Power-to hydrogen, notably “green” hydrogen produced by water electrolysis, is one solution to store renewable electricity, an endeavor towards the limitation of our carbon footprint. While proton-exchange membrane water electrolysers (PEMWE) are energetically-efficient and compatible with transient operation -which is needed to store renewable electricity-, these systems rely on fluorinated proton-exchange membranes and platinum group metals (PGM), which is hardly sustainable [1-2], and their durability is uncertain [3]. On the contrary, alkaline water electrolysers (AWE) are industrially-operated since decades, essentially because they employ limited amounts of PGM [4] (e.g. Ni-based catalysts [5-6]), but their performances are inferior to PEMWEs [7]. Improving their electrodes’ initial activity, robustness in intermittent operation and long-term stability is however possible. Stainless steel supports have historically been used in AWE systems, owing to their robustness and electron-conducting properties. More recently, stainless steels were demonstrated active on their own (without added coatings) for water splitting reactions [8-9]. Because of their non-negligible nickel content, if properly “activated” stainless steels can form a NiFe oxo-hydroxide surface “passivation” layer that is very active for the oxygen evolution reaction (OER) [10-11]. In this presentation, the influence of the initial Fe/Ni ratio (varying between 0.004 and 7.4) and manufacturing process of various Fe-Ni-Cr alloys is evaluated for alkaline OER. Coupling advanced physicochemical characterizations (XPS, STEM-EDS, TEM-ASTAR) to electrochemistry, the active sites’ nature is elucidated. After activation, the nanostructured surface layer reaches a stable composition. While the manufacturing process and the initial surface state of the alloys do not impact the OER performance, the growth and the composition of the NiFe oxo-hydroxide surface layer depends on the initial atomic Fe/Ni ratio, hence driving OER activity. Whatever the initial Fe/Ni ratio of the Fe–Ni alloy, the best oxygen evolution reaction performance (beyond that of commercial IrO2) and durability was obtained for a surface Fe/Ni ratio between 0.2 and 0.4 and includes numerous active sites (high NiIII/NiII capacitive response) and high efficiency (high Fe/Ni ratio). This OER activity is maintained on the long term on such surfaces, proving their practical interest [12]. This knowledge paves the way to active and durable Fe-Ni-Cr alloys oxygen-evolving electrodes for alkaline water electrolysers. REFERENCES [1] S. Park, Y. Shao, J. Liu, Y. Wang, Energy Environ. Sci. 5 (2012) 9331-9344. [2] P.C.K. Vesborg, T.F. Jaramillo, RSC Adv. 2 (2012) 7933-7947. [3] U. Babic, M. Suermann, F.N. Büchi, L. Gubler, T.J. Schmidt, J. Electrochem. Soc. 164 (2017) F387-F399. [4] S. Marini, P. Salvi, P. Nelli, R. Pesenti, M. Villa, M. Berrettoni, G. Zangari, Y. Kiros, Electrochim. Acta 82 (2012) 384-391. [5] F. Dionigi, T. Reier, Z. Pawolek, M. Gliech, P. Strasser, ChemSusChem 9 (2016) 962-972. [6] M. Schalenbach, O. Kasian, K.J.J. Mayrhofer, Int. J. Hydrogen Energy 43 (2018) 11932-11938. [7] K. Zeng, D. Zhang, Prog. Energ. Comb. Sci. 36 (2010) 307-326. [8] H. Schäfer, S. Sadaf, L. Walder, K. Kuepper, S. Dinklage, J. Wollschläger, L. Schneider, M. Steinhart, J. Hardege, D. Daum, Energy Environ. Sci. 8 (2015) 2685-2697. [9] H. Schäfer, M. Chatenet, ACS Energy Letters 3 (2018) 574-591. [10] H. Schäfer, S.M. Beladi-Mousavi, L. Walder, J. Wollschläger, O. Kuschel, S. Ichilmann, S. Sadaf, M. Steinhart, K. Küpper, L. Schneider, ACS Catal. 5 (2015) 2671-2680. [11] F. Moureaux, P. Stevens, G. Toussaint, M. Chatenet, J. Power Sources 229 (2013) 123-132. [12] L. Magnier, G. Cossard, V. Martin, C. Pascal, V. Roche, E. Sibert, I. Shchedrina, R. Bousquet, V. Parry, M. Chatenet, Nat. Mater. 23, 252–261 (2024)