Electrocatalytic　oxidation　as　a　promising　route　to　produce　value-added　products　from　biomass-derived　organics　has　received　increasing　attention　in　recent　years.　However,　the　efficient　conversion　of　concentrated　feedstock　solutions　with　high　selectivity　and　Faradaic　efficiency　(FE)　remains　challenging.　Herein,　we　report　a　cation-defective　Ni-based　electrocatalyst　derived　from　the　surface　reconstruction　of　the　NiCo　Prussian　blue　analogue　(NiCo　PBA)　in　alkaline　media　for　the　efficient　oxidation　of　biomass-derived　organics　in　a　high　concentration　solution.　Taking　5-hydroxymethylfurfural　(HMF)　as　an　example,　the　NiCo　PBA　can　deliver　a　satisfactory　catalytic　performance　in　terms　of　high　HMF　conversion　(97%),　selectivity　to　2,5-furandicarboxylic　acid　(98%),　and　FE　(100%),　even　at　a　concentration　as　high　as　100　mM.　Theoretical　calculations　suggest　that　the　cation　defects　not　only　promote　the　fast　conversion　of　Ni(OH)2　to　electrochemically　active　NiOOH　under　anodic　potential　but　also　enhance　the　adsorption　of　HMF　onto　the　active　sites　and　accelerate　the　spontaneous　chemical　oxidation.　This　study　provides　deep　insights　into　the　structural　evolution　of　PBA-based　catalysts　and　reveals　the　pivotal　factor　that　affects　the　performance　of　electrocatalytic　oxidation,　paving　the　way　to　further　develop　advanced　electrocatalysts　for　efficient　oxidation　reactions　with　a　high　concentration.　©　2024　American　Chemical　Society.