However, in most cases, stoichiometricamounts peroxides like H 2 O 2 and tertbutyl hydroperoxide (TBHP) were used as exogenous oxidizing reagents and oxygen sources, [2] resulting in massive waste and an increase in costs. [3] The strategy that water serves as the oxygen source to produce high-value organic chemicals has drawn more attention due to its low price and cleanliness, [4] yet its inertia under ambient temperatures leads to more energy consumption in practical working conditions. Electrochemical methods with waste-free electrons as redox agents provide a green and mild pathway to watermediated oxidation of organics, [5] where the balance between the oxidation of organics and the competitive oxygen evolution reaction (OER) is deemed as the main problem. [6] Inspired by the tandem catalysis tactic, turning this competition between the OER and the selective oxidation of styrene to benzaldehyde into cooperation should be feasible, but it has rarely been achieved.Single-atom catalysts (SACs) meet with great favor in electro catalysis for their superior activity, higher metal-utilization efficiency, and enhanced selectivity. [7] In the past decade, enormous encouraging SACs have been created and utilized to catalyze the oxygen reduction reaction, the OER, and other heterogeneous reactions. [8] However, the activity of SACs often suffers the limit for complex reactions with multiple intermediates, because the single site is not able to catalyze efficiently all the reactions in the whole system. [9] Although introducing extra active centers into SACs has the potential to break this limit, [10] spatiotemporal transfer of intermediates over catalytic sites and inherent synergetic mechanisms remain unexplored, thus leading to a low product yield and undesirable side reactions. Besides, insufficient cognition on the reconstruction of the catalytic sites in working conditions also increases the difficulty of their design. Therefore, constructing active sites with a specific function to catalyze complex reactions in a tandem system still confronts a great challenge.Herein, we fabricate a single-atom catalyst with Cr atoms atomically dispersed at CoSe 2 support (Cr 1 /CoSe 2 ), in which Co and Cr sites are endowed with a specific function to oxidize water and styrene respectively. Especially, the anchored Cr single atoms could not only serve as the active sites for the Electrocatalytic oxidation of organics using water as the oxygen source is a prospective but challenging method to produce high-value-added chemicals; especially, the competitive oxygen evolution reaction (OER) limits its efficiency. Herein, a tandem catalysis strategy based on a single-atom catalyst with Cr atoms atomically dispersed at a CoSe 2 support (Cr 1 /CoSe 2 ) is presented. Thereinto, Co and Cr sites are endowed with a specific function to activate water and styrene respectively, and the competition between the OER and styrene oxidation is turned into mutual benefits via cooperated active sites. Under a potential of 1.6 V Ag/AgCl , exce...