Abstract

A coupled thermochemical/electrochemical cycle was investigated to produce hydrogen from renewable resources. Like a conventional thermochemical cycle, this approach leverages chemical energy stored in a thermochemical working material that is reduced thermally by solar energy. However, in this concept, the stored chemical energy provides only a fraction of the energy required for effectively splitting steam to produce hydrogen. To push the reaction towards completion, an electrically-assisted proton-conducting membrane is employed to separate and recover hydrogen as it is produced. This novel coupled-cycle concept provides several benefits. First, the required oxidation enthalpy of the reversible thermochemical material is decreased, enabling the process to occur at lower temperatures. Second, removing the requirement for spontaneous steam splitting widens the scope of materials compositions, allowing for less expensive/more abundant elements to be used. Lastly, thermodynamics calculations suggest that this concept can potentially reach higher efficiencies than photovoltaic-to-electrolysis hydrogen production. A novel thermochemical/electrochemical test stand was conceptualized and constructed to prove the concept, and the practical feasibility of the proposed coupled cycle was assessed by validating the individual components of the system: proton conduction across a BaCe0.1Zr0.8Y0.1O3-δ (BCZY18) membrane, thermochemical activity of the CaAl0.2Mn0.8O3−δ (CAM28) working material reduced at 650 °C, and indirect observation of hydrogen production.

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