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HYDROGEN PRODUCTION BY SPLITTING WATER
Hydrogen Production by Splitting Water in an Electrolyzer: A
Computer Simulation Based Study
Dr. Aniruddh Singh
Department of Applied Science and Humanities, Ajay Kumar Garg Engineering College, PO Adhyatmic Nagar,
Ghaziabad 201009
[email protected]
_________________________________________________________________________________________________________
Abstract -- Several methods for conversion of water into its
constituents have been studied in recent years. They
include usage of catalysts, photosynthetic disintegration of
water, thermal and photolytic splitting of water and
electrolytic splitting of water using photovoltaic (PV) cells.
In this paper, we investigate a potential method for
photovoltaic production of hydrogen as a sustainable
means to satiate future energy demand. We perform
computer simulation of a workable model of a hydrogen
producing electrolyzer and present results.
Keywords: Conversion of Water, Hydrogen Production
I. INTRODUCTION
HYDROGEN as a vector of energy is advantageous compared
to other energy sources. Hydrogen can be stored and shipped
over large distances and burns with nonpoisonous fumes. Fuel
cell technology can be used to produce electricity. The process
of photo electrolysis where solar cells are used to generate
voltage across the electrodes of an electrolyzer, seems to be an
ideal candidate which can be integrated with fuel cell
technology to serve future power needs. It certainly seems
profitable to generate hydrogen from pure water with photo
electrolysis as there is no environmental hazard and is also
cost effective compared to other renewable resources like
hydro or wind.
Electrolysis of Pure Water: Pure water does not conduct
electricity, because the numbers of H+ and OH- ions are small
(10-7 mol/L each). When a potential of 2.06 V is applied, the
following reaction occurs at cathode and anode of the
electrolyzer.
Anode Oxidation: H2O = 4 H+ + 4 e + O2
Cathode reduction: 4 H2O + 4 e = 2 H2 + 4 OHOver all reaction : 2 H2O = 2 H2 + O2
Pure water is impractical to use in this process because it is an
electrical insulator. That problem is reported to be
circumvented by the addition of a minor amount of soluble
salts that turn the water into a good conductor . Such salts
have small effects on the reaction because they change the
pH of water.
In this paper, we simulate the electrolysis of pure water with
the help of a computer as a means to produce hydrogen and
also consider some alternative reaction pathways involving
electrolysis of water. This is done to compare the efficiencies
of different reaction pathways and to ultimately propose the
improvement in the production rate of hydrogen from such
processes. As the reactions in such processes have multiple
steps a proper understanding of the results is heavily
dependent on employing correct simulation techniques.
II. THE COMPUTER SIMULATION TECHNIQUE
Studies of reacting systems - whether small scale reactions in
a laboratory or large scale processes, all focus on obtaining a
basic description of the individual steps involved in the
reaction, and the characteristic rate of each step. In most
studies, this information is obtained by experimental analysis.
After carrying out an experiments and analyzing the results, a
mechanism is written down. This modeling of the process can
be used not only to describe the experimental results, but also
to predict behavior of the system under conditions which have
not been studied explicitly.
Understanding the mechanism of a chemical reaction is also
important in context of obtaining deeper insight of the
reaction process. This approach is used to gain mechanistic
information for reacting systems. Two different approaches
are generally employed: algebraic expressions, or rate laws,
derived from the mechanistic steps describing the reaction,
and numerical simulation of a mechanism using a computer.
A rate law is obtained on the basis of the reaction mechanism
involved. The rate law of a complicated reaction scheme
involving multiple steps is governed by a set of coupled
differential equations which determine the time dependence of
each chemical species, and approximations are made to
combine and simplify them.
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AKGEC JOURNAL OF TECHNOLOGY, Vol. 2, No. 1
In this approach, it is assumed that the intermediate steps will
have small effect and can be neglected and the final equations
involve only experimentally measurable quantities. The most
common method used to obtain a rate law is to apply the
steady state approximation to the coupled differential
equations obtained from the reaction mechanism.
These approximations apart from putting restrictions on the
experimental conditions are also prone to introducing errors in
the final calculation. This is particularly true in cases where
the knowledge of the mechanism is incomplete or the
mechanism involve large no. of intermediate steps. They also
include reactions whose mechanisms are too complicated to
yield a rate law; those whose rate laws are too cumbersome to
be tested experimentally reactions which never attain steadystate under the experimental conditions of interest and those in
which physical conditions such as temperature and volume are
not constant.
