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Figure 5.4: Comparison of activity and selectivity for LixN, nitrogenase, and the ideal catalyst. Data from references [8, 12–14]. 14N2. Recent reports confirm that Li (or LixN) electrodes in 0.2 M LiClO4 in 1% ethanol in THF can reduce N2 to NH3 [8–10], confirming an earlier study from the 1990s [11]. As shown in Figure 5.4, both the rate (70 μA/cm2) and the Faradaic selectivity (6.5%) are low, while the overpotential is large (>3 V), demonstrating ample room for improvement. In comparison, nature’s nitrogenase enzyme is far more efficient, with an ammonia production rate equivalent to 3 mA/cm2, ∼1 V over- potential and up to 75% Faradaic selectivity. Other studies in the literature lack rigorous control tests, calling into question their reliability. It is worth perusing upon which of the metrics plotted in Figure 5.4 has the greatest impact on the performance of a real device. There are two different cases: (i) when NH3 is to be used as a fertiliser, and (ii) as an energy vector (solar fuel). For fertiliser applications, improving the selectivity and current density would have the greatest impact. A recent paper recently emphasised the importance of Faradaic selectivity: it estimated that a 5 m2 state-of-the-art solar cell would be sufficient to provide NH3 as fertiliser for a hectare of land, assuming 100% Faradaic selectivity and 1 V overpotential [15]. A 2 V overpotential would require a 45% increase in solar cell area. However, decreasing the selectivity to 1% would require a colossal 100-fold increase in solar cell area, which would be prohibitively expensive. On the other hand, high current densities are also critical to minimise the capital costs. Electrochemical devices run at current densities ∼1.5 A/cm2; taking into surface roughness, a reasonable surface specific current would be ∼10 mA/cm2, far higher than the current densities for N2 reduction shown in Figure 5.4. For energy storage in the form of a solar fuel, overpotential is more important than it is for fertiliser. For instance, even if NH3 was produced with zero overpotential at the anode and cathode, the cost based on average electricity prices alone would be e0.08/kWh; with an overpotential of 1 V it would be equivalent to be e0.15/kWh. For reference, the average cost of methane is e0.03/kWh [16]. 54PDF Image | sustainable production of fuels and chemicals
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