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Hang-Ah Park — 2013-14 Fellow

Project Title: Nano-Photochemical Cell with Spatially Separated Oxidation and Reduction Channels for Hydrogen Generation from Water under Visible Light

One of the most pivotal issues today is the shortage of fossil fuels and environmental pollution resulting from their use. Thus, developments in alternative renewable energy are necessary. Photocatalytic hydrogen generation from water is an environmentally desirable method to produce clean energy because it needs solar energy and water which are abundant, renewable, and innocuous. To split water using light, it is important to design a photocatalyst that can generate hydrogen from water efficiently. Among plenty of photocatalysts, titanium dioxide (TiO2) is a promising material due to its non-toxicity and stability, as well as abundance resulting in a cheap price. However, TiO2 has several limitations such as a large bandgap energy (3.2 eV) and fast electron-hole recombination rate. To solve these problems, synthesis of a hybrid photocatalyst with carbon materials, such as carbon nanotubes or grapheme, is getting more attention.

Another problem of using a single phase photocatalyst is that it is hard to separate the products of water splitting because there is no separation between the reduction and oxidation reactions accelerating recombination of generated charges and products. As a solution, the photoelectrochemical cell composed of a semiconductor as a working electrode connected through an external circuit to a counter electrode has been developed. However, using an external circuit is not necessary, and it would be better not to use it in terms of cost reduction.

Here, we suggest a novel photochemical cell composed of an abundant photocatalyst which can react under a visible light region and micro-channels for separation of the products without an external circuit. As a photocatalyst, TiO2 can be located outside of a CNT which acts as a photosensitizer and separator for TiO2, and platinum co-catalyst particles can be located inside of a CNT. CNTs should spatially separate the reduction and oxidation parts of water-splitting so that higher yield of products from water can be obtained under visible light. Hydrogen and oxygen should be accumulated effectively by developing spatially separated oxidation and reduction channels of the photochemical cell, which can be a promising candidate for renewable and sustainable energy in the future.

We fabricated the TiO2/CNT/Pt hybrid material for the main part of the photochemical cell and carried out characterizations of the materials to confirm the structure using Raman spectroscopy and UV-vis absorbance. In addition, a simple photocatalysis test was conducted via photo-degradation of methylene blue to see whether the material has photo-activity under visible light or not. The result shows our TiO2/CNT/Pt hybrid material has decomposed the methylene blue under visible light irradiation with enhanced photocatalytic activity than pristine TiO2 and TiO2/CNT.

The ultimate goal of the research is to fabricate a novel photochemical cell enabling the separation of hydrogen and oxygen generated from water under visible light irradiation through spatially designed micro-channels so that the next generation renewable energy system can be possible. To realize the goal, we are going to fabricate the whole photochemical cell to produce O2 and H2 separately. In the system, when photogenerated holes react with water molecules to produce oxygen on the surface of TiO2, electrons can transfer through a CNT to platinum particles inside of the CNT. The protons generated by water oxidation can transfer inside of the CNT to produce hydrogen by bonding with electrons on the platinum particles. Generated hydrogen can migrate through the open site of the CNT. Finally, by attaching a glass column on the top membrane, we will be able to obtain hydrogen and oxygen separately from water under visible light. The photochemical cell can contribute to renewable energy field with advantages of high efficiency, visible light response, and simple fabrication and perfect separation of the products.