We studied the stability of our bes

We studied the stability of our best performing region of electrolyte conditions using a series of 2 h, chronoamperometric experiments on a single Ti electrode, as shown in Figure 3B (green). The 0.1 M HNO3/0.3 M KNO3electrolyte was replaced every 2 h for the 8 h experiment, yielding an initial Faradaic efficiency toward ammonia of 78% for the first 2 h period. Faradaic efficiency decreases somewhat for each subsequent 2 h experiment but decreases less with each test, suggesting that selectivity stabilizes over time. FE also remains above 50% throughout the test, with jNH3remaining as high throughout (19, 17, 14, 16 mA/cm2). To explore the cause of loss in FE, we performed XPS on the Ti cathode before and after testing and observed a trace amount of Pt on the surface following the 8 h test (Figure S3). However, we note that an analogous Ti/Nafion/GC test also showed a similar decrease in FE (Figure 3B blue) following an initial high FE of 82%, suggesting that this decrease cannot be attributed to Pt deposition alone. We hypothesize that this decrease in FE may be caused by one or more of several factors, including (1) contamination of the cathode by Pt, C, or N species, (2) the effect of possible hydride and/or oxide species on selectivity (see below), (3) surface active site poisoning, and (4) nanostructuring of the cathode surface under experimental conditions. Further studies are needed to elucidate the exact mechanism of FE loss and determine the full limitations of this system. We found that by testing a Ti/GC system with no Nafion and only a single 20-sccm Ar inlet for four, consecutive 2 h tests, the current was far lower and more stable than in the previously discussed systems, with FE increasing somewhat over time and stabilizing near 66% (Figure 3B red). This data suggests that both mass transport and product access to the anode have a significant effect on selectivity to ammonia and current density over time. On the basis of these results, we believe that the high currents of the higher-mass-transport Nafion systems create harsher electrochemical reaction conditions which accelerate various degradation mechanisms such as those described above.