Strong Performance Improvement of La[sub 0.6]Sr[sub 0.4]Co[sub 0.8]Fe[sub 0.2]O[sub 3−δ] SOFC Cathodes by Electrochemical Activation

Abstract

It is shown that the electrochemical resistance of mixed conducting solid oxide fuel cell ͑SOFC͒ model cathodes can be reduced drastically by a short but strong dc polarization of the cell. The samples investigated are dense thin-film microelectrodes of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ on a yttria-stabilized zirconia solid electrolyte. Relative performance improvements of more than two orders of magnitude can be achieved with a cathodic dc bias of the order of 1 V, applied for a few minutes at fuel cell operating temperature. The positive effect on the electrode performance corresponds to an acceleration of the oxygen surface exchange reaction, initially the resistance determining process. For this surface-related resistance, absolute values as low as 0.065 ⍀ cm 2 at 700°C have been obtained. After such an activation, the resistance slowly increases again on a much larger time scale, indicating the possibility of a steady performance enhancement by a periodic activation with short dc pulses. X-ray photoelectron spectra show that a strong cathodic polarization severely changes the cation concentrations within the outermost surface layer of the electrode, and these field-induced surface compositional changes are assumed to be the main cause of the performance improvement.High-temperature solid oxide fuel cells ͑SOFCs͒ receive considerable attention because they enable a highly efficient conversion of chemical into electric energy. A major goal in SOFC research is the development of cathodes with a sufficiently low electrochemical resistance ͑ϳ0.15 ⍀ cm 2 ͒ at operating temperatures substantially below 800°C. 1-3 It is generally accepted that this is hardly achievable with lanthanum strontium manganite ͑LSM͒-based oxygen electrodes, which are the most thoroughly studied and technically developed SOFC cathodes at the moment. Focus has been shifted toward other materials that offer a sufficient electrochemical performance already in the intermediate-temperature range, and particularly the mixed conducting perovskites of the ͑La,Sr͒͑Co,Fe͒O 3−␦ family are considered as promising candidates.In the literature, activation effects upon dc polarization have been reported for porous LSM and LSM/YSZ ͑yttria-stabilized zirconia͒ composite electrodes, 4-11 as well as for Pt electrodes. 11-13 However, a general agreement on the causes of these activation effects has not been achieved yet, and the reported magnitudes of the effect and the suggested mechanisms strongly vary in the literature. Measurements on morphologically and crystallographically well-defined electrodes are not available but are highly desirable in this context. 11 Moreover, to the best of the authors' knowledge, a significant performance improvement by electrochemical polarization has not been observed for the technologically important class of mixed conducting electrode materials.In this work, a strong electrochemical activation effect is reported for mixed conducting La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3−␦ ͑LSCF͒ electrodes. The performance improvement is achieved...

Geometrically asymmetric electrodes for probing electrochemical reaction kinetics: a case study of hydrogen at the Pt–CsH2PO4 interface

“…At high s/k values, the electrolyte approaches a state of 'perfect' conductivity, and the isolation therefore approaches a value simply determined by the radii of the two electrodes. At low s/k values, the potential distribution within the electrolyte approaches that expected for perfect (reversible) electrodes, and the isolation approaches a value determined by r WE and t. Indeed, as evident from eqn (13), in this limit, r eff CE is simply 1.667t, and hence the isolation, via eqn (14), depends only on the ratio r WE /t. Finally, although the origin of the shallow minimum in x at approximately s/k = 0.1 cannot be readily explained, this feature was confirmed (via extensive computations in this regime) to reflect the real behavior of the system, rather than a computational artifact of mesh resolution or convergence criterion.…”

“…3 Dependence of isolation, x, on overall cell radius, r 0 , for 3 different working electrode radii (with values as indicated). Also shown are the effective counter electrode radii calculated for each of these three geometries from eqn (13), which applies when r 0 is large. Calculations performed for k = 0.615…”

“…In most instances a reference electrode is retained as part of the configuration. [4][5][6][7][8][9] In contrast to this trend, Fleig and co-workers [10][11][12][13][14] have adopted a point electrode geometry that is explicitly reference-less. For dense, surface-limited electrodes, the assumption that the point geometry has effectively eliminated the contribution of the counter electrode is likely valid.…”

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…Short treatments of high cathodic -and anodic to a lesser extent -polarisation were found to dramatically improve the reaction rate at the MIEC/O2 2PB by up to two orders of magnitude depending on the MIEC material 62 . The enhancement was attributed to a modified, non-equilibrium cation stoichiometry observed on the surface of activated electrodes, which has a lasting effect after the treatment 34 . High cathodic polarisation may be expected to even totally decompose the electrochemically active part of the perovskite surface, after which a fresh, more active surface is formed.…”

“…For example the segregation of SrO to the perovskite surface hinders the oxygen reduction reaction by inhibiting electron transfer from the perovskite bulk to oxygen species adsorbing onto the surface and by diminishing the concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice 32 (see Figure 4a). If the segregated layers are converted by strong reduction of the perovskite material (2 V) 34 or removed by dissolution in aqueous HCl solution 14 then the current density increases by orders of magnitude. In spite of this, recent investigations could not establish any clear correlation between the SrO segregation and the surface oxygen exchange rate on RP-type O2-electrodes 15,35 , suggesting that our understanding of the SOC electrode process related to surface reorganisation is still incomplete.…”