Nickel-based Electrocatalysts for Oxygen Evolution in Alkaline Water Electrolysis
Creators: name: Cossar, Emily
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2022
Subjects: Nickel
OER
Alkaline
Electrolysis
Ionomer

description: As atmospheric carbon dioxide (CO2) levels continue to rise due to anthropogenic fossil fuel utilization, the need to develop and employ alternative energy carriers becomes more and more critical. In recent years, interest in hydrogen (H2) has significantly increased as it is a clean and sustainable, alternative fuel, which can be both produced and utilized without greenhouse gas emissions; H2 can be produced via water electrolysis powered by renewable energy sources (RES), such as wind and solar energy, then, H2 can be utilized as a fuel in a hydrogen fuel cell, emitting only water as a by-product. Not only is H2 a clean alternative fuel, but it also provides an economically feasible way of storing renewable energy so that RES supply can be better regulated according to demand. Of the existing water electrolysis technologies, not many offer the ability to produce hydrogen both efficiently and at low cost. The alkaline environment of the more commonly employed traditional alkaline electrolyser allows for the use of non-noble metal electrocatalysts, as well as inexpensive cell materials. This process however suffers from an inefficient cell design. Conversely, the proton exchange membrane water electrolyser (PEMWE) utilizes a solid polymer electrolyte membrane, which allows for a compact, low resistance cell design. However, the harsh acidic environment of this device requires expensive platinum group metal (PGM) catalysts and expensive cell components. Anion exchange membrane water electrolysis (AEMWE) is a promising technology for low-cost, efficient H2 production as it combines the compact cell design of the PEMWE with the favourable alkaline environment of the traditional alkaline electrolyser. The electrochemical water splitting process is limited by the kinetically unfavourable oxygen evolution half-cell reaction (OER), which requires expensive rare catalysts such as iridium, to efficiently carry out the reaction. Nickel (Ni) is a promising inexpensive and abundant catalyst for the OER in alkaline media, due to its high activity and corrosion resistance. A significant increase in OER activity can be achieved by iron (Fe) incorporation into Ni catalysts. The addition of ceria (CeO2), a mixed ionic-electronic conductor with favourable oxygen storage and release properties, can also have a positive effect on catalytic performance. While developing electrocatalysts for improved OER performance is important, evaluating the studied materials as anodes in practical AEMWE devices is imperative as it accounts for the efficiency of the catalysts in electrode layers formed using an anion exchange ionomer (AEI). An AEI is a solid polymer electrolyte that serves as a binder for the particles as well as a hydroxide ion conductor in a catalytic layer of an AEMWE. The main objectives of this thesis are to (i) develop highly active NiFe-based nanoparticle (NP) catalysts with and without CeO2 for the promotion of the OER in AEMWE devices, and (ii) study the effects of commercial AEI type and amount on the efficiency of the produced NiFe-based particles in AEMWE anodes. These objectives will help further understand the behaviour of Ni-based catalysts in AEMWE systems, as well as the effects that catalyst-ionomer interactions can have on anode efficiency in carrying out the OER. The nanoparticles developed in this work were synthesized by an easily scalable chemical reduction method in ethanol using sodium borohydride. Results show that Ni NPs, which are around 4-6 nm in size, with 10 and 20 at% Fe, provide the highest OER performance. Incorporating small amounts of CeO2 into the NiFe materials results in better charge and mass transfer of the catalysts, however it introduces an additional ohmic resistance, which prevails over any OER-promoting interactions between NiFe and CeO2. The best NiFe-based catalysts with and without CeO2 were evaluated as anodes in a single cell AEMWE in combination with the commercial Fumatech Fumion® ionomer as well as the commercial Ionomr Innovations AemionTM ionomer. The single-cell AEMWE analysis of the various catalytic layers shows that Ni90Fe10 with 15 wt% Fumion® shows the best catalytic performance of 1.72 V at 0.8 A cm-2 in 1 M potassium hydroxide (KOH) at 50°C. Ni90Fe10 is also the most stable under operating conditions in comparison to the other tested Ni-based materials. While it was found that using 7 wt% AemionTM provided similar catalytic activity to 15 wt% Fumion®, results show that the AemionTM ionomer interacts with NiFe to inhibit the formation of NiOOH, the OER active phase. The results of this work highlight the complex interactions between Ni-based nanoparticles and anion exchange ionomers towards the OER and provide possible directions for future research and development in high performing Ni-based anodes for AEMWE.