Immobilization Methods for Binding Polyoxometalates onto Surfaces for Water Splitting Applications Open Access

Wilkening, Sarah Marie Lauinger (2017)

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Photoelectrochemical conversion of solar light into hydrogen fuel through water splitting is the technological pathway to fuel with low or no CO2 emissions. The water oxidation half reaction of water splitting requires a durable and efficient catalyst. All-inorganic polyoxometalate (POM) water oxidation catalysts (WOCs) are oxidatively, hydrolytically and thermally stable, and can aid in the multi-electron process required to split water. Homogenous POM WOCs have been studied in a variety of conditions with photosensitizers and sacrificial electron acceptors and are becoming more understood; however, these homogenous catalysts behave differently when bound to a surface. Cost-effective thin-film and nanoparticle semiconductor materials are optimal photoelectrochemical materials that, when combined with POM WOCs, are a potential new hybrid material for water splitting systems.

The general goals of this thesis are to (1) immobilized POM WOCs onto light absorbing surfaces for solar-driven water oxidation; (2) stabilize the POM WOC on the light absorbing surface using an atomic layer deposition protection layer; (3) electrochemically evaluate the stability of these species when immobilized via silylation on metal oxide surfaces or via Nafion on glassy carbon electrodes; and (4) evaluate the stability of these homogenous species through early-time kinetic measurements of oxygen generation and oxidant consumption.

Prior to this work, immobilization techniques resulted in little POM WOC stability on the surface. The first part of this thesis discloses the silylation technique which results in high surface coverage of the POM while maintaining water oxidation activity. In the follow-up research, the immobilized POM is further stabilized with nanometers of protection using atomic layer deposition (ALD). The end of the thesis reports on the early-time kinetic studies by light-driven oxygen generation combined with oxidant consumption of several POM WOCs and the stability assessment of an electrocatalyst immobilized on glassy carbon electrodes for water oxidation.

Table of Contents

Chapter 1: Background of Photocatalysts and Water Oxidation Catalysts

1.1 Introduction and Outlook

1.2 Photocatalysis

1.3 Water Splitting and Challenges in Solar Energy Generation

1.4 Hydrogen Evolution Reaction Catalysts

1.5 Oxygen Evolution Reaction Catalysts

1.6 Overview of Polyoxometalate Chemistry for Solar Fuels

1.7 Heterogenized Systems with POMs

1.8 Goal of This Work

1.9 References

Chapter 2: Immobilization of Polyoxometalate Water Oxidation Catalysts onto Titanium Dioxide

2.1 Abstract

2.2 Introduction

2.3 Experimental

2.4 Results and Discussion

2.5 Conclusion

2.6 References

Chapter 3: Stabilization of Polyoxometalate Water Oxidation Catalysts onto Hematite Using Atomic Layer Deposition

3.1 Abstract

3.2 Introduction

3.3 Experimental

3.4 Results and Discussion

3.5 Conclusion

3.6 References

Chapter 4: Stability Assessment of Water Oxidation Catalysts and Conclusions

4.1 Abstract

4.2 Introduction

4.3 Experimental

4.4 Results and Discussion

4.5 Conclusion

4.6 References

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