CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 57, 2017 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Sauro Pierucci, Jiří Jaromír Klemeš, Laura Piazza, Serafim Bakalis 
Copyright © 2017, AIDIC Servizi S.r.l. 
ISBN 978-88-95608- 48-8; ISSN 2283-9216 

Analysis of Unbalanced Pressure PEM Electrolyzer for High 
Pressure Hydrogen Production 

Dang Saebeaa, Yaneeporn Patcharavorachotb, Viktor Hackerc, Sutthichai 
Assabumrungratd, Amornchai Arpornwichanope ,Suthida Authayanunf* 
aDepartment of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailand 
bSchool of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 
10520, Thailand 
cInstitute of Chemical Engineering and Environmental Technology, Graz University of Technology, Inffeldgasse 25C, Graz 
8010, Austria  
dCenter of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of 
Engineering, Chulalongkorn University, Bangkok 10330, Thailand 
eComputational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, 
Chulalongkorn University, Bangkok 10330, Thailand 
fDepartment of Chemical Engineering, Faculty of Engineering, Srinakharinwirot University, Nakhon Nayok 26120, Thailand 
suthidaa@g.swu.ac.th  

Proton exchange membrane (PEM) electrolyzer is a promising technology and likely to be an important 
hydrogen generator. The ability to produce high purity hydrogen and deliver it at relatively high pressure is an 
important advantage of the PEM electrolyzer technology. In this work, the high pressure PEM electrolyzer 
without the need for external compression is studied. The simulation of the electrolyzer is performed based on 
an electrochemical model with consideration of hydrogen permeation. The effect of cathode pressure and 
membrane thickness on electrolyzer performance is studied. The explosion limit of a hydrogen-oxygen mixture 
in the anode is also taken into consideration. The electrochemical compression shows advantage in term of 
delivering hydrogen at high pressure with having less effect on performance and low power requirement. The 
increase of cathode pressure slightly affects the electrolyzer performance. The high pressure operation at the 
cathode and the use of thin membranes cause hydrogen crossover from the cathode to anode, especially at 
high current density operation.  

1. Introduction 

The hydrogen production from a water electrolysis has several advantages in terms of high purity, simple 
process, no pollution and availability of sources (Herraiz-Cardona, 2013). Industrial water electrolysis cells 
have been established for one hundred years (Leroy, 1983). There are four types of electrolyzers available: 
alkaline, acid, polymer electrolyte membrane and high temperature solid oxide electrolyzers. Proton exchange 
membrane (PEM) or solid polymer electrolyte (SPE) electrolysis is a promising technology and is likely to be 
an important hydrogen generator in a hydrogen economy. The ability to produce high purity and high pressure 
hydrogen is a significant advantage of the PEM electrolyzer technology (Santarelli et al., 2009). In contrast to 
alkaline electrolyzers, caustic alkaline or acidic fluid electrolyte is not necessary for PEM electrolyzers. The 
electrolyte for PEM electrolyzers is a polymer membrane. The most commonly used membrane is Nafion with 
a thickness of less than 0.2 mm (Ursua et al., 2012). In addition, PEM electrolysis has additional advantages 
over alkaline electrolysis such as lower parasitic energy losses and higher purity hydrogen output. Compared 
to the alkaline electrolyte, PEM can be operated at higher current densities due to the higher conductivity of 
the electrolyte (Marangio et al., 2009). The electrodes of PEM electrolyzer typically consist of noble metals, 
e.g., platinum or iridium.   The following reactions take place in a PEM electrolysis cell: 

Anode:   H2O ----› ½O2 + 2H
+ + 2e−                 (1) 

                               
 
 

 

 
   

                                                  
DOI: 10.3303/CET1757270

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Saebea D., Patcharavorachot Y., Hacker V., Assabumrungrat S., Arpornwichanop A., Authayanun S., 2017, Analysis 
of unbalanced pressure pem electrolyzer for high pressure hydrogen production, Chemical Engineering Transactions, 57, 1615-1620   
DOI: 10.3303/CET1757270 

1615



Cathode:   2H+ + 2e− ----› H2    (2) 

