Breakthrough in Hydrogen Research Promotes Future of Clean Energy

Researchers have solved a long-time mystery around a hydrogen reaction that could help promote the wide-scale development of affordable clean, hydrogen-based energy. A team from Penn State discovered the reason for hydrogen spillover, which is a catalytic reaction that occurs between metal nanoparticles and a thermally stable oxide. Specifically, it occurs when hydrogen atom-like equivalents actually spill from the metal to the oxide when mixed with certain catalysts; hydrogen spillover is the term used for the hydrogen-on-oxide species.

Researchers led by Bert Chandler, professor of chemical engineering and chemistry at Penn State, have not only discovered the how and why of hydrogen spillover—something no one has done since its identification in 1964—but also managed to measure the process and create a new hydrogen-spillover system that expends less energy than previously developed systems.

Their findings arm scientists with new knowledge to develop efficient hydrogen activation and storage, a problem that has hindered the use of clean energy based on hydrogen, Chandler said.

“We are now able to explain how hydrogen spillover works, why it works, and what drives it,” he said in a post on Penn State's news site. “And, for the first time, we were able to measure it—that’s key. 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.”

A New System for Hydrogen Storage

Scientists first discovered hydrogen spillover in a platinum-on-tungsten-oxide system in 1964; since then has been observed in different systems. In hydrogen-spillover systems, hydrogen gas splits into hydrogen atom equivalents—a proton and an electron, but in a slightly different arrangement than their typical layout. In this system, the protons stick to the material’s surface while the electrons enter the semiconducting oxide’s near-surface conduction band.

As it is now, conventional hydrogen storage requires significant amounts of energy to keep the hydrogen cool enough to remain a liquid. However, the Penn team has developed a gold-on-titania system that can effectively, efficiently, and reversibly break apart hydrogen molecules into hydrogen atoms at higher temperatures that require less energy.

The researchers hope to use the system to test more advanced chemistry applications, such as converting the atoms for use as clean fuel and hydrogen storage, Chandler said.

Gold catalyzes hydrogen differently than many other metals, requiring almost no thermal energy to initiate a reaction with the hydrogen, he explained. "It only activates that reaction at the interface with titanium oxide substrate," Chandler said. "That means that no hydrogen adsorbs to the gold, so we can quantify all spillover produced because it all goes to the substrate, without leaving any fizz on the gold.”

That last bit is important, he said, because most hydrogen spillover-facilitating systems are messy, as the spillovers can appear to vary their bonding strength to both the nanoparticle and the semiconductor oxide substrate. This is something Chandler dubbed "fizz adsorption," the fuzzy, sticky bonding that conceals true adsorption and masks what’s driving the spillover: thermal energy or entropy, the latter of which has been near impossible to measure in terms of balancing these systems, he said.

Solving the Puzzle

Without the fizz, the researchers realized that the adsorption in their system was weak—which “flew in the face of what everyone knew,” Chandler said. Without thermal energy as a significant variable, the researchers determined that only entropy—or energy dispersing to substates—could be driving the atoms from the gold to the substrate.  

“We got really lucky with our choice of system, which we selected because we were already interested in how gold works as a catalyst,” Chandler said, explaining that previous researchers could measure the amount adsorbed accurately because weak adsorption on the oxide masked the amount of spillover from the metal.

Potential for Hydrogen-Based Energy

The team took six years to measure and re-measure before confirming their findings that entropy drives hydrogen spillover. He added. Researchers published a paper on their work in the journal, Nature Catalysis. The team now aims to investigate material types that could facilitate better hydrogen storage to continue progress toward clean energy development, Chandler said.