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Fast calculation dials in better batteries

A simpler and more efficient way to predict performance will lead to better batteries, according to Rice University engineers.

That their method is 100,000 times faster than current modeling techniques is a nice bonus.

The analytical model developed by materials scientist Ming Tang and graduate student Fan Wang of Rice University’s Brown School of Engineering doesn’t require complex numerical simulation to guide the selection and design of battery components and how they interact.

The simplified model developed at Rice — freely accessible online — does the heavy lifting with an accuracy within 10% of more computationally intensive algorithms. Tang said it will allow researchers to quickly evaluate the rate capability of batteries that power the planet.

The results appear in the open-access journal Cell Reports Physical Science.

There was a clear need for the updated model, Tang said.

“Almost everyone who designs and optimizes battery cells uses a well-established approach called P2D (for pseudo-two dimensional) simulations, which are expensive to run,” Tang said. “This especially becomes a problem if you want to optimize battery cells, because they have many variables and parameters that need to be carefully tuned to maximize the performance.

“What motivated this work is our realization that we need a faster, more transparent tool to accelerate the design process, and offer simple, clear insights that are not always easy to obtain from numerical simulations,” he said.

Battery optimization generally involves what the paper calls a “perpetual trade-off” between energy (the amount it can store) and power density (the rate of its release), all of which depends on the materials, their configurations and such internal structures as porosity.

“There are quite a few adjustable parameters associated with the structure that you need to optimize,” Tang said. “Typically, you need to make tens of thousands of calculations and sometimes more to search the parameter space and find the best combination. It’s not impossible, but it takes a really long time.”

He said the Rice model could be easily implemented in such common software as MATLAB and Excel, and even on calculators.

To test the model, the researchers let it search for the optimal porosity and thickness of an electrode in common full- and half-cell batteries. In the process, they discovered that electrodes with “uniform reaction” behavior such as nickel-manganese-cobalt and nickel-cobalt-aluminum oxide are best for applications that require thick electrodes to increase the energy density.

They also found that battery half-cells (with only one electrode) have inherently better rate capability, meaning their performance is not a reliable indicator of how electrodes will perform in the full cells used in commercial batteries.

The study is related to the Tang lab’s attempts at understanding and optimizing the relationship between microstructure and performance of battery electrodes, the topic of several recent papers that showed how defects in cathodes can speed lithium absorption and how lithium cells can be pushed too far in the quest for speed.

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Water + air + electricity = hydrogen peroxide

The production of hydrogen peroxide can be much safer and simpler through a process developed at Rice University.

A reactor developed by Haotian Wang and his colleagues at Rice’s Brown School of Engineering requires only air, water and electricity to make the valuable chemical in the desired concentration and high purity.

Their electrosynthesis process, detailed in Science, uses an oxidized carbon nanoparticle-based catalyst and could enable point-of-use production of pure hydrogen peroxide solutions, eliminating the need to transport the concentrated chemical, which is hazardous.

By using a solid electrolyte instead of traditional liquid electrolyte, it also eliminates the need for product separation or purification used in current processes, so no contaminating ions will be involved.

“If we have electricity from a solar panel, we can literally get hydrogen peroxide from just sunlight, air and water,” said Wang. “We don’t need to involve organics or fossil fuel consumption. Hydrogen peroxide synthesis by traditional, huge chemical engineering plants generates organic wastes, consumes fossil fuels and emits carbon dioxide. What we’re doing is green synthesis.”

Hydrogen peroxide is widely used as an antiseptic, a detergent, in cosmetics, as a bleaching agent and in water purification, among many other applications. The compound is produced in industrial concentrations of up to 60% solution with water, but in many common uses, the solution is far more diluted.

“Industrial hydrogen peroxide has to be transported in high concentrations to maximize the economics,” Wang said.

“Transportation is hazardous and costly because the concentrated compound is unstable. Hydrogen peroxide also degrades over time, and has to be stored once it gets to its destination.

“Our technology delocalizes the production of hydrogen peroxide,” he said. “As renewable electricity input gets cheaper, air is free and water is also cheap, our product should be competitive in terms of price.

“Instead of storing containers of hydrogen peroxide, hospitals that use it as a disinfectant could in the future turn on a spigot and get, for instance, a 3% solution on demand,” Wang said. “Instead of storing chemicals to disinfect pool water, homeowners can flick a switch and turn on the reactor to clean their pools.”

The Rice reactor is somewhat similar to a fuel cell, with electrodes on either side to process hydrogen (or water) and oxygen (from air), feeding them to catalysts on two electrodes sandwiching an ionically conductive porous solid electrolyte.

“A fuel cell minimizes the production of hydrogen peroxide to produce just water with maximized energy efficiency,” said Rice postdoctoral researcher and lead author Chuan Xia. “In our case, we want to maximize hydrogen peroxide instead, and have tuned our catalyst to do so.”

The low-cost carbon black catalyst, set in a solid electrolyte and oxidized to enhance its reactivity, shifts the oxygen reduction pathway towards the desired chemical at rates and concentrations determined by the applied voltage, air and water feedstock and a steady supply of deionized water. The reaction takes place under ambient temperatures and pressures.

Co-lead author Yang Xia, a second-year graduate student in the Wang lab, said the catalyst proved robust enough to synthesize pure solution of 1%-by-weight hydrogen peroxide over 100 continuous hours in the lab with negligible degradation.

Wang said the lab plans to engineer both larger reactors and plug-and-play components with an eye toward testing with industrial partners. He sees great promise for industrial-scale applications like municipal water purification systems. The Rice lab has tested low concentrations of its product on campus rainwater and proved its ability to remove organic carbon contaminants.

“There are so many potential applications,” he said. “Before this, electrochemical synthesis of hydrogen peroxide was limited by its product separation or purification process, but we’ve solved the big barrier to practical applications.”

Rice graduate student Peng Zhu and academic visitor Lei Fan are co-authors of the paper. Wang is the William Marsh Rice Trustee Chair, an assistant professor of chemical and biomolecular engineering and a 2019 CIFAR Azrieli Global Scholar.

Rice University and the J. Evans Attwell-Welch Postdoctoral Fellowship provided by the Smalley-Curl Institute supported the research.

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