水素製造のためのバイオチャー/リグニン・アシスト型水電解

概要

Hydrogen (H2), as a most promising candidate for energy carrier as well as fuel for fuel cell electric vehicles and power generation, is expected to play a key role in the route to a greener future. Although most of currently available H2 is produced from fossil fuels such as nature gas, the introduction of solar/wind powers gives a hope to produce H2 renewably and largely through water electrolysis. The voltage-based energy efficiency of water electrolysis cell systems is 62–82%, while its further increase is still challenging. Carbon-assisted water electrolysis is proposed to increase the voltage-based efficiency by integrating chemical energy of carbon into that of H2. However, the enhancement of performance for practical use is still required. In this study, lignin and biomass-derived char (biochar) are suggested to be the excellent carbon materials to assist water electrolysis for H2 production with high performance. In this type of electrolysis, both materials are oxidized, in other words, electrochemically gasified at ambient temperature and pure H2 can be produced by integration of electrical energy and chemical energy of biomass. Furthermore, their assisted water electrolysis processes can be effectively combined with other biomass conversion processes.

Chapter 1 of this thesis introduces the existing technologies of H2 production including water electrolysis and biomass gasification. This chapter also includes the main objectives of this study.

Chapter 2 proposes the electrolysis of alkaline water that has dissolved lignin. In electrolysis of alkaline lignin solutions, linear sweep voltammetry (LSV) showed that the oxidation occurred at interelectrode potential of 0.45–0.7 V, which was much smaller than 1.23 V, the standard potential to split water into H2 and oxygen (O2) at 25 °C. It was thus expected that use of lignin potentially but　significantly raised the efficiency of the electrolysis, if defined by H2 production per electricity input. Results from the electrolysis of solutions of potential lignin monomers suggested that the lignin contained functional groups more reactive than those of phenol, guaiacol and syringol while as reactive as those of catechol. The oxidation of lignin was further enhanced by its hydrothermal process (HT) for 4.76 min at 300 °C prior to the electrolysis. HT produced little or no catechols, but caused depolymerization producing guaiacol and its derivatives. It was thus concluded that the depolymerization was responsible for enhanced oxidation reactivity of lignin by HT. Continuous electrolysis of lignin after HT showed higher efficiency than the reference alkaline water, while the improvement was as small as 0.1 (LHV of H2 per electrical energy input). More extensive depolymerization or formation of monomers as reactive as catechol is hence required for significant improvement of the efficiency and effective integration of chemical energy of lignin and electricity into H2. Besides, further investigation on details of reaction process in continuous electrolysis is required to find out the proper interelectrode potential for actual use of this process.

Chapter 3 proposes and demonstrates the biochar assisted acidic water electrolysis. LSV occurred at interelectrode potential as low as 0.5 V. The performance of biochar depended significantly on the carbonization temperature for its preparation. It was found that 850 °C was the best carbonization temperature that provided an optimum combination of specific surface area and carbon type distribution. It was revealed by continuous electrolysis that formation of O-containing functional groups on the biochar surface was predominant over CO2 formation at the anode while H2 was formed obeying the stoichiometry at the cathode. Accumulation of the O-containing groups on the biochar surface decreased its electrochemical reactivity, slowing down the electrolysis. Thermal treatment at 850 °C removed the major portion of O-containing groups from the spent biochar, fully recuperating its electrochemical reactivity. CO2 gasification enhanced the biochar activity, and its effect went far beyond the heat treatment. Based on the above-mentioned characteristics of BAWE, its combination with CO2 gasification as the biochar recuperator as well as syngas producer is proposed.

Chapter 4 summarizes the general conclusions of this work and some perspectives are proposed to improve this study.

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