For such systems, kinetic modeling is best done by a computer
[1]. Numerical simulation of chemical reactions is a powerful
tool to accompany the experiments. Unlike algebraic rate
laws, which are often highly simplified, simulations are able
to handle large amount of data. They also provide a means of
evaluating various hypotheses for further experimental
investigation. A simulation based study can be particularly
valuable in studies of very complex systems. It allows us to
plan our experimental work when necessary.
Two very different computational methods are available for
simulations. The most commonly used is the deterministic
approach, in which the time dependence of species
concentrations is written as a set of coupled differential
equations which are then integrated. A deterministic model
presumes that a reaction is well understood that the complete
time-dependent behavior of a system can be calculated from
the solution of the differential equations. This method works
well for many systems which have small range of rates and
concentration and have negligible instabilities [1].
The stochastic method is a computationally simpler alternative
to deterministic simulations for many types of chemical
systems [1]. For chemical reactions whose complex sets of
differential equations are difficult to solve or have large
ranges of rates or concentrations - it is the only method and is
entirely different from the deterministic one. Rather than
finding a solution which describes the state of the system at all
points in time, changes in a system are modeled by randomly
selecting among probability-weighted reaction steps. The
stochastic method places no constraints on the chemical
processes occurring during the reaction and is highly accurate
and can be simulated on a personal computer.
III. RESULTS AND DISCUSSION
Two sets of simulations were done using Chemical Kinetics
Simulator (CKS) developed by IBM. In the first simulation,
the direct splitting of water in an electrolyser is studied. Here
we study the improvement in the reaction efficiency as a
function of applied voltage. It is found that the efficiency of
production of Hydrogen is insensitive to applied voltage.
In the second simulation, an alternative reaction pathway is
proposed and studied where a form of electro-catalysis is
utilized. Here again the overall improvement in efficiency is
studied as a function of various reaction parameters.
In the direct splitting of water, the following reaction steps
were considered.
H2O => H+ + OHOH- => OH + eH+ + e- => H
2H => H2
2OH => H2O2
The decomposition of hydrogen peroxide is not taken into
account as the decomposition is slow. Thus hydrogen
peroxide and not oxygen is considered as the final product.
The reaction rates for various steps where either taken from
NIST chemical kinetics database[3] or were calculated (steps
involving ionic conductivities) by utilizing conductivity and
electro chemical equivalent data [2].
The plots for two simulation runs ,one for voltage applied
across the electrodes of the order of 2.1 V and another for a
100 times increase in the voltage are shown below.
Figure 1. Concentration (moles/lts) vs Time(sec)
plot for production of H2, voltage applied 2.1V.
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HYDROGEN PRODUCTION BY SPLITTING WATER
In the above scheme the chlorine gas is fed back into the
reaction solution and a first order gas phase reaction with the
solution is involved. This kind of step is adopted for the usual
chlorination of water process.
Since the first reaction in the above scheme is reversible, the
rate constant of the reverse reaction is also taken into account.
As the reverse step has to compete with the 4-th step in the
scheme and since the 4-th step is 100000 times faster the
reverse step has insignificant role to play.
Figure 2. Concentration (moles/lts) vs Time(sec) plot for
production of H2, voltage applied 210V.
The enthalpy changes for all the steps mentioned above were
calculated. The over all reaction was found to be endothermic.
It is therefore mandatory to study the temperature sensitivity
of the reaction scheme as the energy would be absorbed from
the ambient environment.
It is seen from the plots that there is negligible improvement
in the reaction efficiency with increase in the voltage. This is
due to the fact the rate determining step is at least 1000 times
slower then the other steps.
The rate of individual steps were either taken from NIST
chemical kinetics database and anode and cathode reactions
rate were calculated using ECE and conductivity data for
various ions [2].
As reported in the literature, the reaction efficiency is said to
improve with the addition of minor concentration of soluble
salts like NaCl. From the above discussion, it seems that the
procedure of introducing salts will be not so effective as that
has no influence on the rate determining step and merely
improves the rate of other steps. Even if we consider the heat
produced via Joules heating it will not lead to a faster
dissociation rate as thermal dissociation of water requires a
temperature of the order of 2000 – 5000 K, temperature
difficult to obtain on a large scale from Joules heating with
resistivities of the order of few ohm-m provided by the
electrolyte.
