Nanoparticle organic hybrid materials (NOHMs) have been proposed as excellent electrolytes for combined CO ₂ capture and electrochemical conversion due to their conductive nature and chemical tunability. However, CO ₂ capture behavior and transport properties of these electrolytes after CO ₂ capture have not yet been studied. Here, we use a variety of nuclear magnetic resonance (NMR) techniques to explore the carbon speciation and transport properties of branched polyethylenimine (PEI) and PEI-grafted silica nanoparticles (denoted as NOHM-I-PEI) after CO ₂ capture. Quantitative ¹³ C NMR spectra collected at variable temperatures reveal that absorbed CO ₂ exists as carbamates (RHNCOO ⁻ or RR′NCOO ⁻ ) and carbonate/bicarbonate (CO ₃ ²⁻ /HCO ₃ ⁻ ). The transport properties of PEI and NOHM-I-PEI studied using ¹ H pulsed-field-gradient NMR, combined with molecular dynamics simulations, demonstrate that coulombic interactions between negatively and positively charged chains dominate in PEI, while the self-diffusion in NOHM-I-PEI is dominated by silica nanoparticles. These results provide strategies for selecting adsorbed forms of carbon for electrochemical reduction.