Feng-Yuan Zhang

Liquid Water Removal from a Polymer Electrolyte Fuel Cell

Liquid water transport and removal from the gas diffusion layer GDL and gas channel of a polymer electrolyte fuel cell PEFC are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate or stoichiometry with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Visualization of Liquid Water Transport in a PEFC

Using an optical /air polymer electrolyte fuel cell (PEFC), the mechanics of liquid water transport, starting from droplet emergence on the gas diffusion layer (GDL) surface, droplet growth and departure, to the two-phase flow in gas channels, is characterized under automotive conditions of 0.82 A/cm2, 70°C, and 2 atm. It is observed that water droplets emerge from the GDL surface under oversaturation of water vapor in the gas phase, appear only at preferential locations, and can grow to a size comparable to the channel dimension under the influence of surface adhesion. Liquid film formation on more hydrophilic channel walls and channel clogging are also revealed and analyzed.

In situ characterization of the catalyst layer in a polymer electrolyte membrane fuel cell

The catalyst layer CL in the polymer electrolyte membrane fuel cell PEMFC has not been explored in detail due to its complexities and difficulties associated with accessing it. It is important to understand water ingress and egress at the CL as it offers potential for water management by manipulating the evaporation rate. A technique for in situ visual characterization of the CL is presented. This is accomplished by designing and fabricating a catalyst-visible operational fuel cell and developing a microvisualization system. The dynamics of microdroplets on the CL surface including formation, growth, coalescence, and removal were visualized in an operating PEMFC. The liquid water behavior at the interface of the CL and the gas diffusion layer GDL were shown to promote the periodic droplet reemergence on the GDL surface in the flow channels. Mechanisms of water condensation and transport within the CL pores are discussed with respect to pore architecture and wetting properties. It has been shown that reduction of pore size and CL thickness alleviates flooding therein and promotes better catalyst utilization. Evaporation was identified as one of the distinguishing mechanisms of the CL, and one of the future challenges will be to control this mechanism. The experimental results should prove useful in clarifying the role of the CL in water management, and in refining models used to optimize PEMFC performance.

Diode-laser tomography for arcjet plume reconstruction

Diode-laser absorption tomography is described with which the spatial temperature and the atomic number density distribution of a 3-kW class arcjet can be derived simultaneously by reconstruction of the absorption coefficient field of the arcjets argon exhaust plume. One can perform various parameter measurements by changing the arcjets mass-flow rates and discharge currents. The maximum temperature and atomic number density increase with the mass-flow rate and the discharge current. The trend for increase is not always found for a specific input power, although at a fixed mass-flow rate the power increases at that rate.

Investigation of thin/well-tunable liquid/gas diffusion layers exhibiting superior multifunctional performance in low-temperature electrolytic water splitting

Liquid/gas diffusion layers (LGDLs), which are located between the catalyst layer (CL) and bipolar plate (BP), play an important role in enhancing the performance of water splitting in proton exchange membrane electrolyzer cells (PEMECs). They are expected to transport electrons, heat, and reactants/products simultaneously with minimum voltage, current, thermal, interfacial, and fluidic losses. In this study, the thin titanium-based LGDLs with straight-through pores and well-defined pore morphologies are comprehensively investigated for the first time. The novel LGDL with a 400 μm pore size and 0.7 porosity achieved a best-ever performance of 1.66 V at 2 A cm−2 and 80 °C, as compared to the published literature. The thin/well-tunable titanium based LGDLs remarkably reduce ohmic and activation losses, and it was found that porosity has a more significant impact on performance than pore size. In addition, an appropriate equivalent electrical circuit model has been established to quantify the effects of pore morphologies. The rapid electrochemical reaction phenomena at the center of the PEMEC are observed by coupling with high-speed and micro-scale visualization systems. The observed reactions contribute reasonable and pioneering data that elucidate the effects of porosity and pore size on the PEMEC performance. This study can be a new guide for future research and development towards high-efficiency and low-cost hydrogen energy.

