The Nature of Coupled Heat and Mass Transport in Porous Carbon Electrodes

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The goal of this project is to understand coupled heat- and mass-transport processes in porous carbon electrodes for applications in energy-conversion and storage. Energy-conversion devices, such as polymer electrolyte fuel cells (PEFCs), hold great promise for minimizing the environmental impact of the transportation sector. However, water management is still a large problem at low operating temperatures. As liquid water accumulates in the thin, porous carbon layers, current density decreases due to inadequate reactant delivery. One of the challenges in successful PEFC design is understanding the coupled mass and thermal transport phenomena in porous carbon layers to optimize water management and increase power output, improving PEFC performance. The proposed project will determine the fundamental mechanisms of water transport in porous, mix-wettability carbon materials. Greater understanding of evaporative mechanisms via temperature gradients will be achieved on the nano- and micro-scales. The results of the project will advance the understanding of water transport mechanisms under thermal gradients and provide a roadmap of optimal electrode design for a large class of energy-conversion and ?storage technologies, such as fuel cells, redox-flow batteries and solar-fuel generators. The topic of renewable energy will be brought into K-12 classrooms through available energy kits integrated with the PI's energy software platform. The concepts of waste heat, efficiency, cost/benefit analysis, and renewable energy will be taught with hands-on design activities. Additionally, research findings will be disseminated by PI's undergraduate mentoring and incorporation in an electrochemical energy-conversion and -storage course.Water management in mix wettability, porous carbon layers is critical to developing and manufacturing cost-effective PEFCs. To achieve maximum water permeation, and consequently higher fuel cell current densities, it is necessary to understand the interplay between pressure- and capillary-driven liquid-water transport and phase-change induced (PCI) flow due to evaporation/condensation in the porous electrodes and gas-diffusion layers (GDLs). GDLs serve multifunctional roles, and heat and mass transport in GDLs depends on both material morphology and transport properties, such as electrical and thermal conductivity, gas diffusivity, and fluid permeability. Although some aspects of water transport in GDLs have been explored with modeling and experiments, evaporation and PCI flow within these materials are still poorly understood. This fundamental knowledge is lacking primarily due to the challenge of taking experimental measurements and visualizing evaporating water front within these porous materials. Recent reports suggest that PCI flow is even more significant at lower water levels in GDLs, however the physical reasons for this are not fully comprehended. It is imperative to quantify water transport under induced thermal gradients to find exact liquid front distribution within these porous layers. In this project, the evaporation rate-limiting step will be identified with in-situ experimental instrumentation and evaporating water-fronts will be visualized using X-ray computed tomography (X-ray CT). The mechanisms of PCI flow in hierarchical electrodes will be explored by imposing thermal-gradients across the thickness of the porous electrode. Water recirculation is expected and will be visualized and quantified in the through-thickness direction utilizing nano- and micro- X-ray CT. The precise techniques of X-ray CT allow the gathering of an unprecedented level of detailed information on the exact location of water clusters under varied thermal gradients. Simultaneously, heat and mass-transport through these electrodes will measured. Pore-network and continuum models will be used to help interpret the gathered data and predict novel material architectures. This new understanding will be leveraged to identify nano- and micro-scale characteristics of optimal GDL morphologies for heat and mass-transport. Through combined novel experimental and modeling capabilities the PIs will engineer GDL designs to modulate phase-change induced flow and effectively manage water transport in PEFCs, thereby increasing the attainable power density.