electrode we show SOEC test results with long-term degradation rate as low as 0.3-0.4 %/kh at 1 A/cm 2 , 800 C and inlet gas mixture of p(H2O)/p(H2):90/10. This enables SOEC operation of such cell for more than 5 years below thermo-neutral potential at these operating conditions.

A Decade of Solid Oxide Electrolysis Improvements at DTU Energy

“…Evidently cell C is the better performing. Analysis of impedance spectra recorded at these electrolysis test conditions at start of the test reveals that the polarization resistance attributed to the electrochemical reactions in the Ni/YSZ electrodes were 98 mcm 2 and 38 mcm 2 for cell A and C, respectively (8). Both cells experience an initial increase in Rp; for both cells Rp subsequently reaches a constant level.…”

“…Several parameters (e.g. Ni/YSZ ratios, layer thicknesses, particle sizes and sintering temperature) have been investigated to optimize the production of the 4-layer co-sintered half-cells (8,40,41). In recent years processing optimization has also focused on the shift towards -if not completely water based slurries then at least -environmentally friendly production as reported by Foghmoes and co-workers (42).…”

Long Term Testing of Solid Oxide Electrolysis Cells under Co-Electrolysis Conditions

“…Predominant fuel electrode degradation was concluded from the electrochemical analysis, which is thus confirmed by the percolation images taken from hydrogen outlet in Figure 10. A loss of active TPBs seems thus responsible for the observed degradation (13,23,28). A gap in the layers is observed in the outlet, at the interface of CGO and YSZ layers (see marker on Figure 10(b)).…”

“…Degradation is observed at the fuel electrode for both high frequency process (1-10 kHz) and medium frequency process (100-1000 Hz). Previous studies have indicated the loss in percolation of Ni network to be one of the major causes for the Ni-YSZ active electrode degradation (12)(13)(14)28). This phenomenon will be investigated in the microstructural analysis.…”

“…Durability testing of SOEC cells has previously been reported both for steam and co-electrolysis conditions (11)(12)(13)(14). The cells have been tested for thousands of hours under different testing conditions such as temperature, gas composition and applied current (11)(12)(13)(14)(15)(16)(17). It is a highly desirable to operate SOEC at thermoneutral voltage, i.e.…”