Diagnostics of an argon arcjet plume with a diode laser

The diode-laser absorption technique was applied for simultaneous velocity and temperature measurements of an argon plume exhausted by an arcjet. The Ar I absorption line at 811.531 nm was taken as the center absorption line. The velocity and the temperature were derived from the Doppler shift in the absorption profiles and the full width at half-maximum of the plume absorption profile, respectively. From the measured plume velocity and temperature, the total enthalpy of the exhausted plume, the thrust efficiency, and the thermal efficiency of the arcjet were derived, and the performance of the arcjet was examined. The results are demonstrated to agree with results derived by other methods, and the technique can be applied to the measurement of other arcjet systems without much modification.

Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting

Increase 50-time catalyst mass activity from revealing true reactions in proton exchange membrane electrolysis. Better understanding of true electrochemical reaction behaviors in electrochemical energy devices has long been desired. It has been assumed so far that the reactions occur across the entire catalyst layer (CL), which is designed and fabricated uniformly with catalysts, conductors of protons and electrons, and pathways for reactants and products. By introducing a state-of-the-art characterization system, a thin, highly tunable liquid/gas diffusion layer (LGDL), and an innovative design of electrochemical proton exchange membrane electrolyzer cells (PEMECs), the electrochemical reactions on both microspatial and microtemporal scales are revealed for the first time. Surprisingly, reactions occur only on the CL adjacent to good electrical conductors. On the basis of these findings, new CL fabrications on the novel LGDLs exhibit more than 50 times higher mass activity than conventional catalyst-coated membranes in PEMECs. This discovery presents an opportunity to enhance the multiphase interfacial effects, maximizing the use of the catalysts and significantly reducing the cost of these devices.

Investigation of a copper etching technique to fabricate metallic gas diffusion media

A new fabrication technique based on etching is employed to convert a copper foil into a porous structure with an array of micron size pores. The motivation stems from the need to develop a more efficient and controllable gas diffusion medium for fuel cell applications. The influence of mask shape, mask width and etching time was investigated experimentally. A correlation to predict trench width with etching time was derived; normalizing by mask width allows one to collapse the data. The etching rates to obtain micro-scale features, which are of the order of 1–2 µ mm in –1 , are mainly dominated by the mask width due to mass-transport resistance. It is possible to control the pore dimensions, porosity and pore size distributions with this technique. (Some figures in this article are in colour only in the electronic version)

Investigation of titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells

ABSTRACT In a proton exchange membrane electrolyzer cell (PEMEC), liquid/gas diffusion layer (LGDL) is expected to transport electrons, heat, and reactants/products to and from the catalyst layer with minimum voltage, current, thermal, interfacial, and fluidic losses. In addition, carbon materials, which are typically used in PEM fuel cells (PEMFCs), are unsuitable in PEMECs due to the high ohmic potential and highly oxidative environment of the oxygen electrode. In this study, a set of titanium gas diffusion layers with different thicknesses and porosities are designed and examined coupled with the development of a robust titanium bipolar plate. It has been found that the performance of electrolyzer improves along with a decrease in thickness or porosity of the anode LGDL of titanium woven meshes. The ohmic resistance of anode LGDL and contact resistance between anode LGDL and the anode catalyst play dominant roles in electrolyzer performance, and better performance can be obtained by reducing ohmic resistance. Thin titanium LGDLs with straight-through pores and optimal pore morphologies are recommended for the future developments of low-cost LGDLs with minimum ohmic/transport losses.

In situ investigation on ultrafast oxygen evolution reactions of water splitting in proton exchange membrane electrolyzer cells

The oxygen evolution reaction (OER) is a half reaction in electrochemical devices, including low-temperature water electrolysis, which is considered as one of the most promising methods to generate hydrogen/oxygen for the storage of energy. It is affected by many factors, and its mechanism is still not completely understood. A proton exchange membrane electrolyzer cell (PEMEC) with optical access to the surface of anode catalyst layer (CL) coupled with a distinguished high-speed and micro-scale visualization system (HMVS) was developed to in situ investigate OERs. It was revealed in real time that OERs only occur on the anode CL adjacent to liquid/gas diffusion layer (LGDL). The CL electrical conductivity plays a crucial role in OERs on CLs. The large in-plane electrical resistance of CLs becomes a threshold of OERs over the entire CL, and causes a lot of catalyst waste in the middle of LGDL pores. Moreover, the oxygen bubble nucleation, growth, and detachment and the effect of current density on those processes were also characterized. This study proposes a new approach for better understanding the mechanisms of OERs and optimizing the design and fabrication of membrane electrode assemblies.