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.