Surface area determination of porous materials: the BET approach, its limitations, and alternatives.

Surface area is a key property in the characterization of porous materials, and it strongly influences their performance in a variety of applications such as gas storage and separation. To quantify such a textural property, the Brunauer-Emmett-Teller (BET) method has been the most widely used approach, based on gaseous adsorption isotherms of non-reactive gases such as N2 at 77 K and Ar at 87 K. Despite its widespread use, the physical validity and reliability of the BET method for porous materials remain subjects of active debate. These include the selection of the linear region used to compute the BET surface area, violations of its fundamental assumptions (e.g., pore filling in porous materials), and sensitivity to the choice of adsorbates as probe molecules (e.g., N2vs. Ar). By integrating insights from both experimental and computational developments over the past decades, this review comprehensively evaluates the applicability and limitations of the BET method across diverse porous materials. Specifically, current understanding of surface-area determination in porous materials is summarized, including how pore structure, adsorption mechanisms, adsorbate choice, and analysis protocols influence the BET-derived surface area, thereby offering best practices for the BET approach. In addition, this review synthesizes insights into conditions under which the BET analysis may become fundamentally unreliable. A variety of alternative approaches that have been developed to enable more consistent, comparable, and physically meaningful surface-area characterizations are also highlighted. Finally, this review outlines key future directions toward achieving more accurate surface-area characterizations.