Hydrogen Fuel Cell
I–An introduction to PEM hydrogen fuel cell
a. Fuel cell
A fuel cell is an electrochemical conversion device. It produces electricity from fuel (on the anode side) and an oxidant (on the cathode side), which react in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.
Fuel cells are different from electrochemical cell batteries in that they consume reactant from an external source, which must be replenished – a thermodynamically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system.
Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide.
The principle of the fuel cell had been demonstrated by Sir William Grove in 1839, and other investigators had experimented with various forms of fuel cell. The first practical fuel cell was developed by Francis Thomas Bacon in 1959.
There are several kinds of fuel cells, but Polymer Electrolyte Membrane (PEM) fuel cells—also called Proton Exchange Membrane fuel cells—are the type typically used in automobiles. A PEM fuel cell uses hydrogen fuel and oxygen from the air to produce electricity.
b. How does PEMFC work?
A proton exchange membrane fuel cell transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy, as opposed to the direct combustion of hydrogen and oxygen gases to produce thermal energy.
A stream of hydrogen is delivered to the anode side of the membrane electrode assembly (MEA). At the anode side it is catalytically split into protons and electrons. This oxidation half-cell reaction is represented by:
H2→2H+ + 2e-
The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the MEA, thus creating the current output of the fuel cell.
Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This reduction half-cell reaction is represented by:
4H+ + 4e- + O2 → 2H2O
II– Current research
There are two active projects on improving fuel cell’s efficiency in our lab, as described below:
A.The Mechanics of PEM Fuel Cell Stack Compression
The decreasing performance of PEM fuel cell test vehicles can be caused by changes in stack clamping pressure (i.e., compression). When the stack is compressed, the membrane electrode assembly (MEA) typically deforms 50 to 200 nm. This nanoscale deformation causes changes to the porosity of the Gas Diffusion Layer (GDL) which, in turn, alters (i) the permeability and diffusion of the reactant gas and (ii) the transport of the liquid water in the MEA. The proposed module will allow students to assess the effects of compression and GDL nano-deformation on fuel cell performance. A system will be constructed such that the stack compression can be altered without disassembling it. This will be facilitated by a cell equipped with a compression plate with a compression adjuster and a load cell for clamping pressure control. This feature is essential since a cell’s performance is altered considerably if it is disassembled and then reassembled, even if the clamping conditions are accurately reproduced. Students will analyze the influence of nano-deformation on (i) gas fuel flow channels, (ii) fuel flow patterns, (iii) fuel cell stack temperature, (iv) humidity conditions, (v) fuel cell vibration conditions (e.g. amplitude, frequency, and test times) and (vi) for different types of GDL (e.g. carbon cloth and carbon fiber). The porosity of the compressed and uncompressed GDL will be measured using water intrusion porosimetry. The permeability of the compressed and uncompressed GDL can be measured using a capillary flow porosimeter. The pressure drop across the GDL and the gas flow rate through the sample will be measured while the GDL is under the desired compression.
Current student(s) on the project: Bryan Dallas
B.Performance and platinum dissolution in Proton Exchange Membrane Fuel Cells under mechanical vibration
One of the factors that affect the performance of proton exchange membrane fuel cells (PEMFC) is the loss of electrochemically active surface area of the Platinum (Pt) based electrocatalyst due to platinum dissolution and sintering. The intent of the current research is to understand the effect of mechanical vibrations on the Pt particles dissolution and overall PEMFC performance. This study is of great importance for the automotive application of fuel cells, since they operate under a vibrating environment.
Carbon supported platinum plays an important role as an electrocatalyst in PEMFC. Pt particles, typically a few nanometers in size, are distributed on both cathode and anode sides. Pt particle dissolution and sintering is accelerated by a number of factors, one of which is potential cycling during fuel cell operation.
To study the effect of mechanical vibrations on Pt dissolution and sintering, an electrocatalyst (from cathode side) was analyzed by SEM/EDS (Energy Dispersive Spectroscopy) TEM and XRD.
The performance, dissolution and sintering of the Pt particles of 25 cm2 electrocatalyst coated membrane were studied during a series of tests. The experiment was conducted by running three accelerated tests. Each test duration was 300 hours, with different parameters of oscillations: one test without vibrations and remaining two tests under vibrations with frequencies of 20 Hz and 40 Hz (1g and 4g of magnitude) respectively. For each of the tests a pristine membrane was used. The catalyst of each membrane was analyzed by ESEM, TEM and XRD in pristine state and in degraded state (after 300 hours of accelerated test). In order to specify the same area of observation on a catalyst before and after accelerated test, a relocation technique was used.
Current student(s) on the project: Georgiy Diloyan