Implementing sustainable energy conversion and storage technologiesi sh ighly reliant on crucial oxygen electrocatalysis, such as the oxygen evolution reaction (OER) and oxygen reduction reaction(ORR).However,t he pursuit of low cost, energetic efficient and robust bifunctional catalysts for OER and ORR remains ag reat challenge. Herein, the novel Na-ion-deficient Na 2Àx CoP 2 O 7 catalysts are proposed to efficiently electrocatalyze OER and ORR in alkaline solution.T he engineering of Na-ion deficiency can tune the electronics tructure of Co, and thus tailor the intrinsically electrocatalytic performance. Among the sodium cobalt phosphate catalysts, the Na 1.95 CoP 2 O 7 (NCPO5) catalyst exhibitst he lowest DE (E J10,OER ÀE JÀ1,ORR )o fo nly 0.86 V, which favorably outperforms most of the reported non-noblem etal catalysts. Moreover,t he Na-ion deficiency can stabilize the phase structure andm orphology of NCPO5 during the OER and ORR processes.T his study highlights the Na-iond eficient Na 2Àx CoP 2 O 7 as ap romising class of low-cost,h ighly active and robust bifunctional catalysts for OER and ORR.[a] Dr.

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