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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. 1 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. 39 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. 40 AKGEC JOURNAL OF TECHNOLOGY, Vol. 2, No. 1 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. 41