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This book offers comprehensive information on the main techniques for measuring water-oxidation-catalyst (WOC) performance, with a particular focus on the combined use of sacrificial oxidants and dyes within closed-batch reactors. It provides an overview of the latest advances in the synthesis of more efficient WOCs, followed by an analysis of the requirements for sustainable energy production. Readers will find a detailed description of the reaction mechanism used in catalyst assessment systems, which reveals the benefits and limitations of the most common sacrificial oxidant/dye pair. Experimental techniques including electrochemical methods for characterizing novel and non-photoactive WOCs are also described.

Throughout the book, various manganese oxides are used as examples of the techniques reviewed or proposed systems. Cost considerations and technological perspectives of the scale-up of solar-driven hydrogen production are also addressed. Lastly, the book presents lessons learned from the implementation of a large-scale real-world device.

Chapter 1. Introduction to the Water Splitting Reaction

Abstract
This chapter will cover the most important aspects concerning the water splitting reaction by providing an overall description of the main solutions, as reported in the literature, addressing the improvement of the efficiency of solar driven hydrogen production. The natural water splitting system will be considered as a starting point, in order to gain a better understanding of the main challenges that material scientists are facing towards the development of novel catalysts of water oxidation, mainly. The attention will focus on the water oxidation half-reaction, since it is the most complex and demanding process from both the kinetic and thermodynamic points of view. In addition, some general aspects of the photocatalytic water splitting and the principal semiconductors studied for such process will be reviewed.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 2. Steps Towards a Sustainable Hydrogen Production from Sunlight and Water

Abstract
This chapter focuses on some aspects concerning the realization of an actually sustainable H2 production, and especially the need for earth abundant, environmental friendly, solar driven heterogeneous catalysis for the Water Oxidation reaction: the catalyst shall be thermodynamically and mechanically stable to allow cyclic long-term operations. The focus will be mainly on Mn- and Co-compounds, though reference will be made to other compounds, when appropriate.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 3. Sacrificial Oxidants as a Means to Study the Catalytic Activity of Water Oxidation Catalysts

Abstract
An overview of the different sacrificial oxidants used in literature is reported, paying particular attention to the “sacrificial pair” $$\text{Ru}(\text{bpy})_{3}^{2 + } \text{/S}_{2} \text{O}_{8}^{2 - }$$, a photosystem made of a Ru-dye (Tris(bipyridine)ruthenium(II) dichloride, working as “antenna” for visible light) and a final electron acceptor (i.e. the persulfate ion). Such sacrificial oxidant is one of the most common in the literature and it was used in all the experiments described in Chap. 4. Different configurations of batch reactors can be used in the sacrificial-oxidant-driven water oxidation (WO) reaction, and three of them (i.e. the Clark-electrode Cell, the Stripping Flow Reactor and the Bubbling Reactor) are described in detail. The effects of both mass transfer limitations and side reactions on the determination of the two parameters describing the activity of water oxidation catalysts (i.e. the O2 production rate and the total evolved O2) are discussed, evidencing how such undesired phenomena occur to a different extent with the three reactor configurations.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 4. Use of the Bubbling Reactor with the Photosystem for Measuring the Rate of Water Oxidation as Promoted by Different Manganese Oxides

Abstract
In the current chapter, the use of a bubbling reactor for investigating the activity of different manganese oxides (i.e. Mn2O3, Mn3O4 and MnO2) is described. The most important aspects of water oxidation reaction considered are the role of the catalyst specific surface area (SSA), and the effect of the irradiance on the $${\text{Ru}} ({\text{bpy}})_{3}^{2 + } /{\text{S}}_{2} {\text{O}}_{8}^{2 - }$$ system. The former was investigated by preparing and testing samples with the same crystal structure, but with different SSA values. Notably, water oxidation catalyst activity does not strictly increase with SSA, but rather depends on the preparation route, which affects the nature of the surface. The effect of the irradiance was studied by using three irradiances conditions (i.e. 0.3, 0.5 and 1.0 sun) and three different catalyst contents (i.e. 10, 20 and 40 mg), evidencing how the increase in irradiance enhances the degradation processes rather than the O2 evolution. On this basis, a kinetic model imposing steady state conditions on transient species was developed, yielding a simple linear combination of two exponentials as expression of $$R_{{{\text{O}}_{2} }}$$. The results show that the ratio between the kinetic constants of the desired (i.e. O2 formation) and undesired path (i.e. dye degradation) decreases at increasing irradiance, evidencing how the role of parasitic reactions, far from being negligible, tends to be overwhelming.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 5. Electrochemical Measurements as Screening Method for Water Oxidation Catalyst

Abstract
In actual water splitting devices, the WOC will be deposited on an anode surface. Therefore, whatever the results obtained with WOC particles suspended in stirrer tank reactors, it becomes necessary to study WOC performances by means of electrochemical experimental setups. The WOCs deposition on an anode will depend on their physico-chemical nature, therefore several deposition methods, including wet and dry approaches, are found in literature. This Chapter reviews the available electrochemical techniques that can be adopted to study WOCs that are deposited on an electrode. In addition, the parameters used in literature to compare the different WOC materials will be explained. At the end of the Chapter, an example of the performance of different MnOx films will be reported. The water oxidation activity of three MnOx crystalline phases prepared by two deposition techniques will be compared. The aim of the comparison is to determine whether two electrodes having the same crystalline phase behave differently or not when they are deposited by two different techniques.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 6. Scaling Up the Process of Photo-Electrochemical Water Splitting

Abstract
This chapter will address the main issues related to the scaling up of a WS process. Currently, only few examples of pilot-scale water splitting (WS) devices are available in literature: low efficiencies and high costs are the main aspects that limit the use of this technology at industrial level, since results obtained at lab-scale are difficult to reproduce in larger configurations. The main factors that influence the device efficiency, which include activity and stability of catalysts and photoabsorbing materials, synthesis procedures, operational conditions and device configuration, will be examined. By the end of this chapter, a case of a pilot-scale photoelectrochemical (PEC) device will be presented and a prospective development of this technology will be discussed.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli

Chapter 7. Conclusions

Abstract
The current chapter reports the main conclusions that were drawn during the writing process. Suggestions on how to optimize the experimental methodology for a more accurate comparison between different heterogeneous catalysts are given. The difficulties concerning the issues of the scale up and of the efficiency of novel water splitting cells are addressed, as well. Finally, future perspectives of the solar driven hydrogen production technology are foreseen.
Carminna Ottone, Simelys Hernández, Marco Armandi, Barbara Bonelli