Heterogeneous Catalysts

As explained in the previous section, significant progress has been made along the last century in terms of the design of new organometallic complexes for water oxidation.⁷⁴Heterogeneous catalysts are, however, more advantageous due to their huge surface area, which leads to improved efficiency. Furthermore, high temperatures and hard conditions are often applied in industrial catalytic processes, which makes heterogeneous catalysts more convenient, since homo-geneous catalyst are often decomposed, losing their catalytic activity at these conditions.⁷⁵

The heterogeneous catalysts may facilitate the adsorption of the reactants and their subsequent conversion into products. However, the product must be rapidly removed from the surface to regenerate the active sites from the surface. This leads to the concept that the catalytic reaction is a cycle which is made up of elementary phisicochemical processes such as, chemisorption, dissociation or activation, diffusion, recombination and finally desorption.

Chemisorption is defined as the adsorption of reactants or intermediates on the catalyst surface with an interaction energy which is strong enough to form chemical bonds between the adsorbate and the surface and to weaken internal bonds within the adsorbate. This leads to the activation or dissociation of the adsorbate molecule. When the interaction between product molecules and the catalyst becomes too strong the desorption of product molecules becomes rate-limiting, decreasing the overall speed of the reaction. This leads to a balance between a sufficient catalyst-adsorbate bond strength to activate the adsorbate and surface poisoning, which should be avoided.

22 Chapter 1 - Introduction As a result, a volcano-type plot⁷⁶ (Figure 1.7) of the catalyst activity against reactant-interaction strength can be drawn, the catalytic activity increasing up to a particular interaction strength known as thesabatier maximum. This shape of the plot is a consequence of theSabatier principle⁷⁷which states that the rate of a catalytic reaction is maximized at an optimum interaction strength of the reactants with the catalyst, providing a rational strategy to optimize the catalyst features.

Figure 1.7:Sabatier volcano plot as Catalytic activity vs. Adsorption Energy.

Since OER is a surface reaction, the surface morphology also plays a significant role in the electrochemical performance. Thus, the problem is not limited only to find the best material but also the best morphological structure. The surface architecture should be characterized and designed according to the electrocat-alytic mechanism. This becomes even more complicated for multi-component electrocatalysts, where each component and also the position of each active site may have a different role in the overall mechanism.

In this section conventional materials such as metal oxides and mixtures of differ-ent metals with practical potdiffer-ential use are described. Since the OER performance is highly dependent on the material and structure, it is difficult to simply classify the performance in accordance with the electrocatalyst material.

1.4 - Water Oxidation Catalysts (WOC) 23

Metal Oxides

Unfortunately, water oxidation is constrained by the kinetically sluggish oxygen evolution reaction (OER) because it is thermodynamically and kinetically un-favorable for removing four electrons to form oxygen. Consequently, a huge amount of effort has been devoted to develop catalysts for more effective water electrolysis. Metal oxides, including RuO₂and IrO₂-based electrodes, first-row metals (Mn, Co, Ni, Cu) oxides, hydroxide layers, spinels and perovskites have been intensively studied.

First-row Metal Oxides

Due to the similarities with the Mn₄Ca cluster present in the OEC (Figure 1.2), the performance of MnO₂in OER has been studied. However, it’s poor conductivity keeps it behind its counterparts.⁷⁸Adding metallic catalysts contributes to the electrocatalytic activity enhancing significantly the electrical conductivity. Indeed, grafting Au⁰nanoparticles onα-MnO₂improves the electrocatalytic activity about 6 times.⁷⁹Electronic interaction provided by gold promotes the formation of the most active Mn³⁺sites for OER.α-MnO₂ is a promising structure for the OER performance, whileβ-MnO₂has a poor activity due to the absence of di-µ-oxo bridged Mn centers.^80,81

Co₃O₄has a significant good performance as an OER electrocatalyst, although only¹/1000of the exposed sites are responsible for the electrocatalytic activity.^82–84 As proposed in the literature, these minority sites are Co⁴⁺cations connected through bridging oxo species, which can effectively interact with water molecules.

The significant point is that the active sites are subject to change by varying their overpotential. This suggest that the overall system is quite dynamic, and can not be simply predicted based on the initial material or structure. In addition, the presence of Co³⁺and Co⁴⁺provides a good environment for the creation of active sites depending on the ratio. Since OER is firstly based on the adsorption on the catalyst surface, the electronic states play an essential role in its electrocatalytic activity and structure doping, facet growth control and oxygen vacancies have

24 Chapter 1 - Introduction been used to enhance the electro-performance of Co₃O₄for OER.

For crystalline electrocatalyst, the crystal orientation plays an important role in the OER performance due to the different number of active sites on each surface.⁸⁵Since the active sites in Co₃O₄involves a Co³⁺rather than Co²⁺, the OER kinetics can be faster on a Co³⁺rich facet.

