(404e) Hexachloroiridate As an Alternative to Hexacyanoferrate in Electrochemical Mass Transfer Measurements

An important characteristic in the design of
electrochemical reactors is the mass transfer performance. By using what is
known as the limiting current density method, it is possible to determine the
mass transfer performance of a given reactor experimentally [1]. In many cases,
the hexacyanoferrate(III)/hexacyanoferrate(II) redox pair is used for this
procedure. However, it is known that this couple suffers from non-ideal
behavior [2-4]. Over time, the hexacyanoferrate complex will deposit on the
electrodes, forming an additional resistance to mass transfer. In order to obtain
reliable measurements, it is therefore necessary to periodically polish the
electrodes. The situation is especially dire for electrode materials such as
stainless steel, where polishing is required every two hours [5]. For 3D
electrodes this is troublesome, as their fine structures cannot be polished
reliably. Extensive strategies are described in literature in order to alleviate
the problem, mostly with regards to the experimental conditions [6]. Despite
these, we experience that the non-ideal behavior persists (see figure 1). For
this reason we propose the use of the
hexachloroiridate(IV)/hexachloroiridate(III) redox pair as an alternative.

Figure 1: Limiting current density of
hexacyanoferrate(III) as observed in a nickel rotor-stator spinning disk electrolyzer.
a) at the start of the experimental work, b) after a day of work, c) after two
days. Fresh solutions were used each time, with the recommendations of D.A. Szánto
et al. being followed as far as practicable [6].

In this work we show that hexachloroiridate(IV)/hexachloroiridate(III)
is a well-behaved redox couple that can be used in mass transfer experiments
(see figure 2). Furthermore, its behavior is compared to that of the
hexacyanoferrate couple. In contrast to the traditionally used
hexacyanoferrate, we see that hexachloroiridate does not poison the electrodes
(figure 3). With regards to the experimental conditions, some of the pitfalls and
downsides are explored in using this new couple. Here it is shown that the pH
of the solution is vitally important to the performance of the
hexachloroiridate couple, which is not the case for its traditionally used
counterpart. Though, through proper care, we show that the limiting current
density can remain stable for over 16 hours as long as the bulk concentration
is corrected for.

Figure 2: The limiting current density plateau
of hexachloroiridate on platinum. On the left: the reduction of
hexachloroiridate(IV), on the right: the oxidation of hexachloroiridate(III). A
flat current density plateau is found between -0.2V and 0.7V for the reduction,
and 1.0V and 1.2V for the oxidation.

Figure 3: Evolution of the limiting current
over time on platinum. On the left: the hexachloroiridate(IV) reduction, on the
right: the hexacyanoferrate(III) reduction. At 16 hours, the solution was
refreshed. Though the hexachloroiridate complex loses current faster, it
recovers fully after refreshing. The current loss is entirely due to a loss of
bulk concentration. The hexacyanoferrate current does not recover fully, which
indicates some electrode poisoning.

References

[1] T. Mizushina,
"The Electrochemical Method in Transport Phenomena," Advances
in Heat Transfer, vol. 7, pp. 87-161, 1971

[2] C.
Beriet and D.J. Pletcher, "A Microelectrode Study of the Mechanism and
Kinetics of the Ferro-/Ferricyanide Couple in Aqueous Media," J. of
Electroanal. Chem., vol. 361, pp. 93-101, 1993.

[3] M.
Stiebl and K. Jüttner, "Surface Blocking in the Redox System
Pt/[Fe(CN)6]3-,[Fe(CN)6]4-," Journal of Electroanalytical Chemistry, vol.
290, pp. 163-180, 1990.

[4] J.
Kawiak, P.J. Kulesza and Z. Galus, "A search for conditions permitting
model behavior of the [Fe(CN)6]3-/4- system," J. Electroanal. Chem.,
vol. 226, pp. 305-314, 1987.

[5] W.
Taama, R. Plimpley and K. Scott, "Influence of Supporting Electrolyte
on Ferricyanide Reduction at a Rotating Disc Electrode," Electrochimica
Acta, vol. 41, no. 4, pp. 549-551, 1996.

[6] D.A.
Szánto, S. Cleghorn, C. Ponce-de-Léon and F.C. Walsh, "The Limiting
Current for Reduction of the Ferricyanide Ion at Nickel," AIChE J.,
vol. 54, no. 3, pp. 802-810, 2008.