The Engineer's Guide to Practical Soil Microbial Fuel Cell Design
Human-caused climate degradation and the explosion of electronic waste have pushed the computing community to explore
fundamental alternatives to the current battery-powered, over-provisioned ubiquitous computing devices that need constant
replacement and recharging. Soil Microbial Fuel Cells (SMFCs) offer promise as a renewable energy source that is biocompatible
and viable in difficult environments where traditional batteries and solar panels fall short. However, SMFC development is
in its infancy, and challenges like robustness to environmental factors and low power output stymie efforts to implement
real-world applications in terrestrial environments. To overcome these issues, I led a 2-year design journey in collaboration with researchers from the Georgia
Institute of Technology, UC San Diego, UC Santa Cruz, and Northwestern University to uncovers barriers to
practical SMFC design for powering electronics, which we address through a mechanistic understanding of SMFC theory from
the literature. We present nine months of deployment data gathered from four SMFC experiments exploring cell geometries,
resulting in an improved SMFC that generates power across a wider soil moisture range. From these experiments, we extracted
key lessons and a testing framework, assessed SMFC’s field performance, contextualized improvements with emerging and
existing computing systems, and demonstrated the improved SMFC powering a wireless sensor for soil moisture and touch
sensing. We contribute our data, methodology, and designs to establish the foundation for a sustainable, soil-powered future.
Soil Microbial Fuel Cells:
SMFCs are devices that harness specific types of microbes to generate electricity from soil. Exoelectrogens, or
microbes that transfer electrons outside of their cells, break down organic matter and release electrons, which
is captured by an anode. The cathode of the cell is exposed to oxygen, which gets reduced into water. The electrolyte,
or the layer between the anode and cathode, facilitates ion transfer and insulates the anode biofilm from oxygen, which hinders exoelectrogenic activity.
Because the electrodes only serve as surfaces for the microbes to live on, they do not get damaged or used up like traditional
batteries and fuel cells do. SMFCs can run forever as long as there is still organic carbon in the soil, and can be
made from almost any electrically conductive material. What's more, exoelectrogens can be found in nearly every environment,
meaning that there is no need to introduce foreign species into the ecosystem.
Iterative Design for Robustness:
While the theory of SMFCs is fairly simple, the challenge of implementing a robust working cell has impeded research into
soil-powered computing. Previous studies have tried to use low-performance, basic cells that were originally
designed for contaminant removal and had to maintain inundated environments to achieve high power output.
The works that have proposed different SMFCs rarely examine their relative strengths and weaknesses and never
explain their design rationales. This has made SMFC a difficult subject for non-experts to explore. To overcome this,
I devised a framework to break the SMFC down into modules that can be individually tested and improved in a principled manner, which greatly
shortens the design cycle of these devices.
Using this iterative framework, I went through four different versions of SMFCs over the course of two years where I modified their geometry to optimize
their computationally usable energy output for lower moisture environments. I started from the v0 design, which was
based on the MudWatt,
and ended up with an improved v3 design that features a horizontal anode and a vertical cathode that has one side
permanently exposed to air. The v3 prototypes were able to operate in soil with 4% lower volumetric water content (VWC),
recovered faster from drought, and produced higher peak power throughout the 161-day experiment.
Contextualize Improvement In Simulation:
To understand the performance improvement of the v3 cells in terms of computing capabilities, I built a trace-based
runtime simulation of the theoretical number of operations one could achieve with our v3 cell design compared to
the control v0 cell.
The simulation was constructed using real-world SMFC voltage traces collected throughout
our design iterations (161 total days of data collection) alongside datasheet values for three
computing modalities (Advanced,
When the energy in our simulated capacitor reaches a level sufficient
to turn on the modalities, we discharge all of the energy and increment the number of operation by one.
For the Analog modality, we ignore the capacitor and assume that the device turns on when the SMFC voltage exceeds a minimum threshold. More detail
can be found in Section 5.1 of the paper and in the GitHub code.
As a whole, we found that the v3 cell generated on average 68 times more power than needed for the Analog system to
operate, and increased its theoretical runtime by 120% compared to the baseline v0 cell.
In addition, systems powered by v3 cells are able to achieve a roughly 40% increase in total operation
count across the board (43.2% for Advanced and 41.7% for Minimal), further contextualizing our new SMFC design’s improved robustness to VWC.
