Solar Electrochemical Thermal Process (STEP) Ammonia: Optimization of the Electrolysis Conditions

Abstract

In the solar thermal electrochemical process (STEP), sunlight is split into visible (for photovoltaic electricity) and thermal (unused, sub-bandgap) radiation using the full solar spectrum to efficiently drive high temperature electrolyzes. Electrolysis conditions for STEP ammonia are investigated. A mixed molten carbonate/hydroxide electrolyte with iron oxide catalyzes ammonia formation from water (steam) and air (nitrogen) via an iron intermediate.The higher temperature required for effective iron formation needs to be balanced by the lower temperature for effective hydration of the electrolyte. STEP ammonia is illustrated at a nickel anode and steel cathode at 650 °C in Li 1.6 Ba 0.3 Ca 0.1 CO 3 with 6m LiOH and 1.5mThe Haber-Bosch ammonia process uses H 2 as a reactant, principally produced by natural gas steam reformation (CH 4 + 2H 2 O → 4H 2 + CO 2 ). Ammonia production was 1.45x10 8 tons in 2014; 1 emitting 2x10 8 tonnes of the greenhouse gas CO 2 . Ammonia is a critical resource to produce the world's fertilizer. 2 CO 2 -free alternatives are needed to synthesize ammonia directly from air and water. [3][4][5][6][7][8][9] We utilized Fe 2 O 3 as an electrocatalyst in molten hydroxides for such an electrolysis. 4 Metallic iron was determined as the chemical intermediate, 8 and the Fe 2 O 3 ammonia electrocatalyst can be isolated on activated carbon. 9We introduced an alternative solar energy conversion, STEP (solar thermal electrochemical process) to drive CO 2 -free chemical syntheses. 10-12 For example, STEP cement converts limestone to lime without CO 2 emission, 12,13 similarly STEP fuels, 14-18 STEP iron, 19-23 STEP carbon, 14,24-33 etc. 34,35 STEP uses full insolation, including solar thermal, driving hot electrolyses to desired products. Solar to chemical efficiencies as high as 50%have been observed for CO 2 splitting using photovoltaics and applying their (unused) solar thermal lowering the electrolysis potential. 14 STEP ammonia combines our previous STEP iron (electrolysis of iron oxide to iron in molten salts) and ammonia (electrolysis of air, N 2 , and water to ammonia) chemistries as illustrated in Figure 1. Incident sunlight is split into PV visible and thermal (unused, sub-bandgap) radiation. The solar thermal component heats the electrochemical couple, while the solar PV visible component generates electronic charge to drive electrolysis of the heated electrochemical redox couple. The electrolysis forms anodic oxygen, and cathodic iron (from Fe 2 O 3 ) which reacts with water and nitrogen to form ammonia (and Fe 2 O 3 , renewed to iron in the next cycle).In this communication, we focus on the electrolysis component for STEP ammonia. STEP requires high temperature with molten carbonate to deposit and reform the iron catalyst necessary for sustainable iron. [19][20][21][22][23] Molten hydroxide can be added to establish a foundation for proton availability (2MOH ⇌ M 2 O + H 2 O). However, high temperature dehydrates the electrolyte. Here, a "goldilocks" intermediate temperature range is esta...