SOLAR LIGHT DRIVEN MICRO FUEL (H2/O2) GENERATION DEVICE BASED ON THE MICROFLUIDIC CHIP

Y. Pihosh1, 2*, Y. Kajita1, K. Mawatari1, 2 and T. Kitamori1, 2 1The University of Tokyo, Department of Applied Chemistry, JAPAN

2Japan Science and Technology Agency, Core Research for Evolutional Science and Technology, JAPAN ABSTRACT

In this work, we report on the development of a solar light driven micro fuel (H2/O2) generation device based on the microfluidic chip platform which can be later applied in fabrication of portable micro fuel cell devices. We successfully integrated Pt microelectrode and TiO2 nanostructure as a phototoanode with the help of bottom up and top down technological processes in order to create an internal fuel generation and a separation system in the micro scale area. We describe the device concept and demonstrate its working principle under solar light illumination, and successfully verified H2/O2 generation rate by GC-MS in the microfluidic chip. KEYWORDS: Glass micro-chip, Solar water splitting, H2/O2 generation/separation INTRODUCTION

Recently, the claim for renewable energy sources has increased significantly due spreading portable electronic devic-es. To satisfy this claim, a high-density power source has come in need. Micro fuel cells (MFC) are considered a good candidate, because their energy density is higher compared to existing batteries as they use external fuel (H2/O2), but there are some difficulties when making portable MFC. To solve these problems, we propose to combine the fuel genera-tion system with an MFC device, which, however, requires separation of the generated H2/O2 and proton (H+). The pro-posed device will consist of two parts: internal fuel generation and a fuel cell [1]. The both parts are integrated in one closed system based on two microchannels which are bridged by the system of nanochannels arrays (for efficient H+ transfer and separation).

Firstly, two dimensional nanochannels made in fused silica have been proposed as efficient H+ transport media [2]. Through a series of experiments the H+ diffusion rate in nanochannels was directly verified, and the maximum value of H+ diffusion coefficient was detected at the nanochannels size of 180 nm [3], which was~4.5 times higher than the bulk value. This finding of enhanced proton mobility in glass nanochannels has been explained by the proton-transfer layer, which is located within approximately 50 nm from the glass/water interface where water molecules form a loosely struc-tured water network (inhibited molecular motions of water) [2,3]. Next, we proposed a structure of a light-driven micro fuel generation device (Fig.1a) integrated in a microfluidic chip [4]. The measured IPCE value of the previous device under the UV light irradiation was 12%, which is 1.5 times higher than the reported value for bulk experiments based on a TiO2 flat film.

At present work we successfully developed and integrated the TiO2-brush-type nanorods structure, which we reported recently [1], directly into the microfluidic device (Fig.1b) in a form of a photoanode, and improved the IPCE value up to 48%. Finally, the value of the generated H2/O2 has been evaluated with the help of the GC-MS technique. EXPERIMENTAL

Device design, working principle and fabrication Design of the micro fuel generation device based on the microfluidic chip platform is presented in Figure 1(a). The

fuel generation and separation functional module which is integrated in the chip are shown in the blue square. The mod-ule consists of three parts: two microchannels, one with an integrated nanostructured TiO2 photoanode and the other with a Pt cathode; a two-dimensional nanochannels array which bridges the microchannels for efficient proton transport and separation, and hydrophobic modified channels. The latter are used for H2/O2 separation through the Laplace pressure, which we reported recently[5].

The working principle of the proposed micro-fuel generation device is based in the photocatalytic fuel production. Once the solar light is applied to the TiO2 nanostructured micro photoanode, then water can be photocatalytically split into O2, H+ and free electrons (e-). Next, the protons are transferred to the Pt cathode through the system of the na-nochannels array. At the time when H+ and e- have reached the Pt cathode, H2 is catalytically produced, as shown in the inset of Fig 1(a). For autonomous fuel production in the microspace of the proposed device, the generated H2 and O2 gas-es must be separated from the microchannels in order to maintain continuous photocatalytical water splitting. For that reason the other hydrophobic shallow microchannels are used as we described above. The gases generation also takes place near the nanochannels array, however, due to high Laplace pressure between water and the nanochannels, the gas is prohibited to penetrate inside the nanochannels.

