Penn State scientists unlock the key to clean energy storage

"We didn’t invent new chemistry; we just collected the data. It took us six years of measuring and re-measuring. When you make an exceptional claim, you better have exceptional evidence."
Amal Jos Chacko
Representational image of battery storage.jpg
Representational image of battery storage.

Petmal/iStock 

For decades, hydrogen spillover has been a scientific enigma, quietly captivating researchers with its potential for revolutionizing clean energy. 

The phenomenon involves small metal nanoparticles anchored on thermally stable oxides like silica, and accelerate chemical reactions while leaving themselves unaffected. Hydrogen atoms, akin to elusive equivalents, spill over from the metal to the oxide, forming what scientists term "hydrogen spillover." 

While this phenomenon was first described in 1964, its true nature and potential remained shrouded in mystery. However, a recent breakthrough at Penn State has shed light on this long-standing puzzle.

Unlocking the mystery

Bert Chandler, a professor of chemical engineering and chemistry at Penn State, led a research team that has achieved what was once considered impossible. They not only unraveled how and why hydrogen spillover occurs but also provided the first quantitative measurement of this intricate process. 

Their findings, published in Nature Catalysis, open doors to a deeper understanding of hydrogen activation and storage, holding immense promise for the clean energy sector.

Chandler explained that the research team employed a unique gold-on-titania system, demonstrating the efficient and reversible splitting of hydrogen molecules into hydrogen atoms. 

This breakthrough, taking place at higher temperatures, reduced energy requirements significantly when compared to conventional hydrogen storage methods that necessitate cooling hydrogen to keep it in a liquid state.

Implications for clean energy

Hydrogen spillover systems involve hydrogen gas splitting into hydrogen atom equivalents: protons and electrons. In this process, protons attach to the material's surface while electrons enter the semiconducting oxide's near-surface conduction band. This discovery paves the way for innovative applications, including converting these hydrogen atoms into clean fuel and hydrogen storage.

What sets this research apart is the role of entropy, which plays a crucial part in driving the atoms from the metal to the substrate. Unlike previous assumptions that thermal energy was the primary driving force, Chandler's team found that entropy was the key player, providing a more nuanced understanding of the phenomenon.

"We are now able to explain how hydrogen spillover works, why it works, and what drives it. And, for the first time, we were able to measure it — that’s key,” stated Chandler, emphasizing the significance of their findings. “Once you quantify it, you can see how it changes, figure out how to control it and figure out how to apply it to new problems."

“We got really lucky with our choice of system, which we selected because we were already interested in how gold works as a catalyst,” explained Chandler. “We didn’t invent new chemistry; we just collected the data. It took us six years of measuring and re-measuring.”

“Entropy drives hydrogen spillover”

The implications of this discovery are immense, particularly in the field of clean energy development. It represents a testament to the scientific process, where curiosity, persistence, and rigorous experimentation can lead to profound breakthroughs. 

"Science is a self-correcting process. We've known about spillover for a long time, but no one had found the right system to quantify and understand it. We collected the data and figured out how to explain the phenomenon. It turns out, the balance of energies that we use is not always obvious, and entropy can drive things we don't expect," Chandler added.

Study Abstract

Hydrogen spillover involves the migration of H atom equivalents from metal nanoparticles to a support. While well documented, H spillover is poorly understood and largely unquantified. Here we measure weak, reversible H2 adsorption on Au/TiO2 catalysts, and extract the surface concentration of spilled-over hydrogen. The spillover species (H*) is best described as a loosely coupled proton/electron pair distributed across the titania surface hydroxyls. In stark contrast to traditional gas adsorption systems, H* adsorption increases with temperature. This unexpected adsorption behaviour has two origins. First, entropically favourable adsorption results from high proton mobility and configurational surface entropy. Second, the number of spillover sites increases with temperature, due to increasing hydroxyl acid–base equilibrium constants. Increased H* adsorption correlates with the associated changes in titania surface zwitterion concentration. This study provides a quantitative assessment of how hydroxyl surface chemistry impacts spillover thermodynamics, and contributes to the general understanding of spillover phenomena.

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