Novel two-step thermochemical cycle for hydrogen production from water using germanium oxide: KIER 4 thermochemical cycle

This paper proposes a novel two-step thermochemical cycle for hydrogen production from water using germanium oxide. The thermochemical cycle is herein referred to as KIER 4. KIER 4 consists of two reaction steps: the first is the decomposition of GeO2 to GeO at approximately 1400–1800 °C, the second is hydrogen production by hydrolysis of GeO below 700 °C. A 2nd-law analysis was performed on the KIER 4 cycle and a maximum exergy conversion efficiency was estimated at 34.6%. Thermodynamic analysis of GeO2 decomposition and hydrolysis of GeO confirmed the possibility of this cycle. To demonstrate the cycle, the thermal reduction of GeO2 was performed in a TGA with mass-spectroscopy. Results suggest GeO2 decomposition and oxygen gas evolution. To confirm the thermal decomposition of GeO2, the effluent from GeO2 decomposition was quenched, filtered and analyzed. SEM analysis revealed the formation of nano-sized particles. XRD analysis for the condensed-filtered particles showed the presence of Ge and GeO2 phases. The result can be explained by thermodynamic instability of GeO. It is believed that GeO gas disproportionates to ½Ge and ½GeO2 during quenching. 224 ml hydrogen gas per gram of reduced GeO2 was produced from the hydrolysis reaction.