Shanhai Ge

Liquid Water Formation and Transport in the PEFC Anode

Transparent polymer electrolyte fuel cells PEFCs with parallel- and serpentine-channel flowfields have been developed to study liquid water formation and transport on the anode side. In situ observations reveal that liquid water in the anode channels results from condensation of water on cooler and more hydrophilic channel walls and that water available for condensation in the anode comes either from the cathode through membrane transport or from hydrogen consumption. No water droplets can be found on the anode gas diffusion layer GDL surface, in sharp contrast with the cathode side. Moreover, GDL wettability has much influence on water distribution in the anode for the cell operated at low current density 0.2 A/cm 2 . Using hydrophobic GDL at low current density, water is prone to condense on the channel surfaces rather than inside hydrophobic carbon paper GDL. The condensed water then accumulates and results in channel clogging in the anode. In contrast, by using untreated carbon paper as the anode GDL, it is found that channel clogging by liquid water is avoided under similar operating conditions. This finding implies that water distributions in the anode with hydrophilic and hydrophobic anode GDLs differ much from each other at low current density. Finally, experiments suggest that water condensation on the channel surfaces is a primary mechanism for liquid water formation and anode flooding and that elevating the anode plate temperature modestly is effective to avoid surface condensation and mitigate anode flooding.

Lithium-ion battery structure that self-heats at low temperatures

Lithium-ion batteries suffer severe power loss at temperatures below zero degrees Celsius, limiting their use in applications such as electric cars in cold climates and high-altitude drones. The practical consequences of such power loss are the need for larger, more expensive battery packs to perform engine cold cranking, slow charging in cold weather, restricted regenerative braking, and reduction of vehicle cruise range by as much as 40 per cent. Previous attempts to improve the low-temperature performance of lithium-ion batteries have focused on developing additives to improve the low-temperature behaviour of electrolytes, and on externally heating and insulating the cells. Here we report a lithium-ion battery structure, the ‘all-climate battery’ cell, that heats itself up from below zero degrees Celsius without requiring external heating devices or electrolyte additives. The self-heating mechanism creates an electrochemical interface that is favourable for high discharge/charge power. We show that the internal warm-up of such a cell to zero degrees Celsius occurs within 20 seconds at minus 20 degrees Celsius and within 30 seconds at minus 30 degrees Celsius, consuming only 3.8 per cent and 5.5 per cent of cell capacity, respectively. The self-heated all-climate battery cell yields a discharge/regeneration power of 1,061/1,425 watts per kilogram at a 50 per cent state of charge and at minus 30 degrees Celsius, delivering 6.4–12.3 times the power of state-of-the-art lithium-ion cells. We expect the all-climate battery to enable engine stop–start technology capable of saving 5–10 per cent of the fuel for 80 million new vehicles manufactured every year. Given that only a small fraction of the battery energy is used for self-heating, we envisage that the all-climate battery cell may also prove useful for plug-in electric vehicles, robotics and space exploration applications.

In Situ Imaging of Liquid Water and Ice Formation in an Operating PEFC during Cold Start

Cold-start capability and survivability of polymer electrolyte fuel cells PEFCs in a subzero environment remain a major challenge for automotive applications. Its fundamental mechanisms are not fully determined, but it is recognized that product water becomes ice or frost upon startup when the PEFC internal temperature is below the freezing point of water. If the local pore volume of the cathode catalyst layer CL is insufficient to contain all of the accumulated water before the cell operating temperature rises above freezing, solid water may plug the catalyst layer and stop the electrochemical reaction by starving the reagent gases. In addition, ice formation may result in serious damage to the structure of a membrane electrode assembly MEA. In spite of the importance of PEFC cold-start capability and associated MEA durability, very few studies in the literature have focused on PEFC startup dynamics, freeze/thaw cycling,

Cyclic Voltammetry Study of Ice Formation in the PEFC Catalyst Layer during Cold Start

