Free software for
the determination of the
real surface area
of Fuel Cells electrocatalysts
Leandro Borges Viana and Prof. Dr. Eduardo Gonçalves Ciapina
São Paulo State University (UNESP), School of Engineering, Guaratinguetá, SP, Brazil
www2.feg.unesp.br/ciapina
Abstract:
The determination of the real surface area of solid electrodes is of paramount importance in
research and development of new materials for energy conversion and storage devices. In
particular, the assessment of the specific electrocatalytic activity (i.e., the kinetic current in terms
of A cm-2) and other relevant properties of the electrode/electrolyte interface are crucial for the
development of low temperature Fuel Cells electrode materials. Cyclic voltammetry is one of the
most used electrochemical technique to estimate the real surface area by the charge involved in a
given interfacial process. However, the calculation is often conducted in commercial softwares
that are time-consuming and little intuitive to use for this specific purpose. Herein, we describe
simple-to-use graphical software that allows the calculation of voltammetric charges and the
determination of the real surface area of solid electrodes.
Keywords: real surface area, solid electrodes, electrocatalysis, cyclic voltammetry, software.
Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016). Avaliable at www2.feg.unesp.br/ciapina 1 1. Introduction
The knowledge of the real surface area of solid electrodes is of paramount importance for
the understanding and development of new materials for electrochemical energy conversion and
storage devices such as Fuel Cells, electrolyzers, and batteries. Taking the hydrogen-oxygen Fuel
Cell as an example, high surface area electrocatalyst are needed to boost the rate of reaction at
the electrodes and thus the performance of the system [1,2]. To maximize the exposed surface
area available for the reactants, catalysts are often prepared to present particles in the nanometer
scale, typically between 2 and 10 nm. State-of-the-art electrocatalysts are based on nanoparticles
of noble metals such as Pt and Pd as well as its alloys with transition metals (Ru, Sn, Co, etc)
anchored on a high surface area carbon [3–5].
The assessment of the real surface area of a given electrocatalyst is a key step in the
determination of parameters related to an electrochemical reaction (e.g. the electrocatalytic
activity in terms of A cm-2) or fundamental properties of the electrode/electrolyte interface such
as the double layer capacitance (in units of µF cm-2). The experimental determination of the real
surface area of fuel cells electrocatalysts is often made by means of the current-potential
characteristics of the material in a supporting electrolyte by the widely used electrochemical
technique, the cyclic voltammetry (CV) [6–9]. In a typical CV experiment, the electrode potential
is varied at a given scan rate (normally between 5 and 100 mV/s) and the current is recorded. An
example of a CV obtained for a nanostructured platinum electrode immersed in diluted sulfuric
acid is shown in Figure 1. Further details of the experimental procedure is found in ref.[10].
2 Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016) Avaliable at www2.feg.unesp.br/ciapina Figure 1. Cyclic voltammogram obtained for a Pt electrode (Pt black, Johnsom Matthey Co.) in diluted sulfuric acid
(0.5 mol L-1 H2SO4). Scan rate 20 mV/s. Potentials were measured against a reversible hydrogen electrode (RHE)
prepared in the same solution. The shaded red area represents the charge traditionally used to estimate the
electrochemical active surface area.
The current-potential profile depicted in Figure 1 provides information where the
adsorption of hydrogen and oxygen-containing species takes place or the potential window
where no faradaic process occurs (the so-called the double layer region). For Pt-based electrodes,
the “hydrogen region” is traditionally used for the determination of the real surface area and it
will be used here to illustrate the commonly used procedure. Other processes such as the
oxidation of a pre-adsorbed monolayer of carbon monoxide (CO stripping) or the reduction of
surface oxides can also be used for the evaluation of the active area of an given electrode
material. The reader is referred to refs.[6,11,12] for more information.
The adsorption of atomic hydrogen on Pt sites (often called underpotential deposited
hydrogen, Hupd) takes place according to the reaction (1):
Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016). Avaliable at www2.feg.unesp.br/ciapina 3 𝑃𝑡 + 𝐻! 𝑂! + 𝑒 ! ↔ 𝑃𝑡 − 𝐻!"# + 𝐻! 𝑂
(1)
The amount of charge required to form or remove a monolayer of Hupd was determined
(experimentally) to be 210 µC cm-2 on smooth polycristalline Pt[6]. Therefore, if the coulombic
charge related to the removal (the backward reaction) of Hupd is evaluated from a CV experiment
for a given Pt electrode, the real (electrochemical active) surface area can be obtained. Details of
the physicochemical processes and the major assumptions behind the normalization process is
out of the scope of the present paper and can be found in refs [13–15].
The determination of the charge (Q) involved in the Hupd process in a CV experiment is
found by integration of the current-potential curve and then by dividing the calculated value by
the scan rate of the measurement (υ), as follows:
1
𝑄(𝐻!"# ) = 𝜐
!"
𝑖𝑑𝐸
!"
Additionally, given the electrode/electrolyte interface also behaves as a capacitor, the so-called
double layer charging current has to be properly accounted for (subtracted) by a process called
double-layer correction. Traditionally, this is done by assuming a straight line as the baseline in
the whole potential window of interest, as shown in Figure 1.
