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Methods
Fabrication of electrochemical cell for ammonia synthesis
Nafion 211 membrane (DuPont®) was boiled in 3% H2O2 for 1 hour, rinsed by deionised water, boiled in de-ionised water for 2 hours then in 0.5 M H2SO4 for 1 hour. After rinsing with deionised water a few times, the membrane was stored in deionised water for cell fabrication.
Pt/C (E-Tek, 30 wt%) on SGL gas diffusion layer (GDL 10 BC) was used as both electrodes with a Pt loading of 1 mg cm−2. Some 5% Nafion suspension (Aldrich) and isopropanol were mixed with Pt/C catalysts for preparation of the catalytic layer. The membrane electrode assembly (MEA) with a working area of 1 cm2 was fabricated by hot pressing.
The MEA was put in electrochemical cell testing jig using graphite as bipolar plates. 35 wt% ammonia aqueous solution (Alfa Aesar) was pumped to both sides of the cell by a parasitic pump (Waterson Marlon 320) for one day to convert H+-form Nafion 211 membrane into NH4+-form. De-ionised water was then pumped to the cell for one week to clean up the residual ammonia and no ammonia can be detected from the outlets. A d.c. voltage of 40 mV was applied to the cell for 4 hours to activate the MEA and improve the electrode/electrolyte interfaces then air was flowed through both cathode and anode chambers overnight before ammonia synthesis experiments. All the presented experimental data were collected from the same MEA.
Ammonia synthesis and detection
H2 (or water) and N2 (or air) were passed through room temperature water first then filled into the chambers of the cell. The dc potential was applied by a Solartron 1470A electrochemical interface controlled by software Cell Test® for automatic data collection. The order for applied voltage was from low to high. The produced ammonia was collected by dilute H2SO4 (0.001 M). The concentration of  in the absorbed solution was analysed using Nessler's reagent (Aldrich). The produced ammonia was detected using an ammonia meter (Palintest 1000) and the rate of ammonia formation was calculated using the following equation.
 
Where [NH4+] is the measured NH4+ ion concentration, V is the volume of the dilute H2SO4 for ammonia collection, t is the adsorption time and A is the effective area of the cell.
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Author Contributions
S.W.T. and R.L. drafted and J.T.S.I. revised the manuscript. S.W.T. and J.T.S.I. conceptualized the study. R.L. and S.W.T. performed synthesis, characterization and analysis.
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Supplementary Material
Supplementary Information:
Supplementary information
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Acknowledgments
This work was funded by EPSRC SuperGen ‘Delivery of Sustainable Hydrogen' project (EP/G01244X/1).
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References
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