Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer： The blessing and curse of ionomer

Ionomer impregnation represents a milestone in the evolution of polymer electrolyte fuel cell （PEFC） catalyst layers. Ionomer acts as the binder, facilitates proton transport, and thereby drastically improves catalyst utilization and effectiveness. However, advanced morpho- logical and functional characterizations have revealed that up to 60% of Pt nanoparticles can be trapped in the micropores of carbon support particles. Ionomer clusters and oxygen molecules can hardly enter into micropores, leading to low Pt utilization and effectiveness. Moreover, the ionomer thin-films covering Pt nanoparticles can cause significant mass transport loss especially at high current densities. Ionomer-free ultra-thin catalyst layers （UTCLs） emerge as a promising alternative to reduce Pt loading by improving catalyst utilization and effectiveness, while theoretical issues such as the proton conduction mechan- ism remain puzzling and practical issues such as the rather narrow operation window remain unsettled. At present, the development of PEFC catalyst layer has come to a crossroads： staying ionomer-impregnated or going iono- mer-free. It is always beneficial to look back into the past when coming to a crossroads. This paper addresses the characterization and modeling of both the conventional ionomer-impregnated catalyst layer and the emerging ionomer-free UTCLs, featuring advances in characterizing microscale distributions of Pt particles, ionomer, support particles and unraveling their interactions; advances in fundamental understandings of proton conduction and flooding behaviors in ionomer-free UTCLs; advances in modeling of conventional catalyst layers and especially UTCLs; and discussions on high-impact research topics in characterizing and modeling of catalyst layers.

EFFECT OF GROUNDWATER TABLE CONTROL ON WATER SAVING IRRIGATIONSTRATEGIES IN THE QINGTONGXIA IRRIGATION DISTRICT

This paper focuses on the analysis of the effects of groundwater tablecontrol under different irrigation water amounts on the water and salinity balance and on cropyield. Two experimental areas, the Pingluo and Huinong experimental sites, were selected to collectthe required data. The agro-hydrological model Soil-Water-Atmosphere-Plant (SWAP) was used toanalyse the water flows and salt transport processes for different groundwater levels and irrigationscenarios. Six scenarios, which resulted from different groundwater table regimes combined withdifferent irrigation amounts, were simulated. The results show that high ground-water tables due tothe excessive irrigation are the main cause of the large amount of drainage water and low cropyield; reducing irrigation water without a lower groundwater table will not lead to a largereduction of the drainage water, and will reduce the crop yield even more; to lower the groundwatertable is a good measure to control the drainage water and increase crop yield.