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

本项目的目标是了解多孔碳电极在能量转换和存储应用中的耦合传热和传质过程。能量转换装置，如聚合物电解质燃料电池（PEFC），在最大限度地减少运输部门对环境的影响方面具有很大的前景。然而，在低操作温度下，水管理仍然是一个大问题。当液态水在薄的多孔碳层中积聚时，由于反应物输送不足，电流密度降低。成功设计PEFC的挑战之一是了解多孔碳层中的耦合质量和热传输现象，以优化水管理并增加功率输出，从而改善PEFC性能。该项目将确定水在多孔、混合润湿性碳材料中传输的基本机制。通过温度梯度蒸发机制的更好的理解将在纳米和微米尺度上实现。该项目的结果将推进热梯度下的水传输机制的理解，并提供了一个路线图的最佳电极设计的一大类能量转换和？储存技术，如燃料电池、氧化还原液流电池和太阳能燃料发电机。可再生能源的主题将通过与PI的能源软件平台集成的可用能源套件带入K-12教室。废热，效率，成本/效益分析和可再生能源的概念将与实践设计活动一起教授。此外，研究结果将通过PI的本科生指导和电化学能量转换和存储课程的整合进行传播。混合润湿性，多孔碳层中的水管理对于开发和制造具有成本效益的PEFC至关重要。为了实现最大的水渗透，从而更高的燃料电池电流密度，有必要了解压力和毛细驱动的液体-水传输和相变诱导（PCI）流动之间的相互作用，由于在多孔电极和气体扩散层（GDL）的蒸发/冷凝。GDL具有多功能的作用，GDL中的热量和质量传输取决于材料形态和传输特性，如电导率和热导率、气体扩散率和流体渗透率。尽管已经通过建模和实验探索了GDL中水传输的某些方面，但这些材料中的蒸发和PCI流动仍然知之甚少。缺乏这方面的基础知识主要是由于采取实验测量和可视化这些多孔材料内的蒸发水前沿的挑战。最近的报告表明，PCI流量在GDL中的较低水位时甚至更显著，但是对此的物理原因尚未完全理解。必须量化诱导的热梯度下的水传输，以找到这些多孔层内的准确的液体前沿分布。在这个项目中，蒸发速率限制步骤将确定与现场实验仪器和蒸发的水锋将使用X射线计算机断层扫描（X射线CT）可视化。将通过在多孔电极的厚度上施加热梯度来探索分级电极中的PCI流动的机制。水再循环是预期的，并且将利用纳米和微米X射线CT在整个厚度方向上进行可视化和量化。X射线CT的精确技术允许收集关于在不同的热梯度下水团簇的确切位置的前所未有的详细信息。同时，通过这些电极的热量和质量传输将被测量。孔隙网络和连续体模型将用于帮助解释收集的数据并预测新的材料结构。这种新的理解将被用来确定热和质量传输的最佳GDL形态的纳米和微米尺度特征。通过结合新的实验和建模能力，PI将设计GDL设计，以调节相变诱导流并有效管理PEFC中的水传输，从而提高可达到的功率密度。

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.