Prediction of CO2 adsorption of biochar under KOH activation via machine learning

• Machine learning predicts the CO 2 adsorption capacity of KOH-activated biochar. • Multi-layer perceptron shows best prediction accuracy with a test R 2 of 0.961. • The impact of one or two factors on output features is explored via SHAP values. • Micropores and N-containing groups are crucial for CO 2 adsorption at low pressure. • Narrower or more micropores greatly promote CO 2 adsorption at low temperatures. To effectively increase the CO 2 adsorption capacity of biochar, the activation process is often an indispensable link. However, the introduction of an activation process poses challenges in clarifying the relationship between the characterization parameters of biochar, adsorption parameters, and CO 2 adsorption capacity. Herein, a comprehensive dataset encompassing the CO 2 adsorption data of KOH-activated biochar using a “two-sep method” was compiled. Subsequently, ridge regression, multi-layer perceptron, and random forest models were employed to predict its CO 2 adsorption performance. To comprehensively explore the effects of activation conditions, physicochemical properties and adsorption parameters on CO 2 adsorption capacities, partial dependence via Shapley additive explanation (SHAP) values analysis was conducted. The results demonstrate that the multilayer perceptron model exhibits the highest prediction accuracy with a test R 2 value of 0.961. Additionally, it was found that the CO 2 adsorption capacity of activated biochar is primarily determined by micropores and nitrogen-containing groups rather than total pore volume at low adsorption pressure (< 0.3 bar). Moreover, it increases significantly with decreasing average pore size, increasing pore volume, and increasing nitrogen content at low adsorption temperatures (< 20 °C). When the ratio of KOH to biochar is in the range of 1–2 and the activation temperature is ∼ 700 °C, activated biochar with high CO 2 adsorption performance can be obtained. This study may provide valuable insights for the application of activated biochar in CO 2 adsorption.