Water Splitting Process

Figure 1.2:[Left] Photosystem II and [Right] Mn₄Ca-cluster structure.

Reprinted with the permission from Ref.²⁰

In the full oxidized S₄state, the Mn₄Ca cluster catalyzes the oxygen evolution, completing the four-electron water oxidation that splits water into molecular oxy-gen, protons and electrons.^21,22Experimental and computational characterization of the OEC structure and overall structural rearrangement during the stepwise photocatalytic cycle has been crucial to understand the reaction mechanism and for future design of artificial catalytic systems.^23–25

1.2 Water Splitting Process

Inspired by nature,²⁶the water splitting process^27,28is a two coupled half-reactions, which deals with the splitting of two water molecules to finally produce hydrogen as energy carrier^29,30(Eq. 1.2). The separate study of both half-reactions allows the rational tuning of specific catalysts for each case, as well as, the design of devices capable to perform the whole water splitting process.

2 H₂O(l)→2 H₂(g) + O₂(g) (1.2) Innovative ideas have been proposed to achieve more efficient energy conversion or storage systems such as alkaline water electrolysis, fuel cells and metal-air batteries. The design of these systems are complicated,³¹ but the essential ideas are well-known. All of them are based in two-electrode systems, where the cathode part involves hydrogen evolution reaction (HER) and the anode part

6 Chapter 1 - Introduction proceeds with oxygen evolution reaction (OER). The key reason that keeps these systems from the practical use to date is the slow kinetics of the oxygen evolution reaction. OER is a four electron-proton coupled reaction while HER is a two electron-transfer reaction, where it is expected that OER requieres higher energy to overcome the kinetic barrier to occur.

Note that the reactions at the cathode and anode parts for the water splitting reaction are different under acidic (Eq. 1.3) or alkaline (Eq. 1.4) conditions, as shown in the following equations. Acidic conditions requires the presence of second to third row precious-metals (Ru, Ir, Pt), while earth-abundant catalysts are only stable in neutral to alkaline conditions.

Acidic conditions:

Solar-driven water splitting devices^32–34 mainly require the presence of a light absorber, fuel forming electrocatalysts, an electrolyte and a means to sepa-rate the electrochemical cells, which shows its optimal performance in acidic media as proton exchange membrane (PEM). The main proposed devices de-signs are the commercial photovoltaic electrolyzer cells (PV-Electrolyzer), photo-electrochemical cells (PECs) and mixed colloidal devices³⁵(Figure 1.3).

1.2 - Water Splitting Process 7

Figure 1.3:Schematic draw of three electrochemical devices for the light driven water splitting process.

The Photovoltaic electrolyzer cells (PV-Electrolyzer)^36,37design use a commercial PV cell as light absorber, which is connected to a catalyst located in both anode and cathode electrodes. Main advantage is the large availability of PV cells and heterogeneous catalysts. However, the main drawback of such design is the high cost of the PV cells in a large-scale application. On the other hand, in mixed colloidal cells, the light harvester and catalyst are full integrated in discrete nanoparticles and suspended in the electrolyte, being a low-cost alternative to the PV-Electrolyzer cell. However, such design is not competitive enough needing enhancements in terms of stability and efficiency. Finally, photoelectrochemical cells³⁸are in between in terms of efficiency and feasibility of PV-electrolyzer and the mixed colloidal cells.

Photoelectrochemical cells (PECs)^39–41 are a developing technology that has shown significant promise in providing a solution for hydrogen production and further reaching applications as the technology matures. There is still much research required before PECs are a viable technology. The benefits for suc-cess are worthwhile and compelling as fully developed PEC technology offers a limitless energy source while having a very little impact on the environment.

This technique is somewhat similar to the process of electrolysis used in Proton Exchange Membrane fuel cells (PEMs), but instead of using electricity for the water splitting process, light from the sun is used as the energy source to obtain

8 Chapter 1 - Introduction hydrogen. PECs usually consist in a working and counter electrode, one or both being photoactive. An n or p-type semiconductor is generally used as working electrode with a platinum counter electrode. Electron-hole pairs are generated on the working electrode by photon absorption with an energy level equal or higher than the band-gap of the photoanode semiconductor. If an n-type semiconduc-tor is used, electrons are collected in the photoanode being transported to the counter electrode through an external circuit. The photogenerated electrons are consumed to reduce protons into hydrogen (H₂) at the cathode, while holes take part in the oxidation of water into (O₂) and protons at the anode cell. In contrast, p-type semiconductors employed as working electrode photogenerate electrons able to reduce protons into (H₂), while in the counter electrode water is oxidized into (O₂) and protons. The overall reaction is the cleavage of water by sunlight.

For instance, titanium oxide (TiO₂) has been extensively studied, but because of its large band gap (3-3.2 eV), TiO₂absorbs only the ultraviolet part of the solar emission and so has low conversion efficiencies. Numerous attempts to shift the spectral response of TiO₂ into the visible, or to develop alternative oxides affording water cleavage by visible light, have so far failed. As a consequence, a catalyst is often proposed to improve their performance such as a water oxidation (WOC) or hydrogen evolving (HEC) catalyst. The water oxidation catalyst (WOC) is able to regenerate the electron-hole pair by subtracting electrons from the oxidation of water, while the hydrogen evolving catalyst (HEC) is able to speed up the reduction of protons to H₂.