Tag Archives: Redox Cycle

Creating a Redox Materials Database for Solar-Thermochemical Air Separation and Fuels Production

Josua Vieten*, Dorottya Guban, Martin Roeb, Christian Sattler, Institute of Solar Research, DLR (German Aerospace Center), Germany; Patrick Huck, Matthew Horton, Kristin Persson, Lawrence Berkeley National Laboratory, USA; Brendan Bulfin, ETH Zurich, Switzerland

15th Annual NH3 Fuel Conference, Pittsburgh, PA, October 31, 2018
NH3 Energy+ Topical Conference at the AIChE Annual Meeting


Converting heat from renewable sources into other forms of energy is considered an essential factor in the reduction of greenhouse gas emissions. For instance, high temperatures can be reached using concentrated solar power (CSP), and the thus-captured energy can be converted into so-called solar fuels via thermochemical processes. These consist of the partial reduction of a redox material, usually a metal oxide, at high temperatures following the exothermic re-oxidation of this material at a lower temperature level using steam or CO2, which are thus converted into hydrogen or carbon monoxide, respectively. These two gases can be combined to generate syngas for the production of hydrocarbons (see Fig. 1). Through the same process, a stream of mostly inert gas can be produced by re-oxidation with air, allowing air separation using renewable energy sources. Hydrogen production and air separation can also provide the feedstock for ammonia production through the Haber-Bosch process, as the achieved oxygen partial pressures can be kept low enough to avoid catalyst poisoning. [2] Ammonia produced through this method can be used for fertilizer production, or as a fuel for energy storage. Continue reading

Screening Binary Redox Pairs for Solar Thermochemical Ammonia Synthesis Using Machine Learned Predictions of Gibbs Formation Energies at Finite Temperatures

Christopher J. Bartel*1, John R. Rumptz1, Aaron M. Holder1,2, Alan W. Weimer1, Charles B. Musgrave1
[1] University of Colorado Boulder; and [2] National Renewable Energy Laboratory, United States

NH3 Fuel Conference, Minneapolis, November 2, 2017
AIChE Annual Meeting, Topical Conference: NH3 Energy+


Solar thermochemical ammonia synthesis (STAS) is a reduction/oxidation (redox) cycle which enables the production of ammonia (NH3) from air, water, and concentrated sunlight. In this process, a metal nitride (MN) is oxidized by steam to produce a metal oxide (MO) and NH3; the resulting MO is reduced at high temperature (driven by concentrated solar radiation) and subsequently used to reduce atmospheric nitrogen (N2) and reform the MN and restart the NH3 synthesis cycle. The identification of optimal redox pairs (MO/MN) for this process has been historically limited by the lack of thermochemical data (i.e., Gibbs formation energies at finite temperatures) available for these materials, especially nitrides.

Prior work by our group has demonstrated the use of machine learning to enable the prediction of Gibbs formation energies up to very high temperatures (1800 K) using low-cost DFT calculations (e.g., PBE+U), thus eliminating the need for experimentally measured thermochemistry. Utilizing this approach, we’ve screened the reaction energetics and thermodynamic stability of all known binary (i.e., monometallic) MN/MO pairs, increasing the number of redox pairs considered for this process by an order of magnitude. In addition to the consideration of new redox pairs, we also assess the effects of operating conditions and reaction scheme on the viability of candidate materials. Within this work, we gain insights into new candidate materials for NH3 synthesis, the effects of operating conditions on the viability of the overall process, and the correlated stability of metal oxides and metal nitrides. Continue reading