Validate Lab Results With Outdoor Deployment:
Although the v3 cells from our lab experiments have been shown to produce upwards of 50 μW after being dried out, they
were evaluated under controlled settings where the soil was kept flooded for weeks at a time to revive them
back to their maximum output after each drying cycle, which is unrealistic for most applications. To examine
our improved v3 SMFC under field conditions, we deploy a modified v3 cell (v3.1) outside in an irrigated
yard to gauge its performance and understand the impacts of real-world stimuli on SMFC power output.
The scaffold, flange, cap, and anode G-clips were all 3D printed from plant-based PLA plastic to reduce
the number of store-bought parts. All of the membrane and electrode materials, geometries, and configurations
were kept the same as the v3 design, ensuring minimal impact on cell behavior.
The cell was incubated in the lab until it reached a voltage of 600 mV, then deployed in a residential yard at a location with hot-summer Mediterranean
climate (USDA Zone 10a). The surrounding environment is highly arid, with all of the non-desert plants being
regularly irrigated to keep them alive. This location was chosen to observe the effects of irrigation (or the lack
thereof) on SMFCs in naturally dry environments, which has been a highly-motivating application for smart
agricultural sensors. As seen below, the power level of the incubated v3.1 cell dropped significantly after being transplanted outside. However, it still
produces enough power to theoretically turn on the Analog system (i.e., MARS from Arora et. al)
during spikes in moisture levels caused by occasional irrigation (see
shaded red regions for the energy usable by the Analog module). This amount of power is also within the envelope of many common
subsystems (i.e., a real-time clock) of modern computing, showing promise.
Soil-Powered Sensor Using RF Backscatter:
To demonstrate the potential of soil-powered backscatter sensors, I integrated our v3.1 cell with a MARS tag in the lab
to perform wireless touch and soil moisture sensing using only energy from a SMFC. MARS is basically a Colpitts oscillator connected to
an RF switch that modulates the impedance of an antenna. As such, I built our own MARS tag using KiCAD
and modified the design to accomodate a "sensing capacitor" that takes the form of either a bare wire or a coplanar capacitor for touch and moisture sensing respectively.
MARS leverages a passive communication technique known as RF backscatter,
which means it modulates the reflected RF signal of an active transmitter (Tx) to a receiver (Rx) instead of generating
its own signal.
When the capacitance of the sensing capacitor changes, the Colpitts oscillator's frequency also changes, which affects the frequency at which
the antenna's impedance is being modulated, thus mixing our reading into the backscattered signal in the form of a frequency shift.
I used HackRF Software-Defined Radios for Tx and Rx,
and GNU Radio
+ MATLAB for data processing.
One basic capability of the soil-powered backscatter sensor is touch sensing. In the demo, a user grabbed
the sensing wire with their hand twice, which correlates with the two dips in backscatter frequency.
Since the fringing field of a cylindrical capacitor (the wire) is very small, only items very close to or directly
touching the wire will change its capacitance, making it robust to noise. This configuration provides a binary
measurement for whether something is in contact with the wireless sensor, which can be useful for applications
like wildlife monitoring.
Another practical application we enable is soil moisture sensing.
By tuning the geometry of the coplanar capacitor that serves as our moisture sensor, one can adjust the resolution of
the VWC reading and even set a threshold VWC at which the backscatter signal cuts off, alerting the Rx device of
overwatering or extreme rain events. This potentially makes the soil-powered MARS sensor a powerful choice for
VWC sensing and flood detection in wetland and green infrastructure monitoring applications where batteries
and solar panels face issues from corrosion and chemicals leaching out.
Led an interdisciplinary team of 13 researchers across four institutions as the first author to coordinate experiments and deployments.
Designed and manufactured 3D-printable SMFCs using Onshape and SOLIDWORKS.
Created a trace-based simulation and visualizations for the runtime of soil-powered devices using Python.
Built custom data logging hardware and backscatter sensors to
monitor and make use of the low voltage output of the SMFCs.
The study, “Soil-Powered Computing: The Engineer’s Guide to Practical Soil Microbial Fuel Cell Design,” was supported by the National Science Foundation (award number CNS-
2038853), the Agricultural and Food Research Initiative (award number 2023-67021-40628) from the USDA National Institute of Food and Agriculture, the Alfred P. Sloan Foundation, VMware Research, and 3M.