Mode　switching　is　essential　in　a　unitized　regenerative　fuel　cell　system.　The　switching　of　electricity,　reactants,　and　reversible　electrochemical　reactions　occurs,　thereby　affecting　cell　performance.　During　model　switching,　insufficient　reactant　supply　may　cause　severe　concentration,　especially　along　the　width　of　the　catalyst　layer　which　is　not　currently　considered　in　a　low-dimensional　model.　To　fill　this　gap,　a　three-dimensional　simulation　in　a　proton　exchange　membrane　unitized　regenerative　fuel　cell　is　performed　and　validated　to　investigate　the　dynamic　responses　of　current　density　and　mass　transfer.　Results　certify　that　reductions　on　current　density　and　hydrogen　generation　are　severe,　as　the　cell　is　switched　to　a　high　electrolysis　voltage.　The　analysis　of　species　transfer　along　the　width　of　the　porous　layers　indicates　that　the　deterioration　is　attributed　to　the　occurrence　of　serious　concentration　polarization　which　is　caused　by　the　severe　local　water　starvation　on　the　catalyst　layer,　especially　below　the　rib.　A　high　inlet　velocity　for　reactant　and　low　fuel　cell　voltage　are　conducive　for　remitting　local　water　starvation　to　improve　cell　performance　effectively.　However,　a　slight　deterioration　cannot　be　avoided　completely　before　the　water　supply　arrives,　which　is　limited　by　the　high　rates　of　electrochemical　reactions　at　a　high　electrolysis　voltage.