Pore hierarchy in electrode materials has been verified by many literatures that is capable of greatly facilitating the mass transportation between internal active surface and bulk solution, which is critical for capacitive applications such as supercapacitors and capacitive deionization. Herein, by applying in situ foamed Mg chelates as precursors, we managed the scalable fabrication of hierarchically porous carbon (HPC) materials. Particularly, citric acid first reacted with magnesium nitrate to form Mg chelate while the generated gaseous HNO3 molecules bubbled the intermediate carbon framework to produce abundant open pores. The as-made precursors were then submitted to potassium hydroxide activation for high carbonization degree and rich meso-/micropores. Electrochemical measurements revealed that the optimized sample (HPC-2) exhibited very high specific capacitance of 213.5 F g-1 (at 1.0 A g-1) in 1.0 mol L-1 neutral NaCl solution and high rate capability of ~67.5 % at 10.0 A g-1. Furthermore, it showed impressive capacitive deionization performance regarding high removal efficiency (67.1%), large capacity (1865.7 mg g-1 in 2200 mg L-1 NaCl solution), and robust cycling stability.