When examining reduction of BTDN under low CO2 concentration (0.8% CO2, 2 bar; 4.32 mM), we note that there is a gradual recovery of free CO2 during the reoxidation phase of IR-SEC experiments (see Figure ). The release of CO2 from the reduced BTDN-CO2 species is examined in more detail by potential step SEC experiments (Figure ) where BTDN is reduced at -2.1 V vs Fc 0/+ and subsequently reoxidized at -0.8 V Fc 0/+ for two cycles. The relative band intensity at 2342 cm -1 shows almost reversible uptake and release of free CO2 for two cycles. We note the very slow kinetics for release of CO2 (k ≈ 6 × 10 -3 s -1 ). This is consistent with the observation from scanning SEC experiment (Figure ) where there is no distinct reoxidation peak that can be attributed to oxidation of a reduced BTDN-CO2 species, suggestive of a slow oxidation kinetics. At present, it is not possible to distinguish between oxidation kinetic or CO2 release kinetic as the rate limiting step.