Fuel Cell Experiment #1:

The Characteristic Curve of a Fuel Cell

Fuel Cell Experiment #2:

Parameters Influencing the Characteristic Curve

by

David M. Smiadak

School of Engineering

Grand Valley State University

EGR 380 Renewable and Sustainable Energy

Instructor: Dr. M. Szen

October 15, 2008

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1.0 Fuel Cell Experiment 1, the Characteristic Curve of a Fuel Cell

The principle objective of this experiment was to determine the voltage-current characteristic of a fuel cell and plot a power-current diagram. This provides a basic knowledge of the behavior of a fuel cell. The results can be used to size and design fuel cell stacks.

1.1 Data Interpolation

The fuel cell voltage-current relationship was experimentally determined. The experimental results are shown in Table 1.

Table 1: Measured and calculated values for current, voltage and power

Nominal Current Measured Values Calculated

0.0 0.02 8.97 0.18

0.2 0.41 7.88 3.23

0.5 0.69 7.59 5.24

1.0 1.21 7.14 8.64

1.5 1.69 6.49 10.97

2.0 2.2 6.13 13.49

3.0 3.18 5.78 18.38

5.0 5.16 5.15 26.57

7.0 7.15 4.75 33.96

10.0 10.11 4.39 44.38

The voltage-current relationship is plot in Figure 1.

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Figure 1: Voltage-current relationship

The fuel cell power-current relationship was determined based on experimental results and is shown in Figure 2.

Figure 2: Power-current relationship

Based on the characteristic curve the maximum power of the fuel cell occurs with the greatest stack current load, however, if the experiment were to continue, a drop off in performance would

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be predicted. This is one reason why fuel cells are designed to operate at or below the power maximum.

1.2 Question One

For the power of fuel cell stacks, two parameters are significant: the number of cells and the current density (in A/cm2). From the results of your measurement of the stack at a load current of 10 A, determine the voltage and the current density of an individual cell. Note: The active surface of these cells (surface of the electrodes) is 25 cm2. Assuming these values are transferable to larger fuel cells, use your results to specify two fuel cell stacks:

A 1 kWel rated stack with a working voltage Vstack = 24V

A 5 kWel rated stack with a working voltage Vstack = 42V For both stacks, give the following values: cell current, number of cells and active cell surface.

SOLUTION: See Attached.

1.3 Question Two

The power density of a fuel cell (in W/L) is an important characteristic for the capacity of a fuel cell, for example for use in a motor vehicle. Calculate this value for the experimental fuel cell (without fan and end plates) for a power of 50 W. Then compare this value with fuel cells that are used today in automobile prototypes. Here values range from 1 to 2 kW/L are being reached. How might the power density of the experimental fuel cell stack be optimized? Provide some ideas.

SOLUTION: See Attached.

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2.0 Fuel Cell Experiment #2, Parameters Influencing the Characteristic Curve

The principle objective of this experiment was to investigate the effects of reduced air supply, increased internal resistance, and fuel cell temperature on the characteristic curve of the fuel cell.

3.1 Data Interpretation

From the experimental values, the voltage-current relationship was determined. The experimental results are shown in Table 2.

Table 2: Measured values for current and voltage Nominal

Current 1 Measured Values, Fan

at AUTO Calculated Nominal

Current 2 Measured Values,

Fan at 6% Calculated

0.0 0.02 8.97 0.18 0.0 0.02 9.11 0.18 0.2 0.41 7.88 3.23 0.2 0.24 8.22 1.97 0.5 0.69 7.59 5.24 0.5 0.52 7.84 4.08 1.0 1.21 7.14 8.64 1.0 1.00 7.46 7.46 1.5 1.69 6.49 10.97 1.5 1.52 7.16 10.88 2.0 2.20 6.13 13.49 2.0 2.00 6.91 13.82 3.0 3.18 5.78 18.38 3.0 3.02 6.42 19.39 5.0 5.16 5.15 26.57 5.0 5.00 5.79 28.95 7.0 7.15 4.75 33.96 7.0 7.03 5.00 35.15

10.0 10.11 4.39 44.38 7.4 7.39 4.62 34.14 7.6 7.60 4.60 34.96 7.8 7.83 4.13 32.34 8.0 8.02 4.53 36.33 8.2 8.22 4.32 35.51

The voltage-current relationship for both the AUTO fan and 6% fan are compared in Figure 3.

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Figure 3: Voltage-current relationship, fan at AUTO and fan at 6%

The three distinct regions of a fuel cell I-V curve are not distinguishable in the experimentally collected data. The data however should be characterized into three regions, activation, ohmic, and mass transport. Losses in the activation stage are due to electrochemical reactions, losses in the ohmic region are due to ionic and electronic conduction and losses in concentration. Each type of loss should exhibit a unique curve along the I-V plot. The power-current curve for the fan at AUTO is shown in Figure 2. The power-current curve for the fan at 6% is shown in Figure 4.

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Figure 4: Power-current relationship, fan at AUTO and fan at 6%

The power-current curve of the fan at AUTO and the fan at 6% exhibit similar trends as shown in Figure 4.

3.2 Question One

What do you observe about the operation of fuel cells from the shape of the performance curve at reduced air supply?

SOLUTION: The performance curve of the fuel cell was observed for the fan at 6% and the fan at AUTO. When conducting the experiment with the fan at AUTO it was observed that the fan would not remain on constantly, this setting reduced the air supply to the fuel cell stack periodically before adjusting. The fan at 6% remained on constantly even at the reduced speed. From the data collected it was observed that the performance curve for the fan at 6% was slightly greater than the fan at AUTO. It can be concluded that the fuel cells performance is increased if constant air is supplied to the stack rather than periodic bursts.

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3.3 Question Two Calculate the oxygen flow rate needed at an individual cell and the rate of water formation in order to produce an electric current of 10 A. Then determine the theoretically needed volumetric airflow for the entire stack on the assumption that the usable oxygen portion in air is 20%. Consider the number of cells of the stack. Note: Perform the calculation at standard conditions (0C, 1 atm). The molecular standard volume of oxygen is =22.4 L/mol; the Faraday constant is F = 96485 C/mol.

SOLUTION: See Attached.

3.4 Question Three The fuel cell stack actually operates with excess air = 10. What does excess air mean and why is it necessary?

Note: Also consider the temperature behavior of the fuel cell at reduced fan power.

SOLUTION: Excess air is any additional air greater than the theoretical air required to supply a fuel cell stack. It is important that excess air is provided to the fuel cell stack because conditions are less than ideal and it is likely that there will be fuel stack inefficiencies that degrade its processing of supply air over time. Excess air ensures that the fuel cell is adequately supplied.