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"Dirt-powered fuel cell" draws near-limitless energy from soil
A Northwestern University team has demonstrated a remarkable new way to generate electricity, with a paperback-sized device that nestles in soil and harvests power created as microbes break down dirt – for as long as there's carbon in the soil.
Microbial fuel cells, as they're called, have been around for more than 100 years. They work a little like a battery, with an anode, cathode and electrolyte – but rather than drawing electricity from chemical sources, they work with bacteria that naturally donate electrons to nearby conductors as they chow down on soil.
The issue thus far has been keeping them supplied with water and oxygen, while being buried in the dirt. “Although MFCs have existed as a concept for more than a century, their unreliable performance and low output power have stymied efforts to make practical use of them, especially in low-moisture conditions," said UNW alumnus and project lead Bill Yen.
So, the team set about creating several new designs targeted at giving the cells continual access to oxygen and water – and found success with a design shaped like a cartridge sitting vertically on a horizontal disc. The disc-shaped carbon felt anode lies horizontally at the bottom of the device, buried deep in the soil where it can capture electrons as microbes digest dirt.
The conductive metal cathode, meanwhile, sits vertically on top of the anode. The bottom part thus sits deep enough to have access to moisture from the deep soil, while the top sits flush with the surface. A fresh air gap runs down the whole length of the electrode, and a protective cap on top stops dirt and debris from falling in and cutting off the cathode's access to oxygen. Part of the cathode is also coated with a waterproofing material, so that when it floods, there's still a hydrophobic section of the cathode in touch with oxygen to keep the fuel cell running.
In testing, this design performed consistently across different soil moisture levels, from completely underwater to "somewhat dry," with just 41% water by volume in the soil. On average, it generated some 68 times more power than was required to operate its onboard moisture and touch detection systems, and transmit data via a tiny antenna to a nearby base station.
As with other super-long term power generation sources, like betavoltaic diamond batteries made using nuclear waste, the amount of power generated here isn't large enough to go and run a dirt-powered car or smartphone. It's more about powering small sensors that can run over the long term without needing regular battery changes.
"If we imagine a future with trillions of these devices, we cannot build every one of them out of lithium, heavy metals and toxins that are dangerous to the environment," said Yen. "We need to find alternatives that can provide low amounts of energy to power a decentralized network of devices. In a search for solutions, we looked to soil microbial fuel cells, which use special microbes to break down soil and use that low amount of energy to power sensors. As long as there is organic carbon in the soil for the microbes to break down, the fuel cell can potentially last forever."
Thus, sensors like these could be very handy to farmers looking to monitor various soil elements – moisture, nutrients, contaminants, etc – and apply a tech-driven precision agriculture approach. Pop a few dozen of these things around your property, and they should be good to generate data for years, possibly even decades to come.
Perhaps the neatest part here is that all components of this design, according to the research team, can be bought off the shelf at a hardware store. So there's no supply chain or materials issues standing between this research and widespread commercialization.
注释：
anode: n
表示" 阳极；正极"，means "a positively charged electrode by which electrons leave an electrical device"，如：The reverse process may proceed at the anode. 相反的过程也会在阳极上发生。
cathode: n
表示" 阴极"，means "a negatively charged electrode that is the source of electrons entering an electrical device"，如：Cathode rays are attracted by a positive charge. 阴极射线被阳电荷所吸引。
electrolyte: n
表示"电解质；"，means "a solution that conducts electricity"，如：Electrolyte solutions can conduct electricity. 电解质的水溶液是可以导电的。
chow：n; v
表示" 食物；吃"，means ""，如：We had some good chow in that little restaurant. 我们在那家小餐厅吃了一些不错的食物。Wanna get something to chow down on? 想不想弄点东西来吃啊?
stymie: v
表示" 妨碍"，means "hinder or prevent the progress or accomplishment of"，如：His action without authorization has stymied the whole plan. 他擅自采取的行动已经妨碍了整个计划的实施。
cartridge：n
表示"弹药筒;胶卷盒"，means ""，如：He fired a shot, then ejected the spent cartridge. 他开了一枪，然后倒出了用过的弹夹。A film cartridge can only be opened in complete darkness. 胶卷盒只能在完全黑暗的环境中打开。
voltaic: adj
表示" 电流的；伏特的"，means "pertaining to or producing electric current by chemical action"，如：beta voltaic cell β伏打电池
lithium: n
表示" 锂"，means "a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several minerals"，如：The new kids on the block are disposable lithium batteries. 市面上电池的新品种是可丢弃式锂电池。
中文简要说明：
美国西北大学(Northwestern University)的研究团队展示了一种独特的发电新方法，一种书本大小的装置放置在土壤中，就可以开发发电。它的原理是利用微生物分解泥土产生的电力，所以也可以称为「泥土驱动电池」。
新阿特拉斯(New Altas)报导，这是一种细菌电池，已发明了 100 多年。它们的结构与电池相同，都有正极、负极和电解质，但它们不是从化学来源获取电力，而是透过细菌的分解作用，当细菌在吞噬土壤里的有机物时，自然而然地将电子提供给附近的导体。
不过，埋在泥土中的电池如果要保持效用，必须持续它们的水和氧气供应，这在实际执行上有困难，所以过去的泥土电池效率并不高，也就没办法实际应用。
因此，西北大学团队开始设计让微生物可持续获得氧气和水的方法，后来发现形底部采用水平圆盘，上头再立起扁方盒的设计，可以有效的持续作用。底部圆盘是正极，深埋在土壤中，可以在微生物消化泥土时产出足够数量的电子。
同时，导电金属的负极，垂直位于顶部。因此，底部的位置足够深，以便能够从深层土壤中获取水分。至于新鲜空气，可以从方盒中的镂空设计中流通，当然，为了防止灰尘和碎片的干扰，外头可以罩上滤网。
在测试中，不同的土壤湿度，从完全的潮湿土壤，到稍微干燥的土壤。发电效果都差不多，这就是团队想看到的结果，稳定的供电才能够有效使用。
团队主持人杨比尔(Bill Yen)说：「想象一下，如果我们未来拥有数万个这样的设备，我们对锂、重金属的需求就会少了很多。我们利用土壤微生物来推动电池，它们在自然环境中就可以长期运作，只要土壤中存在可供微生物分解的有机碳，电池就有可能永远持续下去。」
不过，千万不要以为这样的电池能够推动电动车或智能型手机。它更多的功能是是为小型的环境传感器供电，通常是气象与环境调查之用，安装的地方往往位在人迹罕至所在，所以科学家希望它们的电池无需定期更换。
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