Though the earth’s remaining stores of nonrenewable energy sources like fossil fuels are becoming less abundant, the deleterious effects of these energy sources are not similarly diminishing. As a result, the search for renewable energy sources has burgeoned over the last few decades. One of the most conspicuous sources of renewable energy is the sun, whose rays have been captured by solar panels to generate electricity. However, converting solar energy to electricity is not the most effective use of the sun’s rays, as the energy harvested by solar panels is not easily stored, making the availability of electricity dependent on the presence of the sun’s rays.
Lately, the renewable energy field has turned its attention to photoelectrochemistry, which is the process that converts light energy to stored chemical energy using a photoelectrochemical cell (PEC). Yale researchers in the Department of Chemistry and Energy Sciences Institute, led by Victor Batista and Gary Brudvig, teamed up with colleagues in Boston College to study the surface facets of the PEC electrode which mediate the fuel-producing reaction. Their research, which found that certain facets of the hematite electrode surface affect both the rate and favorability of the reactions that take place in the PECs, will help lead to solar cells that maximize fuel production while minimizing energy waste.
What plants and PECs have in common
PECs are effectively an attempt by scientists to mimic the process of photosynthesis in plants. During the part of photosynthesis known as the light-dependent reactions, a pigment called chlorophyll harvests light energy from the sun in the part of a plant cell called the chloroplast. Plants use this absorbed energy to separate charges in a complex of proteins collectively known as Photosystem II (PSII). PSII uses “holes” left behind by the charges during separation to accomplish the splitting of water, H2O, into two protons and oxygen gas. The plant ultimately uses the energy to produce glucose, whose bonds store the energy as chemical energy.
Similar to this natural process, PECs absorb light energy from the sun to excite electrons. However, instead of harboring chlorophyll pigments to absorb light energy, PECs use a semiconductor, which acts as a working electrode to provide a source of energy to drive redox reactions. A redox reaction involves reduction, or gain of electrons, in one species and oxidation, or loss of electrons, in another.
In PECs, the semiconductor working electrode utilizes energy originally from the sun to oxidize water, splitting it into protons and oxygen gas in a manner similar to the light-dependent reactions of photosynthesis. However, unlike the chloroplasts of plants which pass electrons down a chain of carriers, PECs have an additional electrode known as the counter electrode, which governs a redox reaction to balance water’s loss of electrons. This counter electrode is where the reduction of protons to hydrogen gas takes place, resulting in the formation of hydrogen fuel–a valuable source of stored energy.
How the surface affects efficiency
However, the absorption of sunlight by a PEC does not always result in the production of hydrogen fuel. In some instances, the electrons fall back to their original energy states and recombine with the positively charged “holes” they left behind when they were originally excited by photons. Ke Yang, a researcher in the Batista lab and an author on the paper, explains that such recombination of charges actually wastes a significant amount of energy, since the absorption of light energy results not in the production of hydrogen fuel, but in the return to the original species.
Both the fuel-producing redox reaction and the energy-consuming recombination reaction occur in any given PEC, though not necessarily at the same rates. The rate of redox and recombination reactions in a PEC depends on the interactions between the semiconductor working electrode, counter electrode, and water. While much is known about the electronic properties of different semiconductors involved at the photoelectrode to water interface, the effect of the interface on the chemical kinetics, or the rates at which molecules interact, and overall PEC performance is less thoroughly understood. Thus, the team of Yale and Boston College researchers sought to examine exactly how this interface affected the kinetics of both the redox and recombination reactions in a PEC.
The researchers used hematite, an iron ore mineral, as the semiconductor for the working electrode. The choice of hematite has both electrochemical as well as practical advantages. The energy associated with the excitation of hematite’s electrons makes it suitable to absorb sunlight at the energy level needed for a PEC to split water. Additionally, Yang explains, hematite is rather inexpensive, abundant, and well-studied, making it a good candidate for study. The researchers chose to look at two varieties of hematite, referred to as hematite {001} and hematite {012}. The two types of hematite differ primarily in their preparation, resulting in distinct morphologies as well as other differences in the facets of the surface exposed to H2O.
One significant difference the researchers found between hematite {001} and hematite {012} is the number of hydroxyl groups, or OH groups, on the semiconductor’s surface that are able to interact chemically with water. There were significantly more OH groups available in hematite {012} than in hematite {001}. The increased number of OH groups was correlated with an increase in the kinetics and thermodynamics of both the redox and recombination reactions, meaning that both reactions occurred at a faster rate and with a more favorable energy in hematite {012} than in hematite {001}. Because hematite {001} and {012} share similar light absorption patterns and a similar degree of crystalline order, the researchers concluded that the differences in rates are a result of the surface differences in the hematite varieties—most notably the OH groups which mediate the reactions occurring in the PEC.
Despite the enhanced kinetics of hematite {012}, hematite {012} is not necessarily the preferable version of the semiconductor for the efficient production of solar fuel. Hematite {012} showed enhanced kinetics and thermodynamics of both the redox and the recombination reactions. This means that while the creation of hydrogen fuel from the redox reactions increased, so did the waste of energy associated with the recombination reaction. Thus, while hematite {012} is more favorable in terms of producing fuel, hematite {001} is favorable in terms of minimizing energy waste. This understanding of the different types of hematite can help improve how hematite PECs are built for maximum efficiency.
Knowing which variation of the semiconductor makes the reactions faster and more energetically favorable is useful for maximizing fuel production while minimizing energy waste. “The work provides information that will be useful for the design and development of photoelectrochemical systems with enhanced efficiency for solar energy conversion,” Brudvig said. The researchers mention the idea of heterogeneous catalysts to accomplish this. “[We hope] to develop better catalysts for chemical applications such as PECs, fuel cells, and batteries,” Brudvig added. This would greatly improve our ability to produce renewable energy for a sustainable future.