The transition from fossil fuels to a clean hydrogen economy will require cheaper and more efficient ways to use renewable sources of electricity to break down water into hydrogen and oxygen.
But a key step in this process, known as the Oxygen Release Reaction or REL, has proven to be a bottleneck. Today, its efficiency is only about 75%, and the precious metal catalysts used to speed up the reaction, such as platinum and iridium, are scarce and expensive.
Now, an international team led by scientists from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have developed a suite of advanced tools to break this bottleneck and improve other processes related to energy, such as finding ways to recharge lithium-ion batteries. faster. The research team described their work in Nature today.
Working at Stanford, SLAC, DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of Warwick in the UK, they were able to zoom in on individual catalyst nanoparticles – shaped like tiny plaques and around 200 times smaller than a red blood cell – and watch them speed up the generation of oxygen inside bespoke electrochemical cells, including one that fits into a drop of water.
They found that most of the catalytic activity took place at the edges of the particles, and they were able to observe the chemical interactions between the particle and the surrounding electrolyte on a scale of billionths of a meter as they increased the voltage. to cause the reaction. .
By combining their observations with previous computational work done in collaboration with the SUNCAT Institute for Interface Science and Catalysis at SLAC and Stanford, they were able to identify a single step in the reaction that limits its speed.
“This suite of methods can tell us where, what and why how these electrocatalytic materials operate under realistic operating conditions,” said Tyler Mefford, a Stanford and Stanford Institute for Materials and Energy Sciences (SIMES) scientist at SLAC who directed the research. “Now that we have explained how to use this platform, the applications are extremely broad.”
Switch to a hydrogen economy
The idea of using electricity to break down water into oxygen and hydrogen dates back to 1800, when two British researchers discovered that they could use the electric current generated by Alessandro Volta’s newly invented cell battery to power the reaction.
This process, called electrolysis, works much like a battery in reverse: rather than producing electricity, it uses electric current to split water into hydrogen and oxygen. The reactions that generate hydrogen and oxygen gas take place on different electrodes using different precious metal catalysts.
Hydrogen gas is an important chemical feedstock for ammonia production and steel refining, and is increasingly targeted as a clean fuel for heavy transport and long-term energy storage. But more than 95% of the hydrogen produced today comes from natural gas via reactions that emit carbon dioxide as a by-product. The production of hydrogen by electrolysis of water powered by electricity from solar, wind and other sustainable sources would significantly reduce carbon emissions in a number of important industries.
But to produce hydrogen from water on a scale large enough to power a green economy, scientists will need to make the other half of the water-splitting reaction – the one that generates oxygen – much more efficient and find ways to make it. work with catalysts based on metals that are much cheaper and more abundant than those used today.
“There aren’t enough precious metals in the world to power this reaction on the scale we need,” Mefford said, “and their cost is so high that the hydrogen they generate can never compete with hydrogen derived from fossil fuels. “
Improving the process will require a much better understanding of how water separation catalysts work, with enough detail for scientists to predict what can be done to improve them. Until now, many of the best techniques for making these observations have not worked in the liquid environment of an electrocatalytic reactor.
In this study, scientists found several ways to get around these limitations and get a sharper image than ever before.
New ways to spy on catalysts
The catalyst they chose to study was cobalt oxyhydroxide, which comes in the form of six-sided flat crystals called nanoplates. The edges were sharp and extremely thin, so it would be easy to distinguish if a reaction was taking place on the edges or on the flat surface.
About ten years ago, Patrick Unwin’s research group at the University of Warwick invented a new technique to put a miniature electrochemical cell inside a nanoscale droplet protruding from the tip of the cell. ‘a pipette tube. When the droplet is brought into contact with a surface, the device images the topography of the surface and the electronic and ionic currents with very high resolution.
For this study, the Unwin team adapted this small device to work in the chemical environment of the oxygen release reaction. Postdoctoral fellows Minkyung Kang and Cameron Bentley moved it from place to place on the surface of a single catalyst particle during the reaction.
“Our technique allows us to zoom in to study very small regions of reactivity,” said Kang, who conducted the experiments there. “We are looking at the generation of oxygen on a scale more than a hundred million times smaller than conventional techniques.”
They found that, as is often the case with catalytic materials, only the edges actively promote the reaction, suggesting that future catalysts should maximize this type of fine and fine characteristic.
Meanwhile, Andrew Akbashev, a researcher at Stanford and SIMES, used atomic force electrochemical microscopy to determine and visualize exactly how the catalyst changed shape and size during operation, and found that the reactions that initially had changed the catalyst to its active state were very different from what had previously been assumed. . Rather than protons leaving the catalyst to trigger activation, hydroxide ions first inserted themselves into the catalyst, forming water inside the particle that caused it to swell. As the activation process progressed, this water and the residual protons were driven out.
In a third set of experiments, the team worked with David Shapiro and Young-Sang Yu at the advanced light source at the Berkeley lab and with a Washington company, Hummingbird Scientific, to develop a cell to electrochemical flow that could be integrated into a scanning transmission radiography. microscope. This allowed them to map the oxidation state of the working catalyst – a chemical state associated with catalytic activity – in areas as small as around 50 nanometers in diameter.
“We can now begin to apply the techniques we developed in this work to other electrochemical materials and processes,” Mefford said. “We would also like to study other energy-related reactions, such as the rapid charging of battery electrodes, the reduction of carbon dioxide for carbon capture and the reduction of oxygen, which allows us to use the hydrogen in fuel cells. “