Behind the Science: Behavior of Highly Diluted Electrolytes in Strong Electric Fields

Behind the Science: Behavior of Highly Diluted Electrolytes in Strong Electric Fields

Author: Anne Deveson

Dr. Anne Deveson, Deputy Editor for Chemistry – A European Journal, talks to Professors Rudi van Eldik (pictured left) and Immo Weber (right), University of Erlangen, Germany, about their article that was recently accepted for publication in Chemistry – A European Journal. The paper takes a look into the problem of corrosion of aluminum heat sinks, which can cause severe fire hazards in long-distance electrical power transmission systems. This can cause losses in the million euro per day range.

Could you briefly explain the focus and findings of your article to a non-specialist and why it is of current interest?

Due to unavoidable corrosion of the aluminum heat sinks as part of the water cooling cycle in high-voltage direct current (HVDC) transmission modules, aluminum hydroxide is released continuously into these systems. Aluminum hydroxide and water exist in an acid-base equilibrium with negatively charged aluminate ions [Al(OH)4], which in turn are attracted by the platinum grading electrodes when they are connected as positively charged anodes. This attraction occurs also in highly diluted solutions at voltages around 10 kV and a constant operation current of 8 mA, where the electric field is sufficiently strong and the electrodes act as centers of maximum field strength. Under these conditions, de-ionized water is electrolyzed, hydronium ions H3O+ are formed as acid on the anode, while hydroxide OH is generated at the cathode. If aluminate anions [Al(OH)4] approach the anode surface, they are neutralized to aluminum hydroxide Al(OH)3 that deposits on the electrode surface. Due to proton tunneling these depositions can grow to a thickness of up to 1 cm before they become insulating by aging and interrupt the electric grading cycle.

If gaseous CO2 is purged through the aqueous system, it reacts with water to form an equilibrium mixture of anionic bicarbonate HCO3 and hydronium ions, a simple buffer system. Firstly, the overall pH is lowered by this reaction to such an extent that aluminum hydroxide dissolves as cationic aluminum aqua complexes, viz. [(H2O)4Al(OH)2]+, that are soluble and do not precipitate under the electrochemical conditions anymore. Secondly, bicarbonate anions are attracted at the anode to such an extent that they form a negatively charged outer surface layer on the electrode, which electrostatically repels the negatively charged aluminate [Al(OH)4] as a competitive process. Both effects are synergistic, such that even trace amounts of dissolved CO2 are sufficient to prevent alumina deposition on the electrodes.

What are the key issues of CO2 pulse-doping and how is it technically applied?

Continuous purging of the cooling water with gaseous CO2, even at a low flow rate, leads to an uncontrollable increase in water conductivity following some delay, which decreases only slowly during the course of time. This accumulation effect is due to the kinetically retarded relaxation of dissolved CO2 in its coupled hydration and dissociation equilibria. Furthermore, when most of the CO2 is dissolved physically it forms a CO2 reservoir. This problem can be overcome by pulse-doping of the cooling water with gaseous CO2, where in sufficiently long intervals dissolved CO2 can relax into the dissociation equilibrium before the next gas pulse is applied. This easily applicable and preventive technique allows simple digital feedback-control by water conductivity only, which can be automatically adjusted in the correct range above a minimum of 0.15 µS cm–1 and below a critical value depending on the individual plant construction in the sense of an easy to implement on-off mechanism.

This method is since February 2014 tested in a commercial HVDC plant in China and formation of alumina depositions on the grading electrodes has so far not been observed. This technique is generally applicable and certainly not restricted to Siemens HVDC transmission modules only.

Alumina deposition on grading electrodes is a significant problem for the power industry. How big an impact do you see your work potentially having?

We believe that our developed solution by CO2-doping will in future prevent fire hazards in HVDC plants. Some of these fires caused service interruptions with losses up to 1 million Euro per day. We, therefore, expect that this simple solution will quickly become a standard technique to prevent alumina deposition on grading electrodes in HVDC transmission modules worldwide.

How long did this investigation take?

The investigation took two years. At the start of the project, we were confronted with many contradicting hypotheses within a community of electrical engineers and material scientists. We quickly recognized that we were dealing with a cluster of problems closely related to each other. We therefore divided this cluster into separate sub-problems in terms of unsolved questions and designed particular experiments to find appropriate answers. Step by step pieces of the puzzle were resolved to come to an overall solution of the problem.

Which steps were these?

