Electrolytic cell | EPFL Graph Search

An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would otherwise not occur. The external energy source is a voltage applied between the cell′s two electrodes; an anode (positively charged electrode) and a cathode (negatively charged electrode), which are immersed in an electrolyte solution. This is in contrast to a galvanic cell, which itself is a source of electrical energy and the foundation of a battery. The net reaction taking place in a galvanic cell is a spontaneous reaction, i.e, the Gibbs free energy remains -ve, while the net reaction taking place in an electrolytic cell is the reverse of this spontaneous reaction, i.e, the Gibbs free energy is +ve. Principles In an electrolytic cell, a current passes through the cell by an external voltage, causing a non-spontaneous chemical reaction to proceed. In a galvanic cell, the progress of a spontaneous chemical reaction

Heat treatment of carbon anodes for the aluminum industry

Mauriz Lustenberger

Aluminum is produced by the electrolytic reduction of alumina dissolved in molten cryolite. This process requires carbon anodes which are consumed during the electrolysis. The anodes contribute roughly with 15% to the total aluminum production cost. If the anode quality is poor the share can as well reach 25 %. It is known that the anode behavior during aluminum electrolysis is significantly influenced by the anode baking process. However, there is a lack of knowledge to predict the anode behavior through given baking parameters. In addition, process control of industrial furnaces is still to a large degree empirical. Often the emission of industrial furnaces causes problems as deposits can accumulate in the exhaust system which leads to fire incidents. This investigation has three major goals: The prediction of the anode behavior during electrolysis for a given heat treatment. The understanding of the process control of industrial scale open-top anode baking furnaces. The emission control of the open-top anode baking furnace. The baking parameters are defined in this thesis as the anode heat-up rate and the baking level. Through pilot baking it is shown that the baking level is basically determined by the final baking temperature although the soaking duration has some influence too. The anode baking level can be estimated with a calculation for any given baking curve. Therefore the impact of the temperature and soaking duration on the real density anode property needs to be determined through a pilot study. The optimum baking parameters regarding the anode behavior during electrolysis depend on the material. Some anode materials react more sensitive to the heat treatment than others do and are therfore more difficult to handle in the industrial baking process. Through pilot scale anode baking it is shown how to predict the anode behavior during electrolysis. Process control of the anode baking in industrial open-top furnaces is complex since the anode baking is indirectly controlled by the combustion process inside the flues. Through measurements the combustion process is analyzed in this thesis. The results show that a furnace operated at its physical limits is difficult to control regarding to emissions. However, the gained knowledge allows to optimize existing furnace process control systems. The furnace emissions are mainly caused by insufficient combustion of the pitch fumes formed during the pyrolysis of the anode binder pitch. It is shown that emission problems cannot always be solved by a well chosen control system setup. In such a situation combustion control is required in addition to temperature control. A technology has been developed to measure online the combustion situation of each flue. For this purpose 2-color pyrometers, together with an intelligent signal processing, are required. This technology is integrated since 10 months in an industrial baking furnace control system. The results are promising. The developed technology is an important step for furnace operation at maximum productivity and minimum emission. A patent has been applied for the combination of online combustion analysis concurrent with temperature measurement (swiss apply No. 01598/02 by Mauriz Lustenberger and Felix Keller, R&D Carbon Ltd. Sierre).

EPFL2004

Modeling and Optimal Control of a Redox Flow Battery

Véronique Amstutz, Alberto Battistel, Julien Léo Billeter, Dominique Bonvin, Timm Faulwasser, Hubert Girault, Heron Vrubel, Daniel Wrang

Vanadium Redox Flow Batteries (VRFB) can be used as energy storage device, for example to account for wind or solar power fluctuations. In VRFBs charge is stored in two tanks containing two different vanadium solutions. This approach decouples the storage capacity and the power supply which is dependent only on the number and size of the cells [1]. A control specific model of a VRFB is proposed, which captures the essential dynamic properties of the battery while ignoring all fluid mechanical elements. The model of the battery is a nonlinear DAE comprising a differential equation for the state of charge SoC and algebraic equations based on the Butler-Volmer equation for the current I and the voltage U, the power P = U∙I being set by the operator. The battery typically operates in constant power mode, but the control system limits its operating range by switching to a constant voltage when upper or lower voltage thresholds are reached. In addition, this VRFB contains a secondary flow circuit, where the electrolyte discharge reactions produce hydrogen and oxygen [2]. Model parameters are estimated by minimizing the difference between the measured and modeled SoC over time, the other states (I, U and P) being compared with measured data. The model is validated using independent measurements, which show good fits. Finally, the dynamic model will be used to formulate and numerically solve the problem of optimal battery operation in different scenarios. [1] C. Blanc, A. Rufer, Chapter 18, in Paths to Sustainable Energy, InTech, 2010 [2] V. Amstutz et al., Energy and Environmental Science 7, 2350-2358 (2014)

2016

Redox flow battery and indirect water electrolysis

Véronique Amstutz

The progressive integration of renewable energy sources such as wind turbines and photovoltaic pannels in the current electrical network, and the rise of the electrical mobility, provoke a change of paradigm in the sector of energy management. The increasing variability of the electricity production and consumption profiles, together with the requirement for a reliable supply of energy, necessitates the implementation of energy storage means of different scales and for a wide range of applications. Redox flow batteries (RFBs) are well adapted for buffering the fluctuations of solar or wind energy production. They present a fast response time, can withstand a large number of charge-discharge cycles and their ouput power is independant of their energy capacity. The main limitation of RFBs resides in their low energy density. The objective of the present work was to test a mean to overcome this low energy density. A new concept was developed, which rests on the addition of a second pathway to discharge the RFB. This pathway allows to generate hydrogen and oxygen, without affecting the functioning of the RFB. This concept was called dual-circuit RFB and was patented. RFBs are based on two liquids electrolytes, each stored in a reservoir, and flowing through an electrochemical cell for their electrochemical conversion. Each electrolyte contains one redox couple and Ce(IV)/Ce(III) and V(III)/V(II) were selected as positive and negative redox couples in the RFB developed here. Charge-discharge curves of a VâCe RFB were measured for characterisation purposes and for the preparation of the charged electrolytes. The latter ones can be discharged electrochemically in the RFB to generate electricity (electrochemical discharge mode), or they can be directed in a secondary circuit where they are discharged for the production of hydrogen or oxygen (chemical discharge mode). The chemical discharge of the negative electrolyte consists of the reaction of V(II) with protons to produce hydrogen and V(III). Mo2C was selected as heterogenous catalyst for the characterisation of the reaction. A conversion close to 100% suggested no loss of current. A kinetic analysis provided some insights into the catalytic mechanism of this reaction. The chemical discharge of the positive electrolyte aimed at the conversion of Ce(IV) to Ce(III) by the oxidation of water to oxygen and also necessitates a catalyst. Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) were evaluated in terms of conversion and kinetics. The composition of the positive electrolyte was shown to be of importance for this reaction. To show the feasibility of this concept, a larger-scale demonstrator system was designed based on a 10 kW (40kWh) commercially available vanadium RFB. A characterisation of this RFB was first performed. The design a suitable Mo2C catalyst and its corresponding catalytic bed is also discussed. As a conclusion, the concept developed and experimentally tested in the present work leads to a RFB system which is characterised by two discharge modes, increasing its energy storage capacity and energy density. The dual-circuit RFB also represents a crossing between electrical grid and hydrogen mobility as the hydrogen produced by surplus electricity could be delivered to fuel cell cars. The application of this system in a local distribution electrical network, close to a wind or solar source seems a promising approach.