Progress and challenges pertaining to the earthly-abundant electrocatalytic materials for oxygen evolution reaction
Introduction
The water splitting or electrolysis for producing hydrogen is restraint by the inherent thermodynamically unfavourable OER that necessitates high energy input for driving the reaction. Increasing the efficiency of the system is challenging in water electrolysis and this requires the development of highly efficient electrocatalysts for the OER. The conversion of intermittent solar or photovoltaic electrical energy to chemical energy using water electrolysis to produce hydrogen fuel is a valuable way of storing the surplus or excess solar energy produced during peak or maximum generation period (e.g. daytime). While at the low generation period (usually at night), the hydrogen fuel could be utilized to regenerate the electricity through the fuel cell which is an energy device that electrochemically converts fuel e.g. H2 to electricity with lower emission of greenhouse gas, higher power density, and higher efficiency [1,2]. When H2 is used as fuel, the only by-product of the reaction is water which can, in turn, serve as feedback into the water electrolysis processes. Fig. 1 presents a schematic illustration of the unique roles of water electrolysis and fuel cell in such an environmental-friendly energy solution [3].
Energy is in high growing demand globally, and the high rate of consuming these various forms of energies derived from the burning of fossil fuels, coal, oil, and gas exploration is causing great depletion to the environment. This has aroused a lot of research that is motivated and directed towards finding other energy sources [4]. The burning of fossil fuels on the other hand produces gasses that are not environmentally friendly [5,6]. Many ideas have been proposed by scientists to bring about an efficient energy conversion technique. Some of these include; alkaline water electrolysis, fuel cells, and metal-air batteries [[7], [8], [9], [10], [11]]. These systems are constructed to have a two-electrode system, in which the cathodic part is hydrogen evolution reaction (HER), while the anodic is the oxygen evolution reaction (OER) [12]. The process of anodic OER has been a bottleneck that has hindered the widespread application and commercialization of these systems. This is due to its sluggish kinetics, causing a high overpotential (η), ηa > ηc as shown in Fig. 2.
Mechanistically, since OER is a four electron-proton coupled reaction, it requires a sufficient amount of energy to subdue the kinetic barrier of OER from occurring. The noble metals or metal oxides of Ruthenium (Ru) and Iridium (Ir) are hitherto the most OER-efficient catalysts [[13], [14], [15]]. Nevertheless, their scarcity and high cost have warranted the need for the synthesis of other alternative OER catalysts. Although they possess high OER activity, however, RuO2 is unstable under high acidity and readily dissolves into the electrolyte due to the formation of high oxidation states [13,14,16]. Consequently, scientists are seriously doing thorough work to discover new families of cost-effective electrocatalysts, which has a performance that can be compared or even better than the noble metal-based materials [17]. One of how this can be achieved is to synthesize catalysts that have smaller dimensions and higher surface-to-volume ratios [18]. As a result of the breakthroughs in so many researches, several species of atoms, compounds, and materials have been investigated to exhibit a higher OER activity and are attracting great attention over the existing noble-metal based materials [19]. These include transitional metal compounds [18] polyoxometalates, carbon nanotubes (CNTs), single atoms, and carbon-based nitrogen-doped materials and complex oxides [20,21]. Many papers have discussed comprehensively, various OER electrocatalytic activities of catalysts with regards to individual forms of materials such as CNTs, single atoms, etc., [8,9,18,20,[22], [23], [24], [25]]. Most of them have not given a generalized discussion, in terms of various species of atoms, compounds, and materials.
Herein, we first described some tutorial guides into the mechanistic study of OER, gave a general overview of electrocatalytic kinetics for OER, with a focus on the various advantages and disadvantages of alternative earthly-abundant OER electrocatalysts that can replace noble-metal-based materials. Besides, the first few parts of this review give some tutorial manuals for describing various parameters needed in electrochemical (EC) OER processes. We do hope they will be useful as guiding principles that inspire the newcomers in this field. Excellent stability of an electrocatalyst remains an important parameter to achieve OER of practical implications and cost-effectiveness. Therefore, the present study also includes various insights into the stability studies and some technological and technical way forwards considering its significance when addressing the challenges associated with these electrocatalysts to meet the industrial and practical applications. Finally, the future perspective of designing and synthesizing efficient, low-cost electrocatalyst, and more reliable methods of evaluating the stability of electrocatalysts are discussed. Thus, providing information and insights that will foster future directions into fabricating efficient electrocatalysts for better OER performance and more industrial applications.
Section snippets
Electrocatalytic kinetics
An electrocatalyst is a catalyst that increases the speed of electrochemical reactions, which involves charge transfer as shown in Eqs.(1) - (10). It can serve as the electrode itself or possibly will have to need an adjustment on its exterior. An electrocatalyst generally works through taking reactants onto its surface to produce the adsorbed intermediates and in so doing improves charge transfer between the electrode and reacting species. The variety of electrocatalytic kinetic parameters
Basic mechanistic principles and electron transfer reaction of OER
This section provides a brief description of the mechanistic principle of OER, the brief theory of single/multiple electron transfer reaction and the corresponding relationships between the Tafel slope and these theories.
Transition metal compounds
The trends and progress in the development of earth-abundant first-row transition metal such as Fe, Mn, Co, Ni, Cu based electrocatalysts have led to their applications for OER. Numerous efforts have been devoted to the different electrocatalytic designs, syntheses, and characterizations to study the effectiveness and performance of those alternative OER electrocatalysts. In the past years, Ni/Co, Ni/Fe, and Co/Fe-based electrocatalyst systems have shown to be viable materials with the future
Stability studies
Hydrogen evolution reaction occurs very easily on a lot of metals at a low overpotential in water splitting because it involves only two (2) electron transfer steps. This is a different case in OER which involves two (2) steps: O-H bond breaking and attendant O-O bond formation. This is a four electron-proton coupled reaction, thus leading to a high overpotential for overall water splitting because the OER requires higher energy (and also higher overpotentials) to overcome the kinetic barrier [
Conclusion and future perspective
In this review, the tutorial manuals describing various parameters needed in electrochemical OER processes and the comprehensive summary of the electrocatalytic kinetics and theoretical modeling for OER have been provided. Also, a mechanistic study of OER, both in acidic and alkaline pH was described. With experimental results, DFT calculation and the single/multiple electron transfer reaction as clearly explained have been used to show support for the synthesis of high activity and good
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgments
Authors acknowledge their Universities and Institutes for the platform to carry out this research. The corresponding author acknowledges the award received from The World Academy of Sciences (TWAS) in partnership with South Africa's National Research Foundation (NRF) and Department of Science and Technology (DST) for the TWAS-NRF Doctoral Fellowships (UID: 105453 & Reference: SFH160618172220) and the NRF: S&F-Extended Support for Scholarships and Fellowships (Reference: MND190603441389, UID:
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