Reassembling mixed nickel iron hydroxide
MNF has a long history with energy studies. When Thomas Edison tinkered with batteries more than a century ago, he also used the same nickel and iron elements in nickel hydroxide-based batteries. Edison observed the formation of oxygen gas in his nickel hydroxide experiments, which is bad for a battery, but in the case of splitting water, production of oxygen gas is the goal.
“Scientists have realized for a long time that the addition of iron into the nickel hydroxide lattice is the key for the reactivity enhancement of water splitting.” Kuai said. “But under the catalytic conditions, the structure of the pre-designed MNF is highly dynamic due to the highly corrosive environment of the electrolytic solution.”
During Lin’s experiments, MNF degrades from a solid form into metal ions in the electrolytic solution — a key limitation to this process. But Lin’s team observed that when the electrochemical cell flips from the high, electrocatalytic potential to a low, reducing potential, just for a period of two minutes, the dissolved metal ions reassemble into the ideal MNF catalyst. This occurs due to a reversal of the pH gradient within the interface between the catalyst and the electrolytic solution.
“During the low potential for two minutes, we demonstrated we not only get nickel and iron ions deposited back into the electrode, but mixing them very well together and creating highly active catalytic sites,” Lin said. “This is truly exciting, because we rebuild the catalytic materials at the atomic length scale within a few nano-meter electrochemical interface.”
Another reason that the reformation works so well is that the Lin Lab synthesized novel MNF as thin sheets that are easier to reassemble than a bulk material.
Validating findings through X-rays
To corroborate these findings, Lin’s team conducted synchrotron X-ray measurements at the Advanced Photon Source of Argonne National Laboratory and at Stanford Synchrotron Radiation Lightsource of SLAC National Accelerator Laboratory. These measurements use the same basic premise as the common hospital X-ray but on a much larger scale.
“We wanted to observe what had happened during this entire process,” Kuai said. “We can use X-ray imaging to literally see the dissolution and redeposition of these metal irons to provide a fundamental picture of the chemical reactions.”
Synchrotron facilities require a massive loop, similar to the size of the Drillfield at Virginia Tech, that can perform X-ray spectroscopy and imaging at high speeds. This provides Lin high levels of data under the catalytic operating conditions. The study also provides insights into a range of other important electrochemical energy sciences, such as nitrogen reduction, carbon dioxide reduction, and zinc-air batteries.
“Beyond imaging, numerous X-ray spectroscopic measurements have allowed us to study how individual metal ions come together and form clusters with different chemical compositions,” Lin said. “This has really opened the door for probing electrochemical reactions in real chemical reaction environments.”
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