Chemical Reaction in Electrode Surface Could Help Design More Efficient Renewable Energy Technologies

Chemical reactions are very significant in designing the most optimal renewal energy technologies. Researchers have finally mapped out how proton-couple electrons transfer at an electrode surface.

Chemical Reaction For Effective Renewable Energy Technologies

In a new study, MIT graduate student Noah Lewis is the lead author of the paper. Ryan Bisbey, a former MIT postdoc; Karl Westendorff, an MIT graduate student; and Alexander Soudackov, a research scientist at Yale University, traced how electrons and protons behave at a surface site and they played a role in energy conversion devices or catalytic reactions.

When a molecule, usually water or acid, transmits a proton to another molecule or to an electrode surface, the proton acceptor is stimulated to pick up an electron as well. This process is known as proton-coupled electron transfer. Numerous energy applications have made use of this type of reaction.

Electron transfer processes that are related to protons are widely occurring. These are frequently crucial phases in catalytic systems, and Yogesh Surendranath, an MIT professor of chemistry and chemical engineering and the study's senior author, noted that they are especially significant for energy conversion processes like fuel cell catalysis or hydrogen production.

This method is employed in an electrolyzer that produces hydrogen gas by taking protons out of water and adding electrons to the protons. When protons and electrons are taken out of hydrogen gas and added to oxygen to make water, a fuel cell produces electricity.

Many other kinds of chemical reactions, such as carbon dioxide reduction (the addition of protons and electrons to convert carbon dioxide into chemical fuels), frequently involve proton-coupled electron transfer. Because they can carefully regulate each molecule's structure and watch as protons and electrons move between them, scientists have learned a great deal about how these reactions happen when the proton acceptors are molecules.

The rate of proton transfer to the oxygen ion at the surface at equilibrium-the condition where the rates of proton donation to the surface and proton transfer back to solution from the surface are equal-was determined by the researchers using this system to measure the flow of electrical current to the electrodes. They discovered that this rate is significantly influenced by the pH of the surrounding solution, with the highest rates occurring at the most acidic pH (0) and the most basic pH (pH 14).

Researchers created a model based on two potential reactions that could happen at the electrode to explain these observations. First, protons are transferred to the surface oxygen ions by hydronium ions (H3O+), which are found in high concentrations in very acidic solutions, producing water. In the second, hydroxide ions (OH-), which are highly concentrated in strongly basic solutions, are created when water transfers protons to the oxygen ions on the surface.

However, because hydronium loses protons more quickly than water, the rate at pH 0 is roughly four times greater than the rate at pH 14.

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Reaction Rate

Remarkably, the researchers also found that the two reactions have identical speeds at pH 10, where the concentration of hydroxide ions is one million times greater than that of hydronium, rather than at neutral pH 7, where the quantities of hydronium and hydroxide are equal. It is suggested by the model that this is because the forward reaction, which involves the donation of protons from hydronium or water, contributes more to the overall rate than the backward reaction, which involves the removal of protons from water or hydroxide.

The scientists note that the new data imply that previous models of how these events happen at electrode surfaces may need to be reevaluated because they assumed that the forward and backward reactions contribute equally to the total rate.

According to Surendranath, the forward and reverse reactions both contribute equally to the reaction rate, and that is the default assumption. Our discovery is really startling because it suggests that we should reexamine the presumption that has been used to study everything from hydrogen evolution to fuel cell catalysis."

The researchers are currently using their experimental setup to explore how adding different types of ions to the electrolyte solution around the electrode may speed up or slow down the rate of proton-coupled electron movement.

"With our system, we know that our sites are constant and not affecting each other, so we can read out what the change in the solution is doing to the reaction at the surface," Lewis said.

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