Direct electrochemical conversion of CO(_2) capture solutions (instead of gaseous CO(_2)) to valuable chemicals can circumvent the energy-intensive CO(_2) regeneration and pressurization steps, but the performance of such processes is limited by the sluggish release of CO(_2) and the use of energy-consuming bipolar membranes (BPMs). Herein, we discovered that an ammonium bicarbonate (NH(_4)HCO(_3))-fed electrolyzer outperforms the state-of-the-art KHCO(_3) electrolyzers largely because of its favorable thermal decomposition property, which allows for a 3-fold increase in the in situ CO(_2) concentration, a maximum 23% increase in formate faradaic efficiency, and a 35% reduction in cell voltage by substituting BPM with an anion exchange membrane (AEM). We then demonstrated an integrated process by combining NH(_4)HCO(_3) electrolysis with CO(_2) capture by on-site generated ammonia from the electroreduction of nitrate, which features a remarkable 99.8% utilization of of CO(_2) capturing agent. Such a multipurpose process offers a sustainable route for the simultaneous removal of N wastes and streamlined CO(_2) capturing and upgrading.