A cyclic voltammetry technique has been developed to investigate the effect of ice formation in the cathode catalyst layer CL on electrochemically active Pt area during and post-subzero startup of a polymer electrolyte fuel cell PEFC. It was found that the Pt area decreases after each cold start and the Pt area loss increases with the product water generated during cold start. We hypothesize that the Pt area loss is caused by ice precipitated between Pt particles and ionomers during cold start. After startup from a subzero temperature and warmup to 25°C with all ice in the CL melted, the cell remains subject to Pt area loss, but this loss at 25°C is substantially reduced. We also find that subsequent cell operation at 70°C and 1 A/cm 2 for 2 h is very effective for recovering the active Pt area and cell performance. Both permanent loss in the active Pt area and cell performance degradation due to structural alteration of the cathode CL by the presence of ice increase gradually with the cold-start cycle number and become more severe for cold start from �30°C than from �10 and �20°C. In addition, startup from a subzero temperature appears to have no influence on the electrochemically active Pt area of the anode CL. It is suggested that the ice amount present in the CL holds a key to determine the temporary Pt area loss due to ice formation as well as permanent performance degradation resulting from cold-start cycling.

Effect of Cathode Pore Volume on PEM Fuel Cell Cold Start

Effect of Cathode Pore Volume on PEM Fuel Cell Cold Start Ashis Nandy, Fangming Jiang, Shanhai Ge,* Chao-Yang Wang,* and Ken S. Chen Electrochemical Engine Center and Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Nanoscale and Reactive Processes Department, Engineering Sciences Center, Sandia National Laboratories, Albuquerque, New Mexico 87185-0836, USA

Reaction temperature sensing (RTS)-based control for Li-ion battery safety

We report reaction temperature sensing (RTS)-based control to fundamentally enhance Li-ion battery safety. RTS placed at the electrochemical interface inside a Li-ion cell is shown to detect temperature rise much faster and more accurately than external measurement of cell surface temperature. We demonstrate, for the first time, that RTS-based control shuts down a dangerous short-circuit event 3 times earlier than surface temperature- based control and prevents cell overheating by 50 °C and the resultant cell damage.

Fast charging of lithium-ion batteries at all temperatures

Significance Range anxiety is a key reason that consumers are reluctant to embrace electric vehicles (EVs). To be truly competitive with gasoline vehicles, EVs should allow drivers to recharge quickly anywhere in any weather, like refueling gasoline cars. However, none of today’s EVs allow fast charging in cold or even cool temperatures due to the risk of lithium plating, the formation of metallic lithium that drastically reduces battery life and even results in safety hazards. Here, we present an approach that enables 15-min fast charging of Li-ion batteries in any temperatures (even at −50 °C) while still preserving remarkable cycle life (4,500 cycles, equivalent to >12 y and >280,000 miles of EV lifetime), thus making EVs truly weather-independent. Fast charging is a key enabler of mainstream adoption of electric vehicles (EVs). None of today’s EVs can withstand fast charging in cold or even cool temperatures due to the risk of lithium plating. Efforts to enable fast charging are hampered by the trade-off nature of a lithium-ion battery: Improving low-temperature fast charging capability usually comes with sacrificing cell durability. Here, we present a controllable cell structure to break this trade-off and enable lithium plating-free (LPF) fast charging. Further, the LPF cell gives rise to a unified charging practice independent of ambient temperature, offering a platform for the development of battery materials without temperature restrictions. We demonstrate a 9.5 Ah 170 Wh/kg LPF cell that can be charged to 80% state of charge in 15 min even at −50 °C (beyond cell operation limit). Further, the LPF cell sustains 4,500 cycles of 3.5-C charging in 0 °C with <20% capacity loss, which is a 90× boost of life compared with a baseline conventional cell, and equivalent to >12 y and >280,000 miles of EV lifetime under this extreme usage condition, i.e., 3.5-C or 15-min fast charging at freezing temperatures.