Using photovoltaic cells to power electrochemical reactions isn't a new science, but researchers from Washington University in St. Louis have demonstrated how the two existing technologies can be used to perform oxidation reactions while generating hydrogen gas as the only byproduct. The method helps eliminate stoichiometric reduction products generated during an oxidation.
"All chemical oxidations have a byproduct," says Kevin Moeller, PhD, professor of chemistry in Arts & Sciences, and lead on the project, "so the question is not whether there will be a byproduct but what that byproduct will be. People have started thinking about how they might run oxidations where the reduced byproduct is something benign.
If you use oxygen to do the oxidation, the byproduct is water, and that is a gentle process."
"But… like all other molecules, oxygen has a set oxidation potential, or willingness to accept electrons. "So whatever I want to oxidize in solution has to have an oxidation potential that matches oxygen's. If it doesn't, I might have to change my whole reaction around to make sure I can use oxygen. And when I change the whole reaction around, maybe it doesn't run as well as it used to. So I'm limited in what I can do.
"Electrochemistry can oxidize molecules with any oxidation potential, because the electrode voltage can be tuned or adjusted, or I can run the reaction in such a way that it adjusts itself. So I have tremendous versatility for doing things," Moeller explains.
In an article in Green Chemistry, the team reports on using the method to directly oxidize an enol ether , a vinyl sulfide and a carbamate at the electrode — without using a chemical reagent.
Because reagents are usually expensive and toxic, Moeller hopes the new process can help clean up reagent recycling. Work also is underway to extend the reactions to the recycling of chemical oxidants.
"We started the work about six-months ago. Initially, we are looking at Pd, CAN, and ScIII because we have experience with those metals. After that, we will turn our attention to a variety of others ranging from Cr to Os to Ru, etc.," says Moeller.
While the solar part of the equation is easy, Moeller says the key challenge is getting industry to adopt electrochemistry as a synthetic tool.
"Electrochemistry is not typically part of a synthetic chemist's background. This unfamiliarity is a significant barrier to its adoption. For this reason, we are working to illustrate the simplicity of the method, while at the same time develop new reactions that will demonstrate how synthetic chemists can think about an electrolysis reaction."
Another challenge is limitations to controlling the current in small-scale reactions.
"Using a photovoltaic, it is difficult to maintain the amount of electricity running through the cell at an ideal level. Hence, we have seen lower chemical selectivity in some of the reactions run."
However, he adds that this limitation is not a problem on larger-scale reactions with solar panels generating electricity that could then be stored and used on demand with conventional electrolysis equipment.
"We are just trying to figure out what is possible. What we know is that the use of electrochemistry can change the way we think about problems in chemical synthesis. For many years, we have either ignored this potential or pushed it onto a 'back-burner.' Maybe now it is time to really begin analyzing the issues and examining the potential of the technique," he concludes.
