Increased use of fossil fuels is causing rising levels of carbon dioxide and other emissions.
Demands and emissions: Higher use of non-fossil fuel sources is needed to address rising energy demands and carbon dioxide emissions. So, sustainable alternatives, such as solar, wind, and geothermal power, are being pursued. Discoveries in these areas will make it vital to have a reliable way to store the electrical energy.
Storing energy as fuel: Why is storing energy important? Many alternative power sources are intermittent. That is, wind turbines do not produce power on still days. The same goes for solar panels at night. So, energy must be stored for use when power is not generated. Also, energy storage could boost the efficiency of continuous power sources, such as geothermal and nuclear. These energy suppliers cannot store power, so the energy is either used or lost.
One of the best ways to store energy is as fuel. Fuels have a much higher energy density compared to batteries or mechanical storage devices. So, converting electrical energy generated by solar and other power sources into chemicals could turn intermittent sources into reliable fuels.
So, what's the problem? The electrocatalysts synthesized today are expensive and are based on precious metals like platinum. However, the enzymes that participate in photosynthesis in Nature are more efficient and use inexpensive, abundant metals such as nickel and iron.
Our solution: Design highly active catalysts for reactions that convert electrical energy into chemical bonds in fuels. Most of these reactions involve moving more than one proton and more than one electron. One hurdle to designing these catalysts is the lack of knowledge about proton relays. Proton relays are clusters of atoms or functional groups that are key in delivering protons to (or from) the active site of catalysts.
Much important research has been carried out on electron transfer reactions and pathways, but protons are almost 2000 times as massive as electrons. As a result, proton transfer reactions occur over much shorter distances, and proton transfer pathways need to be understood in much greater detail to efficiently carry out the multi-proton and multi-electron reactions required for fuel generation from renewable energy sources.
So, our team is working to
- Learn how proton relays accelerate proton transfers
- Couple proton transfers with electron transfers to accelerate reactions
Our approach focuses on three reactions and the electrocatalysts needed to speed those reactions. The three reactions are (the protons are shown in red)
2 H+ + 2 e- ⇌ H2
O2 + 4 H+ + 4 e- ⇌ 2 H2O
N2 + 6 H+ + 6 e- ⇌ 2 NH3
2 H+ + 2 e- ⇌ H2 Creating hydrogen fuel. Adding two protons (2 H+; ideally derived from water) and two electrons to form hydrogen (H2) is the simplest fuel generation reaction. Reading from left to right, this reaction combines two protons and two electrons to form the stable but highly energetic H2 molecule. The reverse process, known as the oxidation of H2 (read the equation from right to left), is important in fuel cells.
The demand for H2 is expected to increase as large amounts are used in processing fossil fuels and removing oxygen from biomass to be converted to fuel. These processes are in addition to the potential use of hydrogen for transportation fuel.
O2 + 4 H+ + 4 e- ⇌ 2 H2O Reducing oxygen in fuel cells. Adding four protons and four electrons to oxygen to form water or H2O is half of the reaction used by most fuel cells to generate power. This reaction is known as a reduction reaction because it adds electrons. The reaction moving from right to left is being intensively studied by solar energy scientists, who want to use sunlight to split apart water and release energy.
N2 + 6H+ + 6 e- ⇌ 2 NH3 Creating ammonia for fertilizer. Adding six protons to nitrogen to give ammonia, moving from left to right, is a reaction of global importance; as creating ammonia for fertilizer consumes about 1 percent of the world's total energy supply. This same reaction moving from right to left is known as the oxidation of ammonia. Oxidation reactions involve losing electrons. This reaction has been considered in fuel cells, but less extensively than hydrogen.
Serving as prototypes. In addition to being of interest for energy applications, these reactions are prototypical systems that will allow us to explore how electrocatalysts must evolve to perform increasingly complex processes (2, 4, and 6 protons). Enlarge Image
Why molecular electrocatalysts? Molecular electrocatalysts, well-defined synthetic materials that make reactions happen, are effective and affordable. Other catalysts, known as heterogeneous catalysts, use platinum. But, molecular electrocatalysts get the job done using nickel and iron, which are abundant and inexpensive. More importantly, molecular catalysts have a well-defined structure and a precise way of doing things. This makes them easier to study and control.
Why the emphasis on proton relays? All electrocatalytic reactions for fuel generation and use involve multiple protons. The simple, controlled movement of both protons and electrons from the solution to the catalyst's active metal site—where the reaction occurs—is essential. Because protons are much more massive than electrons, it is likely that proton transfer will need to be even more carefully controlled and designed than pathways for electrons.
Scientists at Pacific Northwest National Laboratory have shown that proton relays profoundly influence reactions using molecular catalysts. They have also created nickel and cobalt catalysts that use nitrogen-containing groups that hang off the catalyst to relay the protons. These catalysts are said to have pendant amine bases as proton relays. These catalysts produce hydrogen at rates close to those of nickel-iron enzymes, far surpassing other synthetic catalysts. Remarkably, these new catalysts are not inhibited by carbon monoxide, which does degrade the platinum catalyst used in fuel cells.
In parallel with these advances, researchers at the University of Washington, Pennsylvania State University, and the University of Wyoming are developing detailed theoretical and experimental understandings of proton/electron transfer processes.
As the benefits of incorporating proton relays are more widely recognized, we expect a rise in interest from the catalysis community in adapting proton relays into catalysts.
The Center for Molecular Electrocatalysis. We have assembled a team of researchers that have experience in a variety of different disciplines that we believe will allow us to make progress in attacking these problems.