Experimental - Determining Catalytic Activity - ORR Catalysis with Pt-based CSNPs

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03/11/2020 Experimental - Determining Catalytic Activity - ORR Catalysis with Pt-based CSNPs
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ORR Catalysis
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Experimental -
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Experimental -
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Home > Transition to the Nanoscale > 
Experimental - Determining Catalytic
Activity
            Catalytic activity has been mentioned a few times before, but
it has yet to be discussed in any significant level of detail.  The activity
of a catalyst is effectively just the rate of a specific reaction on that
catalyst.  Catalytic activity is reported in two distinct ways in the
literature: specific activity (SA), which is just the rate normalized to the
real surface area of the catalyst, and mass activity (MA), which is just
the rate normalized to the mass of catalyst employed.3  The methods
for determining both types of activity are incredibly similar, and rely
on the use of a rotating disk electrode (RDE) to generate cyclic
voltammograms.  The general setup for an activity measurement is a
three-electrode cell (typically glass or Teflon), as illustrated below. 
The working electrode is an RDE with the catalyst of interest attached,
the counter electrode is typically just a graphite rod, and the reference
electrode is quite frequently a standard Ag/AgCl electrode.  The
reference electrode is isolated from the rest of the cell by a Nafion
membrane in order to prevent the diffusion of chloride ions into the
electrolyte.  Since actual fuel cell stacks tend to employ acidic
electrolytes, the electrolyte of choice is usually 0.1 M HClO4, and
these measurements are normally conducted at room temperature.
 
 
            For the sake of brevity, we will not discuss in detail cyclic
voltammetry or the complicated fluid dynamics governing an RDE. 
Cyclic voltammetry just involves applying a sawtooth potential
waveform and measuring the resulting current as a function of
potential.  For reference, a typical window is 0.05 VRHE to 1.2 VRHE at
50 mV/s.4  An RDE is just like any normal electrode, except it can be
programmed to rotate at any particular frequency (1600 rpm is pretty
standard).  This rotation generates a fluid current that increases the
flux on the working electrode.
 
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03/11/2020 Experimental - Determining Catalytic Activity - ORR Catalysis with Pt-based CSNPs
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            The first step of determining activity involves determining the
potential of the reference electrode with respect to a universal
standard, like the reversible hydrogen electrode (RHE).  The potential
of a particular reference electrode can vary by a couple millivolts
between days, which can drastically impact the reported activity --
hence, it is important to standardize the measurements.  This is
accomplished by saturating the electrolyte with hydrogen gas (~15
minutes for a 150 mL cell), rotating the RDE such that the oxidation of
hydrogen is not limited by diffusion, and determining the potential at
which no current flows.  This is the potential of the reference
electrode with respect to RHE.
 
            The second step involves purging the system with oxygen
(again ~15 minutes for a 150 mL cell), rotating the RDE at various
prescribed speeds (commonly 400 rpm, 900 rpm, 1600 rpm, and
2500 rpm), and recording a CV in the potential window described
above.  A background curve is generated by recording a CV in the
same potential window, but in an environment saturated with argon. 
Subtracting the background curve from each of the oxygen curves is
equivalent to correcting for non-Faradaic currents (such as those due
to charging/discharging of the electrical double layer capacitance). 
The result is a set of ORR polarization curves, such as those shown
below.
 
 
 
 
            There are a few interesting concepts these curves illustrate
nicely.  Lower potentials (~0.05 V to 0.6 V) correspond to higher
overpotentials (since the ORR is a reduction reaction), and hence in
the range of lower potentials, pretty much every oxygen molecule that
comes in contact with the working electrode is reduced immediately. 
Since the fluid flux (and hence the oxygen flux) on the working
electrode increases with increasing rotation rate, it makes sense that
higher (in magnitude) currents are observed in this region at higher
angular frequencies.  The second limiting case, at higher potentials, is
significantly less interesting -- all of the polarization curves converge
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03/11/2020 Experimental - Determining Catalytic Activity - ORR Catalysis with Pt-based CSNPs
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to zero current when the potential is not sufficient to drive the
reaction.  In-between these two limiting regions is where all the magic
happens: in this region, the measured current has two contributing
components, the so-called diffusion and kinetic currents.  The latter
of these is what we care about: it's the additional current generated
due to catalysis of the reaction.  This is the current that defines the
activity of a catalyst.
 
            Catalytic activity, however, is typically not reported as a
current, but rather a current density.In the case of MA, the kinetic
current at a particular potential (standard is 0.9 VRHE) is simply
divided by the mass of catalyst used.  In the case of SA, the kinetic
current is divided by the real surface area of the catalyst.  This,
however, is not just the area of the disk to which the catalyst is
attached -- nanoparticulate catalysts are rough, and hence their real
surface areas are significantly higher than their geomoetric surface
areas.  A rough estimate of real surface area can be obtained by
performing a CO-stripping experiment, in which carbon monoxide is
allowed to adsorb to the surface to form a monolayer (~7 minutes),
the system is purged with argon to remove all of the carbon monoxide
except what is adsorbed to the catalyst (~25 minutes), and then a
series of at least two CVs is run.  A sample CO-stripping curve is
provided below -- the red line corresponds to the first run (the huge
peak is caused by the oxidation of CO to CO2, which promptly diffuses
away from the electrode), and the blue line corresponds to the second
run (equivalent to the argon background from before).  The area of
the CO-stripping peak is essentially just the charge transfer due to the
oxidation events, and this charge can be related to the real surface
area of the catalyst.4
 
 
 
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