Basicity and Electrolyte Composition Dependent Stability of Ni-Fe-S and Ni-Mo Electrodes during Water Splitting.

Jochem H J Wijten,Iván Garcia‐Torregrosa,Eva A Dijkman,Bert M Weckhuysen

doi:10.1002/cphc.201901219

Abstract

Non‐noble metal electro‐catalysts for water splitting are highly desired when we are moving towards a society where green electrons are becoming abundantly available, offering clear prospects to make our society more sustainable. In this work, Ni−Fe−S is reported as a high performing anode material for the water splitting reaction, operating at low overpotentials and showing high apparent stability. Furthermore, Ni−Mo electrodes are developed on metallic foam substrates and optimized in terms of their performance. The Ni−Fe−S material as anode, combined and integrated with Ni−Mo as cathode in a cell configuration, splits water at 10 mA cm−2 and a potential of 1.55 V. Similar to previous reports, we confirm that Mo leaches from Ni−Mo/Ni foam electrodes. Cycling tests and ICP‐AES measurements show that the stability of Ni−Fe−S is apparent, and that in reality S is leaching from the material as was already suggested in literature. We expand on this knowledge and show that the leaching of S is dependent on both pH and the cation used during electrocatalysis. Furthermore, we find that applying an oxidative potential is in truth stabilizing towards S and that the alkalinity causes leaching. S was furthermore mobile and found to segregate towards the surface. Finally, using too low pH values (11 and lower) result in the passivating hydroxide metal layers being destroyed and the Ni−Fe−S dissolving completely.

Highlights

Introduction reaches overpotentials of only290 mV at 10 mA/cm2.[1,7,13,14]Ni Fe S is recently studied as a promising water splitting anode material reaching lower reported overpotential values for oxygen evolution reaction (OER) down to 65 mV.[15,16,17,18,19] It should be noted that this low overpotential was, in part, due to the use of high surface area substrates (Ni foam).[15]
Material stability was studied in detail and we found that S leaches from the Ni Fe S material as a function of pH and electrolyte cations
For Ni Mo, homogeneous, amorphous coatings are formed via electrodeposition on high surface area (5400 m2/m3) Ni foams with a granular morphology containing several large cracks (Figure 1a), which is a typical morphology for these materials.[24,25,26]

Summary

Introduction

Introduction reaches overpotentials of only290 mV at 10 mA/cm2.[1,7,13,14]Ni Fe S is recently studied as a promising water splitting anode material reaching lower reported overpotential values for OER down to 65 mV.[15,16,17,18,19] It should be noted that this low overpotential was, in part, due to the use of high surface area substrates (Ni foam).[15]. Ni Fe S is recently studied as a promising water splitting anode material reaching lower reported overpotential values for OER down to 65 mV.[15,16,17,18,19] It should be noted that this low overpotential was, in part, due to the use of high surface area substrates (Ni foam).[15] Ni Fe S can be used in seawater based electrolytes without forming chlorine.[20]

Results

Conclusion