Zhenye Kang

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.

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 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.

An inkjet-printed capacitive sensor for water level or quality monitoring: investigated theoretically and experimentally

Ink printing is utilized to develop a coplanar capacitive senor with microscale electrodes, which exhibits high sensitivity. The sensor’s capacitance has been theoretically and experimentally correlated to the water resistivity. As a cost-effective, portable and reusable sensor, its application for water level measuring and quality monitoring has been demonstrated.

Micro/nano manufacturing of novel multifunctional layers for hydrogen production from water splitting

Liquid/gas diffusion layers (LGDLs) function as a medium to transport heat, electrons, and reactants/products in/out of the PEMECs simultaneously with minimum losses of electrical interfacial, thermal, and fluidics. As multifunctional layers, they are located between the catalyst layer (CL) and bipolar plate (BP), and play a critical role in enhancing the performance of water splitting in proton exchange membrane electrolyzer cells (PEMECs). With the help of the advanced micro/nano manufacturing methods, novel LGDLs with planar surface, small thickness and well-controllable pore morphologies are fabricated. The new thin LGDLs with different pore morphologies are also in-situ tested in PEMECs and a superior performance has been achieved. This novel thin LGDLs can be a new guide for future research and development towards high-efficiency and low-cost hydrogen energy.

Visualization on rapid and micro-scale dynamics of oxygen bubble evolution in PEMECs

In proton exchange membrane electrolyzer cells (PEMECs), the oxygen generated at the anode side by electrochemical reactions is forced out of cells through the micro channels and liquid/gas diffusion layer, meanwhile, the water flows through the same pathways to reach the reaction sites in an opposite direction, during this process, the complex two-phase counter flow is formed. The two-phase transport conditions strongly affect the PEMEC performance. In this study, a long-working-distance, high-speed, and micro-scale visualization system coupled with a specific designed transparent PEMEC are introduced to investigate the two-phase flow in situ. Optical data can be used to aid in understanding the effects of two-phase flow on the performance of PEMECs. The two-phase flow phenomena in horizontal micro channels of an operating PEMEC is revealed in this study. In addition, the dynamics of gas bubble evolution are also identified. More quantified results will be studied in the future.

Additive manufactured micro-sensor from silver nanoparticles for measuring shear stress and pressure

Additive manufacturing is considered as a revolution in manufacturing. One of this type of manufacturing, three-dimensional (3D) inkjet printing is capable of quickly printing conductive tracks on flexible materials, and have attracted considerable attentions in recent years. To take advantage of this technology, flexible micro-sensors for measuring shear stress and pressure are fabricated from silver nanoparticles with digital models by 3D inkjet printing rapidly. The influence of printing layers and sintering temperature on the chemical and physical properties of micro-sensors are investigated. Surface morphology and electrical resistivity are also examined ex-situ. Test results show the thickness and electrical resistivity of AM micro-sensors deceased significantly as the increase of sintering temperature, which achieve 1.41 µm and 7.64×10−8 Ω·m at the sintering temperature of 250 °C, shear stress and pressure. The novel AM micro-sensors provide a new route to the economic and rapid development of shear stress and pressure sensing.