Tuning the key determinants of 2-dimensional transition metal electrocatalysts for oxygen evolution reaction
Creators: name: De Silva, Gusthinnadura Oshadie
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 310607
subject: Nanobiotechnology
classificationCode: 340304
subject: Optical properties of materials
classificationCode: 401807
subject: Nanomaterials

description: Electrochemical water splitting is considered as an efficient, sustainable, and eco-friendly approach to produce hydrogen gas as an alternate fuel, which has the potential to meet the global energy demand. The electrochemical water splitting involves an oxidation reaction in which the oxygen evolution reaction (OER) occurs on the anode surface and a reduction reaction in which the hydrogen evolution reaction (HER) occurs on the cathode surface. The mass production of hydrogen gas is currently limited due to the high activation energy barrier involved in both OER and HER. However, the OER reaction has a higher activation energy barrier compared to HER. Therefore, designing and developing efficient OER electrocatalysts is vitally important to achieve the commercially viable production of hydrogen gas. Significant efforts have already been devoted to developed electrocatalysts for OER. However, there are many gaps that can be filled by further research. Among these, the ability to rationally design the OER electrocatalysts is of foremost important, as many factors such as physicochemical, electrical, and electrochemical properties of catalysts are expected to affect their performance. The key focus of this thesis is to investigate some of the crucial factors that can remarkably influence the performance of catalysts on OER. 

In a catalyst, the minority impurity phases typically present alongside the main catalytic phase, can play either synergistic or antagonistic roles during catalytic activity. During the synthesis of β-Co(OH)2, which is commonly known as a promising layered electrocatalysts for OER, the material can transform into CoOOH phase. Chapter 2 is focused on evaluating the influence of CoOOH impurity phase on the catalytic activity of 2-dimensional (2D) β-Co(OH)2 nanosheets. New synthesis methods were developed to obtain pure β-Co(OH)2 -phase material as well as that with the co-existence of CoOOH phase. This study revealed that the presence of CoOOH impurity on Co(OH)2 can reduce the thermodynamic barrier of OER while adversely impacting upon the kinetics of the reaction. The reduction of the thermodynamic barrier could be attributed to the presence of both Co2+ and Co3+ with the co-existence of CoOOH in β-Co(OH)2. Conversely, the significantly improved kinetics in pure β-Co(OH)2 could be attributed to better accessibility of electrochemically active sites compared to the material containing CoOOH impurity phase. This study emphasizes upon the importance of controlling reaction conditions while synthesising electrocatalysts to minimize the antagonistic effect on catalytic activity. 
Further, the modulation of the micro-environment of the electrocatalysts can be a promising strategy employed in designing catalysts. Chapter 3 demonstrates the importance of engineering the micro-environment between the 2D layers of β-Ni(OH)2 in promoting OER electrocatalysis. It is shown that the intercalation of different molecules between the atomic layers of a 2D electrocatalyst can be a smart strategy to achieve a control over local micro-environment during catalysis. The work involved synthesis of β-Ni(OH)2 nanosheets using triethylamine (TEA) as a hydrolysing agent. These TEA molecules could form clathrate-type structures with H2O molecules that facilitated improving the interlayer hydration and providing facile access to the reactant water molecules to the electrode during OER. The evaluation of OER performance revealed that the hydrated 2D β-Ni(OH)2 had a significantly lower overpotential and Tafel values compared to the non-hydrated β-Ni(OH)2, indicating remarkable performance of the former. This study underlines the importance of achieving a preferable micro-environment around the electrocatalyst, as demonstrated through a TEA-mediated interlayer hydration strategy in improving the catalytic performance towards OER.  

Developing a rational approach for designing efficient electrocatalysts for OER and HER is one of the most sought-after, yet most challenging method during catalyst development. The influence of different characteristics of electrocatalysts that simultaneous play either synergistic or antagonistic roles on the catalytic activity profoundly increases the complexity for designing high-performance electrocatalysts. The rational catalyst design approaches mostly rely on high-throughput data to develop structure and function relationships. However, an access to large data libraries remains difficult in the electrocatalysis field. The work presented in Chapter 4 is aimed to overcome this limitation. In this work, the strengths of multivariate correlation analyses were evaluated to quantify the individual contribution of key factors that influence the overall catalytic activity. To demonstrate the proof-of-concept, four different Co3O4 catalysts were synthesized, characterized, and analysed for their ability to catalyse OER. The characterisation of these catalysts provided a data library for nine different morphological, electrochemical, and electronic properties that acted as input parameters in the subsequent study. The output parameters or dependent variable that relied on the original nine independent variable were the overpotential and Tafel slopes of these catalysts while driving OER. A series of statistical tools employed in this study allowed to develop the relationships between properties and activity, and to quantify the contribution of each property of the catalyst towards the OER activity. The discovery of key determinants of OER activity in cobalt-based electrocatalysts from this work is likely to expedite new catalyst development process with further improved performance for OER. A generalisable approach presented in this work is also likely to benefit rational design of other (electro)catalysts for important reactions.

Overall, through studying the performance of nickel and cobalt-based electrocatalysts that are important transition metal electrocatalysts for OER, this thesis has deeply dived into three critical, yet often neglected aspects of electrocatalysts. These include potential impact of a minor impurity phase in catalysts; the importance of engineering the micro-environment of catalysts; and rational design of electrocatalysts through understanding the relationship between properties and function even with a small dataset, by employing appropriate statistical tools. All these outcomes are likely to prompt new research in the field of design and discovery of high performance electrocatalysts for OER.