First, we had to deal with the problem under which pH conditions a basic or acidic aluminum solution is deposited as alumina on either the anode or cathode. The outcome of these experiments might be easy to predict, but are the classical electrochemical laws still valid at such high voltages in diluted solutions? Everything had therefore to be checked under static high voltage conditions. The construction of the HV test set-up and generation of the alumina depositions under close to technical conditions took almost half a year. Challenging problems were gas tightness and pH measurements in almost deionized water. Development of the CO2 pulse-doping technique also took some time. Subsequently, deposition inhibition experiments with tetramethylammonium p-tosylate (TMAT) to prove the repulsion model, took almost two months, for which TMAT had to be prepared on a kg scale for an overall water volume of ca. 25 L in the HV test set-up. Powder X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy, as well as quantum chemical calculation, were also employed.

In addition, we also checked whether there are any crucial interactions of CO2 with the PVDF tubings of the HVDC transmission modules, which is not included in the paper. While PVDF adsorbs pure gaseous CO2 almost as a Henry isotherm, saturated aqueous CO2 solutions at higher temperatures showed no visible effects on PVDF after experiments performed in an autoclave for periods of a week. To secure the operating safety of this whole process is presently a major aspect of the ongoing industrial testing phase.

How did it come to the collaboration with Siemens?

Due to our long-standing expertise in the thermodynamic and kinetic behavior of metal complexes in solution, in particular also aluminum complexes, we were approached by Siemens in the spring of 2011 for a collaboration to solve the alumina deposition problem. The project was originally initiated by Dr. Ralph Puchta, at that point working on his Habilitation in our group in charge of quantum mechanical calculations, together with Dr. Immo Weber, who developed and implemented a currentless alumina anodization of magnesium at KUM GmbH, Erlangen, Germany. Soon after, the project management and coordination was taken over by Dr. Weber as a result of his industrial experience. He also planned and conducted the majority of the experiments in very close cooperation with the Siemens team and other collaborators.

Could you explain the motivation behind the study?

The basic strategy behind such cooperation projects is to accomplish together with the customer, here in particular Siemens, the fastest possible and cost optimized product or process development by directly linking-up state-of-the-art research competence in an interdisciplinary network. Whereas in industry, from an engineer’s point of view, development, ruling and optimization of a practical technological process must be the primary goals, in academia we are mainly focused on the scientific understanding of natural phenomena or experiments, and to pass this knowledge on to the next generation. We were fortunate to all together have managed to combine these apparently opposite motivations in the sense that we shortened the time gap between fundamental research and industrial application from the usual 10–15 to two years. Such projects bring academic scientists back to reality!

What is the broader impact of this paper for the scientific community?

Certainly, high voltage electrolysis appears to be rather exotic, even if it can be performed only in diluted solutions. Under technical conditions (U > 10 kV, I = 8 mA, water conductivity κ = 0.3 µS cm–1) the electric field at the electrode surfaces is so strong that Wien effects must operate, increasing ion mobility and affecting acid dissociation equilibria. Obviously, all interfering thermodynamic equilibria in the HVDC water cooling system are affected by comparable factors and in that way alumina can still be deposited under such extreme conditions. These polarization effects can now be studied in more detail under truly static conditions, which have been restricted to resonance methods up to now. This fundamentally important field has been explored only by a handful of scientists since the discovery by Max Wien in 1927/28.

Furthermore, alumina depositions found in HVDC converter plants often show a well-shaped geometry, as well as a very hard and dense consistency. Instead of disposing it, this ion conducting material of low specific weight could offer many opportunities for technical applications. Double-alignment of superconducting crystal grains in strong magnetic fields was achieved by Bill C. Giessen in 1990–1993. To the best of our knowledge, electrolytic deposition of cationic or anionic metal complexes in strong, but homogenous electric fields to achieve regular surface textures, have not been studied. Such experiments might lead to many applications in materials science.

Do you have any plans for future work extending from this study?

First of all, the growth of the alumina depositions needs to be understood in more detail. This can be well achieved by extended quantum mechanical calculations. Furthermore, can a regular alumina deposition be formed in a strong homogenous electric field on a regular metal surface from diluted solutions? These are fundamental aspects that need to be clarified.

Instead of pulse-doping with gaseous CO2, alternative application methods are also highly desirable for various reasons. Aluminum is only reversibly adsorbed by the ion exchanger, so here the development of an irreversible binding material is also of vast interest. These are only two of many practical aspects to be followed up on.

To accomplish these aims, we are currently seeking further funding from the electrical industry. In addition, we offer our expertise in CO2 pulse-doping within the limits of consultation to operators of HVDC plants. This invention can be freely adopted since it was not patented.

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