Functional prototype

This year we decided to build a soil-based microbial fuel cell(SMFC) wireless sensor that can generate electricity whenever the genetically engineered Shewanella oneidensis MR-1 JG700. These are the components to build a SMFC wireless sensor

Components

Graphite fiber electrode

For our device to detect salicylic acid, we need to inoculate Shewanella on the anode. In our work, we ordered cathode and anode from Keego Technologies LLC, Stanford, USA. Soil was patted down in MFC up to 1 cm to make a smooth surface and anode was placed on the top of the soil, finally soil sample was added. The cathode was placed on the top of the soil. The graphite fiber electrode allows the bacteria to make more contact with the electrode so that the bacteria can transfer much more electrons to the electrode than graphite electrode. We use titanium wire instead of the copper one since the corrosion resistance of the former is better than the latter in soil and will do less harm to the environment.

External Power source

To deploy our device for a long time period, the wireless sensor network requires an autonomous electrical source in the wild that can recharge and store energy for extended periods of time. We used a solar portable charger to charge to power the wireless sensor. We would like to give a special thanks to Professor You-Yin Chen, a Lecturer in the Department of Biomedical Engineering, for his guidance. We integrated the battery into our device.

Potentiostat

The potentiostat is used to provide and maintain a voltage potential between the working electrode(WE) and reference electrode(RE) of the SMFC. It also interfaces with the counting electrode to provide a measurement for its current flow. This measurement is recorded and sent to a phone app. The working electrode, reference electrode, and counter electrode are made of Au, Ag/AgCl, and Pt respectively. We powered this system by a 3.3V battery. We designed this system with operational amplifiers (OPA), due to their robustness in different operating conditions. We would like to thanks Professor You-Yin Chen, a Lecturer in the Department of Biomedical Engineering, for helping us designing the potentiostat. The design is shown as the diagram below

Microcontroller

The voltage value measured from the potentiostat is fed into an Media Tek LinkIt ONE. The anolog voltage measurement is converted using an analog-to-digital converter and stored in the device.

Whole picture of the functional prototype

Data server and App

The SMFC had to be able to transmit data because it will not be practical for someone to frequently go to the device to check the measured voltage. Thus, we designed an Apple watch app that can receive the voltage data. We build a server with PHP and MYSQL to store the data and HTTP protocol to transfer data. The code for Apple watch app is written in SWIFT and the server was built and tested in XAMP server.

Test the Function of sMFC

Challenge: The one factor that affect that power output the most is the water content in the soil. If the water content is not enough, the electrolyte will be quenched in the soil. This quenching effect will substantially decrease the power output of the microbial fuel cell.

All the tests are carried out in lab with Mudwatt Microbial fuel cell(Keego Technologies LLC, Stanford, USA) and agricultural soil

Experiment

We tested the power output of SMFC with different water content. As we can see in the graph below, there is a threshold of approximately 0.23 for water content which must be exceeded in order for the SMFC to function. In addition, power generation increases dramatically with increased water content. The water content of the soil used to grow potatoes is 30-35%, which is above the water content threshold. Moreover, the water content of soil used to plant potatoes is very stable in potato farm, which can ensure that there will not be big fluctuation of the power output of SMFC.