High-Throughput Equilibrium Analysis of Active Materials for Solar Thermochemical Ammonia Synthesis

Abstract

Solar thermochemical ammonia (NH3) synthesis (STAS) is a potential route to produce NH3 from air, water, and concentrated sunlight. This process involves the chemical looping of an active redox pair that cycles between a metal nitride and its complementary metal oxide to yield NH3. To identify promising candidates for STAS cycles, we performed a high-throughput thermodynamic screening of 1,148 metal nitride/metal oxide pairs. This data-driven screening was based on Gibbs energies of crystalline metal oxides and nitrides at elevated temperatures, G(T), calculated using a recently introduced statistically learned descriptor and 0 K DFT formation energies tabulated in the Materials Project database. Using these predicted G(T) values, we assessed the viability of each of the STAS reactionshydrolysis of the metal nitride, reduction of the metal oxide, and nitrogen fixation to reform the metal nitrideand analyzed a revised cycle that directly converts between metal oxides and nitrides, which alters the thermodynamics of the STAS cycle. For all 1148 redox pairs analyzed and each of the STAS-relevant reactions, we implemented a Gibbs energy minimization scheme to predict the equilibrium composition and yields of the STAS cycle, which reveals new active materials based on B, V, Fe, and Ce that warrant further investigation for their potential to mediate the STAS cycle. This work details a high-throughput approach to assessing the relevant temperature-dependent thermodynamics of thermochemical redox processes that leverages the wealth of publicly available temperature-independent thermodynamic data calculated using DFT. This approach is readily adaptable to discovering optimal materials for targeted thermochemical applications and enabling the predictive synthesis of new compounds using thermally controlled solid-state reactions.

High-Throughput Analysis of Materials for Chemical Looping Processes

“…[18,25,[61][62][63] We previously applied a thermodynamic screening approach based on the descriptor G δ (T) to rapidly identify active materials for solarthermal ammonia synthesis in which ~1,100 oxide and nitride pairs were analyzed. [7] Here, we demonstrate that this approach can be applied to any CL process that operates at or near equilibrium conditions with temperatures up to at least 1800 K, irrespective of the active material class (e.g. oxide, halide, etc.…”

“…Chemical looping processes are a promising route for improving energy efficiency, [1][2][3][4] leveraging renewable energy sources, [5][6][7][8][9][10][11] facilitating chemical conversions, [12][13][14][15][16][17][18][19][20] and reducing undesirable emissions across many sectors of the chemical industry. [21][22][23][24][25][26] In a chemical looping (CL) process, the overall reaction is separated into multiple (typically two) subreactions, each mediated by the redox chemistry of a "looped" active material.…”

“…oxide, halide, etc. ), the number of subreactions, [7] or the number of reactant and/or product species. This includes well-studied CL processes (e.g.…”

Thermodynamic and First-principles Assessments of Materials for Solar-driven CO2 Splitting Using Two-step Thermochemical Cycles

“…Particularly for ΔSsolid of the CeO2/CeO2-δ redox pair (δ=0.03), it was demonstrated that the relative differences between calculated and measured values are below 12%, although it is found in references [20,23] that this method might over-estimate ΔSsolid. Regarding ΔHsolid of pure and Sm-doped ceria pairs (δ=0.03), the relative differences are also below 12%, demonstrating sufficient accuracy of this method which is expected to be more efficient enabled by the advancement and power of highthroughput computational tools [15][16] . a Entropy and enthalpy units are J (0.5 mol O2) -1 K -1 and kJ (0.5 mol O2) -1 ; b Obtained from measured reduction entropy minus 0.5SO2…”

“…Although considerable materials [4] have been examined including ceria [5][6][7][8] , ferrites [9][10][11] and perovskites [12][13][14] , the state-of-the-art solar-to-chemical energy conversion efficiency was as low as ~5% for solar thermochemical CO2 splitting [5] . Therefore, computational assessments of materials are still of great importance [15][16][17] . Intuitively, the thermodynamically suitable redox materials can be identified based on the fundamental concept that the change in the Gibbs free energy, ΔG, should be negative for the two cycle steps (reactions 2 and 3).…”