Cathode materials for protonic ceramic fuel cells: Bulk defect chemistry and surface reaction kinetics

Fuel cells based on ceramic proton conductors receive growing interest because these electrolytes offer a higher ionic conductivity compared to oxide ion conductors, in particular at 300-600 °C. To extend the oxygen reduction reaction to water beyond the three-phase boundary, cathode materials for such cells should have a certain proton conductivity. Since the perovskites considered as cathode material exhibit perceptible oxygen vacancy, proton and hole concentrations, proton uptake can occur by water incorporation (acid-base reaction) and by hydrogen incorporation (redox reaction) [1]. The presence of three mobile carriers can lead to a complex two-fold stoichiometry relaxation kinetics, requiring four diffusion coefficients for complete description [1,2]. The different regimes of proton uptake are explored by thermogravimetry (pH2O changes in different pO2) for perovskites such as La0.5Sr0.5FeO3- and Ba0.5Sr0.2Fe0.8Zn0.2O3- (BSFZ). The obtained maximum proton concentrations are significantly lower than in BaZrO3 electrolyte materials at same T and pH2O [3]. The proton mobility in BSFZ extracted from the transient behavior is comparable to that in BaZrO3 electrolytes. Correlations between cation composition and amount of incorporated protons are discussed. The kinetics of oxygen reduction to water is measured by impedance spectroscopy at dense thin-film BSFZ microelectrodes on proton-conducting Ba(Zr,Y)O3 as substrate. The dependence of surface reaction resistance on electrode area demonstrates that the proton conductivity of BSFZ in the range of » 10-3 S/cm at 400 °C [4] indeed suffices to transport protons from the Ba(Zr,Y)O3- electrolyte through the dense BSFZ film to the gas interface. The reduction of O2 to water can in principle proceed without oxygen incorporation into the cathode material. The values of the pO2 and pH2O dependence of the effective rate constant indicate that molecular oxygen species participate in the rate determining step, and that protonated oxygen species appear only after this step [5]. [1] D. Poetzsch, R. Merkle, J. Maier, Adv. Funct. Mater. 25 (2015) 1542 [2] R. Merkle, R. Zohourian, J. MAier, Solid State Ionics (2016) doi:10.1016/j.ssi.2015.12.011 [3] D. Poetzsch, R. Merkle, J. Maier, Farad. Disc. 182 (2015) 129 [4] R. Merkle, D. Poetzsch, J. Maier,ECS Transact, 66(2) (2015) 95 [5] D. Poetzsch, R. Merkle, J. Maier, J. Electrochem. Soc. 162 (2015) F93

Mixed-conducting cathode materials for protonic ceramic fuel cells: Proton uptake and defect interactions

A cathode in a proton-conducting ceramic fuel cell (PCFC) should meet several criteria including high catalytic activity, electronic conductivity, sufficient proton conductivity, phase stability, etc. to achieve good performance. The proton conductivity allows the oxygen reduction reaction to extend from the triple phase boundary to the whole surface of the cathode (so-called bulk path ). Please click Additional Files below to see the full abstract

Stoichiometry relaxation in oxides with mobile oxygen vacancies, protons and holes: Temperature dependence and trapping effects

Materials with three mobile carriers (oxygen vacancies, protons, holes) are of interest as cathode materials for proton-conducting ceramic fuel cells. The incorporation of protons into such mixed-conducting oxides can occur by acid-base hydration of oxygen vacancies as well as by hydrogen uptake.[1,2] Depending on conditions, one-fold or two-fold conductivity relaxation after pH2O steps is observed.[2-6] Exact analytical relations for these processes were derived in ref. [2]. For a complete description four diffusion coefficients are required, comprising direct as well as indirect terms. The complex non-monotonic kinetic behavior is related to the fact that in a three carrier system the electroneutrality condition does not lead to a simple coupling between the carrier fluxes. Numerical simulations for a wide range of materials and conditions are presented.[2,6] They allow us to identify the conditions for the transition from one-fold to two-fold relaxation, and give a natural explanation for the moving boundary phenomenon observed in ref. [3]. The simulations also show that the assignment of the temperature dependence of the effective diffusivities for acceptor-doped (Ba,Sr)(Zr,Ce)O3-d perovskites (large set of experimental data compiled in [7]) is far from trivial. When the transference number of oxygen vacancies exceeds that of the protons, the faster of the two effective diffusivities approaches the oxygen chemical diffusion coefficient, not the hydrogen chemical diffusivity as one might expect intuitively. Furthermore, the effect of trapping reactions on the relaxation kinetics is investigated. [1] D. Poetzsch, R. Merkle, J. Maier, Faraday Discussions 182 (2015) 129 [2] D. Poetzsch, R. Merkle, J. Maier, Adv. Funct. Mater. 25 (2015) 1542 [3] J. H. Yu, J. S. Lee, J. Maier, Angew. Chem. Int.Ed. 46 (2007) 8992; Solid State Ionics 181 (2010) 154 [4] H. I. Yoo, J. Y. Yoon, J. S. Ha, C. E. Lee, Phys. Chem. Chem. Phys. 10 (2008) 974 [5] E. Kim, H. I. Yoo, Solid State Ionics 252 (2013) 132 [6] R. Merkle, R. Zohourian, J. Maier, Solid State Ionics (2016) doi:10.1016/j.ssi.2015.12.011 [7] G. R. Kim, H. H. Seo, J. M. Jo, E. C. Shin, J. H. Yu, J. S. Lee, Solid State Ionics 272 (2015) 6

