Converting chemical energy into electrical energy can be done in different ways, but for chemical energy stored in various fuels (alcohols, hydrogen, hydrocarbons) electrochemical (fuel) cells are the most direct, and have the potential to be very efficient.
The electrochemistry side of fuel cells is rather straightforward. Hydrogen, possibly with some carbon, on one side combines with oxygen on the other to produce water and possibly carbon dioxide. The half reactions are separated so that the energy difference between reagents and products can be harvested as electrical energy.
The industry is currently challenged by the need to design systems and materials that allow efficient electron transfer (catalysts) while being very long lived (e.g. avoiding CO poisoning). This materials development for fuel cells requires a great deal of electrochemical testing, from measuring device impedances to rotated ring disk electrode catalyst studies.
DC testing of fuel cells/materials includes RRDE work, cyclic voltammetry and polarization curves. RRDE is a common collector/generator setup used for catalyst testing. It requires a bipotentiostat and rotator. Cyclic voltammetry is a powerful physical/analytical electrochemical technique which is good for measuring and comparing the thermodynamics, kinetics, and mechanisms of various reactions. Polarization curves are frequently run on complete devices and are a standard test for fuel cell evaluation.
AC testing of fuel cells—i.e. electrochemical impedance spectroscopy (EIS)—has become more common as the technique has become more widely available. EIS generally gives much more information about the system than does a polarization curve, but it also requires a bit more expertise on the part of the researcher to implement and analyze successfully.
EIS can identify problems that limit a fuel cell’s efficiency, it can help optimize a cell, and it can determine anodic and cathodic process mechanisms. It is particularly good for measuring the equivalent series resistance (ESR) of fuel cells—ESR can be a major source of power loss in a low impedance device. Because of the modeling capability of EIS, you can also extract information on kinetics and mass transport in the fuel cell, both of which are crucial factors to fuel cell performance. EIS is useful in both research and QC applications. EIS of fuel cells runs into some of the same low impedance device and setup limitations that also show up in batteries and supercapacitors.
Many available Gamry systems can cover the various needs in fuel cell research and materials development.
A Reference 3000 with Auxiliary Electrometer, PWR800 and EIS300 control software, and the Reference 30k Booster for extra current, is an ideal solution for fuel cells that operate up to 30 A total current, and up to 20 V in the stack. It is capable of running both DC and AC testing, and with a second Reference instrument and a rotator can do bipstat RRDE work as well. When testing fuel cell stacks, in addition to the total stack measurement, the auxiliary electrometer allows the simultaneous measurement of up to 8 individual cells within the stack, whether running a polarization curve or doing impedance spectroscopy.
Because the Reference 3000 is a floating potentiostat, it can be isolated from earth ground, in conjunction with an electronic load. In this setup, the e-load provides a large DC offset current through one path, and the Reference 3000 superimposes an AC current
on top of that through another path. The fuel cell is subject to both currents, but because the Reference 3000’s ground is isolated from that of the e-load, and because the e-load, operating as a current source has near infinite impedance, the AC and DC currents are separate. There are several advantages of this setup, not the least of which is that the AC is no longer subject to the low bandwidth of the electronic load. Still, this setup is not necessarily easy and can become costly if many stations are required.
For lower cost impedance testing of very high current systems (>30 A), Gamry’s FC350 can be used in conjunction with an electronic load. For users who already have a compatible electronic load, or who need more than 30 A but do not need the additional functionality provided by a Reference 3000, this is a good option. Unlike the Ref3000/e-load setup mentioned above, this setup has the AC and DC components of the current shared by both the fuel cell and the electronic load. This does mean that bandwidth will be limited to that of the load, and that current inaccuracies of the load will affect the measurement accuracy (see expertise section for how to use a shunt). Still, for researchers concerned with value impedance testing of high current systems, the FC350 can’t be beat.
Gamry has been at the forefront of affordable, high performance EIS measurements for more than 15 years. We have also been making floating instruments since our inception. We understand how difficult these measurements can be and design our instruments and our app notes to help users make them as well as possible.
When it comes to fuel cells, most of the basic electrochemical measurements are fairly simple. The exception to this is getting good EIS data on working cells/stacks, particularly on systems that operate at high currents. In the “Gamry Systems” section, we detail several ways to perform these tests. One involves a shared path for AC and DC current and the other involves isolated AC and DC loops that sum up across the fuel cell. In both of these tests, but particularly the first, it may be advisable to use a shunt.
A shunt is a low but known impedance device that is sometimes used for measuring high currents. Placing a 0.1 mΩ shunt after a fuel cell which operates at 100 A will lead to a 10 mV drop, which can be precisely (and accurately) measured. It is much easier to measure 10 mV with precision than it is to measure 100 A. The addition of a shunt to a fuel cell/electronic load path allows a precise measure of the current going through the system. When selecting a shunt to use, try to target one that will lead to a total voltage drop of ~10 mV when the fuel cell is at maximum operating current.