Improvements of CoPx Stability By Cathodic Protection in Discontinuous and Interrupted Operations of Alkaline Water Electrolysis

Abstract

This paper presents a study of the degradation phenomena of Cobalt phosphides (CoPx), which is attracted as the hydrogen evolution reaction (HER) electrode for alkaline water electrolysis cells(AWEs).1-9 Although the HER activity of CoPx showed 100-fold HER activity enhancements over a nickel catalyst, the kinetic current for the HER after discontinuous and interrupted operations was observed to decrease as shown in Figure 1. During the shut-down of the AWEs, the cathode potential changed from -1.03V to -0.8V (vs. Hg/HgO), which is close to the equilibrium potential of Co oxidation reaction to Co(OH)2. Once Co(OH)2 was formed on CoPx electrode, the kinetic degradation was not recovered even though voltage sweep was applied. We employed cathodic protection to mitigate degradation of the CoPx electrode by shifting the potential of the electrode below the open circuit voltage of Co(OH)2. For the cathodic protection case, Mn was co-deposited with the CoPx electrode as a sacrificial species, which reduces the electrode degradation rate by negative polarization of the electrode. The results showed that by the co-depositing even small amount of Mn into the CoPx electrode could restrain HER performance loss during the repeat of discontinuous operation of the AWEs. That is, controlling the performance loss with the cathodic protection is applicable to ensure the stability of the CoPx electrodes of the AWE system for the integration with renewable powers. Figure 1. Stability improvements for CoPx HER electrodes in 5 h discontinuous operation by the cathodic protection Acknowledgements The authors gratefully acknowledge financial support for this work by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2015M1A2A2074657). R eferences [1] A. Goryachev L. Gao, Y. Zhang, R. Y. Rohling, René. H. J. Vervuurt, A. A. Bol, J. P. Hofmann, E. J. M. Hense, ChemElectroChem 2018, 5, 1230-1239[2] Q. Liu, J. Tian, W. Cui, P. Jiang, N. Cheng, A. M. Asiri, X. Sun, Angew., Chem. Int. Ed. 2014, 53, 6710–6714.[3] E. J. Popczun, C. G. Read, C. W. Roske, N. S. Lewis, R. E. Schaak, Angew. Chem. Int. Ed. 2014, 53, 5427–5430.[4] N. Jiang, B. You, M. Sheng, Y. Sun, Angew. Chem. Int. Ed. 2015, 54, 6251–6254.[5] Z. Huang, Z. Chen, Z. Chen, C. Lv, M. G. Humphrey, C. Zhang, Nano Energy 2014, 9, 373–382.[6] F. H. Saadi, A. I. Carim, E. Verlage, J. C. Hemminger, N. S. Lewis, M. P. Soriaga, J. Phys. Chem. C 2014, 118, 29294–29300.[7] F. H. Saadi, A. I. Carim, W. S. Drisdell, S. Gul, J. H. Baricuatro, J. Yano, M. P. Soriaga, N. S. Lewis, J. Am. Chem. Soc. 2017, 139, 12927–12930[8] S. Oh, H. Kim, Y. Kwon, M. Kim, E. Cho and H. Kwon, J. Mater. Chem. A, 2016, 4, 18272-18277[9] J. F. Callejas, C. G. Read, E. J. Popczun, J. M. McEnaney and R. E. Schaak, Chem. Mater., 2015, 27, 3769–3774. Figure 1