Synthesis and Properties of Nanostructured Iron Oxide Photoanodes; Mechanisms of the Pulse Electrolysis and Photo-Electrolysis
2015
Mārtiņš Vanags

Defending
03.06.2015. 14:30, Rīgas Tehniskās Universitātes Materiālzinātnes un Lietišķās ķīmijas fakultāte, Paula Valdena iela 3/7, 272. auditorija

Supervisor
Andris Šutka, Jānis Kleperis

Reviewers
Jānis Grabis, Uldis Rogulis, Donāts Erts

Interface between solid/liquid is studied for two in practice widely used the renewable energy technologies - water electrolysis and photolysis. Importance of these both is determined by the need to accumulate the energy in its most effective way – energy carrier hydrogen. Inductive voltage pulse generating circuit is developed in this work to study the efficiency of water pulse electrolysis. From experimental results found that by providing inductive voltage pulse to water electrolysis cell, it is possibly separation of two distinct processes. The first is the rapid charging (up 1,5μs) of geometric capacitance of electrolysis cell, which accumulates the pulse energy, and second is subsequent slow discharge period between pulses (up to 50μs), when stored energy is used for water electrolysis. Thus, for the first time it is demonstrated that inductive voltage pulse applied to water electrolysis cell is separating charging process of cell’s geometric capacitance from charging electrode’s electric double-layer and charge transfer processes. A method is developed to compare direct current and pulse water electrolysis methods using effective values of current and voltage pulses. In order to obtain improved electrodes for photo-electrolysis of water, different methods are used and compared: spray pyrolysis, cathodic and anodic galvanization. Using these methods α-Fe2O3 thin films are derived for application as anode in water photolysis cell. It is estimated that pure α-Fe2O3 thin films produced with electroplating method has increase in the photocurrent at least three-fold comparing by the spray pyrolysis. It is shown that physical and photo-electrochemical properties of thin α-Fe2O3 film change by changing the pulse frequency at the cathodic voltage pulse plating method - increasing pulse frequency will reduce the average crystallite size, surface morphology becomes more homogeneous and photocurrent rises up. Markedly higher photocurrent is obtained using different amplitude pulses with opposite polarity in plating process. For the first time in photo-electrode studies thin films of α-Fe2O3 are doped with yttrium ions. It is estimated that Y3+ ion incorporate into the place of Fe3+ in hematite crystal structure reducing the concentration of oxygen vacancies, thereby decreasing concentration of intrinsic charge carriers and increasing the barrier layer thickness. Flat band potential at interface semiconductor/electrolyte is affected by Y impurity – shift to cathodic potential direction is observed, resulting in effective separating of light-induced electron-hole pairs. At higher concentrations Y begin to act as recombination center for light-induced charge carriers, reducing photo-activity.


Keywords
Impulsu elektrolīze, Elektriskais dubultslānis, Hematīts, Fotoanods

Vanags, Mārtiņš. Synthesis and Properties of Nanostructured Iron Oxide Photoanodes; Mechanisms of the Pulse Electrolysis and Photo-Electrolysis. PhD Thesis. Rīga: [RTU], 2015. 164 p.

Publication language
Latvian (lv)
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