Following with first row transition metal oxide derivatives, Cu is cheaper than Co and has a well-defined coordination chemistry and a wide redox capability.

Copper oxide^86,87 has a moderate activity for the OER performance, and this activity can be tuned by the surface morphology. However, the electrocatalytic activity of copper oxides is significantly lower than cobalt.

Perovskites

Perovskites^88–90with structure ABO₃is a metal oxide class of OER catalysts that has been intensively studied. Their physicochemical and catalytic properties can be tunned up by substituting ions of the same or different oxidation states in the A or B structural sites. The OER electrocatalytic activities of substituted perovskites (A_1-xA^’_xBO₃), where A is a lanthanide usually (La), A^’is an alkaline earth metal such as Sr, and B is a first-row transition metal, are reported.

Recently, theegfilling of the surface transition metal cation has been shown to greatly influence the binding of OER intermediates on the perovskites surface and their OER activity. The intrinsic OER activities exhibit a volcano type dependence on the occupancy ofeg orbitals of surface transition metal cation in an oxide.

Indeed, the highest OER activity among all oxides studied as predicted by theeg

activity descriptor is Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF).⁹¹

Spinels

The general formula is A^’B^’₂O₄where A^’ and B^’ atoms are usually made up of group 2, group 13 and first-row transition metal elements. This structure shows two crystallographic octahedral (O_h) and tetrahedral (T_d) sites. That leads to two types of spinel. The normal spinel has a configuration of (A²⁺_Td)(B³⁺_Oh)O₄and

1.4 - Water Oxidation Catalysts (WOC) 25 the other is the inverse spinel (A²⁺_Oh)(B³⁺_Td)(B³⁺_Oh)O₄, according to the crystal field theory⁹²the difference in metal coordination results in a different d-band splitting egt2gfor T_dandt2geg) for O_hsites.

Most spinel oxides are iron-based and cobalt-based compounds with other tran-sition or alkaline metals as dopants agents. In the case of iron-based systems (MFe₂O₄) where M=Co, Ni, Cu or Mn the OER activity trend is (CoFe₂O₄)>

(NiFe₂O₄)>(CuFe₂O₄)>(MnFe₂O₄).^93–95Nevertheless, the situation becomes different for cobalt-based systems (M_xCo_3-xO₄) where adding M=Li, Ni or Cu would be beneficial except for Mn in terms of OER activity.^96,97The reason why Mn lowers the OER activity in cobalt-based spinels is attributed to the suppression of the Jahn-Teller distortions.⁹⁸

Nobel Metal Oxides

Noble metal oxide catalysts based on RuO₂, IrO₂are important OER catalysts, in acidic conditions.^99–102These metal oxides are in the top of the volcano plot (Figure 1.8), which means that the intermediates binding energies correlate in a good compromise for the OER. These two precious metal oxides adopt a rutile structure, where Ru or Ir are located in the center of an octahedral site with oxygen in the corners, being shared between octahedrons, both RuO₂and IrO₂ are considered as benchmark electrocatalysts owing to their high electrocatalytic activities toward OER. However, the OER performance are highly influenced by the synthesis methodology and catalyst nature.

26 Chapter 1 - Introduction

Figure 1.8:Activity trends towards OER for metal oxide materials.

The performance of RuO₂is better than IrO₂, but it is not stable in conventional electrolytes. Experimentalists have shown that the OER performance of elec-trochemically deposited RuO₂is better than that of the chemically synthesized RuO₂, but its stability is worse. Moreover, IrO₂ films exhibits excellent OER activity reaching a higher current density than RuO₂nanoparticles with the same overpotential conditionsη^OER = 0.275V.

To improve the stability of RuO₂, doped bimetallic oxide systems (Ru_xIr_1-xO₂) have been proposed. This systems turned out to be very effective and only a small amount of Ir was incorporated into the catalyst, which could significantly suppress the deterioration without sacrificing much performance of OER.¹⁰³ As can be seen, there is a lot of controversy about which is the best option taking into account electrochemical activity and chemical stability of those heteroge-neous catalyst, that’s why both experimental and theoretical studies are needed to find the optimal candidate.

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