Over 12% Light to Hydrogen Energy Conversion Efficiency of Hydrogen Generation from Water: New System Concept, Concentra
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Over 12% Light to Hydrogen Energy Conversion Efficiency of Hydrogen Generation from Water: New System Concept, Concentrated Photovoltaic Electrochemical Cell (CPEC) Katsushi Fujii1, Shinichiro Nakamura2, Kentaroh Watanabe3, Behgol Bagheri1, Masakazu Sugiyama4, Yoshiaki Nakano3 1
Global Solar plus Initiative, c/o RCAST, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904 JAPAN 2 RIKEN Research Cluster for Innovation, Nakamura Lab., 2-1 Hirosawa, Wako, Saitama, 351-0198, JAPAN 3 Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904 JAPAN 4 School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, JAPAN ABSTRACT Energy storage is a key technology for establishing a stand-alone renewable energy system. Current energy-storage technologies are, however, not suitable for such an energy system. They are cost ineffective and/or are with low energy-conversion efficiency. Hydrogen generation and storage from water by sunlight is one of these technologies. In this study, a simple concept of hydrogen generation from water by using sunlight, “concentrated photovoltaic electrochemical cell (CPEC)” is proposed. It is experimentally shown that the CPEC operates stably and achieves conversion efficiency from light to hydrogen energy of over 12%. INTRODUCTION Facing the exhaustion of fossil fuels, “renewable-electricity generation” by solar and/or wind power is becoming indispensible. In the true sense of an alternative energy to fossil fuels, energy storage of renewable energy is also important to complement its capricious nature. Hydrogen generation by water splitting is one of the promissing techniques for energy storage along with heat storage by concentrating solar power (CSP) and rechargeable batteries. Studies on hydrogen generation using water splitting have been focused on two main technologies. One is electrochemical water splitting by using electricity generated from polycrystalline silicon (c-Si) solar cells. The energy required for water splitting is at least 1.23 V corresponded to the Gibbs energy change. The open-circuit voltage (Voc) of a c-Si solar cell is around 0.6 and 0.7 V. This voltage is not high enough to provide 1.23 V, which is required to split water. Thus, the water splitting system has to be consisted to pursuit the energy conversion efficiency [1]. In addition, the energy conversion efficiency of c-Si solar cells from light to electricity is usually 10 – 15%. Considering with the conversion efficiency of electrochemical cell, the energy conversion efficiency from light to hydrogen is expected not to be high. The other technology is the photoelectrochemical water splitting using semiconductor particles or electrodes [2,3]. The required Gibbs energy change for water splitting of 1.23 V can be converted to the light wavelength of 1000 nm. The light wavelength less than 750 nm of the sunlight contains over 50% energy of sunlight. Thus, it is expected to use sunlight energy of over 50% to gener
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