Reversible　solid　oxide　cells　(RSOC)　have　attracted　much　attention　due　to　their　unrivaled　efficiency.　As　one　of　the　most　promising　energy　storage　and　conversion　technologies,　the　RSOC　will　have　to　switch　frequently　between　fuel　cell　and　electrolysis　mode　to　follow　the　nature　of　intermittent　renewable　energy　sources　and　meet　time-varying　electricity　demand.　In　this　paper,　a　3D　dynamic　model　of　a　single　repeating　unit　was　developed　and　validated.　Based　on　this　model,　the　interaction　between　the　transient　external　electrical　performance　with　the　temporal-spatial　evolution　of　the　reactant　and　product　concentrations　was　investigated　during　the　RSOC　switching　from　SOFC　to　SOEC.　Once　the　mode　switching　occurs,　the　power　density　decreases　to　a　lower　value　and　then　asymptotically　recovers　to　a　new　quasi-steady　state.　The　recovery　time　of　the　concentration　field　within　porous　electrodes　determines　the　total　transient　time.　In　addition,　detailed　parametric　studies　for　the　key　design　and　operational　parameters　are　conducted　to　find　out　their　impact　on　the　power　density　overshoot　(PDO)　and　relaxation　time　(τ0).　The　results　suggest　that　shortening　the　cell　length　can　effectively　reduce　PDO　and　τ0.　With　the　hydrogen　molar　fraction　of　the　incoming　fuel　gas　(xH2)　equal　to　0.2　and　0.8,　respectively,　the　maximum　values　of　PDO　and　τ0　are　attained.　Compared　with　the　maximum　value,　when　adopting　0.4　≤　xH2　≤　0.6,　the　PDO　and　τ0　can　be　reduced　by　40　%　and　25　%,　respectively.　Extending　the　mode　switching　time　can　effectively　reduce　PDO　and　τ0.　The　power　density　drops　directly　into　the　quasi-steady　state　without　oscillations　when　the　switching　time　is　more　than　0.3　s,　and　the　τ0　is　slightly　shorter　than　the　mode　switching　time.　©　2022　Elsevier　Ltd