A recorded research talk for ECS Webinar Series (Nov. 13th 2024) can be found here, Operando NMR Methods for Redox Flow Batteries and Ammonia Synthesis
Redox flow batteries
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Large-scale energy storage is becoming critical to balance the intermittency between renewable energy production and consumption. Redox Flow Batteries (RFBs), based on inexpensive and sustainable redox-active molecules, are promising storage technologies. A RFB (figure on the left) consists of two tanks of redox-active electrolytes, one catholyte and one anolyte, and its capacity can be scaled up by increasing the volume of the tanks. The electrolytes flow through an electrochemical cell where redox reactions happen. One of the distinct features of RFBs is the decoupling of their energy storage and power generation, which requires decoupled in situ monitoring.
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We have developed operando NMR to probe the electrolyte in the flow path, or in the battery cell. A wide range of redox processes can be readily studied. Operando NMR unravelled the decomposition of redox-active electrolytes and guided the regeneration of active species. ​Coupling NMR and EPR, we have demonstrated the possibility of multi-modal characterisations. The figure presents the animated operando 1H NMR and EPR spectra of 10 mM DHAQ as a function of electrochemical cycling. Probing the electron and nuclear spins simultaneously allows reaction mechanisms to be determined and quantified.
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1. Nature 2020, 579, 224-228.
2. J. Am. Chem. Soc. 2021, 143, 1885-1895.
3. Nature Chemistry 2022, 14, 1103-1109.
Ammonia synthesis
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Ammonia is one of the most widely produced commodity chemicals, providing 40-50% of nitrogen for humans. Its production consumes 1-2 % of global energy with large carbon footprint. Making use of electrons from renewables, lithium-mediated electrochemical synthesis is a promising approach to decarbonise ammonia synthesis.
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We develop and apply in situ NMR methods to study Li-mediated electrochemical ammonia synthesis and probe the individual reaction steps, as illustrated in the scheme. Key reaction details have been directly observed. First, the electrochemical plating of metallic Li and the concurrent corrosion has been unravelled. The corrosion was found to be exacerbated by the addition of a proton donor, ethanol. Second, nitrogen splitting by Li metal has been observed and the effect of ethanol was investigated, as shown in the figure above. Third, protonolysis of lithium nitride to lithium and ammonium ions has been monitored. Guided by the insights from the NMR study, we aim to improve the efficiencies of the synthesis.
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1. ChemRxiv, DOI 10.26434/chemrxiv-2024-cpf4.
CO2 reduction
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Operando analysis is crucial for understanding the selectivity and stability of the electrochemical CO2 reduction reaction (eCO2RR). Existing operando techniques normally adapt single-compartment cells operating at low currents. However, high current densities on the order of 100 mA cm-2 are required for practical applications, and under these conditions, selectivity and reaction pathways can differ.
We develop an inline operando NMR method compatible with high-current reaction conditions. Demonstrating on a copper-catalyzed eCO2RR at a current of 100 mA cm-2, our NMR study revealed a fast decrease of Faradaic efficiency for formate and ethanol within the first few hours of reaction (figure above), accompanied by a pH decrease from 14 to 8 within the first hour and a continuous concentration increase of bicarbonate. At 200 mA cm-2, the bicarbonate concentration reached the saturation point of 3.34 M within five hours. Water crossover was simultaneously observed and quantified via a deuteration technique and showed a strong current dependency. Our NMR observations revealed a highly dynamic environment of copper-catalyzed eCO2RR at high currents and will further aid the design and optimization of this reaction. Using a common flow cell and a benchtop NMR system, the new operando approach is accessible by non-NMR experts and readily applicable to a wide range of catalysts, electrolyte compositions and reactor designs for eCO2RR.
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1. ChemRxiv, DOI 10.26434/chemrxiv-2024-mdvvl-v2.
Lignin oxidation
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Exciting research projects are in the making, stay tuned!
In situ NMR probe development
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In situ NMR is a valuable tool for studying electrochemical devices, including redox flow batteries and electrocatalytic reactors, capable of detecting reaction intermediates, metastable states, time evolution of processes or monitoring stability as a function of electrochemical conditions. We develop in situ NMR hardware, e.g. a parallel line detector for spatially selective in situ electrochemical NMR spectroscopy. The detector consists of 17 copper wires and is doubly tuned to 1H/19F and X nuclei ranging from 63Cu (106.1 MHz) to 7Li (155.5 MHz). The flat geometry of the parallel line detector allows its insertion into a high electrode surface-to-volume electrochemical flow reactor, enabling a detector-in-a-reactor design. This integrated device is named “eReactor NMR probe”. The new eReactor NMR probe offers a general method for studying flow electrochemistry, and we envision applications in a wide range of environmentally relevant energy systems, for example, Li metal batteries, electrochemical ammonia synthesis, carbon dioxide capture and reduction, redox flow batteries, fuel cells, water desalination, lignin oxidation etc.
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1. J. Magn. Reson. 2024, 361, 107666
2. PCT/EP2024/080237, 25th Oct. 2024