When we think about the kinds of fuels we encounter in our daily lives, the first that come to mind may be gasoline or propane. These everyday fuels have a commonality. However, an additional commonality they share is that they are all derived from oil, a non-renewable resource that takes millions of years to form. This reliance on finite fossil fuels raises concerns about their long-term availability and the environmental impact of their extraction and combustion. However, there is a form of energy that is not only abundant but has been known about for decades: Hydrogen gas. Hydrogen can be easily stored in tanks and transported to locations where energy is needed. As a comparison, 2.2 lbs of hydrogen contains the same amount of energy as 6.2 lbs of gasoline.1 Hydrogen can be generated by "splitting" water into its elemental components, hydrogen and oxygen, by applying a voltage. What is unique about solar water splitting is that the applied electrical potential can be generated from sunlight energy. Semiconducting electrode materials that are activated by solar energy are called photoelectrodes. Fully transitioning to a hydrogen-based fuel economy requires research into semiconductor photoelectrodes that can be used to produce hydrogen at a large scale.
The Center for SoftPhotoElectroChemical System (SPECS), directed by Professor Erin L. Ratcliff at the University of Arizona, is taking a different approach, seeking to use carbon-based (organic) polymer materials as photoelectrodes. These organic polymers offer a unique advantage over inorganic materials for electrodes: They can be formulated into an ink and rapidly printed using commercial printing techniques, the same kinds of techniques used to produce newspapers and even graphic T-shirts. This capability makes the production process less expensive to build and drives down the cost of the end photoelectrode through the scale at which it is produced. While the cost of production is low, there are still chemical questions and unknowns that need to be addressed before the eventual implementation of organic-based photoelectrodes.
To address these unknowns, SPECS is working diligently to understand the fundamental mechanisms at the meeting point between water and polymer. Understanding this meeting point is critical to enable polymer electrodes to generate fuels like hydrogen from water. When immersed in a water solution with added salts (electrolytes) for electrical conductivity, polymers act like a giant sponge. Water and ions that make up the electrolyte will enter the polymer lattice, causing the polymer electrode to swell. Additionally, as sunlight strikes the polymer, charge carriers form on the polymer backbone, which participate in the water splitting reaction. These processes all occur simultaneously, shown in Figure 1. By untangling the roles each of these processes have in the overall water splitting reaction, polymer electrodes can be better understood and help make photoelectrodes that can produce the most amount of hydrogen for the lowest cost.
SPECS is utilizing a combination of computational and experimental resources across six academic institutions and one national laboratory. Three main thrusts are at the core of SPECS: (1) Understanding the mechanisms that govern the transport of charge carriers, such as electrons, across the polymer backbone is being led by Prof. Chad Risko (University of Kentucky); (2) Investigating how the local environment experienced by a charge carrier affects its ability to participate in a redox reaction with water is being led by Prof. Tim Lian (Emory University); and (3) Identifying the specific properties within the polymer itself that must be tuned to increase its longevity as an electrode is being co-led by Prof. Seth Marder (University of Colorado Boulder) and Prof. Jianguo Mei (Purdue University). These areas of research will contribute to the development of more efficient and durable photoelectrodes for converting sunlight into clean fuel.
As this newly formed Center begins answering these questions and conducting research, the path towards a reliable and scalable photoelectrode to convert clean fuel from sunlight will begin to be paved. With the potential of printable semiconductors and advances in renewable energy technologies, we can envision a future where sustainable and abundant hydrogen fuel replaces finite fossil fuels, reducing our dependence on oil and mitigating the environmental impact associated with traditional energy sources.
