Indirect ocean capture of atmospheric CO 2 : Part I. Prototype of a negative emissions technology

•Indirect ocean capture, a negative emissions technology, is experimentally tested.•The electrochemical process removes CO2 from air by shifting the pH of seawater.•Acid/base for the process is made via bipolar membrane electrodialysis (BPMED).•Electricity consumed by BPMED dominates the energy cost of the process.•Efficient BPMED at high acid/base concentration would greatly reduce energy cost.