Annual Report 2017 – Electrochemistry Laboratory due to the poor proton acceptability in a neutral electrolyte [5]. Referring to Figure 1d both ,BSCF and PBCO, show a similar decreasing trend of current density (~ 60 % and -68 %, respectively, of its’ initial activities) as the stability trend in the alkaline electrolyte. While they share a similar stability trend at pH 9, PBCO distinguishes itself as a more appealing OER catalyst due to its higher activity by a factor of 3.4 than that of BSCF. From these observations it is clear that a higher mass activity can initially be achieved by PBCO under a quasi-neutral pH.
Electrocatalysis & Interfaces From the above findings, important remarks are drawn relating thermodynamically stable phases of perovskite catalysts to their respective functional stability. First, the loss of initial activity during the stability test corresponds to the loss of perovskite structure as a result of A- and B-site cation dissolutions. This is evident from the stability tests of PBCO-FS in pH 9 and 13, and BSCF-FS in pH 9. Second, even though the cation dissolution is driven by thermodynamic forces, one should keep in mind that the dissolution is not an immediate process but occurs at a kinetically controlled rate.
Acknowledgement The authors gratefully acknowledge the Swiss National Science Foundation through its Ambizione Program and the NCCR Marvel, CCEM through the project RENERG2, the Swiss Competence Center for Energy Research Heat and Electricity Storage (SCCER HaE) through the Commission for Technology and Innovation, Switzerland, and Paul Scherrer Institute for financial contributions to this work, respectively.
References
Figure 2. Performance comparison of BSCF-FS, PBCO-FS, and IrO2 anode OER catalyst in a technical alkaline exchange membrane water electrolyzer: a) Polarization curves and b) voltage vs. time at steady state current density of 500 mA cm-2 obtained for membrane electrode assemblies (MEAs) having BSCF-FS, PBCO-FS, and IrO2 as anodic electrode under non-carbonated condition. c) and d) present the same results under carbonated condition (1 %wt KHCO3). The adverse performance of BSCF relative to PBCO in pH 9, which is evidently contrary to that in pH 13, indicates that a dependence exists between the catalytic behavior and the pH level. As to unravel this relationship, DFT calculated Pourbaix diagrams are used as aid to understand the thermodynamic nature of the catalyst materials in aqueous electrochemical environments. According to the Pourbaix diagrams, BSCF in alkaline conditions above pH 12.6 is predicted to be meta-stable with a stability threshold of 0.5 eV atom-1. Meanwhile, PBCO is found to be thermodynamically unstable in both pH regimes.
[1] E. Fabbri, A. Habereder, K. Waltar, R. Kötz, T.J. Schmidt, Catal. Sci. Technol. 4 , 3800–3821 (2014). [2] E. Fabbri, M. Nachtegaal, T. Binninger, X. Cheng, B.J. Kim, J. Durst, F. Bozza, T. Graule, R. Schäublin, L. Wiles, M. Pertoso, N. Danilovic, K.E. Ayers, T.J. Schmidt, N at. Mater. 1 6 (9), 925– 931 (2017). [3] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G.L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A.P. Seitsonen, A. Smogunov, P. Umari, R.M. Wentzcovitch, J Phys-Condens Mat. 21 (2009). [4] A. Jain, S.P. Ong, G. Hautier, W. Chen, W.D. Richards, S. Dacek, S. Cholia, D. Gunter, D. Skinner, G. Ceder, K.A. Persson, Appl. Mater. 1 (2013). [5] Y. Surendranath, M. Dinca, D.G. Nocera, J. Am. Chem. Soc. 131 , 2615–2620 (2009).
Yet, PBCO-FS shows an initially higher OER in KHCO3 as compared to BSCF-FS. Here, one has to recognize that the dissolution mechanisms during OER are also certainly kinetic processes which obviously cannot be deduced from Pourbaix diagrams alone since they only describe the catalysts under equilibrium conditions. In this respect, it could be postulated that PBCO may exhibit a slower initial dissolution process than BSCF at pH 9 and, hence, may initially demonstrate a higher mass activity. This difference in activity is once more confirmed by adopting these perovskites as anodic electrode materials in alkaline exchange membrane water electrolyzers (AEMWE). Figure 2 shows AEMWE performances for membrane electrode assemblies (MEA) having BSCF, PBCO, and commercial IrO2 as anodic electrode.
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