REAL-TIME QUANTIFICATION METHOD FOR HYDROGEN
Jan Senn, Christoph Grimmer, Stephan Nestl, Viktor Hacker
Institute of Chemical Engineering and Environmental Technology, NAWI Graz,
Graz University of Technology, Inffeldgasse 25C, 8010 Graz, Austria
[email protected]
Keywords: real-time hydrogen quantification, electrochemical method
INTRODUCTION
Hydrogen based on renewable resources is considered to be one of the major energy
carriers of the future [1]. Therefore proper and selective methods of hydrogen
determination and quantification are necessary in order to develop future technologies [2].
The most common methods include mass flow controller/meter [1,3], gas chromatography
[4] and water displacement in a gradual cylinder [5]. Unfortunately all of those methods
exhibit flaws: impreciseness regarding impurities/humidity (mass flow meters), poor time
resolution (GC, water displacement) or no selectivity (water displacement).
Therefore an alternative, selective electrochemical approach for quantifying hydrogen in
real-time within a mixture of gases (except oxygen) is presented, which provides especially
for dynamic processes more reliable data with a time resolution of below one second.
METHOD
At the test electrode a potential of 430 mV vs. RHE is applied. Because of this
overpotential hydrogen reacts instantly to H+ releasing 2 electrons according to:
H2 → 2H+ + 2e-
(1)
E0 = 0.00 V vs. SHE
Formed protons are carried through the membrane to the counter electrode where they
react back to hydrogen. The corresponding current is measured and analysed (conversion
factor: 1 A ≡ 7.05 ml min-1) [2]. Meanwhile the counter electrode which is flushed with
hydrogen acts as pseudo reference electrode.
We use an in-house made and previously published fuel cell with an active area of 25 cm 2
as electrochemical cell [6]. The cell consists of steel end plates, current collectors made of
gold bathed copper, meander like flow graphite fields and a commercial membrane
electrode assembly (MEA) [6].
RESULTS AND DISCUSSION
To determine the hydrogen crossover through the membrane, nitrogen was used as carrier
gas (CG) at the test electrode and hydrogen was applied at the counter electrode (30 ml
min-1 respectively). At an average background current (resulting from hydrogen crossover)
of 27 mA (1.1 mA cm -2) a noise of approx. 5 mA was detected coinciding with literature [7].
With a mass flow controller (Bronkhorst F-201CV-020) hydrogen was supplied to the test
electrode 0-20 ml min-1 (Fig. 1). The MFC gas flow matches the electrochemical
measurements. The results show that at a given flow rate of zero the MFC delivered minor
quantities of H2 and after commencing the H2-supply the MFC reacts by providing too
much gas at once, visible as a distinct peak of oversupply (Fig. 1a). At higher mass flows
more hydrogen than expected was detected due to relative inaccuracies of the MFC. At
high CG flow, changes of H2 flow are detected very precisely.
a) 25
b) 11
H2 flow [ml/min]
H2 flow [ml/min]
20
15
10
5
0
0
10
20
30
Time [min]
40
50
10
9
8
7
19
21
Time [min]
23
25
Figure 1: Electrochemical detection of hydrogen supplied by a MFC, stepwise increased
flow rate of 0-20 ml H2 min-1, black: electrochem. data; gray: data from MFC.
CONCLUSION
The presented electrochemical method is capable to detect and quantify hydrogen flow
rates in real-time, independent of moisture, impurities and gas mixtures and with high time
resolution. By comparing electrochemical data with data from the MFC the high accuracy
and fast response of this approach is confirmed.
ACKNOWLEDGMENTS
Financial support by the Austrian Federal Ministry of Transport, Innovation and Technology
(BMVIT) and The Austrian Research Promotion Agency (FFG) through the program a3plus
and the IEA research cooperation is gratefully acknowledged.
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