The oxygen evolution reaction (OER) has drawn significant interest in the field of renewable and sustainable energy in recent years, with potential applications for hybrid electric vehicles (HEV) among other areas. Perhaps its most important feature is the fact that it is a necessary ‘step’ in the evolution of H2 gas by water electrolysis, bringing with it a significant potential (E ≈ 1.23 V). To overcome this potential barrier with minimum overpoetential η, an effective electrocatalyst is required to facilitate the reaction. NixFey Layered Double Hydroxide (LDH) has been shown to exhibit efficient catalysis of the OER, demonstrating a more competitive overpotential than previously studied electrocatalysts based on rare earth metals such as iridium. NixFey LDH has other advantages over these catalysts such as earth abundance, cost and stability (in operating conditions). Downsizing of the NixFey LDH can result in an even smaller value of η. This enhanced catalytic activity is a result of an increase in edge-site density in dispersions with smaller average size. The aim here is to improve NixFey LDH catalysis based on this relationship.
In this work, synthesis of high quality, planar NixFey LDH platelets with regular hexagonal morphology was achieved using a wet chemistry method at low temperature (100 oC). Using platelets synthesized in this way, OER catalysis has been demonstrated with η = 0.36 V, a competitive value when compared to many state-of-the-art OER electrocatalysts in identical conditions (5 mVs-1, quoted at current density j = 10 mAcm-2). Next, the project concentrated on the post-synthetic treatment of NixFey LDH dispersions to reduce lateral platelet dimensions and further improve edge-site density for electrocatalytic optimization. This study comes in accordance with the growing number of high-performance electrocatalysts being studied for OER.
Two primary techniques are employed here to improve NixFey LDH catalytic activity. Firstly, size selection of the material using centrifugation. Operating in the relatively low centrifuge rate range of 500 – 3000 rpm, it is possible to isolate hexagon dispersions with average lateral platelet size from 0.8 μm down as low as 0.4 μm. Secondly, tip sonication of dispersions is employed, with the aim of breaking LDH platelets and hence exposing more edge sites. In contrast to the centrifugation method, this will essentially destroy the materials hexagonal morphology but with an expected trade off of enhanced catalysis with respect to OER.
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