Unitized regenerative fuel cells (URFCs), which oxidize hydrogen to water to generate electrical power under the fuel cells (FCs) mode and electrolyze water to hydrogen under the water electrolysis (WE) mode for recycling, are known as clean and sustainable energy conversion devices. In contrast to the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on the hydrogen electrode side, the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the oxygen electrode side requires highly efficient bifunctional oxygen catalysts. Conventional precious metal catalysts combine Pt and IrO₂ with excellent ORR and OER activities to achieve bifunctional electrocatalysis performance, but the scarcity and high cost of precious metals have restricted their applications. Although platinum group metal (PGM)-free bifunctional catalysts circumvent the problems of high price and scarce resources, they suffer from insufficient activity and poor stability. Therefore, much attention has been paid by researchers on developing efficient, durable and low-cost bifunctional oxygen catalysts. In this review, we mainly introduce the recent advances in bifunctional oxygen catalysts for URFCs focusing on the catalyst design, activity, and durability. First of all, the fundamental understanding of the ORR and OER mechanisms is essential prior to discussing the development of bifunctional oxygen catalysts. Starting from activity descriptor-based approaches in the identification of catalyst activity, this review summarizes the alternative catalyst design strategies confronted with the unfavorable scaling relationship existing among the binding energies of different oxygen-containing reaction intermediates during ORR and OER. Subsequently, in addition to introducing the design strategies of conventional PGM-based bifunctional catalysts, the recent progress of PGM-free bifunctional catalysts, including perovskite oxides, spinel oxides, other transition metal compounds, and carbon-based (non-metal) catalysts, is presented in terms of their structure-property relationship. Various strategies have been developed by researchers to optimize the performance of PGM-free bifunctional catalysts, such as nanostructuring, defects engineering, heteroatom doping, phase and composition modulation, support coupling and morphology engineering, etc. Some PGM-free bifunctional catalysts reported in the literature show promising ORR and OER activities superior to Pt+IrO₂ in an alkaline environment. In general, although great progress has been made on PGM-free bifunctional electrocatalysts, their cycling durability is still far from that of precious metal catalysts, and few of them have been applied in acidic environments. Therefore, much more efforts are needed to improve the stability of PGM-free bifunctional catalysts. Lastly, the challenge and future development of designing optimal bifunctional oxygen catalysts are discussed.

Band alignments of electrode-water interfaces are of crucial importance for understanding electrochemical interfaces. In the scenario of electrocatalysis, applied potentials are equivalent to the Fermi levels of metals in the electrochemical cells; in the scenario of photo(electro)catalysis, semiconducting oxides under illumination have chemical reactivities toward redox reactions if the redox potentials of the reactions straddle the conduction band minimums (CBMs) or valence band maximums (VBMs) of the oxides. Computational band alignments allow us to obtain the Fermi level of metals, as well as the CBM and VBM of semiconducting oxides with respect to reference electrodes. In this tutorial, we describe how to obtain the band alignments using ab initio molecular dynamics simulations. To be simple, we introduce the protocol of computational band alignments through two selected charge-neutral interfaces, i.e., Cu(100)- and SnO₂(110)-water interfaces. It should be bear in mind that one can also apply this protocol to electrified interfaces. The band alignments at charge-neutral interfaces have different meanings for metals and semiconducting oxides. For metals, the alignments amount to Potentials of Zero Charge of metals, under which the metal-water interfaces possess zero net charge. For semiconducting oxides, the alignments show the positions of CBMs and VBMs under a special pH and potential. The special pH is named as Point of Zero Charge and the special potential is called Flat-Band Potential. The oxides-water interfaces have zero net charge if they are at the special pH and potential. It is worth noting that neither the positions of CBMs nor VBMs are directly interpreted as applied potentials. In the protocol, we refer computed levels to standard hydrogen electrode (SHE), and thus directly compare the levels with those from electrochemical experiments. With PBE functional, the computed Fermi level of Cu(100) is -0.726 V with respect to SHE and matches the experimental determination of -0.73 V (SHE). The CBM and VBM of SnO₂(110), however, are computed as 1.76 V and 0.6 V (SHE), respectively, which fails to match the experimental values of 3.747 V and 0.147 V (SHE), respectively. We attribute the failure to the delocalization error of density functional theory. Because of the error, DFT tends to spatially delocalize one-electron orbitals, which occasionally has negligible influences on the Fermi level of metal, but significantly underestimates the band gaps of semiconducting oxides.

Due to the small size at least in one dimension (< 25 μm), ultramicroelectrode (UME) has small electric-double-layer capacitance, low IR drop, rapid mass transfer rate, fast response, high signal/noise ratio and high spatiotenporal resolution. UME is qualified not only to study the kinetics of fast electrode processes, but also to act as the probe of scanning electrochemical microscopies to obtain the localized chemical or electrochemical reactivity of the substrates. Thus, UMEs play a significant role in various research domains of electrochemistry, and have become an important electrochemical experimental method. Herein, we will introduce the basic principles, a simple fabrication method and voltammetric experimental protocols of UME, providing a guide to carry out the UME experiments.

Electrochemical scanning tunneling microscopy (ECSTM) plays an important role in the field of electrochemistry, which can obtain potential-dependent structural information of electrode surface with high spatial resolution and observe some reaction processes in electrolyte solutions, and provide a powerful way to understand the interfacial structure and electrode processes from the perspective of high spatial resolution. In this article, the study of electrodeposition of Cu on Au (111) by ECSTM is taken as an example to introduce the experimental methods required for ECSTM and share our experience with other electrochemical groups. Firstly, the working principle of STM is introduced so that readers can understand the imaging principle of STM. Secondly, we describe the process in detail and the points for attention during ECSTM experiments, which include the cleanings of the electrochemical cell and O-ring, the preparation and encapsulation of the ECSTM tip, the preparation and cleaning of the working electrode, and the selections of the counter electrode and reference electrode. Thirdly, the ECSTM study on the initial stage of Cu electrodeposition on Au(111) has been taken to demonstrate the experimental procedure of ECSTM, and shown its ability to obtain image with high spatial resolution. We have analyzed the two interfacial structures obtained by ECSTM at two different potential regions according to the cyclic voltammetric curve of Cu UPD on Au (111) electrode. The high-resolution atomic image obtained at a relatively positive potential is assigned to the image of Au atoms. Further, it has been demonstrated that the ($\sqrt{3}$×$\sqrt{3}$)R30° structure can be observed after the completion of the submonolayer of Cu on Au(111) in the sulfuric acid solution. However, the ($\sqrt{3}$×$\sqrt{3}$)R30° structure obtained between the two pairs of current peaks of cyclic voltammetric curve should be assigned to the adsorption of sulfate anions, which occupy the centers of honeycomb lattice formed by Cu adatoms. The ECSTM images demonstrate the ability of this technique to achieve high-resolution imaging in electrochemical environments. The analysis shows that caution should be taken when analyzing ECSTM images due to the lack of chemical recognition ability and that it is important to combine ECSTM with other experimental techniques or theoretical methods to analyze the obtained data.