At the anode, water is oxidized to produce oxygen, electrons and protons. Protons transport across the 
membrane to the cathode and react with electron to produce hydrogen (Eqs(1) and (2)). PEM electrolyzers 
are commercially available for low scale production applications. The hydrogen purity is typically above 99.99 
vol.% (in some cases up to 99.999 vol.%) without the need of an auxiliary purification equipment (Bhandari et 
al., 2014). Moreover, low gaseous permeability of the polymeric membranes lowers the risk of flammable 
mixtures formation. PEM electrolyzers operate at a temperature of around 80 °C and a pressure of up to 
15 bar. Typically, PEM electrolyzers have production capacities of 0.06–30 Nm3H2/hr. Specific energy demand 
is typically in the range of 6–8 kWh/Nm3 H2 and their efficiencies are in the range of 67–82% (Bhandari et al., 
2014).  
A solid electrolyte allows for a compact system design with strong/resistant structural properties, which high 
operational pressures (equal or differential across the electrolyte) are achievable (Medina and Santarelli, 
2010). Some commercial models have claimed to reach pressures up to 350 bar (Ayers et al., 2010). System 
with high pressure operation (Grigoriev et al., 2010) of an electrolyzer (sometimes called electrochemical 
compression), requires less energy to compress and store hydrogen. High pressure PEM electrolyzers to 
deliver hydrogen at pressures of 1.4 MPa and 35 MPa were developed by Proton Energy Systems and 
Mitsubishi, respectively (Barbir, 2005; Witschonke, 2014). In addition, high pressure oxygen production from 
water electrolysis cell stack has also been developed and it can generate oxygen at pressures over 20 MPa 
(Roy et al., 2011).  
Typically, there are two main technology developments for high pressure electrolysis. The former is to keep 
the whole electrolyzer stack in a pressure vessel, and the feeding water is pumped to the same pressure of 
the produced hydrogen (Janssen et al., 2004). The latter is emerging in new applications (Cropley, 2005; 
Degiorgis et al., 2007), at the present in very small sizes, with an unbalanced pressure across the cell (anode 
near ambient pressure, whereas the cathode side is maintained at the hydrogen delivery pressure). PEM 
electrolyzer can produce high purity-hydrogen and deliver it at a relatively high pressure but the high pressure 
operation at the cathode causes hydrogen crossover from the cathode to anode and thus deteriorates the cell 
performance. However, the study of unbalance pressure PEM electrolyzer is still limit and the effect of 
membrane thickness on cell performance at high pressure operation should be considered. In this work, the 
PEM electrolyzer system to produce high pressure hydrogen with consideration of hydrogen permeation is 
theoretically investigated. The electrochemical and hydrogen crossover models are coded by Matlab. The 
effect of cathode pressure and membrane thickness on electrolyzer performance is presented when the 
explosion limit of a hydrogen-oxygen mixture in the anode is also taken into consideration. 

2. PEM electrolyzer model 

The electrochemical model of a high pressure PEM electrolyzer is used to investigate its performance and 
efficiency. The unbalanced pressure between the anode (2 bar) and cathode (10-100 bar) are applied to 
produce high hydrogen pressure and the effect of a hydrogen permeation is taken into account. In the 
electrochemical model, the cell voltage can be calculated by adding the open circuit voltage, the maximum 
voltage, by various losses (i.e., activation, concentration and ohmic losses) as shown in Eq(3). 

ohmdacEV               (3) 

The open circuit voltage can be calculated by the fraction of proton back-permeation to total proton 
permeation (k) and Gibb’s free energy (G) as shown in Eqs(4)-(8). The hydrogen permeation is shown in 
Eq(7) (Kim et al., 2013). 

F

G
kE

2
1


 )(              (4) 

aW

aOcH
m

p

pp
RTGG

,

/
,,ln
21

           (5) 

perHaH

perHa

nn

n
k

,

,









            (6) 

m

aHcH

H

H
perH

h

pp

H

D
n

,,
,


             (7) 

1616

http://www.sciencedirect.com/science/article/pii/S095965261300509X#fd5
http://www.sciencedirect.com/science/article/pii/S095965261300509X#fd6
http://www.sciencedirect.com/science/article/pii/S0360319912012669#bib1


perHaH n
F

i
n , 

2
           (8) 

The activation loss and ohmic loss are shown as follows: 

cc

c

aa

a
ac

i

i
h

F

RT

i

i
h

F

RT

,,
arcsinarcsin

00 22 
           (9)

mem
memeohm

i
hir


                                                     (10) 




























m
memmem

T

1

303

1
12680032600051390 exp)..(                                 (11) 

In addition, the concentration loss can be calculated from Fick’s law as shown in Table 1.  

Table 1: Concentration loss model  

Parameters Equations 

Concentration loss 

cH

Hcc

aO

Oaa
d

C

C

F

RT

C

C

F

RT

,,

,

,,

, lnln
00 24

  

O2 concentration at catalyst surface 
220 O

aneff

ane
chOaO n

D
CC 

,

,
,,,




 

 
an

an

anOHO

O
chO

RT

P

nn

n
C 




,
,

22

2

2 


 

H2 concentration at catalyst surface 
 

220 H
cateff

cate
chHcH n

D
CC 

,

,
,,,




 

cat

cat

catOHH

H
chH

RT

P

nn

n
C 




,
,

22

2

2 


 

 
It is noted that details of the model describing the voltage losses used in this study can be found in Marangio 
et al. (2009). The simulation model of high pressure PEM electrolyzer is solved by Matlab. 

