There is a need to understand the water dynamics of alkaline membrane fuel cells under various operating conditions to create electrodes that enable high performance and stable, long-term operation. Here we show, via operando neutron imaging and operando micro X-ray computed tomography, visualizations of the spatial and temporal distribution of liquid water in operating cells. We provide direct evidence for liquid water accumulation at the anode, which causes severe ionomer swelling and performance loss, as well as cell dryout from undesirably low water content in the cathode. We observe that the operating conditions leading to the highest power density during polarization are not generally the conditions that allow for long-term stable operation. This observation leads to new catalyst layer designs and gas diffusion layers. This study reports alkaline membrane fuel cells that can be operated continuously for over 1000 h at 600 mA cm −2 with voltage decay rate of only 32-μV h −1the best-reported durability to date.

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Proton exchange membrane fuel cells have emerged as one of the leaders for the replacement of fossil fuel powered internal combustion engines. Water removal from the cell is one of the top concerns regarding fuel cell performance for transportation applications. During lower power output or high temperature operation, water removal in the vapor phase can dominate. The rate of water vapor diffusion through the porous cathode gas diffusion layer (GDL) of the fuel cell is limited by the porosity and tortuosity formed by the solid fiber matrix. In this work an experimental apparatus is designed to measure the rate of water vapor diffusion across the GDL to determine an effective diffusion coefficient. The effects of microporous layer (MPL) coating, GDL thickness, and polytetrafluoroethylene (PTFE) loading on the diffusion coefficient is demonstrated. Commercially available diffusion media are tested and include Mitsubishi Rayon Corp. Grafil U-105 series, SGL Sigracet® 25, 35, and 10 series, and Toray TGP-H-120 series. Standard corrections, such as the Bruggeman correction, used in fuel cell literature are found to overpredict the effective diffusion coefficient for the GDL. The MPL was found to produce a significant resistance to water vapor diffusion due to its smaller pore diameters, lower porosity, and an increase in tortuosity. The GDL Grafil U-105 A produced a higher effective diffusion coefficient of 0.070 cm 2 /s compared to the SGL 25BC value of 0.063 cm 2 /s. Confocal scanning laser microscope images indicated that the MPL for the Grafil U-105 A sample is possibly thinner, thus explaining some of the reduction in diffusion resistance. Thickness was found to have no influence on the effective diffusion coefficient for samples without MPL. PTFE causes a rapid decrease in effective diffusion coefficient from 0.095 cm 2 /s for TGP-H-120 0% PTFE to 0.024 cm 2 /s for TGP-H-120 40% PTFE. Comparison to other studies from the literature show good agreement with the present work thus validating the dynamic method for use in diffusion coefficient measurements in fuel cell diffusion media. v

Enhancement of plant drought stress tolerance by plant growth-promoting rhizobacteria (PGPR) has been increasingly documented in the literature. However, most studies to date have focused on PGPR-root/plant interactions; very little is known about PGPR's role in mediating physiochemical and hydrological changes in the rhizospheric soil that may impact plant drought stress tolerance. Our study aimed to advance mechanistic understanding of PGPR-mediated biophysical changes in the rhizospheric soil that may contribute to plant drought stress tolerance in addition to plant responses. We measured soil water retention characteristics, hydraulic conductivity, and water evaporation in soils with various textures (i.e., pure sand, sandy soil, and clay) as influenced by a representative PGPR (Bacillus subtilis strain UD1022) using the HYPROP system. We found that all PGPR-treated soils held more water and had reduced hydraulic conductivity and accumulative evaporation, compared to their corresponding controls. We discuss three mechanisms, due to B. subtilis incubation or production of extracellular polymeric substances (EPS), that are potentially responsible for the changes in hydraulic properties and soil evaporation: (i) EPS have a large water holding capacity; (ii) EPS alter soil matrix structure and connectivity of pore space; (iii) EPS modify the physicochemical properties of water (surface tension and viscosity). These results clearly demonstrate PGPR's ability to increase water availability to plants by slowing down evaporation and by increasing the time available for plants to make metabolic adjustments to drought stress.Plain Language Summary PGPR is a group of beneficial bacteria known to improve plant growth by, e.g., reducing pathogenic infection and/or promoting drought/salt tolerance. Despite the important role PGPR could potentially play in reducing drought stress to plants, we lack a complete understanding on the mechanisms through which PGPR mediate plant tolerance to drought. This study aimed to advance mechanistic understanding of PGPR-mediated biophysical changes in soil through microbe-soil interactions, to complement better understanding gained from previous studies that focused on microbe-plant interactions. Through laboratory measurements and imaging of water retention in soil, we show that a representative PGPR (B. subtilis UD1022) can increase soil water retention and reduce soil water evaporation. This effect is likely caused by the PGPR's ability to produce extracellular polymeric substances, which have high water holding capacity and can induce changes in soil physical properties. These changes lead to slower evaporation from soil, which can make more water available to plants as well as increase the time available for plants to make metabolic adjustments to drought stress. Our results provide scientific support to recent efforts in promoting application of rhizobacteria isolates as ''underground resource'' to contribute to solving globally challenging issues, e.g., water resource shortage and ...

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