Tailored Nickel-Iron Layered Double Hydroxide Particle Size for Optimized O.E.R. Catalysis
Authors: Tyndall Daire , Nicolosi Valeria , Jaskaniec Sonia , Coelho João 
Published: February 2020

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) in the form of hydrogen fuel cells and metal-air batteries, among others. Perhaps the most significant feature of OER is the fact that it is a necessary ‘step’ in the evolution of H2 gas by water electrolysis, bringing with it an associated potential (E ≈ 1.23 V). To overcome this potential barrier with minimum overpoetential (η), an effective electrocatalyst is required to facilitate the reaction. Nickel-Iron Layered Double Hydroxide (NiFe-LDH) has been shown to exhibit efficient catalysis of the OER, demonstrating overpotentials in composite systems which are competitive with previously studied electrocatalysts based on rare earth metals such as ruthenium and iridium[1], as well as being competitive in an economic perspective. NiFe-LDH has other advantages over rare earth catalysts such as earth abundance, cost and stability (in operating conditions). The nature of the material in question is that the catalytic active sites are located at the edges of NiFe-LDH hexagonal crystals. This means that particle size will play an important role in its catalytic ability.
Synthesis of high quality, planar NiFe-LDH platelets with regular hexagonal morphology was achieved using a wet chemistry method at a relatively low temperature (100 oC) using triethanolamine (TEA) as a ‘capping agent’ to allow homogeneous coprecipitation of both the nickel and iron metal centers within brucite-like layers. 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 the same conditions[3] (5 mVs-1, quoted at current density j = 10 mAcm-2). The work aims to develop methods of post-synthetic treatment of NiFe-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[4].
Size studies have revealed lateral dimensions between 0.4 – 1 µm for the as-produced NiFe-LDH platelets with mean lateral size 0.74 µm (± 0.2). Using only centrifugation as a separation technique, in the range 500 to 3000 rotations per minute (rpm), it was possible to isolate platelet dispersions with mean lateral size down to 0.4 µm (± 0.2) (see figure). While an improvement in overpotential was observed for this method, a much more effective and economical method of size reduction can be achieved by using high-frequency tip-sonication to mechanically ‘break’ the LDH platelets in order to expose a higher density of active edge sites. The goal of this work is to understand the extent to which this technique can be used for the benefit of OER catalysis. To do this we essentially need to understand how small can we efficiently isolate ‘broken’ particle dispersions without compromising the NiFe-LDH’s intrinsic catalytic ability or compatibility with previously successful composite materials.
[1] Gong M.; Li Y.; Wang H.; Liang Y.; Wu J. Z.; Zhou J.; Wang J.; Regier T.; Wei F.; Dai H. Journal of the American Chemical Society
2013
135 (23), 8452-8455.
[2] Song F.; X. Hu, Nat. Comm, 2014 Volume 5, 447.
[3] Tahir M.; Pan L.; Idrees F.; Zhang X.; Wang L.; Zou J.; Lin Wang Z. Nano Energy, Volume 37, 2017, Pages 136-157.
[4] Eftekhari A. Materials Energy Today, Volume 5, 2017, Pages 37-57.

Figure 1