Crabmeat is delicious-that's an unassailable, garlicky buttery truth. But here's something you might not know about your predilection for boiled crustacean: Every year, about six to eight million tons of crab, shrimp, and lobster shell waste is produced worldwide, with most of it dumped straight back into the ocean or into landfills, depending on the country.
The issue is not so much the colossal waste tossed aside at the dinner table as it is the lost opportunity. The hardy exoskeletons of crabs and their marine ilk are rich in useful, versatile chemicals like calcium carbonate, which has medicinal and industrial uses, and chitin, the second-most abundant natural polymer found on Earth.
Over the years, scientists have mined chitin from crustacean shells for everything from tissue engineering to making biodegradable plastic. Because it and its sister polymer chitosan are considered eco-friendly and non-toxic, there's been much interest in incorporating these chemical compounds into batteries of all things.
In a paper published earlier this month in the journal Matter, one group of engineers at the University of Maryland did just that, crafting an impressive chitin-zinc battery that's biodegradable, but still holds considerable electrical juice. As the world moves toward a more sustainable future, the hope is that a rechargeable, crab-derived battery may be a viable alternative to, or even a replacement for, lithium-ion batteries, which are increasing in demand (hello EVs!). At the same time, lithium itself is becoming a scarce resource.
"This is a really exciting application of chitin, taking advantage of its renewability and potential for binding to zinc to make novel electrode materials," Mark MacLachlan, a professor of supramolecular materials at the University of British Columbia, who was not involved in the study, tells Popular Mechanics in an email.
The spark that turns on your phone is thanks to some chemistry happening deep within a battery.
Chemical reactions called redox reactions produce a steady drumbeat of electrical current in the form of moving electrons. These travel through a circuit consisting of the battery's electrodes (made of two different conductive metals), its chemical electrolyte (a gel or liquid-like material containing charged particles called ions), and whatever appliance or device the battery is hooked up to.
There's a sundry of metals and electrolytes used in a battery. For instance, in your common household alkaline battery, the positive electrode (the cathode) is made of manganese oxide and the negative electrode (or anode) is made of the trace mineral zinc. The electrolyte in between is potassium hydroxide. In lithium-ion batteries, lithium typically ponies up with another metal like cobalt at the cathode and carbon at the anode, and also makes up the electrolyte.
Lithium-ion batteries are the rising star of the energy world for being lightweight yet packing a wallop of energy (and they're rechargeable, to boot). No surprise, these batteries have found their way into almost all electronic devices and electric vehicles over the last few decades, but mining lithium exacts a hefty and devastating toll on the environment. Not only that, lithium-ion batteries aren't easily degradable or recyclable, which doesn't make them entirely compatible with sustainability-at least in their present form.
"Rechargeable batteries as green energy sources are essential to reduce our reliance on fossil fuels and reduce the emission of greenhouse gas. However, with the surging demand for electric vehicles in recent years, vast quantities of batteries are being produced and consumed, raising the possibility of environmental problems," Meiling Wu, the new study's first author and a postdoctoral fellow at the University of Maryland, says in an email to Popular Mechanics. "For example, polypropylene and polycarbonate separators, which are widely used in Li-ion batteries, take hundreds or thousands of years to degrade and add burden to the environment."
Scientists have toyed around within chitin in past years, looking for ways to incorporate the organic substance-which is structurally similar to cellulose in plants-into batteries. In 2016, MacLachlan and his lab at the University of British Columbia discovered that when you cook up chitin with nitrogen at temperatures as high as 1,652 degrees Fahrenheit, it transforms the former crab shells into a carbon-nitrogen material suitable for electrodes.
But Wu and her colleagues at the University of Maryland went for a different approach. Their focus was on zinc-based batteries, which are a common disposable, yet non-rechargeable battery. Because the mineral is more readily available compared to lithium (read: cheaper to produce and supply) and less toxic (zinc batteries use water as an electrolyte), there's been much interest in zinc as our silvery-white, energy knight.
Swapping out lithium for zinc isn't smooth sailing, though. In batteries as an electrode, zinc has an annoying tendency to form irregularities on its surface. These irregularities form as electrons move through and bubble into little bumps that snowball into larger and larger ones called dendrites, which disrupt the battery's electrical current.
Since chitosan, a chemically treated derivative of chitin, interacts well with water and is able to hold it down from floating around willy-nilly, Wu and her colleagues believed they could use it to make a battery separator-a semi-permeable membrane that keeps the opposite charged electrodes apart-that's also biodegradable.
So how did they do it? The researchers took a coin-sized film of chitosan and bathed it in a solution filled with zinc to get the mineral to stick to the film. Next, they wrung out the chitosan-zinc film, and squeezed it tightly to densely pack it. Unlike prior attempts to densify chitosan films, this technique allowed for comparatively large pores (up to five micrometers in size), allowing free movement of ions, Jodie Lutkenhaus, a professor in chemical engineering at Texas A&M University, who was not involved in this study, tells Popular Mechanics.
"This is key because when you think about a crustacean shell, you think of it being really hard and dense, and that's not good for conducting ions," says Lutkenhaus, who has done research on organic-derived batteries.
To complete the build, a zinc anode was placed on top of the chitosan-zinc separator along with a cathode made of an organic compound called poly(benzoquinonyl sulphide), or PBQS. When Wu and her colleagues ran their Krabby battery in the lab, they saw none of that dendrite formation on the zinc anode. Impressively, it generated an electrical current of 50 milliamperes per square centimeter for 400 hours (or 1,000 charging cycles), which is close to what small lithium batteries are capable of.
Next, they buried the chitosan-based electrolyte, not the entire battery itself, to see how long it would take to degrade. Wu says they found it took about five months for dirt-dwelling microorganisms to eat away at the electrolyte, which is much quicker compared to conventional electrolytes sitting in landfills and definitely good for the environment.
"It doesn't mean the battery device itself will degrade within five months," she says. "Actually, the electrolyte is packed in a closed cell, which is separated from air and organisms. This kind of device can work for a much longer time."
While you're not going to see crab-shell batteries anytime soon, the researchers hope their batteries may become commonplace, used in electronic devices like your cellphone to dedicated storages for renewable energy that release to a commercial grid.
Considering chitosan only costs about $1.70 per gram and you don't need much of it to make their prototype (only about 20 micrograms for a 20-millimeter coin-sized battery), that may give the chitosan-zinc battery a price point advantage over lithium-ion batteries, which suffer from climbing demand and accompanying costs.
However, with the soaring temperatures of climate change hitting marine life the hardest, there's the question of what will happen to costs if batteries made from chitosan take off. It's also worth thinking about how truly green using it would be in the future-we have to consider the carbon footprint of activities associated with retrieving chitin and turning it into chitosan.
"I think the next important step would be to do a techno-economic analysis to examine what is the global production of chitosan and if we're going to, say, apply it to grid-scale energy storage, do we have enough access to it?" says Lutkenhaus. "If we start using it, will it change the price because of the demand . [Also] wrapped up in that is the life cycle analysis . the carbon footprint of capturing crustaceans, shipping, and processing them."
Thankfully, says Wu, we do have other sources of chitin, such as in insects and the cell walls of fungi, so it's not entirely a dud if a marine source falls to the wayside. And while no one so far has attempted synthesizing chitin in the lab, who knows what the future might hold. Right now, it looks like our briny seafood waste could be a solution to our current energy problems.