Michel W. Barsoum, distinguished professor at Drexel’s College of Engineering, and his research team have chemically engineered a novel 2-D nanomaterial that has the potential to significantly enhance next generation energy storage devices. The class of materials, called MXenes, are the first materials that are hydrophilic, have conductive properties comparable to metals, and the plasticity and moldability of clay. The marriage of these properties in a single material and the relatively low cost to produce make it a front-runner in the race to create high performance devices.
The original goal behind the project when it began in September 2010 was to create a better anode than graphite. An anode is the negative conductor of a battery; it releases electrons that flow toward the cathode. A device that is powered by a battery, such as a cell phone, works because the individual parts in the phone are connected to the anode and cathode of the battery. When charges are released from the anode, they travel through a single pathway to the cathode via components of the phone such as the screen, lights and speakers, thus powering the device. Currently, the best material used commercially to make anodes is graphite, and in order to create better batteries with the ability to store more energy per unit of volume, it’s key to find a material that has a high capacity to store charge.
The newly discovered MXenes were derived from a class of materials that were also discovered at Drexel by Barsoum in 1996, called MAX phases. MAX phases are substances with a hexagonal atomic structure similar to graphite but made with a transition metal, an A-group element (one of the elements in columns 13-18 of the periodic table), and carbon or nitrogen. Initially, Barsoum and his research team intended to use MAX phases to create a better anode, since they have conductive properties comparable to metals, but quickly found that they didn’t work with metals that are prevalent in current batteries (such as lithium) due to MAX phases’ chemical structure.
Michael Naguib, a doctoral student who was a member of Barsoum’s research team, figured out a way to chemically remove the A-group element from MAX phases using toxic hydrofluoric acid so that lithium would have a site to bond. The method worked, and a new class of materials was discovered and named MXenes. While testing the properties of the newly discovered materials, the team found that MXenes could bond effectively with polymers and retain the required conductive properties. This meant that the material could be given any desired mechanical property, such as high compressive strength, by simply bonding it to a second material. This makes MXenes more versatile and practical for a variety of applications.
However, the fact that concentrated hydrofluoric acid was needed to manufacture MXenes from MAX phases was an issue that needed to be solved, since HF is extremely hazardous and unsafe to handle, which would increase manufacturing costs among other problems. A solution was found by Michael Ghidiu, a doctoral student at Drexel also part of Barsoum’s team. Rather than using hydrofluoric acid, Ghidiu used lithium fluoride salt and hydrochloric acid, which are common substances found in most high school chemistry labs and much safer to handle than hydrofluoric acid.
When the etching of the A-element was complete, he washed the material in water. He expected to find the same black film that was created when MAX phases were mixed with HF but instead found the MXenes created from this particular process was clay-like in terms of plasticity and moldability. This was a huge leap forward in terms of applications for which MXenes could be used.
“Clay is hydrophilic so it can be molded, heated and made into anything you want when mixed with water, making it a very useful material,” Barsoum said. “However, clay doesn’t conduct electricity. Graphene is conductive but it’s hydrophobic. MXenes are essentially conductive clays, which testing showed renders them excellent electrodes for supercapacitors and potentially batteries. The hydrophilicity allows us to use aqueous electrolytes. From an environmental point of view, being able to work with water is a huge advantage, especially manufacturing-wise. It’s also less expensive than using non-aqueous electrolytes. The clay-like characteristics also allow us to roll it with a rolling pin [and slice it up] like dough to produce electrode in minutes rather than waiting several days. This material also holds together well. Most materials are conductive enough and don’t hold together enough. With graphite you have to add a conductive binder and glue that will prevent the material from flaking.”
The new hydrophilic MXene was pressed to see if it could be rolled into a thin film, since the initial goal was to create a thin film that could be used as an anode. It was then tested on a supercapacitor to see if it maintained its electrical properties. Capacitors only need an anode unlike a battery, which requires both an anode and a cathode that could keep up with the anode’s performance. The team found that it was significantly more effective in terms of energy storage per unit of volume than current materials and stored more energy than the MXenes that were created using HF.
“Current commercial supercapacitors only have the ability to store 200-300 Farads per cubic centimeter,” Yury Gogotsi, a co-inventor of this technology, said. “A supercapacitor made using MXenes has the ability to store 900-1000 Farads per cubic centimeter. In applications where volume is at a premium, such as cell phones that are lighter, smaller and especially thinner than today’s generation, the ability to store a lot of charge in a small volume is very important and 1000 F per cubic centimeter is not a small number.”
The material also didn’t show any degradation after 10,000 cycles of charging and discharging, which indicates that the material is highly durable. So far, the results seem to indicate that the material is a better anode than graphite for lithium batteries, but Barsoum claims that they’re still a few years away from using the material to create an advanced battery.
“We tested the material as a supercapacitor electrode and found that it is a better anode than graphite,” Barsoum said. “However, a battery has two parts: an anode and a cathode. We created a better anode, but if we use current cathodes, the performance of a battery will be limited by the slower of the two components. We need to create a cathode that has equal performance to see the capabilities of an MXene anode.”
Barsoum intends to introduce the material to the marketplace in supercapacitors. He believes that the unique properties of the material make it a practical candidate for a wide variety of applications. Some of the major industries it could potentially affect include cell phones, energy storage for renewable energy, such as solar and wind and most notably batteries of future electric vehicles. There is still a lot of work to be done before that future becomes a reality but Barsoum is confident that it will be achieved within a couple of years.