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Combining the self‐sacrifice of a highly crystalline substance to design a multistep chain reaction towards ultrathin active‐layer construction for high‐performance water splitting with atmospheric‐temperature conditions and an environmentally benign aqueous environment is extremely intriguing and full of challenges. Here, taking cobalt carbonate hydroxides (CCHs) as the initial crystalline material, we choose the Lewis acid metal salt of Fe(NO3)3 to induce an aqueous‐phase chain reaction generating free CO32− ions with subsequent instant FeCO3 hydrolysis. The resultant ultrathin (∼5 nm) amorphous Fe‐based hydroxide layer on CCH results in considerable activity in catalyzing the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), yielding 10/50 mA ⋅ cm−2 at overpotentials of 230/266.5 mV for OER and 72.5/197.5 mV for HER. The catalysts can operate constantly in 1.0 M KOH over 48 and 45 h for the OER and HER, respectively. For bifunctional catalysis for alkaline electrolyzer assembly, a cell voltage as low as 1.53 V was necessary to yield 10 mA cm−2 (1.7 V at 50 mA cm−2). This work rationally builds high‐efficiency electrochemical bifunctional water‐splitting catalysts and offers a trial in establishing a controllable nanolevel ultrathin lattice disorder layer through an atmospheric‐temperature chemical route. [ABSTRACT FROM AUTHOR]