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

该项目的目标是了解多孔碳电极中的热质耦合传输过程，以应用于能量转换和存储。聚合物电解质燃料电池(PEFC)等能量转换设备在将交通部门的环境影响降至最低方面具有很大的前景。然而，在运行温度较低的情况下，水管理仍然是一个大问题。由于液态水在薄而多孔的碳层中积累，由于反应物输送不充分，电流密度降低。成功设计PEFC的挑战之一是了解多孔碳层中耦合的质量和热传输现象，以优化水管理和增加功率输出，从而改善PEFC的性能。拟议的项目将确定水在多孔性、混合润湿性碳材料中传输的基本机制。将在纳米和微米尺度上更好地理解通过温度梯度蒸发的机理。该项目的结果将促进对温度梯度下水传输机制的理解，并为燃料电池、氧化还原液流电池和太阳能燃料发电机等一大类能量转换和储存技术提供最佳电极设计的路线图。可再生能源的主题将通过与国际能源社的能源软件平台集成的可用能源包进入K-12课堂。将通过实践设计活动讲授余热、效率、成本/效益分析和可再生能源的概念。此外，研究成果将通过PI的本科生指导和整合在电化学能量转换和存储课程中传播。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。为了获得最大的水渗透率，从而获得更高的燃料电池电流密度，有必要了解压力和毛细管驱动的液水传输与由于多孔电极和气体扩散层(GDL)中的蒸发/冷凝而引起的相变诱导(PCI)流动之间的相互作用。GDL具有多功能的作用，GDL中的热和质量传输取决于材料的形态和传输特性，如导电性和导热性、气体扩散率和流体渗透性。尽管已经通过模拟和实验探索了GDLS中水分传输的某些方面，但这些材料中的蒸发和PCI流仍然知之甚少。这些基本知识的缺乏主要是因为在这些多孔材料中进行实验测量和可视化蒸发水锋是一项挑战。最近的报告表明，在地下水位较低的情况下，PCI流甚至更显著，然而造成这种情况的物理原因还没有完全被理解。为了在这些多孔层中找到准确的液体前沿分布，必须对诱导温度梯度下的水的传输进行量化。在这个项目中，蒸发限速步骤将用现场实验仪器识别，蒸发水锋将用X射线计算机层析成像(X-射线CT)可视化。通过在多孔电极厚度上施加温度梯度来探索分层电极中的PCI流的机制。水再循环是预期的，并将利用纳米和微型X射线CT在穿透厚度方向进行可视化和量化。X射线CT的精确技术使人们能够在不同的温度梯度下收集关于水团的准确位置的前所未有的详细信息。同时，将测量通过这些电极的热量和质量传输。孔网络和连续介质模型将被用来帮助解释收集的数据和预测新的材料结构。这一新的认识将被用来确定热和质量传输的最佳GDL形态的纳米和微尺度特征。通过结合新颖的实验和建模能力，PI将设计GDL以调节相变诱导流动，并有效地管理PEFC中的水传输，从而提高可获得的功率密度。

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.