Understanding the mechanisms and fundamentals inside the EESs are critical to improve their performances by boosting the favorable processes and suppressing the detrimental ones. Various ex situ techniques have been used for studying EESs, which provide valuable spatial and temporal information. [6-8] Nevertheless, ex situ methods are limited by the time delay and potential impacts during the operation of removing samples from their initial environments to characterization locations. [9] The results might sometimes be missing or even misleading. This elicits the requirement for in situ/operando methods to conduct real-time observations on reactions and processes inside a system. In situ analytical techniques can provide more realistic information on the ongoing chemical and physical processes in an electrochemical cell. [10] Furthermore, correlating in situ information with electrochemical results obtained synchronously from the same device enables to build a direct cause-and-effect relationship, leading to authentic conclusions. [11] Remarkable progress has been made with advanced characterization techniques including optical, [12] X-ray, [13] electron microscopy, [14] neutron techniques. [15] The techniques work on very different principles. Optical microscopy is based on transmitting light to construct images for the samples. It has a wide effective characterization scales from centimeter to lower than micrometer. X-ray characterization techniques utilize electromagnetic radiation to magnify the samples. X-ray methods, including diffraction, absorption spectroscopy, photoelectron spectroscopy and X-ray microscopy, provide crucial information from mesoscale morphology to microscale crystal structure and elemental content of electrode materials. [16] Electron micro scopy probes the samples with electron beams. [17] The techniques based on neutron are quite similar as electron, and neutrons are preferable for analyses of light elements such as lithium ion (Li +). Also, Neutrons can go in more penetration depth with a weaker interaction with matter. [18] With the employment of these techniques, the knowledge of mechanisms of EESs has been significantly enhanced. On the other hand, in situ/operando experiments are restricted under certain circumstances, as they generally require special designs and modifications of the unit structures. Compared with other techniques, optical microscopy is preferable for in situ/operando studies considering the basic equipment needed, nonvacuum manipulation, easy access and operation, as well as its nondestructive nature. [19] However, inspections based on optical microscopy require access within the systems for transmitting light. This review discusses a range of in situ/operando techniques based on optical microscopy reported in literatures for studying electrochemical energy systems. Compared to other techniques (scanning probe microscopy, electron microscopy, X-ray microscopy), optical microscopy offers many advantages including the simplicity of the instrument and operation, co...

Different dimensional supports for non-precious-metal single-atom catalysts play significant roles in their fabrication and electrocatalytic activities for oxygen redox reactions.

In recent years, the use of additive manufacturing (AM) has been demonstrated in the fabrication of components in polymer electrolyte membrane fuel cell, solid oxide fuel cell (SOFC), microbial fuel cell (MFC) and laminar flow-based fuel cell (LFFC). Various AM technologies have been successfully demonstrated in fuel cell manufacturing include material extrusion, powder bed fusion, vat photopolymerization and binder jetting. One of the unique advantages of AM is the ability to handle sophisticated design with features ranging from macro to micro scales, which are inaccessible by conventional manufacturing technologies. Well-designed complex 3D structures were reported to have the potential for increasing the performance of fuel cells. Therefore, AM presents itself as a promising fabrication method to promote the development of fuel cells. Besides, AM also showed its specialty in time-saving, flexibility, and on-demand manufacturability in the fabrication of fuel cell components. In spite of the prospects of AM in fuel cells, more studies and researches are required to overcome challenges, such as availability of material, manufacturing quality, and the costs. This review focuses on the advantages and applications of additive manufacturing that enable improvement in fuel cell performance. The critical challenges and directions for future development are also highlighted.

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