Thus, it is clear that after acquiring the CV, electrochemists and materials scientists need to
use some graphical software to plot and analyze the data. Since the charge transferred between
the electrode and the electrolyte is related to the magnitude of a given electrochemical process,
the integration of some specific region of the cyclic voltammograms is needed. The usual
4 Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016) Avaliable at www2.feg.unesp.br/ciapina procedure should contain more or less the following sequences:
The choice of the potential region for integration
The removal of the background current (i.e., the
double layer charge correction)
Numerical integration of the current-potential curve
The division of the result of the integration by the
scan rate to obtain the voltammetric charge
The use of a suitable conversion factor for the
assessment of the surface
Most scientists make use of current available computer software to perform such
calculations such as ORIGIN (Origin lab Co.), IGOR PRO (Wavemetrics Co), SIGMA PLOT
(Systat Software Inc.), EXCEL (Microsoft Co.), among others. However, despite their powerful
suite, the amount of procedures that should be performed in order to assess the charge (the
integral of the current-potential curve divided by the sweep rate) is may be time-consuming and
little intuitive. Moreover, in some cases, the elevated prices of the licenses (about $ 500 or more)
prevent its use by students or in developing countries.
Aiming at proving electrochemists and materials science researchers a fast, easy and a visual
way to assess voltammetric charges and the electrochemical active surface area of solid electrodes
we developed a free and simple-to-use software entitled “ADVC” (the initials of Area
Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016). Avaliable at www2.feg.unesp.br/ciapina 5 Determination from Voltammetric Cycles), described in the following.
2. Materials and Methods
2.1. Software description:
The software described in this work was written on JAVA®, which is a programming
language and computing platform already present at almost every computer[16]. The software
was designed to provide a graphical interface for plot and analyze current-potential curves from
cyclic voltammetry experiments. The user may visualize the cyclic voltammogram, to compute
coulombic charges, and to estimate the electro-active area of the electrode. The potential range
for the integration of the voltammetric charges can be selected directly from the CVs, simply by
clicking in the desired initial and final potentials or by typing the desired values in the
corresponding boxes. The scan rate of the experiment needs to be informed by the user.
Whenever an adequate conversion factor is available, the electrochemical active surface area can
be estimated.
2.2. Data format:
Data should be space-separated double-column (potential – current) format in the *txt
extension. Points should be used to indicate decimals; if commas are used, an option is present at
the menu “Options” that allows the user indicate the decimal separator used. Subsequent
voltammetric cycles need to be separated by a return character (i.e., an empty line).
6 Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016) Avaliable at www2.feg.unesp.br/ciapina 3. Results and Discussion
A typical view of the software is shown in Figure 2. The main window is divided into 2 parts.
On the right side, the values of the potential and current of the experiment (loaded by the user)
are exhibited. A third column is available for the results of the current density, that is, the current
normalized by the electrochemical active surface area of the electrode. After loading the data the
user can generate a plot of the data by hitting the “Plot data” button and the result will appear in
the right side of the screen).
Figure 2. The main window of the ADVC software. The voltammetric data are shown in the worksheet at the right
side of the main window. In the plot area, the potential range used in the calculation of the voltammetric charges is
presented in a distinct color, along with the corresponding baseline.
Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016). Avaliable at www2.feg.unesp.br/ciapina 7 An interesting feature of the software is that if several voltammetric cycles were carried out
and recorded in a single file, an option is included that just by hitting the “cycle number” button,
the user can switch among distinct cycles. The potential range for the integration and scan rate
can be informed for the evaluation of the coulombic charge. At the bottom, there is a field for
the conversion factor to be used in the area calculation.
3.1. Charge calculation:
The numerical integration is carried out by the trapezoidal rule. The user has 2 distinct
options for the baseline: i) constant current and ii) current as a linear function of the potential.
After defining the type of the baseline, the user has to indicate the initial and final potential for
integration and the scan rate used in the experiment. An interesting feature of the software is
that it shows the integration interval used in the calculation by changing the color of the
experimental points in the graphic area (see Figure 2), so that the user can double-check the
potential range used for integration.
3.2. Real surface electrode area calculation:
After computing the charge the user only needs to inform the conversion factor to be used.
Commonly used conversion factors for Pt and Pd are 210 µC cm-2 (Hupd) and 424 µC cm-2 (PdO
reduction peak), respectively [6,17]. The “normalize” button can be used to plot the CV in terms
of the current density, that is, the current divided by the active surface area of the electrode. The
user has the option to print or save the graphic in the JPEG or PNG format.
IMPORTANT NOTE:
The AGVC software comes with no warranty. It is free for non-commercial use only.
8 Viana L.B.; Ciapina, E.G. Free software for the determination of the real surface area of Fuel Cells electrocatalysts (2016) Avaliable at www2.feg.unesp.br/ciapina 4. Conclusions
We developed a free graphical software that allows the determination of voltammetry
charges and the real surface area of solid electrodes. By the aid to the described software,
electrochemists and materials science researchers have a fast, easy and a visual way to assess
voltammetric charges and the electrochemical active surface area of solid electrodes. The
software may be obtained at the website www2.feg.unesp.br/ciapina.
Acknowledgments
Prof. E. G. Ciapina thank CNPq [Process # 476690/2013-7] and PROPe/UNESP for the
financial support. Mr. Leandro Viana thanks PROPe/UNESP for the scholarship
[PIBIC/Reitoria program].
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