A hydrogen economy requires primary sources to provide the power required to produce hydrogen from water, fossil fuels, biomass or similar compounds. Water-splitting processes are particularly attractive, since water is abundant around the planet, the unique products from water splitting are hydrogen and oxygen and a variety of primary energy sources can be used to produce the hydrogen. External electric power can be used to produce hydrogen by conventional water electrolysis [1], but it firstly requires the production of electricity to realize the electrochemical splitting of the water molecule. A more efficient and potentially more cost effective approach is to use thermal power to drive a thermochemical water splitting process. Such cycles break the hydrogen-oxygen bounds through a series of heat-driven chemical reactions, with recirculation of intermediate substances of different type. By this approach the external heat which drives the chemical reactions is converted into chemical energy of the hydrogen produced from water splitting.

Among the various water splitting processes, the thermo-electrochemical Hybrid Sulfur (HyS) is one of the most appealing cycles. It has only fluid reactants and is comprised of only two global reaction steps: a low temperature electrochemical exothermic section, operating at temperatures on the order of 100 °C, and a high temperature thermal section operating at max temperatures of about 800 °C. In the electrochemical section SO₂ and H₂O are combined together to produce electrochemically H₂ at the electrolyzer cathode and H₂SO₄ at the electrolyzer anode. Sulfuric acid is recycled inside the plant, concentrated and decomposed, in the high temperature thermal section, into SO₂, O₂ and H₂O. Oxygen is then separated from SO₂ and water and extracted as byproduct of the process.

Solar driven HyS process is being studied by the Greenway Energy and the University of South Carolina within the DOE HydroGEN program (part of the DOE Energy Material Network initiative) [2], partnering with the Idaho National Laboratory, the Savannah River National Laboratory and the National Renewable Energy Laboratory. The project mostly focuses on the high temperature sulfuric acid decomposition section, developing new optimized catalyst formulations (i.e. achieving higher conversion, lower performance lifetime degradation and lower costs), to be integrated in novel solar reactor systems. A new bimetallic catalyst formulation is being developed and synthesized, with the catalyst deposited on TiO₂ support. Compared with the current state of the art sulfuric acid decomposition catalysts, developed by Idaho National Laboratory during the DOE Nuclear Hydrogen Initiative [3], the proposed catalyst shows the potential for higher SO₃ conversion yields and very limited sintering effects. Test results, under sulfuric acid environment and operating temperatures on the order of 800 °C, will be shown and discussed.

The new catalyst will be placed in a novel solar reactor, being developed within the HydroGEN project. The proposed concept is based on a solar cavity receiver system, realizing the sulfuric acid decomposition and internal heat recovery in a single unit, directly heated by the incident solar radiation. This system, along with the actual H₂SO₄ decomposition into SO₂, allows: (1) efficient internal heat recovery from the decomposition products and (2) connections with the metallic interfaced HyS equipment (e.g. tubing, valves etc) at low temperatures. Preliminary results from detailed transport model analysis will be shown, demonstrating effective decomposition and internal heat recovery.

The integration of the high temperature decomposition section with the low temperature electrochemical section of the HyS process has also been studied from a process modeling point of view. A new SO₂ electrolysis concept is proposed and discussed, with a vapor SO₂ mixture feeding the electrolyzer. Preliminary results show the potential to achieve high efficiencies, with actual voltage values lower than the traditional target of 600 mV and current densities higher than the target of 500 mA/cm² [4]. A new process flowsheet, initially developed within the current HydroGEN project, will be presented and discussed, achieving thermochemical efficiencies ≥ 35%.

References

[1] J. Ivy, Summary of Electrolytic hydrogen production. Milestone Report. NREL/MP-560-36734; 2004.

[2] https://www.h2awsm.org/ (Accessed February 2018).

[3] Petkovic, L.M. et al. Applied Catalysis A: General 338 (2008) 27.

[4] Corgnale, C., Gorensek, M. and Summers, W. Solar hybrid sulfur cycle water-splitting process. SRNL-STI-2015- 00546, Revision 0, (2015)