By precisely tuning the ratio of phosphoric acid used during pre-activation, the researchers identified an optimal mesopore proportion—around 40%—that allows CO₂ molecules to diffuse rapidly into abundant micropores, maximizing storage capacity. The optimized material, designated PKBC-3, reaches a static adsorption capacity of 3.43 mmol/g and a dynamic capacity of 3.02 mmol/g, advancing the design of next-generation biomass-derived adsorbents.
Rising atmospheric CO₂ levels—now 52% higher than pre-industrial concentrations—underscore the urgent need for cost-effective carbon capture technologies. While chemical amine scrubbing is widely used, it suffers from high energy consumption and corrosion issues. Solid adsorbents such as activated carbon and biochar have emerged as promising alternatives due to their tunable porosity, stability, and low manufacturing cost. However, traditional pyrolysis methods used to produce biochar often yield limited pore development and slow mass-transfer rates. Microwave-assisted pyrolysis has shown potential for dramatically improving pore formation, but achieving the optimal balance between micropores (capacity) and mesopores (kinetics) remains an unresolved challenge. Addressing these limitations motivated the development of a refined pore-engineering strategy.
A study (DOI: 10.48130/scm-0025-0004) published in Sustainable Carbon Materials on 27 October 2025 by Yaning Zhang’s team, Harbin Institute of Technology, presents a scalable path for converting agricultural residues into high-performance CO₂ adsorbents, offering dual environmental benefits: carbon capture and waste valorization.
Using a combination of nitrogen adsorption at 77 K, pore size distribution analysis, fixed-bed breakthrough experiments, and kinetic/adsorption isotherm modeling, the study systematically examined how the H₃PO₄ impregnation ratio (0:1–4:1) regulates the pore architecture and CO₂ adsorption performance of corn straw–based biochar. Nitrogen adsorption revealed that increasing the H₃PO₄/biomass ratio from 0:1 (KBC) to 3:1 (PKBC-3) first optimizes and then degrades the structure: the specific surface area rose from 1,340 to about 3,040 m²/g, total pore volume from 0.6 to 1.9 cm³/g, and micropore volume from 0.5 to 1.1 cm³/g, with mixed Type I/IV isotherms indicating coexisting micro–mesoporosity. Pore size distributions showed that moderate H₃PO₄ levels (2:1–3:1) generate a hierarchical system of 0.5–1.0 nm micropores embedded in 2–4 nm mesopores, whereas over-activation at 4:1 causes wall thinning, pore collapse, and reduced effective surface area. Mesopore proportion analysis identified a dual-threshold behavior: above ~30% mesopores, hierarchical pores begin to form; between 2:1 and 3:1, mesopores stabilize near 40% while micropore volume remains ≥1.0 cm³/g; at 4:1, mesopores drop and micropores shrink by 52%. CO₂ isotherms fitted by Langmuir and Freundlich models showed that adsorption capacity (qm up to 3.43 mmol/g for PKBC-3) is governed by micropore volume, while excessive mesopores (>42%) weaken affinity and cause capacity loss. Breakthrough and kinetic analyses further demonstrated that ~40% mesopores minimize diffusion barriers, giving PKBC-3 the highest dynamic capacity (3.02 mmol/g), longest breakthrough time, lowest activation energy (8.29 kJ/mol), and fastest adsorption rate, whereas too few (<30%) or too many (>50%) mesopores shift the bottleneck to mass transfer or site scarcity, respectively.
The optimized hierarchical pore system enhances both capacity and kinetics—key requirements for post-combustion capture of flue gas containing 12–15% CO₂. With rapid diffusion channels and abundant binding sites, the PKBC-3 biochar demonstrates strong potential for fixed-bed capture units, industrial gas purification, and modular carbon-capture systems. The study also introduces a quantitative design principle: maintaining mesopores at ~40% to balance adsorbate diffusion and micropore filling. This framework can guide the development of tailored adsorbents for greenhouse gas mitigation, heavy-metal removal, and VOC adsorption.
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References
DOI
10.48130/scm-0025-0004
Original Source URL
https://doi.org/10.48130/scm-0025-0004
Funding information
This work was supported by the National Natural Science Foundation of China (Grant No. 52476005), and Heilongjiang Provincial Key R&D Program 'Unveiling the Leader' Project (Grant No. 2023ZXJ02C04).
About Sustainable Carbon Materials
Sustainable Carbon Materials is a multidisciplinary platform for communicating advances in fundamental and applied research on carbon-based materials. It is dedicated to serving as an innovative, efficient and professional platform for researchers in the field of carbon materials around the world to deliver findings from this rapidly expanding field of science. It is a peer-reviewed, open-access journal that publishes review, original research, invited review, rapid report, perspective, commentary and correspondence papers.