Nanostructured photoelectrochemical solar cell for nitrogen reduction using plasmon-enhanced black silicon

Abstract

Ammonia (NH3) is one of the most widely produced chemicals worldwide. It has application in the production of many important chemicals, particularly fertilizers. It is also, potentially, an important energy storage intermediate and clean energy carrier. Ammonia production, however, mostly uses fossil fuels and currently accounts for more than 1.6% of global CO2 emissions (0.57 Gt in 2015). Here we describe a solar-driven nanostructured photoelectrochemical cell based on plasmon-enhanced black silicon for the conversion of atmospheric N2 to ammonia producing yields of 13.3 mg m(-2) h(-1) under 2 suns illumination. The yield increases with pressure; the highest observed in this work was 60 mg m(-2) h(-1) at 7 atm. In the presence of sulfite as a reactant, the process also offers a direct solar energy route to ammonium sulfate, a fertilizer of economic importance. Although the yields are currently not sufficient for practical application, there is much scope for improvement in the active materials in this cell.

Figure 1. Schematic of fabrication and SEM and TEM images of the electrodes.

(a) Schematic illustration of fabrication of the cell. (b) Cross-sectional view and (c) magnified view of SEM images of GNPs-coated black silicon nanostructure. (d) Corresponding TEM and (e) HRTEM images of GNPs-coated silicon nanowire. The well-resolved lattice spacings of 0.31 and 0.23 nm correspond to the Si {111} and Au {111} atomic planes, respectively. SEM, scanning electron micrograph; HRTEM, high-resolution TEM; TEM, transmission electron micrograph.

Figure 2. Photoelectrochemical nitrogen reduction.

(a) Schematic diagram of the cell. (b) Yield of ammonia over 24 h obtained on different substrates: (i) P-type silicon, (ii) bSi, (iii) GNP/bSi, (iv) GNP/bSi/Cr and (v) Au/Si/Cr after illumination with two suns and (vi) GNP/bSi/Cr in dark. (c) The time-dependence of ammonia yield obtained after illumination with two suns (error bars are the s.d. of at least three replicates of independent measurement). (d) The light intensity-dependence of ammonia yield obtained after illumination for 3 h (error bars are an estimate of the combined errors of measurements).

Figure 3. Quantum efficiency and high-pressure yield obtained on a GNP/bSi/Cr photoelectrochemical cell.

(a) Quantum efficiency of ammonia synthesis on a GNP/bSi/Cr photoelectrochemical cell as a function of wavelength (error bars are estimates of the combined errors of measurements). (b) Yield of ammonia in three hours as a function of nitrogen gas pressure at two suns illumination in a fixed volume glass reactor (error bars are the s.d. of at least three replicates of independent measurement).