Power/Performance Bits: Nov. 8

Scrap metal batteries; 3D integration of memory and logic; storing solar energy.

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Scrap metal batteries

A research team at Vanderbilt University used scraps of steel and brass – two of the most commonly discarded materials – to create a steel-brass battery that can store energy at levels comparable to lead-acid batteries while charging and discharging at rates comparable to ultra-fast charging supercapacitors.

The researchers found that when scraps of steel and brass are anodized using a common household chemical and residential electrical current, the metal surfaces are restructured into nanometer-sized networks of metal oxide that can store and release energy when reacting with a water-based liquid electrolyte.

The team determined that these nanometer domains explain the fast charging behavior that they observed, as well as the battery’s stability. They tested it for 5,000 consecutive charging cycles – the equivalent of over 13 years of daily charging and discharging – and found that it retained more than 90% of its capacity.

A prototype high-performance battery made from scrap metal and common household chemicals. (Source: Daniel Dubois/Vanderbilt University)

A prototype high-performance battery made from scrap metal and common household chemicals. (Source: Daniel Dubois/Vanderbilt University)

Unlike lithium-ion batteries, the steel-brass batteries use non-flammable water electrolytes that contain potassium hydroxide, an inexpensive salt used in laundry detergent.

“We’re forging new ground with this project, where a positive outcome is not commercialization, but instead a clear set of instructions that can be addressed to the general public. It’s a completely new way of thinking about battery research, and it could bypass the barriers holding back innovation in grid scale energy storage,” said Cary Pint, assistant professor of mechanical engineering at Vanderbilt.

The team’s next step is to build a full-scale prototype battery suitable for use in energy-efficient smart homes.

3D integration of memory and logic

Engineers at UC Santa Barbara have developed a design for a functional nanoscale computing device. The concept involves a dense, three-dimensional circuit operating on an unconventional type of logic that could, theoretically, be packed into a block no bigger than 50nm on any side.

“Novel computing paradigms are needed to keep up with the demand for faster, smaller and more energy-efficient devices,” said Gina Adam, postdoctoral researcher at UCSB’s Department of Computer Science. “In a regular computer, data processing and memory storage are separated, which slows down computation. Processing data directly inside a three-dimensional memory structure would allow more data to be stored and processed much faster.”

A figure depicting the structure of stacked memristors with dimensions that could satisfy the Feynman Grand Challenge. (Source: UC Santa Barbara)

A figure depicting the structure of stacked memristors with dimensions that could satisfy the Feynman Grand Challenge. (Source: UC Santa Barbara)

Key to this development is the use of a logic system called material implication logic combined with memristors. Unlike conventional computing logic, in this form of computing, logic operation and information storage happen simultaneously and locally. This greatly reduces the need for components and space typically used to perform logic operations and to move data back and forth between operation and memory storage. The result of the computation is immediately stored in a memory element.

In addition, the researchers reconfigured the traditionally two-dimensional architecture of the memristor into a three-dimensional block.

“Since this technology is still new, more research is needed to increase its reliability and lifetime and to demonstrate large scale three-dimensional circuits tightly packed in tens or hundreds of layers,” Adam said.

Storing solar energy

Researchers at Stanford University built a new system to store electricity generated by high-efficiency solar cells and were able to capture and store 30% of the energy captured into stored hydrogen, beating the prior record of 24.4%.

“This milestone brings us much closer to a sustainable and practical process to use water-splitting as a storage technology,” said Thomas Jaramillo, an associate professor of chemical engineering and photon science at Stanford. “Improving efficiency has a remarkable impact on lowering costs. We have to continue work on finding more ways to lower the costs to compete with conventional fuels.”

The solar cell they used is very different – and more expensive – than typical rooftop solar arrays. While typical rooftop arrays are based on silicon, the team employed solar cells that use indium gallium phosphide (InGaP), gallium arsenide (GaAs), and gallium indium nitride arsenide antimonide (GaInNAsSb). Called triple-junction solar cells, each material is tuned to capture blue, green or red light. Through this precision, triple-junction solar cells convert 39% of incoming solar energy into electricity, compared with roughly 20% for silicon-based, single-junction solar cells.

The PV-electrolysis system consists of a triple-junction solar cell and two PEM electrolysers connected in series. (Source: Nature Communications, doi:10.1038/ncomms13237)

The PV-electrolysis system consists of a triple-junction solar cell and two PEM electrolysers connected in series. (Source: Nature Communications, doi:10.1038/ncomms13237)

The focus, however, was not how much energy they captured, but how much energy was stored through water splitting. To store electricity captured from sunlight, the team looked in particular at water-splitting catalysts.

Most photovoltaic-powered water-splitting reactions use a single electrolysis device. Instead, the team combined two identical electrolysis devices in such a manner to produce twice as much hydrogen, making use of the higher-efficiency solar cells. With this system, 30% of the energy originally collected by the triple-junction solar cells had been stored in the form of hydrogen gas.

The team’s focus will shift to reducing the cost of the system, as the price of the materials they used is prohibitive for an industrial process.