Synergistic effects of water and carbon dioxide on the reversible thermoelectric behaviors of polyethyleneimine/single-walled carbon nanotubes composites

Polyethyleneimine (PEI) doped single-walled carbon nanotubes (SWCNTs) composites (PEI/SWCNTs) as n-type organic thermoelectric (TE) materials are important candidates for fabricating flexible TE generators. However, the mechanism of unstable TE behavior for PEI/SWCNTs in the air has been not revealed, which hinders the potential application. In this study, we have investigated the influence of CO2, H2O/N-2, H2O/CO2, and air on the composition of PEI and TE properties of PEI/SWCNTs. The change of chemical structure shows H2O and CO2 have a series of reactions with the amino of PEI to form ammonium salts, which causes the changes in the types of carriers. The Seebeck coefficient shows that the synergistic effect of H2O and CO2 promotes PEI/SWCNTs changing from n-type to p-type. However, the Seebeck coefficient in dry air shows O-2 almost has no influence on the carrier type of PEI/SWCNTs and the n-type PEI/SWCNTs film can be stable for three months in dry air. Noticeably, reversible transformation between n-type and p-type has been realized via regulating the H2O and CO2. The revealed mechanism offers a facile strategy to realize the reversible regulation of TE properties of PEI/SWCNTs and provides a direction for fabricating air-stable n-type PEI/SWCNTs with high TE performance.