Ten-percent solar-to-fuel conversion with nonprecious materials

Abstract

Direct solar-to-fuels conversion can be achieved by coupling a photovoltaic device with water-splitting catalysts. We demonstrate that a solar-to-fuels efficiency (SFE) > 10% can be achieved with nonprecious, low-cost, and commercially ready materials. We present a systems design of a modular photovoltaic (PV)-electrochemical device comprising a crystalline silicon PV minimodule and low-cost hydrogen-evolution reaction and oxygen-evolution reaction catalysts, without power electronics. This approach allows for facile optimization en route to addressing lower-cost devices relying on crystalline silicon at high SFEs for direct solar-to-fuels conversion.

Keywords: artificial leaf; earth abundant; multijunction; renewable; solar cell.

Fig. 1.

(A) Schematic of the experimental setup and electrode geometry for the PV–EC device. (B) Block diagram for an electrochemical load driven by a PV device. The direct electrical connection in which the two half-reactions occur on surfaces equipotential with the terminals of a solar cell describes both wired and wireless water splitting and constrains the currents and voltages of the PV device and the EC system to be identical. (C) J–V curves of the individually measured PV and EC components making up the PV–EC device. The gray curves represent the J–V curve for the PV modules composed of either three (dashed) or four (solid) single junction c-Si solar cells measured under AM 1.5 illumination. The red dashed curve shows the ideal J–V curve obtained for NiB_i and NiMoZn catalysts based on previously reported Tafel analysis. The solid red curve curves show the J–V curve of the NiB_i and NiMoZn electrodes measured in a two-electrode experiment (0.5 M KB_i / 0.5 M K₂SO₄, pH 9.2). The point of intersection represents the J_OP and the SFE of the coupled system.

Fig. 2.

(A) Current under chopped illumination representing J_OP for the PV–EC device (0.5 M KB_i / 0.5 M K₂SO₄, pH 9.2). The chopped illumination illustrates the recovery in SFE and illustrates the reproducibility in measuring J_OP. (B) J_OP measured for over 7 d of operation showing no decrease in SFE over operation time. Spikes in J_OP are due to addition of solution to maintain the solution level and pH. The orange dashed line is a smoothed curve of the data. (C) Gas quantification for the NiMoZn cathode and NiB_i anode in 0.5 M KB_i / 0.5 M K₂SO₄ pH 9.2 solution. The black lines represent 100% Faradaic efficiency based on the charge passed during electrolysis. The green circles represent the H₂ and O₂ measured by gas chromatography. Red arrows indicate when electrolysis was stopped. GC analysis was conducted until the moles of gas measured in the headspace reached a steady state. The lag period in gas generation is due to the buildup of gases in the head space of the EC cell.