Two plots are shown below. The first plot is concentration vs
time graph for the second reaction pathway when the applied
voltage is 2.1 V. The concentration of hydrochloric acid is
about 1 mole/l.
It can be seen from the plot that the production of hydrogen
under the above scheme is rapid and there is already one order
of magnitude improvement in the reaction efficiency. In the
next plot the reaction efficiency is further improved by an
order of magnitude by a 10 fold increase in the voltage.
As a second step our aim is to show that the reaction rate of
production of hydrogen can be improved if one involves a
catalyst, in our case an electro catalyst. In addition to
enhancing the electrical conductivity of the reaction solution
the electro catalyst will increase the rate of the rate
determining step which in our case is the dissociation of
water.
The following reaction pathway was considered for the second
set of simulation.
H2O + Cl2 =>
Cl- => Cl +
H+ + e- =>
HOCl + Cl
2H => H2
2Cl => Cl2
2OCl => O2
HOCl + H+ + CleH
=> H+ + ClO + Cl-
+ Cl2
Figure 3. Concentration (moles/lts) vs Time (sec) plot for
production of H2 for the new reaction scheme, voltage applied 2.1V.
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VI. APPENDIX
The rate of the cathode and anode reactions were calculated as
follows. From Faraday’s law of electrolysis, one obtains:
dm/dt = - zi
where, the term on the left hand side denotes the rate of
decrease in the total mass of the of the ion present in the
solution which participate in the cathode or anodic reaction. z
is the electrochemical equivalent of the ion and i is the total
charge per unit time passed across the electrodes.
Figure 4. Concentration (moles/l) vs. Time (sec) plot
for production of H2 for the new reaction scheme with 10
increase in voltage.
fold
The above simulation is done for a 1 litre of water which
contains about 56 moles of H2O molecules. As can be seen in
the last plot one can obtain of the order of 52 moles of H 2 or
about 1164 litres of H2 when the applied voltage is of the
order of 20 Volts and the reaction will complete within 2000
sec.
IV. CONCLUSION
In the study conducted above, we have shown with the help of
a stochastic chemical kinetics simulation, a substantial overall
improvement in the electro- dissociation of water when the
newly proposed reaction scheme is taken into account. One
can further study the effects of other reaction parameters like
concentration of input material and temperature on the
reaction kinetics to refine the present study. A study of this
kind is important when we consider the potential of producing
hydrogen via a non fossil fuel consuming route ( for example
photovoltaic dissociation of water). We hope to follow up on
these lines with the help of the simulation package (Chemical
Kinetics Simulator) [1] used in this work.
In a cell of dimensions of the order of 100 cm, if we know
the conductivity per mole of a particular ion, then on
application of V volts across the electrodes, the charge carried
by the ion in unit time is VCc where C is the conductivity per
mole of the ion and c is its molar concentration. On the other
hand, when a mass equal to dm is liberated then it is
equivalent to a molar concentration of dm/g , where g is the
gram mole equivalent . Thus, one can set the rate equation as
follows:
dc/dt = -[zgVC]c
Putting in the values of the known quantities in the square
bracket, one can calculate the rate of the cathode or anodic
reactions as is done in the present simulation.
Dr. Aniruddh Singh obtained PhD in
Theoretical Nuclear Physics from Jamia Millia
Islamia, New Delhi.
He obtained BSc Hons in Physics from Delhi
University and MSc Physics from IIT Kanpur.
His PhD thesis is in the field of Variational
Monte Carlo methods as applied to light nuclei
and hypernuclei. He has over five years of
teaching experience and two years research
experience in industry. Currently, he is an
assistant professor with the Department of
Applied Sciences, Ajay Kumar
Garg
Engineering College, Ghaziabad.
V. REFERENCES
[1] Reference Manual, Chemical Kinetics Simulator,
International Business Machine Corp. 1996.
[2] Physical Chemistry, Oxford University Press, Peter
Atkins, Julio de Paula.
[3] NIST Chemical Kinetics Database:
http//www.kinetics.nist.gov.
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