High　carbon　catalyst　loadings　are　commonly　used　for　the　catalyst　layer　(CL)　in　air-cathodes　to　obtain　a　performance　comparable　with　that　using　platinum.　This　results　in　a　much　thicker　CL,　which　severely　limits　mass　transfer.　In　this　study,　we　developed　a　porosity-gradient　CL　to　passively　enhance　mass　transfer　in　the　air-cathode　of　microbial　fuel　cells　(MFCs)　for　the　first　time.　Computational　results　demonstrated　that　a　cathode　CL　with　increasing　porosity　(CL-IP)　and　decreasing　porosity　(CL-DP)　from　the　water　to　the　air-facing　side　exhibited　improved　transport　of　oxygen　and　OH-,　respectively,　alleviating　concentration　overpotentials　in　the　CL　Experimental　results　also　showed　that　an　MFC　that　included　a　cathode　with　CL-IP　achieved　a　maximum　power　density　of　1781　+/-　92　mW　m(-2),　which　was　higher　than　that　achieved　with　CL-DP　and　a　homogeneous　CL　(1614　+/-　72　and　1183　+/-　205　mW　m(-2)).　(C)　2019　Elsevier　B.V.　All　rights　reserved.