There are several possible factors that can affect the overpotential of your nitrogen-doped carbon catalyst for HER. Some of them are:

• The degree of nitrogen doping and the type of nitrogen species. Different nitrogen species, such as pyridinic-N, pyrrolic-N, graphitic-N, and oxidized-N, have different effects on the electronic structure and catalytic activity of carbon materials1. Generally, graphitic-N is considered to be the most active site for HER, while pyridinic-N and pyrrolic-N can act as electron donors and improve the conductivity of carbon materials2. The degree of nitrogen doping can also influence the surface area, porosity, and defect density of carbon materials, which can affect the mass transfer and charge transfer processes during HER3.
• The morphology and structure of the carbon material. The morphology and structure of the carbon material can determine the exposure and accessibility of the active sites, as well as the stability and durability of the catalyst. For example, carbon nanotubes, graphene, and carbon fibers have different dimensions, shapes, and edge effects, which can influence the HER performance4. Moreover, the presence of defects, such as vacancies, edges, and heteroatoms, can modulate the electronic and chemical properties of carbon materials and create more active sites for HER5.
• The interaction between the carbon material and the nickel foam substrate. The interaction between the carbon material and the nickel foam substrate can affect the electrical contact and the charge transfer resistance between them. A good contact can enhance the electron transfer and reduce the ohmic loss, while a poor contact can increase the overpotential and lower the current density6. The interaction can also influence the stability and corrosion resistance of the catalyst, especially in acidic media7.

Therefore, to reduce the overpotential of your nitrogen-doped carbon catalyst, you may need to optimize the above factors, such as the nitrogen doping method, the carbon precursor, the carbonization temperature, the loading amount, and the deposition technique. You may also compare your catalyst with some reported nitrogen-doped carbon catalysts for HER, such as those in , , and , and analyze the differences and similarities. I hope this helps. 😊

The BET (Brunauer, Emmett and Teller) theory is commonly used to evaluate the gas adsorption data and generate a specific surface area result expressed in units of area per mass of sample (m2/g). The amount of gas adsorbed depends on the exposed surface are but also on the temperature, gas pressure and strength of interaction between the gas and solid. In BET surface area analysis, nitrogen is usually used because of its availability in high purity and its strong interaction with most solids.Brunauer-Emmett-Teller (BET) analysis is a physical characterization technique that provides quantitative data on the specific surface area and porosity distribution of solid materials. As the BET principle employs gas adsorption data it is equally applicable to porous and non-porous materials regardless of particle size and shape.

https://onlinelibrary.wiley.com/doi/10.1002/smtd.201800173

https://www.iitk.ac.in/che/PG_research_lab/pdf/resources/BET-TPX-Chemi-reading-material.pdf

https://www.nature.com/articles/s41598-020-59994-1

https://www.sciencedirect.com/science/article/abs/pii/S1309104218301661

N2 adsorption and BET will give you the surface area of the adsorbent, but tell you nothing about the composition and amount of gases adsorbed in the sample from flue gases. What has been adsorbed can be found by thermal desorption (TPD) and taking the evolved gases into a mass-spec for identification. MS signals are not the easiest to quantify, so weight loss is often used to do the quantification and match up with, for example, gravimetric TPD, i.e. TGA