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Summary: Temperature uniformity (TU) of the cathode catalyst layer (CCL) and the power density are two key parameters to describe a fuel cell durability and performance. To achieve desired TU and output performance, a one‐dimensional, non‐isothermal, two‐phase model coupled with a CCL agglomerate submodel is developed and validated in terms of the polarization curve, ohmic impedance and total oxygen transport resistance. Then, the ideal ionomer‐gradient distribution across the triple‐layer CCL is attained by a combination of Non‐dominated Sorting Genetic Algorithm II (NSGA II) and TOPSIS. The results present that the gradient‐decline distribution of ionomer towards the microporous layer (MPL) contributes to the larger power density and effective utilization of ionomer. The fuel cell can also operate at higher power with a top voltage range (TVR). After optimization, the ideal ionomer‐gradient distribution affects the distribution of the total heat source by affecting the local oxygen transport resistance across the ionomer film and the volumetric current density (Jc). As a result, the maximum temperature point in CCL is more likely to occur near the MPL and the CCL temperature becomes more uniform. Besides, the optimum ionomer‐gradient values tend to be different in various operating voltages and RH cases. However, low voltage and high RH conditions are more highly sensitive to the ionomer‐gradient design. It is possible to maintain a proper TU and desired output performance by optimizing the CCL gradient structure, especially for the stationary PEMFC that operates with a particular condition for a long time. Research Highlights: A 1D, non‐isothermal, two‐phase model is validated from multiple aspects.Multi‐objective optimization is performed by combining the NSGA II and numerical model.More uniform CCL temperature and higher power density are achieved after optimization. [ABSTRACT FROM AUTHOR]

The spatial distribution of platinum (Pt) within the cathode catalyst layer (CCL) is vital for the electrochemical reactions and mass transport in fuel cells. Though important, the transient effects of these distributions are seldom explored. This study examines the impact of Pt distribution on transient electrical behaviors, including voltage and local current distribution (LCD) uniformity, using a transient, two-dimensional, two-phase, non-isothermal fuel cell model that incorporates catalyst agglomerate. Three Pt distribution types are investigated: uniform, MPL-side biased, and PEM-side biased. Results indicate a voltage undershoot occurs during current loading. Compared to the homogeneous CCL, PEM-side bias reduces this undershoot by 12.5% due to shortened proton transfer paths and decreased ohmic loss, while MPL-side bias increases it by 18.8% due to the inverse effect. Additionally, Pt distribution affects both oxygen transport and reaction resistance within the agglomerate, influencing LCD uniformity. Under loading conditions, gradient CCLs show inferior LCD uniformity than homogeneous ones. Peak non-uniformity values of 0.22, 0.59, and 0.71 are observed for homogeneous, MPL-biased, and PEM-biased CCLs, respectively. From the perspective of voltage and LCD uniformity, the MPL-side biased CCL is not found to enhance dynamic characteristics, whereas the PEM-side bias improves voltage undershoot but at the cost of LCD uniformity. This study provides a novel perspective on fuel cell dynamics, emphasizing the transient effects of Pt distribution and their potential for optimizing dynamic performance by adjusting the Pt gradient. • Transient behaviors of gradient catalyst layers are studied for the first time. • Pt distribution position impacts proton transfer path and transient ohmic loss. • Reaction resistance changes drive transient current density uniformity variations. • Homogeneous Pt distribution yields optimal transient current density uniformity. [ABSTRACT FROM AUTHOR]

A gradient cathode catalyst layer (CCL) with promoted mass transfer and Pt utilization could enhance the output performance of proton exchange membrane fuel cells (PEMFCs). Since operating conditions affect reaction and mass transfer rate inside the PEMFC, the gradient CCL can only exert its full potential within a narrow operating range. Assessing the impact of gradients, operating conditions, and their interactions on output performance is vital to maximizing the benefits of gradient CCLs. Therefore, a global sensitivity analysis was conducted using Sobol's approach based on a one-dimensional, two-phase, non-isothermal PEMFC model. Five parameters, including Pt gradient (α), ionomer gradient (β), temperature (T), anode relative humidity (RH a), and cathode relative humidity (RH c), were selected as model inputs. The results show that RH c has the most significant impact on output performance among the operating parameters. The advantage provided by the gradient design is presented in mass transport-constrained situations. The impact of α on current density is more significant than β. Meanwhile, regarding the interaction effect of operating conditions and gradients, α and RH c provide the most significant pairwise interaction effect. Finally, the mechanisms underlying these interactions were analyzed, and some options were proposed to maximize the benefits of gradient CCLs. • Impacts of parameters and their interactions on cell performance are assessed. • Gradient designs provide advantages in mass transport-constrained situations. • Cathode relative humidity is the most significant operating parameter. • Interactions between operating conditions and Pt gradient were investigated. [ABSTRACT FROM AUTHOR]

• Phase-change-induced flow is simulated by a 1D, two-phase, non-isothermal model. • Dimensionless parameter (Ts) is used to evalute the relation in CD flow and PCI flow. • Optimization is performed by combining the PSO algorithm and numerical model. • Optimal ionomer-gradient design boosts the current density and affects Ts. Temperature distribution affects water transport by phase-change-induced (PCI) flow in the cathode catalyst layer, where water generates and heat releases. To elucidate the role of PCI flow in water removal, a 1-D, non-isothermal, two-phase model coupled with an agglomerate submodel is established and a dimensionless parameter, Ts , evaluating the relation between the capillary-driven flow and PCI flow is also employed. The effects of operating temperature and inlet relative humidity (RH) on the Ts value are investigated in detail. The results show that raising the operating temperature (from 333.15 K to 363.15 K) contributes to increased Ts and extension of the PCI flow dominating area. Compared with other RH cases, Case-D has the highest Ts due to the adsorption/desorption process between equilibrium water content and dissolved water content as well as the changes of phase-change heat in the cathode catalyst layer. But the widest PCI flow dominating area emerges at Case-C. Lastly, the well-designed ionomer-gradient distribution is identified by the Particle Swarm Optimization algorithm, namely reduction along the through-plane direction in the cathode catalyst layer with three sub-layers, which in turn increases the local porosity. As a result, the overall mass transport capability is enhanced, as reflected by the decrease of Damkohler number and increase of effectiveness factor, thus promoting the output performance. Also, this optimization facilitates the PCI flow and expands the PCI flow dominating area. Overall, it is realizable to avoid water flooding by reasonably adjusting the operating conditions as well as optimizing the catalyst layer gradient structure. [ABSTRACT FROM AUTHOR]