Fig.1 Characterization results of physical property parameters of
electrode material. (a)-(c) are the characterization results of
BET :(a) N2 isothermal adsorption and
desorption curves of electrode materials composed of different
formulations;(b) pore volume distribution of electrode
materials composed of different formulations;(c) pore volume
distribution of electrode materials composed of different
formulations.(d)-(f) is the SEM image of the electrode material
with the formed composition of Gr0.5CB0.5/PTFE0.5.
In order to determine the pore
structures and specific surface area of electrode materials with the
range of 0-200nm, N2 adsorption measurements are
performed. The N2 isothermal adsorption and desorption
curves of different carbon-based catalytic materials determined by BET
are shown in Fig.1a, and specific specific surface area and pore volume
data are shown in Table.S1. Fig.1a shows that the geometric morphologies
of graphite (Gr) and carbon black (CB) are greatly different. The pore
structure is almost absent in graphite, the specific surface area is
only 4.38m2•g−1, and the pore volume
is close to 0. After adding PTFE and pore-forming agent, the specific
surface area increased to
18.67m2•g−1 and the pore volume
increased to 0.08cm3•g−1, indicating
the formation of some type of secondary pore structure.
From Fig.1a、Table.S1, it can be also found that the BET surface area of
CB is 206m2•g−1 . The porous carbon
sample exhibit a typical type I pattern for the
N2 adsorption, which represents a sharp increase at low
relative pressure (P/P0<0.1) due to the
formation of microporous structure. Moreover, the
N2 adsorption–desorption isotherm curves for CB also
show a typical type IV isotherm curve with an obvious hysteresis,
implying the presence of mesoporous structure with a range of pore size
from 20-80nm (Fig.1b). Furthermore, even as our BET results show that
the specific surface area of CB/PTFE0.5 sample is obvious smaller than
that of CB (Table.S1). But from (Fig.1b and 1c) we can see that the pore
volume of per gram CB/PTFE0.5 sample is almost equal to that of CB owing
to the formation of much more mesopores. If normalized to 1g CB
material, the CB0.5Gr0.5/PTFE0.5 sample also possess more mesopores
volume than CB alone when PTFE and pore former are added to CB.
From Fig.1d-1f, we can know that after PTFE and pore-forming agent are
properly added, the photo of surface morphology of different electrode
materials under the scanning electron microscope. The brighter particles
seen in Fig.1f are particles bonded together with carbon black, whose
size is about 30-50nm.There are also some individual smooth dark gray
polymer phases, ranging in size from 0.1 to 1μm. Fig.1d shows the
crystalline flake structure of the graphite, which is added to improve
the mechanical strength of the electrode. In addition, many black pore
structures (0.5-5μm) can be seen in Fig.1d to Fig.1g photos. These
structures have different heights and heights in the field of electron
microscopy, and even cause difficulty in photo focusing, which reflects
that the surface roughness increases after emulsion and pore-forming
agent are added. In summary, SEM images show that the carbon black,
graphite and polymer phases not only form relatively uniform dispersion
systems, but also form many micron-scale pore structures ranging from
0.5 to 5μm.
In brief, based on the characterization results of BET and SEM, we
believe that the addition of PTFE not only acts as a bonding molding
agent, but also acts together with pore-forming agent. It changes the
original pore size distribution of carbon materials, and increases the
number of 20-80nm pores. What’s more, it forms a large number of micron
pores, and combines with CB’s unique nano-scale pore structure to form a
nano-micron multi-stage pore structure. Later performance tests show
that this unique hierarchical pore structures form an efficient gas
transport and dispersion network system. Meanwhile, the dispersed
polymer phase improves the affinity of the catalyst surface to oxygen,
providing a suitable place for the rapid diffusion and migration of
reactants and products.
3.2 Surface defects and electrochemical active surface
area