Hydrogen-water　are　the　primary　reactant-product　pair　in　the　fuel　electrode　of　reversible　solid　oxide　cells.　A　different　dependence　of　polarization　resistance　on　pH2O　in　fuel　cell　and　electrolysis　modes　has　been　proven　experimentally,　revealing　different　rate-limiting　steps　in　fuel　electrodes　in　these　two　modes.　Despite　extensive　studies　on　solid　oxide　fuel　cells　or　solid　oxide　electrolysis　cells,　existing　literature　is　still　hard　to　interpret　this　phenomenon.　To　understand　the　reaction　mechanism　of　reversible　solid　oxide　cells　in　H2/H2O　atmosphere　during　current　direction　switch,　we　develop　an　elementary　reaction　mechanistic　model　of　a　nickel-patterned　electrode　button　cell　coupling　charge　transfer　reactions,　surface　heterogeneous　chemical　reactions,　and　surface　diffusion.　We　use　a　two-step　hydrogen　spillover　mechanism,　i.e.,　H(Ni)+O2−(YSZ)　↔　(Ni)+OH−(YSZ)　+e−　and　H(Ni)+OH−(YSZ)　↔　(Ni)+H2O(YSZ)+e−,　to　describe　charge　transfer　reactions.　This　model　can　well　interpret　this　phenomenon,　and　further,　reveal　the　inherent　relationship　between　the　operating　condition　and　cell　performance.　Model　calculation　reveals　that　the　limitation　of　OH−(YSZ)　surface　coverage　(<1%)　results　that　the　charge　transfer　reaction　generating　OH−(YSZ)　dominates　electrochemical　reaction　rate　no　matter　which　mode　a　cell　operates.　To　enhance　the　electrochemical　performance　of　fuel　electrode,　it　is　the　key　to　understand　how　to　enhance　the　surface　concentration　of　OH−(YSZ).　©　2019　Elsevier　Ltd