Gas bubble behaviour and ohmic potential drop during water electrolysis

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Energy research
Energy policy

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• Publié par: EUROPEAN-COMMISSION
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 4 Mo

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Commission of the European Communities
energy
GAS BUBBLE BEHAVIOUR AND OHMIC POTENTIAL
DROP DURING WATER ELECTROLYSIS
Report
EUR 7610 EN
Blow-up from microfiche original Commission of the European Communities
energy
GAS BUBBLE BEHAVIOUR AND OHMIC POTENTIAL
DROP DURING WATER ELECTROLYSIS
Final Report
C.W.M.P. SILLEN, L.J.J. Janssen,
J. GERAETS and A.G. GOEDKOOP-KOOIJMAN
Research financed by the Commission of the European Communities
within the frame of the first R&D programme in the field of Energy (1975-1979)
Subprogramme "Production and Utilisation of Hydrogen"
Eindhoven University of Technology
Eindhoven
Contract no. 597-78-EH/NL
1981 EUR 7610 EN Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Information Market and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person
acting on behalf of the Commission is responsible for the use which might
be made of the following information
(c)ECSC-EEG-EAEC Brussols . Luxembourg 1981 April 1981
lile work has been carried out under the supervision of a working
group :
Prof. E. Barendrecht
Ing. A.G. Goedkoop-Kooijirtan
Dr. L.J.J. Janssen
Ir. C.W.M.P. Sillen
Dr. S.J.D. van Stralen (Chairman)
Prof .dr. D. A. de Vries
Acknowledgement: M.C. Boschman, J.G.M. Niessen,
ing. J.H.H. Rutten and Ir. W.M. Sluyter. ABSTRACT
In future the advanced alkaline water electrolysis may be an
attractive method to produce hydrogen in the future.
Research_Program
The aim of the hydrogen program of EG is minimizing of energy
losses at the water electrolysis. This investigation contributes
to the fundamentals for an advanced water electrolysis. The
subjects of the present study are the following: the behaviour
of bubbles at perforated electrodes pressed against diaphragms
of different materials, the ohmic resistance of a layer of
solution between diaphragm and working electrode and the bubble
size and distribution in the solution leaving the electrolytic
cell. Using electrodes with gaps in it pressed against a diaphragm,
it is very important that the gaps remain free of gas bubbles.
Whether or not the gaps are filled up by bubbles, depends on
material of diaphragm and on the geometry of electrode.
It has been found that the best electrode, in this respect, of
the ones which have been investigated is the expanded metal gauze
electrode. However, pressing this electrode type against the diaphragm,
the latter is often damaged, since this electrode has gaps mostly
with very sharp edges. For perforated plate electrodes it is observed
that the round gaps are easily filled up with large bubbles,
fvery unfavourable), if a diaphragm of nafion, K2Ti60i3/PTFE, Polysulfone
or ZrO /ίΊϊΈ is used. Asbestos or PAci/PS-diaphragms act differently:
no large bubbles fill up the gaps. The occurrence of large bubbles
in the gaps is probably caused by the poor wettability of PTFE and
Polysulfone.
Potential_dro£
The ohmic potential drop across the solution layer between diaphragm
and working electrode was determined vs. current density, solution flow
velocity, distance between diaphragm and working electrode, distance between backside of working electrode and backwall of
compartment of working electrode, the height in electrolytic cell,
electrode type, nature of electrode surface and of gas evolved..
From the experimental results the following dimensionless correlation
is deduced
AR* = KiReni(l + K2HQ2 + K3DU3)
wmwm
where AR* denotes the resistanceincreaseofsolution layer
