Ultrathin titanium flow distributors made via laser micromachining achieve record fuel cell power densities, surpassing European aerospace targets set for the end of this decade.
(Nanowerk Spotlight) In 2014, Toyota launched the Mirai, the world’s first mass-produced hydrogen fuel cell vehicle. The car represented a genuine engineering achievement, yet it also exposed a stubborn limitation: the fuel cell stack, which converts hydrogen and oxygen into electricity with only water as exhaust, remained heavy and bulky relative to its power output.
Commercial fuel cell systems today deliver around 1 W/cm² of active area and about 5 kW/L of volumetric power density. Aviation applications, where weight penalties are especially severe, demand nearly double the current gravimetric power density of 4-5 kW/kg.
The bottleneck lies in a deceptively simple component: the bipolar plate. These plates distribute reactant gases across the fuel cell, collect electrical current, and manage heat and water. They account for roughly 80% of a fuel cell stack’s mass and 60% of its volume.
Engineers have traditionally machined serpentine or parallel channel patterns into graphite or metal sheets, but this approach limits how efficiently gases reach the catalyst layer where electrochemical reactions occur. Regions between channels become starved of reactant, creating dead zones that drag down performance.
Metal foams emerged as a promising alternative. Their highly porous, interconnected structures offer superior gas distribution and thermal conductivity. Nickel foams gained particular attention, and recent work combining nickel foam with ultrathin carbon nanofiber layers demonstrated significant volumetric power density gains. However, nickel’s density of 8.9 g/cm³ makes it poorly suited for aircraft. Titanium, at 4.5 g/cm³ and highly corrosion-resistant, offers an attractive substitute. The problem is that titanium foams with the tailored architectures needed for fuel cells do not exist commercially.
Overview of different approaches explored in this study for designing and fabricating flow-field/bipolar plates for PEFCs: (a) standard PEFC schematic, (b) conventional bipolar plate with serpentine flow-field, (c) nickel foam flow-field, (d) LPBF-fabricated titanium lattices flow distributor, and (e) laser-patterned titanium flow distributor. Various geometries and patterns were explored to enhance reactant distribution, water management, and overall PEFC performance. (Image: Reproduced from DOI:10.1002/aenm.202504454, CC BY) (click on image to enlarge)
This digital-first approach allowed rapid iteration and refinement without costly physical prototyping. Their best-performing design achieved a peak power density of 1.62 W/cm² and projected stack-level metrics exceeding 10 kW/L and 9 kW/kg. These figures surpass the European Union’s 2030 targets and outperform current commercial systems.
The team built a three-dimensional computational model comparing conventional serpentine flow fields against porous alternatives. Serpentine designs produced highly non-uniform oxygen distribution across the catalyst layer. Porous distributors doubled oxygen concentration at the flow field mid-plane and tripled it at the interface between the catalyst and gas diffusion layer, the porous membrane sandwiched between the flow plate and catalyst where gases diffuse toward the reaction site. Average current density rose by 40%.
Further optimization explored partitioned and graded porosity configurations. Adding two partition walls to guide gas flow pushed current density uniformity from 89% to 93%. A graded design, varying porosity across sixteen zones to redirect flow toward underserved corners, reached 95% uniformity.
The researchers validated their simulations experimentally, beginning with graphene-coated nickel foam. Compressed to 0.25 mm thickness, this material delivered a peak power density of 1.52 W/cm² at 80 °C and 50% relative humidity, roughly 50% better than the serpentine baseline. The graphene coating reduced interfacial electrical resistance between foam and gas diffusion layer, addressing a key weakness of bare metal foams.
For weight-sensitive applications, the researchers designed titanium lattice structures mimicking foam geometry, then fabricated them via laser powder bed fusion. This additive manufacturing technique builds parts layer by layer from metal powder using a focused laser beam. The team tested four architectures: Kelvin cells, Diamond, Face-Centered Cubic, and Gyroid.
Initial prints deviated substantially from specifications, with samples weighing 66% more than designed. Standard printing parameters deposited excessive energy into the fine lattice features. The team refined laser power, scan speed, and beam compensation to tailor the process to miniature geometry, achieving mass deviations of just 10-19%.
The Gyroid structure delivered the highest absolute performance at 1.36 W/cm². However, the Diamond lattice weighed only 290 mg compared to the Gyroid’s 638 mg. When normalized by mass, the Diamond offered superior specific power, making it more practical for aerospace use. Durability testing showed only 6% reduction in maximum power density after 900 accelerated stress cycles.
While additive manufacturing showed promise, the team achieved their strongest results through laser micromachining. This technique used an ultrafast pulsed laser with 310 fs pulse duration to selectively ablate material from 200 μm thick titanium foils, leaving behind micropillar structures standing just 150 μm tall. The precision enabled graded patterns that the computational models predicted would optimize flow distribution.
The researchers tested three pillar sizes while maintaining identical contact area with the gas diffusion layer. Pillars measuring 250 μm struck the best balance between reactant distribution and water drainage, achieving 1.28 W/cm². Smaller 50 μm pillars suffered from water accumulation, while larger 1000 μm pillars disrupted optimal gas transport.
Building on their digitally optimized designs, the team fabricated a non-homogeneous distributor with spatially varying pillar spacing. This design achieved 1.62 W/cm² and a limiting current density, the maximum current the cell can sustain before performance collapses, of 4580 mA/cm² under fully humidified conditions at 80 °C.
Several factors drove these gains. Non-uniform pillar spacing directed reactants toward corners that conventional designs neglect. Wider gaps allowed pillars to press 80 μm deep into the gas diffusion layer during cell assembly, increasing contact area and shortening gas transport paths. Enhanced gas velocity also promoted evaporation of liquid water that would otherwise flood the cell.
The ultrathin 0.15 mm profile translated to projected stack-level volumetric power density exceeding 10 kW/L and gravimetric power density around 9 kW/kg. These figures exclude endplates, the heavy structural caps that seal each end of a fuel cell stack, meaning real-world system weight would be somewhat higher. Even so, both metrics surpass the Toyota Mirai and exceed EU/UK 2030 performance targets.
Porous configurations with smaller features performed markedly better at low humidity or intermediate temperatures around 100 °C, potentially simplifying fuel cell systems by reducing humidification equipment and radiators.
Of the three fabrication approaches examined, laser micromachining of titanium foils showed the greatest promise for aviation, enabling extremely thin structures with graded designs that substantially boosted performance. Scalability remains a challenge, though multi-beam systems or roll-to-roll techniques could address this limitation. Future work will focus on validating these designs in multi-cell stacks and exploring more economical materials including aluminum and stainless steel.
By demonstrating that digitally optimized porous architectures can exceed performance targets previously thought years away, this research establishes a credible pathway toward lighter, more powerful fuel cells suitable for aerospace applications.
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