Formation of Efficient Water Oxidation Electrocatalyst on Gibeon Meteorite and Stainless Steel Electrodes
Authors: Le Formal Florian , Guijarro Néstor , Bourée Wiktor S. , Prévot Mathieu S. , Sivula Kevin 
Published: February 2020

Electrocatalytic water oxidation remains a technology challenge, considering the high overpotential required to drive this 4-electrons reaction, which limits the performance of devices involving this process. However, recent investigations on abundant 3d transition metal electrocatalysts, in particular NiFe, FeCo or NiCo, have shown impressive reduction of overpotentials and demonstrated cooperative function of the different metals when alloyed.

Examining the electrocatalytic performance of naturally-occurring metallic minerals is of interest for energy conversion applications given their unique atomic composition and formation history. Herein, we report the electrocatalytic function of an iron-based Gibeon meteorite for the oxygen evolution reaction (OER) in alkaline medium.1 This meteorite originates from sub-saharan Africa and has the largest strewnfield known with pieces discovered over 20 000 km2 in Namibia and a total collected mass exceeding 21 tons. After ageing under operational conditions in an alkaline electrolyte, an activity matching or possibly slightly superior to the best performing OER catalysts emerges, with stable overpotentials as low as 270 mV (for 10 mA cm−
2) and Tafel slopes of 37 mV decade−
1. The Faradaic efficiency for OER was quantified as unity and no deterioration in performance was detected during the 1000 hours of OER operation tested at 500 mA cm−
2. Spectroscopic analyses suggest an operando surface modification involving the formation of a 3D oxy(hydroxide) layer with a metal atom composition of Co0.11Fe0.33Ni0.55, as indicated by Raman and XPS studies and trace Ir as indicated via elemental analysis. The growth of the catalyst layer was self-limiting to <200 nm after ca. 300 hours of operation as indicated through XPS depth profiling and cyclic voltammetry. The unique composition and structure of the Gibeon meteorite suggest that further investigation of Ir–Co–Ni–Fe systems or other alloys inspired by natural materials for water oxidation are of interest.

Indeed, metal electrodes based on nickel and manganese containing stainless steel also evidence the in-situ formation of a highly catalytic layer for OER in alkaline electrolyte, with overpotentials recorded below 250 mV. The formation of this layer is induced by selective dissolution of metal ions in solution and results in a porous, layered double hydroxide-type structure. The composition of the active layer is shown to be enriched in nickel and manganese while iron and chromium contents are drastically decreased as compared to the initial stainless steel composition. Remarkably, the final composition shows little or no dependence on the initial composition and highlight the synergetic function of different metal atoms. These results emphasize and rationalize the potential use of inexpensive and earth abundant metals for alkaline OER catalysis.

[1] Le Formal, F. L.; Guijarro, N.; Bourée, W. S.; Gopakumar, A.; Prévot, M. S.; Daubry, A.; Lombardo, L.; Sornay, C.; Voit, J.; Magrez, A.; Dyson P.J.; Sivula K. Energy Environ. Sci. 2016, 9, 3448-3455