wm
between diaphragm andworkingelectrode due to bubbles,
divided by the resistanceofthislayer in absence of
bubbles
Re Reynolds number
H reduced height in electrolytic cell
n empirical experimental constant depending on nature of
electrode surface, nature of gas evolved and whether
or not the bubbles can enter a backchamber behind the
working electrode
K empirical constant depending on nature of both electrode
surface and gas evolved.
A remarkable result is that AR* is independent of current density
wm '
in the current densityrangefrom1­ 10 kAm­2.
The size and size distributionofbubbles in the solutionleaving
the electrolytic cellweredetermined for a hydrogen evolving
electrode at variouscurrentdensities and solution flowvelocities.
The main part (70%) of bubbles has a radius smaller than 30 ym.
The maximum of the bubble distribution curve is reached at a bubble
radius of about 10 ym.
It has been found that the experimental results can be represented
by the dimensionless correlation
AR£ ­ KkfRe
where AR* ­ (Ç ♦ \9θ»\,0
Κι ­ empirical constant
f <■ gas void fraction
Re ■ Reynolds number List of symbols
a empirical constant ( )
d, diameter of bubble (m)
d. distance between front side of working electrode
and tip of capillary i
d ,e between diaphragm and back wall of
compartment of working electrode
dmb,eff «««*** distance d^; d^ eff = 0.2^ + ^ (m
d distance between front side and back side of)
working electrode
d^e between back side of working electrode and
back wall of conpartment of workinge (m)
cl distance between front side of working electrode
and diaphragm (m)
D diffusion coefficient (m s"1)
f gas void fraction <-)
F Faraday constant (C kmol" *>
g gravity (m s"2)
h height in electrolytic cell or distance from a point
of working electrode to its lower edge (m)
h total length of working electrode (m)
H reduced length of workinge or reduced
height in electrolytic cell (-)
-2
i current density (Am )
It
Κ enpirical constant ( )
ηl exponent constant )
Ρ absolute pressure (Pa) bubble nunber fraction q» (-)
(-) cumulative bubble number fraction q
(-) e bubble volume fraction
specific resistance of two­phase mixture solution­
gas bubbles (ß m)
specific resistance of the solution without presence
of bubbles (Ω m)
bubble radius (m)
­1
R. at f - 0 and v, * 0 m s (m)
*b,0
R resistance of solution layer with presence of bubbles
wm
between diaphragm and working electrode CO m2)
R resistance of solution layer without presence of
wm,p
bubbles between diaphragm and working electrode (Q m2)
Re Reynolds nuirber :-)
(m)
**b *b " \,o
(-)
AVSb,o
resistance increase of solution layer between diaphragm AR
wm
and working electrode due to presence of bubbles;
AR = (AV ­ AV )/i CQ m2)
wm wm wm,p "
AR AR __ for solution without presence of bubbles (a m2)
wm,p wm
(0 m2) AR^^ at h * 0 m
(Q m2) AR^atd^­Om
,ΟΟΟ
ia m2) AR^ at h = 0 m and cl «= 0 m AR
wm
reduced AR^; AR^ = AI^/R^^ (-)
t (s) time
T (°K) temperature
gas flow velocity; v = V /w (d. ­ d ) (ms­1)
solution flow velocity; v. - ViA^td^, ­ d) (m s"1)
(V) cell voltage V gas flow s'1) (m3 g ^~
V solution flow (m3 s"1)
V kilomolar volume (m3 knol"1)
m
(V) AV ohmic potential drop
(V) AVcl drop between anode and cathode
(V) AV. AV between tip of capillary i and working electrode
AV. AV. for solution without presence of bubbles (V)
i/P i
AV AV between diaphragm and working electrode (V)
VnU
AVV for solution without presence of bubbles (V)
wm,p wm
AV° AV at h = 0 m (V)
wm
lV°°V at d = 0 m (V) A'
wm wm wm
AV° AV at h - 0 in (V)
wni/pm
AV°°V at cl = 0 m (V)
wm,p wm vem
w width of compartment of working electrode (m)
wh of working electrode (m)
w
( ) ζ constant in Faraday law
(kg nf3> ρ mass density
(kg m"3) ρ gas mass density
g
(kg nf3>
ρ, solution mass density
μ.n viscosity (kg s"1 m"1)