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用于能量转换和储存的热和质量耦合传输过程。能量转换设备，如聚合物电解质燃料电池（PEFCs），在最大限度地减少交通部门对环境的影响方面有着巨大的希望。然而，在低运行温度下，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递现象，以优化水管理并增加功率输出，从而提高PEFC的性能。拟议的项目将确定水在多孔、混合润湿性碳材料中运输的基本机制。通过温度梯度对蒸发机制的进一步了解将在纳米和微观尺度上实现。该项目的结果将促进对热梯度下水输送机制的理解，并为大规模的能量转换和？存储技术，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工具包带入K-12教室。余热、效率、成本/效益分析和可再生能源的概念将通过实践设计活动教授。此外，研究成果将通过PI的本科生指导传播，并纳入电化学能量转换和存储课程。混合润湿性、多孔碳层中的水分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，从而提高燃料电池的电流密度，有必要了解压力和毛细管驱动的液态水输送和相变诱导（PCI）流动之间的相互作用，这是由于多孔电极和气体扩散层（gdl）中的蒸发/冷凝造成的。gdl具有多种功能，gdl中的热量和质量传递取决于材料的形态和传输性质，如导电性和导热性、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中水分输送的某些方面，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由于在这些多孔材料中进行实验测量和可视化蒸发水前沿的挑战。最近的报告表明，在gdl中，PCI流量在较低水位时更为显著，但其物理原因尚未完全理解。定量研究诱导热梯度下的水输运是确定这些多孔层内准确的液锋分布的必要条件。在这个项目中，蒸发速率限制步骤将通过现场实验仪器确定，蒸发水面将使用x射线计算机断层扫描（x射线CT）可视化。分层电极中PCI流的机制将通过在多孔电极的厚度上施加热梯度来探索。利用纳米和微x射线CT，预计将在整个厚度方向上对水循环进行可视化和量化。x射线CT的精确技术可以收集到前所未有的详细信息，了解不同热梯度下水簇的确切位置。同时，将测量通过这些电极的热量和质量传递。孔隙网络和连续体模型将用于帮助解释收集到的数据和预测新的材料结构。这一新的认识将被用于确定热和质量输运的最佳GDL形态的纳米和微观尺度特征。通过结合新颖的实验和建模能力，pi将设计GDL来调节相变诱导流量，并有效地管理pefc中的水运，从而提高可实现的功率密度。

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.