(1) 100-170°C: Cu3Al2F12(H2O)12 → Cu3Al2F12(H2O) This new green oxyfluoride Cu3Al2OF10 was isolated by calcination of r-Cu3Al2F12(H2O)12 under ambient air for 1 h at 320°C (wt % = 34.2%, 34.5%) and was characterized by transmission electron microscopy (TEM), nitrogen sorption (Fig. ) and FTIR spectroscopy (Fig ). The poorly crystallized/amorphous state was confirmed by the diffuse electron diffraction pattern obtained by selected area electron diffraction (SAED) (Fig. ). The release of H2O and HF molecules during the thermal treatment induced a slight mesoporosity (SBET = 10 m 2 .g -1 ) as shown by the type II hysteresis of the N2 adsorption/desorption isotherms (Fig. ). The BJH pore-size distribution analysis confirms an average pore diameter less than 10 nm, in good agreement with that observed in the TEM images. Scanning electron microscopy coupled to elemental dispersive X-ray spectroscopy (EDS-SEM) confirmed that the Cu/Al ratio is close to 1.5 before and after calcination (Fig S4 ). Following the synthesis and characterization, Cu3Al2OF10 was investigated as a NO3R catalyst for NH3 production (Fig. ). In a neutral phosphate-buffered electrolyte (pH = 7), the catalyst features a redoxwave centered around 0.6 VRHE prior to a slow onset of reductive current attributed to the hydrogen evolution reaction (HER) (Fig. ). Restructuring of the catalyst during cycling is evident through the change of the first and second cyclic voltammogram (CV) but afterwards, the CV shape remains constant (Fig. ). Upon the addition of 1.0 M KNO3, the catalytic current initiates around 0.2 VRHE, reaching 1300 mA.cm -2 by -0.6 VRHE. The inset in Fig. shows a magnification of the CVs of Cu3Al2OF10 (after the 2 nd CV cycle) in the presence and absence of NO3 -. Through steady-state chronoamperometry coupled with ammonia quantification via 1 H-NMR, we determined that the FE maximizes around -0.4 VRHE at 57% (Fig. ). The mass activity on the other hand (using the total mass of catalyst loaded onto the electrode, 1 mg.cm -2 so that 1 mA.cm -2 = 1 A.g -1 ), continues to increase throughout the entire experimental range, reaching a value of 537 A.g -1 at -0.6 VRHE. NMR spectra were recorded after experiments without negative bias or KNO3 (Fig. ). The absence of an NH3 signal in these control experiments provides evidence that the nitrogen in NH3 is coming from NO3 -. At the same time, using NO3 as the N-source lead to the expected double peak signature of 15 NH4 + further confirming the previous statement.