By Rowenna Hoskin | Science Editor
While you may see wind farms and solar panels dotted around the country, renewable energy is not currently wide spread across the globe. Biofuel is used to supplement petrol in some countries, but the world is currently a carbon-burning society.
The necessity of moving away from fossil fuels and towards the adoption of renewable and sustainable energy is at the front of many scientific minds, including that of Feng Lin, Assistant Professor of Chemistry at Virginia Tech College of Science. His main focus is the capacity of the water molecule to fuel our lifestyles.
He proposes focusing efforts on the hydrogen economy; specifically the splitting of water molecules to produce hydrogen which would then power society’s electrical needs.
The mass production of hydrogen consists of the splitting of water molecules (two hydrogen molecules and one oxygen molecule) to create a usable fuel, hydrogen, and breathable oxygen. With a planet that is around 71% water – and increasing due to global warming – fuel created from water is great news.
The team used freshwater for this study, but Lin suggests that sea water would work and is more widely available – the sea contains 96.5% of the world’s water.
In a new study published in Nature Catalysis, Lin and his team have solved key fundamental barriers in electrochemical water splitting. Chunguang Kuai, one of Lin’s former graduates is first author of the study and is working alongside Lin and other graduates Zhengrui Xu, Anyang Hu, and Zhijie Yang to configure the reassembly, revivification and reuse of a catalyst needed for the process.
Catalysts are substances that increase the rate of the reaction without being consumed within the chemical reaction. One way catalysts do this is to decrease the amount of energy needed for the reaction to start.
Although water appears to be a basic molecule, the process of splitting it into its atoms is quite difficult. Despite this, Kuai and Lin’s lab have succeeded.
The process of moving one electron from a stable atom can be energy intensive; the reaction Lin’s lab has created requires the moving of four electrons in order to oxidise oxygen to produce oxygen gas.
“In an electrochemical cell, the four-electron transfer process will make the reaction quite sluggish, and we need to have a higher electrochemical level to make it happen,” Lin said. “With a higher energy needed to split water the long-term efficiency and catalyst stability become key challenges.”
In order to reach this high energy requirement, Lin’s lab introduced a common catalyst called Mixed Nickel Iron Hydroxide (MNF); this lowers the energy threshold for the reaction. While MNF reacts well in the reaction, it has a short lifespan due to its high reactivity causing a performance decrease.
“Scientists have realised for a long time that the edition of iron into Nickel Hydroxide lattice is the key for the reactivity enhancement of water splitting,” Kuai said. “But under the catalyst conditions, the structure of the pre designed MNF is highly dynamic due to the highly corrosive environment of the electrolytic solution.”
The key limitation is the fact that during the experiment, the MNF degrades from its solid form into metal ions in the electrolytic solution.
MNF has a long history in the world of energy studies; Thomas Edison used nickel and iron elements in nickel-hydroxide based batteries.
To combat this problem, the team discovered a new technique; the periodic reassembling of MNF to its original state allowing the reaction to continue as normal.
Kuai and Lin observed the fact that the dissolved metal ions reassemble into their ideal MNF catalyst form if the electrochemical cell is flipped from a high electrocatalytic potential to a low and reducing potential for a period of two minutes.
The lab also found that the reaction worked best when they synthesised novel MNF as thin sheets, making it easier to reassemble than as bulk material. This is because thin sheets have a higher surface area to volume ratio.
The team conducted synchrotron X-ray measurements at the Advanced Photon Source of Argonne National Laboratory and at Standford Synchrotron Radiation Lightsource of SLAC National Accelerator Laboratory to corroborate their findings. This X-ray uses the basic premise as common hospital x-rays, 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 x-ray provided Lin with high levels of data under 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 imagining, numerous x-ray spectroscopy 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.”
The team are now developing their machine learning approaches to identify material properties highlighting electrochemical performances.
The horizon of renewable energy looks as if it will be a watery one. If the lab can mass produce hydrogen safely, society may one day run entirely on hydrogen fuel created from the most abundant molecule on the planet.
Science and Technology Rowenna Hoskin