Solar fuel production for sustainable energy supply

Sunlight is the most abundant renewable energy source, offering the potential to reduce global dependence on fossil fuels. One way to harness that energy is in fuels such as hydrogen and methanol, which store chemical energy converted from solar. Worldwide attention has focused on techniques such as photoelectrochemical splitting of water into hydrogen and oxygen using sunlight, conversion of water (H₂O) and carbon dioxide (CO₂) into solar fuels, and artificial photosynthesis.¹Our current research and development goal is to photoelectrochemically and electrochemically convert waste steam (CO₂ and H₂O) from army field generators to small organic hydrocarbon fuels for US Army applications.

On the battlefield, fuel is the leading logistics driver, and the army generally opts for high energy density liquid fuels, which are easiest to transport and store. These fuels are necessary for soldier power systems, ground vehicles, and auxiliary power units, and they are transported by convoys. On-site fuel generation from renewable sources would reduce the casualties caused by fuel convoys traveling in hostile environments.

Since Fujishima and Honda reported water splitting with a titanium dioxide-based photoelectrochemical cell (PEC) in 1972,² there have been extensive studies of photocatalytic and photoelectrochemical water splitting with photoactive semiconductors. One area that has seen rapid progress, opening a new avenue to improve solar-to-chemical energy conversion efficiency for semiconductor photocatalysts and PECs, is surface plasmon resonance (SPR). In plasmonic metal semiconductor heterojunction photocatalysts, plasmons can promote charge separation in the semiconductor by three underlying mechanisms:³ photonic enhancement (light scattering and trapping),⁴ direct hot electron transfer,⁵ and plasmon-induced resonant energy transfer (PIRET) from a metal to a semiconductor.⁶ Plasmonic metal semiconductor photocatalysts rely on a structure that enables strong coupling between the plasmon and the semiconductor by at least one of the three mechanisms. A recent study⁶ showed that effective incorporation of the plasmonic nanostructure in the semiconductor can significantly enhance the PEC performance. In that investigation, a vertically aligned hematite nanorod array coupled to a plasmonic gold nanohole array pattern served as a plasmonic photoanode in a PEC for solar water splitting (see Figure 1). Under solar radiation, both the propagating surface plasmon polariton (SPP) and the localized surface plasmon resonance (LSPR) were excited, leading to photonic and PIRET enhancement in charge separation in the hematite nanorods. As a result, the plasmon increased the photocurrent of the photoanode about 10-fold.

Figure 1. A photoelectrochemical cell (PEC) with the hematite (Fe₂O₃) semiconductor nanorod array coupled to a gold (Au) nanohole array pattern. (a) Scheme for the plasmonic photoanode with the hematite nanorod array on the Au/titanium (Ti) nanohole array. SiO₂: Silicon dioxide. FTO: Fluorine-doped tin oxide. (b) Current-voltage (J-V) curves for hematite nanorod array photoanode with (red line) and without (black line) the Au nanohole array pattern under the illumination of AM 1.5G full-spectrum solar light with a power density of 100 mW cm⁻². Ag/AgCl: Silver chloride electrode. (c) Surface plasmon polariton (SPP) enhancement by trapping incident light as waveguided modes in the hematite nanorod at wavelengths corresponding to increased transmission in the Au nanohole array. (d) Localized surface plasmon resonance (LSPR) enhancement at wavelengths corresponding to the LSPR of the Au nanohole array. The incident field is localized at the edges of the hole. (Reprinted with permission from the Nature Publishing Group).⁴

A redox reduction of CO₂ to hydrocarbon fuels, which involves multielectron transfer and proton transport, requires a large activation energy. Hence, conversion of CO₂ remains a major challenge. To meet the army's aim of photoelectrochemical and electrochemical conversion of waste stream (CO₂ and H₂O) from field generators to small organic hydrocarbon fuels, we are exploring two technical approaches. The first is an electrochemical reaction pathway—see Figure 2 (left)—and the second a photoelectrochemical pathway: see Figure 2 (right). Figure 2 (left) shows an electrochemical reactor that uses electrical energy from a photovoltaic device to convert CO₂ and water into methanol. The functional electrocatalysts lower the overpotential of electrochemical reduction of CO₂ and target the selective conversion. This system shares many technological features with fuel cells, which convert chemical to electrical energy through electrochemical reactions. Therefore, like fuel cells, the technology is easy to scale up and down. Figure 2 (right) shows the photoelectrochemical pathway, in which a C-1 hydrocarbon product forms through CO₂ reduction with H₂O. We aim to design bifunctional catalysts for conversion of CO₂ and H₂O to small organic fuels. To further enhance the catalytic activity toward CO₂ reduction, we will couple the semiconductor to the plasmonic nanostructure as the photoanode.

Figure 2. Schematic of (left) electrochemical cell and (right) photo-electrochemical cell for carbon dioxide (CO₂) reduction to fuel reaction pathway (CO₂) +water + sunlight #methanol).

If successful, the PEC system will use sunlight to convert the exhaust gas from the army field generators into hydrocarbon fuels for energy supply. It is possible to combine hydrocarbon fuels with microturbines or fuel cells for energy generation, resulting in a net positive energy system from the waste exhaust. This also improves the overall power system efficiency and effectiveness of operation. In addition, it is possible to subsequently convert the small organic fuels to form a jet propellant-like fuel that can be used in many ground vehicles, aircraft, and auxiliary power units. This will significantly reduce the resupply logistics for fossil fuels, resulting in a reduced requirement for convoy escorts.

The authors wish to thank the US Army and CERDEC (W911NF-14-2-0116) for financial support.