Soft Materials for Photoelectrochemical Fuel Production

Abstract

Polymer semiconductors are fascinating materials that could enable delivery of chemical fuels from water and sunlight, offering several potential advantages over their inorganic counterparts. These include extensive synthetic tunability of optoelectronic and redox properties and unique opportunities to tailor catalytic sites via chemical control over the nanoenvironment. Added to this is proven functionality of polymer semiconductors in solar cells, low-cost processability, and potential for large-area scalability. Herein we discuss recent progress on soft photoelectrochemical systems and define three critical knowledge gaps that must be closed for these materials to reach their full potential. We must (1) understand the influence of electrolyte penetration on photoinduced charge separation, transport, and recombination, (2) learn to exploit the swollen polymer/electrolyte interphase to drive selective fuel formation, and (3) establish co-design criteria for soft materials that sustain function in the face of harsh chemical challenges. Achieving these formidable goals would enable tailorable systems for driving photoelectrochemical fuel production at scale.

Figure 1

Schematic illustration of the polymer-electrolyte interphase, a term specifically chosen to describe the longer length scale phenomena that occurs with polymer-electrolyte interaction. Illustration shows processes occurring in a two-component polymer blends (donor and acceptor) and polymer-electrolyte system that can generate solar fuels, including light absorption to form an exciton, excitation energy transfer (EET) and exciton diffusion, photoinduced electron transfer (PET), and diffusion of charges to the edge of the polymer. The major species present are defined in the legend, with electrolyte ions omitted for simplicity. The scientific questions noted in the text connect to (a), (b), (c), and (d). Scientific questions are illustrated at several points: (a) To what extent do solvents swell the bulk of the film, and what impact does this have on structure and electronic properties? (b) Does the presence of polar solvent molecules modify the energy landscape and the coupled dynamics of charge separation? (c) Does solvent stabilization of charges at the interface prolong their lifetime and ability to do electrochemical work, possibly through surface attached catalysts targeted to specific reactions (d)?

Figure 2

Schematic illustrations of (a) existing technology of photovoltaic electrolysis (PV-EC) and (b) the long-term goal of photoelectrochemical devices (PEC), as well as timelines of major developments in (c) hard (inorganic) PEC devices (referring to refs (−42)); (d) soft (organic) semiconductors and related technologies (referring to refs (−49)), and (e) highlighted considerations around energy justice in basic energy science(referring to refs (,−52)).

Figure 3

Comparisons between soft (organic) and hard (inorganic) semiconductors utilized as photocathodes for HER to illustrate major differences. (a, b) Generalized density of states (DOS) of the two classes, illustrating the possible reaction tailoring based on orbital overlap for soft systems compared to hard systems. (c, d) Illustrations of interactions between electrolytes (solvent, solvated reactants, and ions) and surfaces of soft and hard systems. (e, f) Illustrative band diagrams for soft and hard PEC systems, assuming a buried junction hard photocathode. Acronyms used are valence band minimum (VBM), conduction band maximum (CBM), electron quasi-Fermi level (E_Fn), catalyst (cat), electron donor polymer (D), electron acceptor polymer (A), indium tin oxide (ITO), exciton (Ex), photoinduced electron transfer (PET).

Figure 4

Advancement of the soft photoelectrochemical systems for solar fuels necessitates a multifaceted approach targeting detailed understanding at the molecular level of charge transfer, charge transport and durability to control energy and matter across various spatiotemporal scales. Illustration by Al Hicks, NREL.