Electrocatalyzing Oxygen Evolution Reaction with Nifeooh Aerogels

Abstract

Increasing the production capacity of electrical energy to fulfill the continuously rising global demand, while simultaneously trying to avoid greenhouse gas emissions in the process, and being environmentally sound, is one of the largest challenges of this era.One way to achieve it is to rely on hydrogen for energy storage. Nowadays, most of the hydrogen produced is mainly from fossil fuels, and the emission of detrimental gasses is only shifted. To get to a true green hydrogen, it is necessary to produce it in emissions-free processes.One method to achieve this is to use renewable energies in combination with electrochemical water electrolyzers, in which two distinct chemical reactions take place: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). Both reaction require catalysts to execute at high rates, and while the HER is considered to be relatively facile and takes place at low overpotentials, the OER requires relatively high overpotentials and high loadings of precious metal catalysts. It is considered the bottleneck reaction. The OER is a four electrons oxidation reaction per generated O2 molecule, and proceeds in four distinct reaction steps. This leads to a very sluggish reaction kinetics and high overpotentials to reach viable current densities.In recent years, more and more non-precious metal OER catalyst have been developed. Most notably is the family of mixed nickel-iron oxyhydroxides (NiFeOOH), which are relatively cheap, selective and efficient catalysts in alkaline media, and their performance has been increased by optimizing the Ni:Fe ratio, adding a third metal that either further increase the performance of the catalyst or/and its stability and other methods. One challenge that still remains is to increase the NiFeOOH surface area, and by that the electrochemically active site density (EASD).In this regard, one class of materials that has been attracting the attention of materials’ scientists in recent years are aerogels. Aerogels can be made from many different materials, such as silicates, carbons, metal organic materials, bio-inspired molecules, metals, and metal oxides. They consist of distinct units which form a porous 3D covalent framework (COF). Because of their diversity, aerogels have many different applications, e.g. as insulators, sensors, or catalysts.In this presentation we will report the synthesis of NixFeyOz aerogels, with a modified easy synthetic method via an epoxide route. These aerogels show much higher utilization of the material and overall increase in mass activity when catalyzing the OER when compared to other NiFeOOH derived materials. They were tested for their OER electrocatalytic activity and to the best of our knowledge these are the first aerogel materials that propagate OER themselves, rather than being used merely as support material for OER catalysts. The catalytic activity depends largely on the Ni:Fe ratio and not the surface area, which can lead to mass transport limitations when too high, showing an optimum for the ratio and the surface area.