Heterogeneous Water Oxidation
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Water oxidation is one of the half reactions of
water splitting Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. A version of water splitti ...
: 2H2O → O2 + 4H+ + 4e
Oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
(generation of dioxygen) 4H+ + 4e → 2H2 Reduction (generation of dihydrogen) 2H2O → 2H2 + O2Total Reaction Of the two half reactions, the oxidation step is the most demanding because it requires the coupling of 4 electron and proton transfers and the formation of an oxygen-oxygen bond. This process occurs naturally in plants
photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
to provide protons and electrons for the
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
process and release oxygen to the atmosphere, as well as in some
electrowinning Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution via a process commonly referred to as leaching. Electrorefining uses a similar process to remove impurities from a ...
processes. Since hydrogen can be used as an alternative clean burning fuel, there has been a need to split water efficiently. However, there are known materials that can mediate the reduction step efficiently therefore much of the current research is aimed at the oxidation half reaction also known as the Oxygen Evolution Reaction (OER). Current research focuses on understanding the mechanism of OER and development of new materials that catalyze the process.


Thermodynamics

Both the oxidation and reduction steps are pH dependent. Figure 1 shows the standard potentials at pH 0 (strongly acidic) as referenced to the normal hydrogen electrode (NHE). 2 half reactions (at pH = 0)
Oxidation2H2O → 4H+ + 4e + O2E° = -1.23 V vs. NHE Reduction4H+ + 4e → 2H2E° = 0.00 V vs. NHE Overall2H2O → 2H2 + O2E°cell = -1.23 V; ΔG = -475 kJ/mol
Water splitting can be done at higher pH values as well however the standard potentials will vary according to the
Nernst equation In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempera ...
and therefore shift by -59 mV for each pH unit increase. However, the total cell potential (difference between oxidation and reduction half cell potentials) will remain 1.23 V. This potential can be related to
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy as the recommended name; symbol is a thermodynamic potential that can be used to calculate the maximum amount of Work (thermodynamics), work, other than Work (thermodynamics)#Pressure–v ...
(ΔG) by: ΔG°cell = −nFE°cell Where n is the number of electrons per mole products and F is the
Faraday constant In physical chemistry, the Faraday constant (symbol , sometimes stylized as ℱ) is a physical constant defined as the quotient of the total electric charge () by the amount () of elementary charge carriers in any given sample of matter: it ...
. Therefore, it takes 475 kJ of energy to make one mole of O2 as calculated by thermodynamics. However, in reality no process can be this efficient. Systems always suffer from an overpotential that arise from activation barriers, concentration effects and voltage drops due to resistance. The activation barriers or
activation energy In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...
is associated with high energy
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
s that are reached during the electrochemical process of OER. The lowering of these barriers would allow for OER to occur at lower overpotentials and faster rates.


Mechanism

Heterogeneous OER is sensitive to the surface in which the reaction takes place and is also affected by the pH of the solution. The general mechanism for acidic and
alkaline In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...
solutions is shown below. Under acidic conditions water binds to the surface with the irreversible removal of one electron and one proton to form a platinum hydroxide. In an alkaline solution a reversible binding of hydroxide ion coupled to a one electron oxidation is thought to precede a turnover-limiting
electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change. These reactions involve electrons moving via an electronically conducting phase (typi ...
step involving the removal of one proton and one electron to form a surface oxide species. The shift in mechanism between the pH extremes has been attributed to the kinetic facility of oxidizing hydroxide ion relative to water. Using the
Tafel equation The Tafel equation is an equation in electrochemical kinetics relating the rate of an Electrochemistry, electrochemical reaction to the overpotential. The Tafel equation was first deduced experimentally and was later shown to have a theoretical ...
, one can obtain kinetic information about the kinetics of the electrode material such as the
exchange current density In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler–Volmer equation and other electrochemical kinetics expressions. The Tafel equation describes the dependence of current for an electrolytic process to ...
and the Tafel slope. OER is presumed to not take place on clean metal surfaces such as platinum, but instead an oxide surface is formed prior to oxygen evolution.


Catalyst Materials

OER has been studied on a variety of materials including: * platinum surfaces *transition metal oxides *first-row transition metal
spinels The spinels are any of a class of minerals of general formulation which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a cubic close-packed lattice and the cations ...
and
perovskites A perovskite is a crystalline material of formula ABX3 with a crystal structure similar to that of Perovskite, the mineral perovskite, this latter consisting of calcium titanium oxide (CaTiO3). The mineral was first discovered in the Ural Moun ...
. Recently Metal-Organic Framework (MOF)-based materials have been shown to be a highly promising candidate for water oxidation with first row transition metals.; Preparation of the surface and electrolysis conditions have a large effect on reactivity (defects, steps, kinks, low coordinate sites) therefore it is difficult to predict an OER material's properties by its bulk structure. Surface effects have a large influence on the kinetics and thermodynamics of OER.


Platinum

Platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
has been a widely studied material for OER because it is the catalytically most active element for this reaction. It exhibits
exchange current density In electrochemistry, exchange current density is a parameter used in the Tafel equation, Butler–Volmer equation and other electrochemical kinetics expressions. The Tafel equation describes the dependence of current for an electrolytic process to ...
values on the order of 10−9 A/cm2. Much of the mechanistic knowledge of OER was gathered from studies on platinum and its oxides. It was observed that there was a lag in the evolution of oxygen during electrolysis. Therefore, an oxide film must first form at the surface before OER begins. The Tafel slope, which is related to the kinetics of the electrocatalytic reaction, was shown to be independent of the oxide layer thickness at low current densities but becomes dependent on oxide thickness at high current densities


Iridium oxide

Iridium oxide (IrO2) is the industry standard OER catalyst used in polymer electrolyte membrane electrolysis due to its high stability. It was first proposed in the 1970s as an OER catalyst, and has been widely researched and implemented since then.


Ruthenium oxide

Ruthenium oxide Ruthenium oxide may refer to either of the following: * Ruthenium(IV) oxide, RuO2 * Ruthenium(VIII) oxide, RuO4 {{Short pages monitor