Development of efficient fuel cell and electrochemical cell devices to retrieve energy in a renewable manner lies in the molecular level understanding of the catalytic chemical transformation processes at surfaces and interfaces. This article demonstrates the use of operando soft x-ray photoelectron spectroscopy to correlate the chemical state of electrode and the electrochemical performance of electrochemical devices. Examples are shown for oxygen reduction reaction on the Pt cathode catalyst in proton exchange membrane fuel cells, and oxygen evolution on the IrO2 anode in water electrolyzer.
Perovskite solar cells (PSCs) are the most promising next generation photovoltaic technology, whose energy conversion efficiency has increased rapidly in a few years. However, in contrast to the rapid increase in efficiency, the fundamental understanding of the device working principles has hardly progressed. It is therefore important to clarify the details of photovoltaic conversion processes for a deeper understanding of the energy loss mechanism. Here, we present a study of carrier separation processes in the PSCs by operando profiling of electrical potential distribution using the cross-sectional Kelvin probe force microscopy. We found that the position of the carrier separation in the PSCs depended on the device structures and the composition of the perovskite film. Our experimental findings showed the importance of the hetero-interfaces in the photovoltaic conversion processes.
It has been one of the key issues to understand the lithium transport in the cathodic materials during the lithium ion battery operation at high charge-discharge rate. Operando transmission electron microscope (TEM) observation is a power method to solve this problem, since such dynamics has been suggested to be a kind of nonequilibrium behaviors. We developed nanobatteries consisting of LiMn2O4 nanowires cathode, liquid ion electrolyte and Li4Ti5O12 crystalline anode for operando TEM observation. By taking electron diffraction patterns of the cathode in the vicinity of the interface, we found that the cubic phase of the LiMn2O4 nanowire changed into two phases of the cubic and orthorhombic phases and finally into the tetragonal phase during discharging process. It suggests that a lot of lithium ions must be accumulated in the vicinity of the interface with the electrolyte. We also investigated the lithium ion transport inside the nanowire cathode.
Solid oxide fuel cells (SOFCs) are expected as a next-generation energy conversion device having high efficiency. In order to improve the performance, durability and reliability of SOFCs, further understandings of the electrode reactions are necessary. For this purpose, the introduction of spectroscopic analytical techniques, combined with conventional electrochemical measurements, would be effective. In a last decade, we have been investigating the electrode reactions in SOFCs by applying the operando X-ray absorption fine structure (XAFS) measurements. In this paper, our recent research activities using operando XAFS analysis are reviewed.
Hydrogen permeation from a metal surface was visualized by two-dimensional mapping of hydrogen ion desorbed by scanning electron beam irradiation. The sample used was stainless steel containing dislocations of martensite ; the sample was 100 µm thick, which is nearly the same as the grain size. There was a good correlation between the patterns of the permeation sites and the grain shape observed in secondary electron images.
Near ambient and ambient pressure photoelectron spectroscopy measurement that uses with hard X-rays (NAP- and AP-HAXPES) were conducted at the BL36XU of SPring-8. To increase the pressure limit in the NAP-HAXPES measurement, we replaced the front cone with our home-made one with an aperture diameter of 30 µm from a standard 300 µm size aperture. By using this equipment, a photoelectron spectrum of the gold foil under atmospheric pressure was successfully measured. In this paper, we summarize the investigations of the electronic states of platinum nanoparticles in a polymer electrolyte fuel cell (PEFC) under the voltage operating conditions using the NAP-HAXPES system. The Pt 3d spectra of the Pt/C cathode catalysts clearly show different behaviors with increasing voltage compared with decreasing voltage. Subsequently, we found the electric potential variation of the electrode in a PEFC by non-contact measurements based on the kinetic energy shift in component potential.
According to the law of friction, the maximum static frictional force is proportional to the normal force. A carbon nanotube (CNT) is a nano-size, flexible material. Vertically-aligned (VA)-CNTs, growing with a high density and vertically to the substrate, can yield great friction force because of a large true contact area. To elucidate the property of nanoscale friction, we measured the VA-CNT's friction force. We used two types of VA-CNTs ; grown with Fe and Co as the catalysts. In the small normal force range, coefficient of static friction was higher than that in the bigger force range. This result is different from the law of friction indicating that coefficient of static friction force is constant. The Fe-catalyzed-CNTs yielded greater friction force than the Co-catalyzed-CNTs did. This can be explained by the morphology and crystallinity of those CNTs. Because the Fe-catalyzed-CNTs can contact the substrate surface closely, they can cause greater friction force.