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Transport is amongst the most difficult sectors to decarbonise and carbon capture and storage (CCS) presents a promising solution. However, its application in vehicles is limited by the size and weight of the capture system. The hollow fibre adsorption unit (HFAU) is more compact than mature carbon capture technologies, providing a feasible route to on-board CCS. This work studies the optimization of sorbent-loading inside an HFAU via the synthesis of five HFAUs with increasing sorbent-loading per unit. The HFAUs were formed by impregnating α-Al2O3 hollow fibre supports with CaO. The performance of the HFAUs was tested under five CO2 adsorption/desorption cycles (i.e. adsorption at 650°C, 1 atm; desorption at 900°C, 1 atm). A feed gas with CO2 concentrations mimicking vehicle exhaust streams (14 vol% CO2 balanced in air) was used for adsorption. The HFAU subjected to three impregnations (i.e. HFAU3) achieved the highest total capacity and breakthrough capacity of 5% CO2 per unit length, demonstrating that the CaO-loading can be optimised to maximise performance whilst reducing sorbent requirements, unit size and costs. The performance of HFAU3 was compared against a packed bed adsorption unit (PBAU) using an equivalent mass of CaO tested under the same reaction conditions. HFAU3 achieved a breakthrough capacity of 5% CO2 per unit volume that was 3.3 times greater than that of the PBAU due to HFAU3 achieving an intensified contact between the CaO and CO2. This work provides critical information to advance the design of future HFAUs and validates their potential for on-board CCS.