Concentration and mobility of protons in H+-SOFC cathode materials

Concentration and mobility of protons in H+-SOFC cathode materials

D. Poetzsch, R. Merkle

Owing to their higher conductivity at intermediate temperatures (400-700°C) proton-conducting ceramics are feasible for fuel cells operating in this range. A further advantage is that the oxidation product H2O is formed at the cathode and thus, the fuel is not diluted. Typically, the same materials are investigated as cathode as in the case of oxide-ion conducting electrolytes (YSZ). However, it is hardly known so far if these perovskites have a sufficient proton conductivity to allow for the oxygen reduction reaction to proceed on the whole electrode surface by the so-called "bulk path" (Fig. 1a). In redox-active materials, protons can be incorporated by hydration of oxygen vacancies:

and / or by redox reaction consuming electron holes:

Thus a defect model comprising all three defects must be established. For the example of Ba0.5Sr0.5Fe0.8Zn0.2O3-d , proton concentration and mobility are determined from thermogravimetry on dense ceramic samples (Fig. 1b). The estimated proton conductivity exceeds 10-4 S/cm and thus indicates the "bulk path" is possible.

<p>Fig. 1: (a) possible reaction paths for oxygen reduction on proton-conducting electrolyte; (b) thermogravimetry for Ba<sub>0.5</sub>Sr<sub>0.5</sub>Fe<sub>0.8</sub>Zn<sub>0.2</sub>O<sub>3-</sub><sub>d</sub>, showing absolute weight changes (-&gt; proton concentration) as well as the transient behavior (-&gt; proton diffusivity)</p> Zoom Image

Fig. 1: (a) possible reaction paths for oxygen reduction on proton-conducting electrolyte; (b) thermogravimetry for Ba0.5Sr0.5Fe0.8Zn0.2O3-d, showing absolute weight changes (-> proton concentration) as well as the transient behavior (-> proton diffusivity)

The proton concentration and proton diffusivity extracted from the thermogravimetry measurements on dense BSFZ pellets, a proton conductivity of about 10-3 S/cm is obtained at 400 °C. For 100 nm thick electrodes, this value is expected to suffice for enabling oxygen reduction on the whole electrode surface. The diameter dependence measured for dense thin-film BSFZ and BSCF microelectrodes confirms that oxygen reduction indeed proceeds via the bulk path.

Publications:

D. Poetzsch, R. Merkle, and J. Maier
Investigation of oxygen exchange kinetics in proton-conducting ceramic fuel cells: Effect of electronic leakage current using symmetric cells
Journal of Power Sources 242, 784–789 (2013)   DOI: 10.1016/j.jpowsour.2013.05.108

D. Poetzsch, R. Merkle, and J. Maier 
Proton uptake in the H+-SOFC cathode material Ba0.5Sr0.5Fe0.8Zn0.2O3-δ: transition from hydration to hydrogenation with increasing oxygen partial pressure 
Faraday Discussions, in press, 2015 DOI: 10.1039/C5FD00013K

R. Merkle, D. Poetzsch, and J. Maier 
Oxygen Reduction Reaction at Cathodes on Proton Conducting Oxide Electrolytes: Contribution from Three Phase Boundary Compared to Bulk Path 
ECS Transactions 66(2), 95–102 (2015) DOI: 10.1149/06602.0095ecst

D. Poetzsch, R. Merkle, and J. Maier 
Oxygen Reduction at Dense Thin-Film Microelectrodes on a Proton-Conducting Electrolyte: I. Considerations on Reaction Mechanism and Electronic Leakage Effects 
Journal of The Electrochemical Society 162(9), F939–F950 (2015) DOI: 10.1149/2.0951508jes

D. Poetzsch, R. Merkle, and J. Maier 
Proton conductivity in mixed-conducting BSFZ perovskite from thermogravimetric relaxation 
Physical Chemistry Chemical Physics 16(31), 16446–16453 (2014) DOI: 10.1039/C4CP00459K

 
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