A photo of the fabricated microfluidic device for internal H2/O2 generation is presented in Fig. 1(b). The present de-vice has been realized on a fused silica plate utilizing the top down and bottom up technological processes: firstly, a two-dimensional nanochannels array (400nm wide and 200nm deep), microchannels (600µm wide and 6 µm deep) and hy-drophobic (300µm wide and 1µm deep) channels prepared by the standard electron beam lithography and lift-off pro-cesses, and, secondly, a TiO2-brush-type nanostructured photoanode and a Pt cathode integrated by our original Glancing Angle Deposition technique (GLAD) [6]. The SEM images shown in the inset of Fig. 1(b) present the central part of the

978-0-9798064-6-9/µTAS 2013/$20©13CBMS-0001 608 17th International Conference on MiniaturizedSystems for Chemistry and Life Sciences27-31 October 2013, Freiburg, Germany

microfluidic device and a high-magnified area of the integrated TiO2 photoanode and Pt cathode before the bonding. Once the integration has been completed, a second glass plate was bonded to this substrate using the low temperature bonding technique (at 100°C) [7]. After the chip fabrication, we performed a few kinds of experiments to demonstrate the device working principle under the solar light illumination.

Figure 1. (a) Design of the micro fuel generation device based on the microfluidic chip (the magnified central part pre-sents the fuel generation and the separation module with the explanation of its main functionality); and (b) Photo of the fabricated micro fuel generation device (the blue square represents the SEM images of the fuel generation and the sepa-

ration module with high-magnified SEM images of the integrated TiO2 photoanode and Pt cathode). RESULTS AND DISCUSSION

The photoelectrochemical (PEC) characterization of the fabricated device was conducted according to the standard PEC characterization protocol [8] in the 0.5M NaClO4 solution (pH=7) by using a three-terminal potentiostat with an Ag/AgCl reference electrode and the integrated TiO2-brush-type nanostructured photoanode and Pt microelectrode as a working and a counter electrode, respectively. At first, we measured incident photon-to-current efficiencies (IPCE) from 250 to 600 nm under a calibrated monochromatic light at a constant bias of 0.25V vs Ag/AgCl. The maximum IPCE (~45%) in the UV region was observed for the present device (Fig. 2a). This indicates that the UV light was effectively used even on one 500nm thick micro photoanode. In order to confirm the device stability, the photoresponse (I) over time (t), Amperometric I-t curve were collected with light on/off cycles under the solar simulated light (100mW/cm2) using the same three electrode configuration experimental set-up and the same applied bias voltage (Fig. 2b). In that case we observed a typical I-t profile that consists of the I spike (to ~ 90 µA/cm2), which, upon initial illumination, was followed by relaxation to the steady state level of 55 µA/cm2, thus indicating an excellent stability of the integrated photoanode. After 40 seconds of the I-t Amperometric test, the photocurrent value started to periodically sweep (Fig.2b). This inter-esting trend can be explained as follows: when the generated gases bubbles cover the electrodes, they interfere the photo-reaction which results in the photocurrent decrease. Then, the bubbles were removed from the electrodes and the photo-current increased again to the previous value. In order to confirm our assumption on the I-t sweeping characteristic, we carefully observed the formation of H2 and O2 bubbles on the surfaces of the Pt cathode and TiO2 photoanode, respec-tively, and this process has been recorded with a high speed camera through the microscope (Fig.2c). When the gasses bubbles reached the appropriate size, the photocurrent dropped, and once the bubbles have reached the edge of the hy-drophobic shallow channels, they separated there due to Laplace pressure, and the photocurrent value stabilized again.