3. Results and discussion 

The performance of high temperature PEM electrolyzer is studied under the cathode pressure varying from 10 
to 100 bar. The effect of cathode pressure on the voltage and current density relationship is shown in Figure 1. 

 
Figure 1: Voltage and current density relationship of high pressure PEM electrolyzer at different cathode 

pressures 

1617



It is observed that the voltage slightly increases with increasing cathode pressure. This results in a higher 
power demand when hydrogen is produced at high pressure. In addition, the hydrogen concentration at the 
anode and hydrogen permeated flux caused by the hydrogen crossover phenomena is shown in Figure 2. It is 
found that an increase in cathode pressure results in the increase of both the hydrogen concentration and 
hydrogen concentration flux at the anode. This is because a high pressure difference between the cathode 
and anode promotes the hydrogen permeation from the high pressure to low pressure side (the anode 
pressure is specified at 2 bar). The hydrogen permeated flux increases with the increasing current density 
because the high amount of hydrogen is produced at high current densities and this results in high 
concentration gradient between the cathode and the anode. Although high hydrogen is permeated to the 
anode at high current density, more oxygen is also produced at high current density. Therefore, high hydrogen 
concentration at the anode is observed at low current density up to 6000 A/m2 and the operation at high 
current density has slightly effect on hydrogen permeation as shown in Figure 2. However, the hydrogen 
fraction in the anode mixture stream consisting of oxygen and hydrogen is below the explosion limit at all of 
the studied operational cathode pressures. 

 
Figure 2: Effect of current density and pressure on anode hydrogen concentration (solid line) and hydrogen 

permeated flux (dash line) in the high pressure PEM electrolyzer. 

 
Figure 3: Voltage and current density relationship of the high pressure PEM electrolyzer at different membrane 

thickness. 

 
Furthermore, the effect of membrane thickness of 50, 127 and 178 m for Nafion 112, Nafion 115 and Nafion 
117, respectively, on the performance of high pressure PEM electrolyzer is investigated and the simulation 
results are shown in Figure 3. From the results, it is found that thinner membranes provide higher performance 
at high pressure operation. The use of thick membrane result in the increase of ohmic loss and thus the high 

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

6.00E-05

7.00E-05

8.00E-05

0

1

2

3

4

0 5000 10000 15000
H

2
p

e
r
m

e
a

te
d

 f
lu

x
 (

m
o

l/
m

2
.s

)

A
n

o
d

e
 H

2
 (
m

o
l%

) 

Current density (A/m2)

Pc = 100 bar

Pc = 50 bar

Pc = 10 bar

1618



voltage operation is observed. However, the membrane thickness also affects the hydrogen permeation from 
the cathode to anode. The effect of membrane thickness on hydrogen fraction on anode stream and hydrogen 
permeated flux at high cathode pressure (100 bar) is shown in Figure 4.  The opposite trend is observed when 
the results are compared with effect of membrane thickness on electrolyzer performance in Figure 3. It is 
found that the use of thick membrane has the positive effect on the reduction of hydrogen crossover. 
Hydrogen easily permeates from the cathode to anode when thin membrane is applied. In addition, the 
hydrogen fraction in the anode stream is over the explosion limit at membrane thickness of 50 m with low-
current density operation but the hydrogen is below the explosion limit (5.7% at 100 bar) at all studied current 
density when the membrane thickness of 127 and 178 m is used.  

 
Figure 4: Effect of current density and membrane thickness on anode hydrogen concentration (solid line) and 

hydrogen permeated flux (dash line) in the high pressure PEM electrolyzer. 

4. Conclusions 

The simulation of a high pressure PEM electrolyzer without the need for external compression based on 
electrochemical model is studied for clean hydrogen production. High pressure operation of the PEM 
electrolyzer brings the advantage of delivering hydrogen at a high pressure with the acceptable hydrogen 
permeation. The concentration of hydrogen at the anode increases with increasing pressure at the cathode 
and reducing membrane thickness and current density. Compared to thinner membrane, the use of thick 
membrane provides lower performance. However, the high hydrogen permeation is promoted with the use of 
thin membrane and high cathode pressure operation. 

Acknowledgments  

Support from Srinakharinwirot University, Thailand research fund (DPG5880003) and the Ernst Mach-Grants-
ASEA-UNINET is gratefully acknowledged. 

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0.00E+00

5.00E-05

1.00E-04

1.50E-04

2.00E-04

0

1

2

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p
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 f

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o
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.s

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n

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d

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 H

2
 (
m

o
l%

) 

Current density (A/m2)

0.178 mm

0.127 mm

0.050 mm

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