Structural design of microfluidic fuel cells: A review

Microfluidic fuel cells (MFCs) exploit microscale laminar flow to control reactant delivery and, in many membraneless configurations, maintain fuel-oxidant separation without a conventional ion-exchange membrane. Their compact, membrane-free architecture makes them attractive for portable and integrated power sources, yet performance remains highly sensitive to channel geometry, electrode arrangement, flow distribution, and interfacial transport. Existing reviews have largely addressed device categories or individual optimization strategies in isolation, leaving the links between structural design, transport phenomena, and engineering trade-offs insufficiently resolved. This review reassesses co-flow, counter-flow, single-flow, triple-flow, vapor-feed, and passive MFCs within a unified structure-transport-performance framework, focusing on depletion boundary-layer development, reactant crossover, mixed-potential losses, bubble-induced blockage, hydrodynamic resistance, and current-density nonuniformity. Channel and electrode designs are compared according to their ability to replenish reactants, stabilize the liquid-liquid interface, promote bubble discharge, and alleviate concentration polarization, along with associated penalties such as pressure drop, reduced fuel utilization, and fabrication complexity. Stack architectures are further evaluated in terms of flow maldistribution, shunt currents, and cell-to-cell variation. Beyond cell-level performance, application scenarios, manufacturing reproducibility, durability, cost, and commercialization barriers are discussed. Future progress will rely on validated multiphase modeling, additive manufacturing, physics-informed AI, and standardized testing protocols. Structural optimization should therefore be treated as a multi-objective design problem rather than a pursuit of maximum power density alone.