Prototype reactor for highly selective solar-driven CO 2 reduction to synthesis gas using nanosized earth-abundant catalysts and silicon photovoltaics

Prototype reactor for highly selective solar-driven CO2 reduction to synthesis gas using nanosized earth-abundant catalysts and silicon photovoltaics Félix Urbain1*, Pengyi Tang1,2, Nina M. Carretero1, Teresa Andreu1,5, L. G. Gerling3, Cristobal Voz3, Jordi Arbiol2,4, and Joan Ramon Morante1,5 1 IREC, Catalonia Institute for Energy Research, Jardins de les Dones de Negre 1, 08930 Sant Adrià de Besòs, Barcelona, Catalonia, Spain


Solar-to-Syngas Conversion Efficiency Calculation
Considering an active electrode area of 10 cm 2 for CO 2 RR and OER, respectively, the bias-free operation CO 2 RR current of 5 mA/cm 2 , which we measured (Fig. 5), corresponds to an electric charge of 50 mC, i.e. 31.2*10 16electrons (assuming 1 s of interval duration).These charges are used produced H 2 and CO which both need 2 electrons to be formed.Over the course of the biasfree measurement, a faradaic efficiency of 76 % for the H 2 production and 16 % for the CO production was measured.Consequently, for the number of molecules we can derive that: Considering the respective molar masses, we obtain 0.394 µg of H 2 and 1.16 µg of CO.
Taking into account the higher heat value (HHV) and lower heat value (LHV) for H 2 and CO of 286 kJ/mol and 283.5 kJ/mol, respectively, the specific power of the produced products can be obtained as 143330 kW/kg for H 2 and 10125 kW/kg for CO.
Using these values allows to calculate the total bias-free power output of the device.η STS is given by where the input power (for 16 cm 2 illuminated solar cell area) is the incident light intensity (1600 mW).Thus, the solar-to-syngas efficiency of our device was 4.26 %.

FiguresFig
Figures Fig. S1 Potential-time curves during electrodeposition of Zn on Cu-foams at -40 mA.

Fig. S4
Fig. S4 Cyclic voltammetry (CV) curves of Cu-foam decorated cathodes measured in the filter-press cell with electrolyte flow (20 ml/min) and CO 2 (green curve) or Ar (purple curve) flow (20 ml/min).The electrolyte was 0.5 M KHCO 3 in both measurements, which were conducted at a scan rate of 10 mV/s.The black squares represent the average current density values from hour-long potentiostatic experiments carried out at different applied potentials (see Fig. 4(a)).

Fig. S5
Fig. S5 (a) SEM image of the Cu-Zn cathode after CO 2 RR at -0.8 V RHE for one hour.(b) With high magnification.(c) XRD pattern of the bare Cu-foam, of the as-prepared Cu-Zn cathode, and of the Cu-Zn cathode after one hour electrolysis.

Fig. S6
Fig. S6 Faradaic efficiencies of CO and H 2 production from CO 2 RR at different potentials for a flat Zn foil cathode measured in 0.5 M KHCO 3 .The error bars are standard deviations obtained from 3 experimental repeats.All measurements were conducted at a CO 2 and electrolyte flow rate of 20 ml/min.

Fig. S7
Fig. S7 Left: Schematic drawing of the layer stack used for the deposition of Si heterojunction solar cells.Right: Linear sweep measurement of the Si/Ni foam photoanode in 1 M KOH solution under 100 mW/cm 2 of simulated AM1.5 illumination.The inset illustrates the Si/Ni foam photoanode structure under illumination.Details on the fabrication of the Si heterojunction solar cells can be found in Supplementary Ref. 1.

Fig. S9
Fig. S9Two-electrode current density-voltage characteristics with a Si heterojunction solar cell/Ni foam photoanode and a Zn catalyst coated Cu foam cathode in a bipolar membrane configuration using 1 M KOH as anolyte and 0.5 M KHCO 3 as catholyte solution.The measurements were conducted under simulated AM1.5 illumination at a scan rate of 10 mV/s before and after the bias-free stability testing.The illuminated area of the PV cells was 16 cm 2 , while the active electrode/liquid areas of the cathode and anode were 10 cm 2 .