Sep 14, 2010
Imagine filling up the fuel tank of your sedan and being able to drive from Boston to Atlanta on a single tank of gas1. Imagine a ride so quiet; all you can hear as you drive through the mountains are the road and the wind. While this may sound like a vision of the future, methanol fuel cells can make this possible.
As global demand for low carbon emission energy sources grows, there has been a considerable increase in fuel cell research. Presently, most commercially available fuel cells use a gaseous fuel source (typically hydrogen or methane) and are intended for either large-scale electricity generation or for automobiles. Fuel cells using liquid fuels offer distinct advantages to those using gaseous fuels: liquid fuels are safer to transport and store than gaseous fuels and they require less infrastructure for large-scale adoption.
In an ideal proton-exchange membrane fuel cell, protons pass freely from the anode to the cathode, while fuel molecules remain separated by a membrane. Fuel cells running on liquid fuels have historically suffered from a lack of selectivity: the membrane separating the anode and cathode that allows protons to pass freely also allows fuel to pass through, decreasing efficiency. Drs. Avni Argun and Nathan Ashcraft, working in Paula Hammond’s polymer materials lab in MIT’s Department of Chemical Engineering, have invented a way to manufacture inexpensive, highly-selective polymer membranes that will enable high-efficiency liquid-fuel fuel cells.
A fuel cell is an electrochemical device that produces direct current (DC) electricity from electrochemical reactions between atmospheric oxygen and fuel. Unlike combustion engines, which indirectly convert the chemical energy within fuel into electricity, fuel cells directly convert the energy into electricity, resulting in significantly higher efficiency. While combustion engines have theoretical efficiency limits set by the Carnot cycle, limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe. A perfectly designed device could achieve over 90% efficiency. Although fuel cells still emit carbon dioxide during operation, this amount is about two-thirds the amount per unit of electricity generated by typical fuels2.
With no moving parts, fuel cells are very reliable. Some commercially available systems have statistical uptimes of greater than 95%3. High-temperature fuel cell technologies require the fuel cell core to be replaced every five years, which represents the largest maintenance commitment for a new fuel cell installation.
Since fuel cells facilitate electrochemical reactions, an ideal fuel is one that combines a high heat of combustion with a low activation energy4. Methanol is the most attractive liquid fuel because of its high energy density and low energy threshold (less than 5 eV) for separating protons and electrons to drive the electrochemical reaction.
In a proton-exchange membrane (PEM) fuel cell, such as the kind that Argun and Ashcraft study, the electrochemical reaction occurs across a thin membrane that separates the anode from the cathode. Protons and electrons separate at the anode. The protons flow across a membrane to the cathode, while electrons are channeled through a resistive circuit, providing electric power to an application. An ideal membrane has high proton conductivity, low fuel crossover (fuel leaking from the anode to the cathode via diffusion), and is inexpensive to manufacture. Higher proton conductivity increases a fuel cell’s peak power density, while low fuel crossover increases the efficiency. Traditional membranes for methanol fuel cells use a proprietary material, Nafion®, which has high proton conductivity and moderate fuel crossover.
Argun and Ashcraft have developed a process for manufacturing a polymer membrane that increases fuel cell efficiency by 50% by decreasing fuel crossover by a factor of 200. This was accomplished using a layer-by-layer (LBL) assembly process whereby a membrane composed of many layers is gradually built up. Using alternating solutions of charged polymers, the LBL assembly is a technique commonly used to deposit polymer coatings, and Dr. Argun’s work is a rare example of using this technique to create a membrane. The layers of polymer behave as a membrane through which protons can pass but fuel molecules cannot.
Surprisingly, the first consumer applications using methanol-based fuel cells are not for large-scale electricity generation. In late 2009, Toshiba and Viaspace introduced small, portable charging packages for electronic devices such as mobile phones5. The next step is to directly incorporate these packages into a portable electronic device. Other companies developing methanol-based fuel cell systems include:
- MTI Micro(Albany, NY): MTI Micro has prototyped micro-charging products that aim to replace lithium ion battery packs. Their products target the mobile device market, emphasizing the longer-lasting power and almost instantaneous recharge (the time to pour fuel into the device).
- SFC Smart Fuel Cell(Munich, Germany): SFC offers methanol-based fuel cells with fuel cartridges. Their units are designed to operate in tandem with a battery, keeping the battery fully charged.
- Lilliputian systems(Woburn, MA): Later this year, Lilliputian systems plans to introduce a cigarette-pack-size charger based on a portable fuel cell that runs on butane.
- DyPol (Cambridge, MA): DyPol plans to manufacture low-cost, high-efficiency, lightweight membranes for methanol fuel cells. They are initially targeting military applications such as portable radios and unmanned aerial vehicles.
In order to better tailor these novel membranes to large-scale electricity generation, researchers in the Hammond group at MIT seek to strengthen the mechanical properties of the membrane by embedding strong polymer fibers. Just like steel bars reinforce concrete, the addition of embedded fibers would allow for higher operating temperatures, harsher operating conditions, and higher concentrations of methanol. While there remains room for improvement in fabricating a low-resistivity polymer/catalyst interface, these novel membranes represent a significant advance in fuel cell research.
Presently, the vast majority of commercially available fuel cells require gaseous fuels. Dr. Argun and his colleagues believe their work on direct methanol fuel cells will reduce carbon emissions without the need for expensive infrastructure.
- Assuming 100 miles per gallon. A typical car presently uses 25 mpg and 15% efficiency at present; fuel cells would have 60% fuel efficiency.
- Based on eGRID factor in California. Carbon dioxide emissions are 320 g/kWh when using the California grid, compared to 218 g/kWh when using fuel cells. Figures are published by the U.S. Environmental Protection Agency (EPA).
- Based on discussions with representatives from Fuel Cell Energy and UTC Power.
- By contrast, if a fuel undergoes a combustion reaction, high heat of combustion becomes the most important criterion for selection.