Until just recently, researchers had not discovered that anodes used in microbial fuel cells (MFCs) were, intrinsically, negative in charge and could be changed to a positive charge by means of ammonia treatment (Cheng and Logan 2006). An ammonia treatment performed on MFC anodes produced appreciable electric power increases due to a few proposed factors (Cheng and Logan 2006).
- The positively charged anode more readily excepted a microbial community biofilm of gram-negative bacteria used in MFC technology.
- The efficiency of electron transfer from the bacteria to the anode had increased.
This particular blog entry will discuss the procedure of ammonia treatment that is currently being used by researchers part of the Logan Group – Penn State University. A detailed procedure was originally typeset in a step-by-step format by Douglas Call (Environmental Engineer Ph.D. candidate at Penn State University). Specifics of the procedure were shown to me by Valerie Watson (another Environmental Engineer Ph.D. candidate at Penn State University).
High-temperature Lindberg/Blue M furnace used in ammonia gas treatment (Penn State University Fenske Building- Photo by Eric A. Zielke)
- The first step involves placing your anode material (carbon paper, carbon cloth, carbon brushes, carbon granules, ect.) into a quartz “boat.” This “boat” is then placed in the heat resistant glass tube portion of the furnace.
- The glass tube lid is then cleaned with a Kimwipe wet in acetone (…it doesn’t have to be a Kimwipe,… obviously), and connected to the glass tube portion of the furnace. The glass tube end is connected to ammonia, 5% NH3 in helium, and argon gas supplies (Note: Cheng and Logan (2006) use nitrogen gas instead of argon gas; however, these gases both serve the same purpose of removing oxygen from the glass tube).
- After complete connection, the argon gas is allowed into the furnace chamber at 40-60psi with a gas flow of 150cc for 20 minutes before the furnace is turned on.
- Next, the controls are programed to allow the furnace to rise to 700°C within 30 minutes, maintain that temperature for 1 hour, then cool down and turn off at 20°C (programming the controls would be specifically different for each high temperature furnace used in a heat treatment process).
- Once the furnace reaches 700°C, the argon gas is turned off and the ammonia is feed into the glass furnace chamber at 150cc for 1 hour. After 1 hour, the ammonia is turned off and the argon is passed through, again, at 150cc as the furnace cools down.
- Once the furnace is cooled down to 200°C, the cooling process can be expedited by lifting the furnace lid.
- At 50°C, the anode material can be removed, and is ready to be autoclaved before it is used in a MFC. Note: autoclaving is only used in pure culture research being performed on MFCs and is not necessary when using a mixed culture from domestic wastewater as the microbial community biofilm.
Reading this procedure, a person may beg the question as too the appropriateness of using such an energy intensive process. For example,
Is manufacturing a MFC anode worth all of this electrical energy (most likely produced by coal fire power plants) necessary to raise that furnace to 700°C, just to increase the power production of the MFC? In other words, is the power production increase in a MFC substantial enough to justify this energy intensive process?
Of course, the electrical energy could come from a renewable energy resource, such as with wind, photovoltaic, or hydro (note: micro hydro power is much more sustainable then hydro power). Another point, though, is that a MFC does more then just produce electricity. A MFC treats wastewater. Chances are that the energy needed to produce a MFC are much less then the energy it takes to produce all the chemicals and products, otherwise offset by a MFC, that are needed in at conventional wastewater treatment facility (note: I have never done this calculation,… I’m just making an educated guess).