A method for treatment of seawater comprising: feeding water through an anode compartment of an electrochemical acidification cell, via an anode fluid inlet and an anode fluid outlet, to generate hydrogen ions; feeding water through a cathode compartment of the electrochemical acidification cell via a cathode fluid inlet and a cathode fluid outlet; electrically generating dihydrogen gas in the cathode compartment from water; flowing seawater through a central compartment of the electrochemical acidification cell, via a seawater inlet and a seawater outlet, at a selected flow rate, wherein the central compartment is between the anode and cathode compartments and separated from the anode and cathode compartments by cation exchange membranes, the cation exchange membrane separating the central compartment from the cathode compartment being permeable to calcium, magnesium and sodium cations in the seawater; subjecting the seawater in the central compartment to an ion exchange substitution reaction to exchange the hydrogen ions, produced from water in the anode compartment, for sodium ions in the seawater, wherein the sodium ions transfer from the central compartment to the cathode compartment; lowering a pH of the seawater in the central compartment to less than 6.5 using an electrically driven membrane process; and forming carbonic acid in the central compartment and decomposing the carbonic acid to carbon dioxide gas, wherein the dihydrogen gas generated in the cathode compartment and carbon dioxide formed in the central compartment are simultaneously extracted in a molar ratio that is increased by increasing the applied current. feeding water through an anode compartment of an electrochemical acidification cell, via an anode fluid inlet and an anode fluid outlet, to generate hydrogen ions; feeding water through a cathode compartment of the electrochemical acidification cell via a cathode fluid inlet and a cathode fluid outlet; electrically generating dihydrogen gas in the cathode compartment from water; flowing seawater through a central compartment of the electrochemical acidification cell, via a seawater inlet and a seawater outlet, at a selected flow rate, wherein the central compartment is between the anode and cathode compartments and separated from the anode and cathode compartments by cation exchange membranes, the cation exchange membrane separating the central compartment from the cathode compartment being permeable to calcium, magnesium and sodium cations in the seawater; subjecting the seawater in the central compartment to an ion exchange substitution reaction to exchange the hydrogen ions, produced from water in the anode compartment, for sodium ions in the seawater, wherein the sodium ions transfer from the central compartment to the cathode compartment; lowering a pH of the seawater in the central compartment to less than 6.5 using an electrically driven membrane process; and forming carbonic acid in the central compartment and decomposing the carbonic acid to carbon dioxide gas, wherein the dihydrogen gas generated in the cathode compartment and carbon dioxide formed in the central compartment are simultaneously extracted in a molar ratio that is increased by increasing the applied current.