First principles calculations of oxygen reduction reaction at fuel cell cathodes

This study was partly supported by M-ERA-NET project SunToChem (EK, YM). The computer resources were provided by Stuttgart Super-computing Center (Project DEFTD 12939). Authors thank E. Heifets, M. M. Kuklja, M. Arrigoni, D. Morgan, R. Evarestov, and D. Gryaznov for fruitful discussions.The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, in particular for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO3-type materials are used for this purpose. The dominating surface terminations are (001) AO and BO2, with the relative fractions depending on materials composition and ambient conditions. Here, results of recent large-scale first principles (ab initio) calculations for the two alternative polar (La,Sr)O and MnO2 (001) terminations of (La,Sr)MnO3 cathode materials are discussed. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller compared to MnO2, which leads to drastically decreased estimated ORR rates. Thus, it is predicted for prototypical SOFC cathode materials that the BO2 termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases, which slows down cathode kinetics.Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²https://www.sciencedirect.com/science/article/pii/S245191031930169

Defect thermodynamics and lattice site basicity of proton and mixed conducting oxides

The extent of hydration of acceptor doped proton conducting oxides, typically described by dissociative hydration (1) has been correlated to various materials properties such as cation electronegativityand is argued to reflect the oxides’ basicity. 1,2 The reaction is, however, amphoteric; lattice oxygen ions are protonated while oxygen vacancies are hydroxylated, suggesting that the extent of hydration rather is governed by the basicity of the lattice oxygen ions – and the acidity of the oxygen vacancies. Recently a number of mixed conducting perovskites with redox-active and typically more acidic elements on the perovskite\u27s B-site have been shown to protonate according to (2) indicating that the hydration properties of e.g. novel cathode materials can be tailored by optimizing the oxide ion and vacancy basicity/acidity. In this contribution we introduce the oxides’ proton and hydroxide affinity (PA and HA) as a measure of the oxide ion basicity and vacancy acidity, respectively, and show how these parameters can be determined from first principles DFT calculations. The PA and HA, and thermodynamics of Eq. 1 are calculated for a selection of binary and perovskite structured oxides, and discussed in relation to the oxide’s electronic, structural and bonding properties. The calculated affinities of the binary oxides generally follow the expected periodic trends and are shown to correlated with the position of the O2p bonding states, reflecting the relationship between the oxide’s electronic structure and basicity. We furthermore assess a series of perovskite structured oxides and discuss correlations between their defect thermodynamics/ion affinities and electronic structure, basicity and A-O and B-O bond characteristics The research leading to these results has received funding from the Research Council of Norway (Grant nᵒ 272797 “GoPHy MiCO”) through the M-ERA.NET Joint Call 2016. [1] T. Norby, M. Widerøe, R. Glöckner and Y. Larring, Dalton Transactions, 19, (2004) [2] K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333–5

Proton uptake in the mixed ionic and electronic conductors Ba1-xSrxFeO3-d

Cathode materials for proton-conducting ceramic fuel cells (PCFC) should combine electronic conductivity with adequate proton conductivity and thereby extend the water formation process from the triple phase boundary to the entire surface of the porous cathode. A variety of such materials including perovskite-structured Ba0.95La0.05FeO3 has been studied experimentally with regard to proton uptake, revealing a systematically lower proton concentration than in electrolyte materials and a peculiar interaction between electronic charge carriers (i.e. holes) and ionic charge carriers (i.e. protons). [1] Please click Additional Files below to see the full abstract

Interdependence of Oxygenation and Hydration in Mixed-Conducting (Ba,Sr)FeO3-δPerovskites Studied by Density Functional Theory

Financial support by the German–Israeli Foundation for Scientific Research and Development (grant I-1342-302.5/2016) and the Latvian Council of Science (grant lzp-2018/1-0147 (D.G., E.A.K.)) is gratefully acknowledged. The authors further thank Guntars Zvejnieks for help with CRYSTAL code calculations.Protonic-electronic mixed-conducting perovskites are relevant as cathode materials for protonic ceramic fuel cells (PCFCs). In the present study, the relation between the electronic structure and the thermodynamics of oxygen nonstoichiometry and hydration is investigated for BaFeO3-δ and Ba0.5Sr0.5FeO3-δ by means of density functional theory. The calculations are performed at the PBE + U level and yield ground-state electronic structures dominated by an oxygen-to-metal charge transfer with electron holes in the O 2p valence bands. Oxygen nonstoichiometry is modeled for 0 ≤ δ≤ 0.5 with oxygen vacancies in doubly positive charge states. The energy to form an oxygen vacancy is found to increase upon reduction, i.e., decreasing concentration of ligand holes. The higher vacancy formation energy in reduced (Ba,Sr)FeO3-δ is attributed to a higher Fermi level at which electrons remaining in the lattice from the removed oxide ions have to be accommodated. The energy for dissociative H2O absorption into oxygen vacancies is found to vary considerably with δ, ranging from ≈-0.2 to ≈-1.0 eV in BaFeO3-δ and from ≈0.2 to ≈-0.6 eV in Ba0.5Sr0.5FeO3-δ. This dependence is assigned to the annihilation of ligand holes during oxygen release, which leads to an increase in the ionic charge of the remaining lattice oxide ions. The present study provides sound evidence that p-type electronic conductivity and the susceptibility for H2O absorption are antagonistic properties since both depend in opposite directions on the concentration of ligand holes. The reported trends regarding oxygenation and hydration energies are in line with the experimental observations.Latvian Council of Science lzp-2018/1-0147; German–Israeli Foundation for Scientific Research and Development grant I-1342-302.5/2016; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART