Accessible triple-phase boundary length: A performance metric to account for transport pathways in heterogeneous electrochemical materials

Abstract

The performance of materials for electrochemical energy conversion and storage depends upon the number of electrocatalytic sites available for reaction and their accessibility by the transport of reactants and products. For solid oxide fuel/electrolysis cell materials, standard 3-D measurements such as connected triple-phase boundary (TPB) length and effective transport properties partially inform on how local geometry and network topology causes variability in TPB accessibility. A new measurement, the accessible TPB, is proposed to quantify these effects in detail and characterize material performance.The approach probes the reticulated pathways to each TPB using an analytical electrochemical fin model applied to a 3-D discrete representation of the heterogeneous structure provided by skeleton-based partitioning. The method is tested on artificial and real structures imaged by 3-D x-ray and electron microscopy. The accessible TPB is not uniform and the pattern varies depending upon the structure. Connected TPBs can be even passivated.The sensitivity to manipulations of the local 3-D geometry and topology that standard measurements cannot capture is demonstrated. The clear presence of preferential pathways showcases a non-uniform utilization of the 3-D structure that potentially affects the performance and the resilience to alterations due to degradation phenomena in electrochemical energy storage and conversion devices. our understanding of the detrimental effects of microstructural coarsening, contamination or redox cycling in SOFC/SOEC materials [6,8,[16][17][18][19][20][21][22][23].A main limitation of approaches based on averaged (e.g. effective) properties is that all the TPBs are treated as equally accessible. The visual inspection of 3-D imaging data suggests that this assumption is questionable and that significant local information is lost. In contrast, the TPB tortuosity and TPB critical pathway radius have been recently proposed for the characterization of the transport pathways to TPBs [24]. This approach, based on image processing, provides new insight into the factors that control the electrochemical performance of heterogeneous materials, but it is based on purely geometric concepts that highlight two specific and mostly local properties of the reticulate pathways, i.e. the shortest path length and the smallest constriction that must be passed through to access a TPB. Simulations based on lattice Boltzmann or finite element method that use as computational domain the imaged and meshed 3-D structures [4,25] are capable of quantifying accurately the access to TPB, including the combined effects of local 3-D geometrical features and topology of the microstructure. However, attempts to define dedicated TPB properties that inform on material performance and durability have been limited so far [4], partly because of the high computational requirements.An analytical electrochemical fin model (ECF) has been developed as a screening tool for material design [26][27][28][29]. The method consists in represen...

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