Immobilization of Polyoxometalates for Electrochemical Water Oxidation Público

Abstract

Electrochemical cells capable of efficiently splitting water into its elemental constituents, H2 and O2, the former being a potent green fuel, are a significant area of scientific focus today. One important component of such a cell is the anode, which should be stable toward to the harsh conditions needed for water oxidation. In addition, to overcome the exceedingly large overpotential required to oxidize water, a suitable catalyst, which is immobilized on the anode, is needed. In pursuit of these aims, the polyoxometalate water oxidation catalysts [Co4(H2O)2(XW9O34)2]^10-, where X = V(V) and P(V) have been immobilized and characterized on various anode materials. In two different systems, the solubility of the immobilized catalyst was found to greatly influence its activity and stability. When the water soluble salt of [Co4(H2O)2(PW9O34)2]^10- is immobilized on cationic TiO2 film anodes, and is used to oxidize water, the catalyst decomposes to an unknown cobalt oxide species. In another system, when water-insoluble [Co4(H2O)2(VW9O34)2]^10- is immobilized in a carbon paste anode, under turnover conditions, no catalyst decomposition is observed under turnover conditions, whereas a water-soluble salt of the same catalyst will decompose under these conditions.

Table of Contents

Table of Contents

Abstract.................................................................................................................. iv

List of Abbreviations............................................................................................ vii

List of Figures........................................................................................................ ix

List of Schemes........................................................................................................ x

Chapter 1 General Introduction............................................................................ 1

Background............................................................................................................................................... 2

Polyoxometalates................................................................................................................................... 5

POM WOCs................................................................................................................................................ 6

Chapter 2 Electrostatic Immobilization of [Co4(H2O)2(PW9O34)2]^10- with the Viologen Bis(2-phosphonoethyl)-4,4'-bipyridinium................................................................................... 8

Introduction.............................................................................................................................................. 9

Experimental......................................................................................................................................... 12

Results and Discussion....................................................................................................................... 15

Conclusions............................................................................................................................................. 26

Chapter 3 Carbon Paste Electrodes for Water Oxidation: Immobilization of the WOC [Co4(H2O)2(VW9O34)2]^10-.................................................................................................... 27

Introduction........................................................................................................................................... 28

Experimental......................................................................................................................................... 30

Results and Discussion....................................................................................................................... 32

Conclusions............................................................................................................................................. 40

References and Notes........................................................................................... 41

List of Figures

Figure 2.1. Photograph of the one-compartment cell for bulk electrolysis. Co4P2 modified working electrode (left), Ag/AgCl (3 M NaCl) reference electrode (center) and Pt foil auxiliary electrode (right). 14

Figure 2.2. CV trace of TiO2@FTO. Scan rate: 100 mV/s.................................... 17

Figure 2.3. CV trace of electrode 1. Conditions: 0.1 M pH 8 sodium borate buffer, 0.2 M KCl, scan rate = 50 mV/s.................................................................................................................... 18

Figure 2.4. CV traces of Co4P2 modified electrode 1 (red), Zn4P2 modified electrode 1 (orange), and unmodified electrode 1 (dashed black). Conditions: 0.1 M pH 8 sodium borate buffer, 0.5 M KNO3. Scan rate: 100 mV/s..................................................................................................................... 21

Figure 2.5. XPS spectra series for Co4P2 modified electrode 1 before (top panels) and after (lower panels) one-hour BE at 1150 mV. The left panels are W 4f and the right panels are Co 2p. 24

Figure 2.6. Electrochemical data series for Co4P2 modified electrode 1. Panel A : one-hour bulk electrolyses at 1150 mV in the absence of 0.03 mM bpy. Panel B: one-hour bulk electrolysis at 1150 mV in the presence of bpy. Panel C: CV traces for electrode from Panel A, before (red) and after (blue) one-hour bulk electrolysis at 1150 mV. Panel (D): CV traces for electrode from Panel B, before (red) and after (blue) one-hour bulk electrolysis at 1150 mV. Note: The anodic limit was 1150 mV in the CV traces in Panel D. Scan rate: 50 mV/s for CV traces................................................................................................ 25

Figure 3.1. CV traces for unmodified CPE (blue) and both Cs & TBA Co4V2 modified CPEs (red). Scan rate: 100 mV/s............................................................................................................ 34

Figure 3.2. Bulk electrolyses of TBA Co4V2 modified CPE first run (blue), second run (red) and of an unmodified CPE (green). The large spikes in the current are due to inefficient removal of O2(g) bubbles during the experiment.................................................................................................................... 35

Figure 3.3. FT-IR spectra of the TBA salt of Co4V2 pre-BE (black) and post-BE (red), extracted from the CPE............................................................................................................................... 36

Figure 3.4. RDE CV traces of BASi CPE (blue) and ceresin wax modified CPE (red). Scan rate: 100 mV/s.................................................................................................................................... 39

List of Schemes

Scheme 1.1. General scheme for a PV driven electrochemical water splitting cell. 4

Scheme 2.1. Immobilization of Co4P2 for electrochemical water oxidation. The positively charged viologen is covalently attached to TiO2 and will attract the negatively charged POM molecules (not drawn to scale).................................................................................................................................. 11

About this Master's Thesis

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School	Laney Graduate School
Department	Chemistry
Degree	MS
Submission	Master's Thesis
Language	English
Research Field	Chemistry, Inorganic
Palavra-chave	Electrochemistry Water Oxidation Polyoxometalates
Committee Chair / Thesis Advisor	Hill, Craig L, Emory University
Committee Members	Dyer, Brian, Emory University MacBeth, Cora E., Emory University

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