609

In the next step of the device evaluation performance we meas-ured the fuel generation rate. After the gases generation and separation, the bubbles started coming out from the shallow hydrophobic micro-channels and were collected in the separated traps. Gas chromatog-raphy mass spectroscopy (GC-MS) was used to analyze the trapped gas-es and confirmed them to be hydro-gen and oxygen. The volume of hy-drogen collected in the trap varied linearly with the irradiation time, and showed a stable continuous hy-drogen production at a constant rate. Under present experimental condi-tions we obtained a hydrogen pro-duction rate ~ 55nL/min, and the oxygen production rate ~ 24nL/min, which is in good connection with the theory (Fig. 2d). CONCLUSION

In summary, we established the first necessary step for creation of an internal fuel (H2/O2) generation de-vice based on the microfluidic chip

driven just by solar light. The nanostructured TiO2 based photoanode integrated in the device demonstrated an excellent IPCE (~48%) and stable photocurrent time characteristic. In addition, we have shown the possibility of the H2 and O2 generation and separation in the present device under the solar light illumination, which was confirmed by the GC-MS technique. The fabricated device with the assembly of the MFC device will open a new route to creating a self-recharging high-energy portable power source for electronic devices. ACKNOWLEDGEMENTS

This work was supported by Japan Science and Technology Agency, Core Research for Evolutional Science and Technology. REFERENCES [1] Y. Pihosh, K. Mawatari, I. Turkevych, T.H.H. Le, Y. Kajita, M. Tosa and T. Kitamori, Hierarchical TiO2 brush type

nanostructures for efficient photoelectrochemical water splitting, Proc. Micro Total Analysis System, pp. 148-150, (2012).

[2] T. Tsukahara, A. Hibara, Y. Ikeda and T. Kitamori, NMR study of water molecules confined in extended nanospac-es, Angew. Chem. In. Ed. 46, pp. 1180-1183, (2007).

[3] H. Chinen, K. Mawatari, Y. Pihosh, K. Morikawa, Y. Kazoe, T. Tsukahara and T. Kitamori, Enhancement of pro-ton mobility in extended-nanospace channels, Angew. Chem. In. Ed. 51, pp. 3573-3577, (2012).

[4] Y. Kajita, Y. Pihosh, K. Mawatari and T. Kitamori, Development of light-driven H2/O2 generation chip for micro fuel cell devices, Proc. Micro Total Analysis System, pp. 2005-2007, (2012).

[5] A. Hibara, S. Iwayama, S. Matsuoka, M. Ueno, Y. Kikutani, M. Tokeshi and T. Kitamori, Surface modification method of microchannels for gas−liquid two-phase flow in microchips, Anal. Chem. 77, pp. 943-947, (2005)

[6] Y. Pihosh, I. Turkevych, J. Ye, M. Goto, A. Kasahara, M. Kondo and M. Tosa, Photocatalytic properties of TiO2 nanostructures fabricated by means of glancing angle deposition and anodization, Journal of the Electrochem. Soc. 156, pp. K160-K165, (2009).

[7] Y. Xu, C. Wang, Y. Dong, L. Li, K. Jang, K. Mawatari, T. Suga and T. Kitamori, Anal. Bioanal. Chem. 402, pp. 1011-1018, (2012).

[8] Z. Chen, T.F. Jaramillo, T.G. Deutsch, A. Kleiman-Shwarsctein, A.J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E.W. McFarland, K. Domen, E.L. Milled and H.N. Dinh, Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting pro-tocols, Journal of Materials Research 25, pp. 3-16, (2010).

CONTACT *Y. Pihosh, tel: +81-3-5841-7232; pihosh@icl.t.u-tokyo.ac.jp

Figure 2. (a) IPCE spectra of the micro fuel generation device at 0.25V vs Ag/AgCl. (b) The measured photocurrent time action spectra of the same device under solar light irradiation (at 0. 25V vs Ag/AgCl); (c) Snap shot of the H2/O2

gas generation and separation process in our device during solar light illumina-tion (100mW/cm2); (d) Production rate of H2 and O2 as a function of time (exper-

imental condition where the same as in (b) and (c)).

610