Dasom Jeon

Despite intensive studies over decades, the development of electrocatalysts for acidic water splitting still relies on platinum group metals, especially Pt and Ir, which are scarce, expensive, and poorly sustainable. Because such problems can be alleviated, Ru‐based bifunctional catalysts such as rutile RuO₂ have recently emerged. However, RuO₂ has a relatively low activity for hydrogen evolution reactions (HER) and low stability for oxygen evolution reactions (OER) under acidic conditions. In this study, the synthesis of a RuO_x‐based bifunctional catalyst (RuSiW) for acidic water splitting via the electrochemical evolution from Ru‐based polyoxometalates at cathodic potentials is reported. RuSiW consists of the nanocrystalline RuO₂ core and Si,W‐codoped RuO_x shell. RuSiW exhibits outstanding HER and OER activity comparable to Pt/C and RuO₂, respectively, with high stability. Computational analysis suggests that the codoping of RuO_x with W and Si synergistically improves the HER activity of otherwise poor RuO₂ by shifting the d‐band center and optimizing atomic configurations beneficial for proper hydrogen adsorption. This study provides insights into the design and synthesis of unprecedented bifunctional electrocatalysts using catalytically inactive and less explored elements, such as Si and W.

Facile removal of adsorbed gas bubbles from electrode surfaces is crucial to realize efficient and stable energy conversion devices based on electrochemical gas evolution reactions. Conventional studies on bubble removal have limited applicability and scalability due to their reliance on complex and energy/time‐intensive processes. In this study, a simple and versatile method is reported to fabricate large‐area superaerophobic electrodes (up to 100 cm²) for diverse gas evolution reactions using the gel‐like aerophobic surface system (GLASS). GLASS electrodes are readily and uniformly fabricated by simple spin‐coating and cross‐linking of polyallylamine on virtually any kinds of electrodes within 5 min under ambient conditions. Intrinsically hydrophilic gel overlayers with interconnected open pores allow the physical separation of bubble adhesion and catalytic active sites, reducing bubble adhesion strength, and promoting the removal of gas bubbles. As a result, GLASS electrodes exhibit greatly enhanced efficiency and stability for diverse gas evolution reactions, such as hydrogen evolution, hydrazine oxidation, and oxygen evolution reactions. This study provides deeper insights into understanding the effect of the hydrophilic microenvironment on gas evolution reactions and designing practical electrochemical devices.

Although development and utilization of efficient catalysts with earth-abundant and cheap elements is desired, precious noble metal-based catalysts are still widely used and studied due to the urgent need to address energy and environmental issues. Polyoxometalates (POMs) can be excellent candidates in this context. In this study, we found that oxo-bridged tetraruthenium polyoxometalate (RuPOM) exhibits excellent electrocatalytic activity for both oxygen evolution and reduction reactions (OER and ORR) with minimal use of noble elements and can be used for the development of efficient seawater batteries (SWBs). The deposition of RuPOM on a desired electrode with conducting carbon Ketjen black (KB) by the simple slurry coating method imparted bifunctional OER/ORR activity to the underlying electrode. Although the mass activity was similar, RuPOM/KB mixtures exhibited superior activity even compared to commercially available Pt/C when comparing the activity per noble metal element. Based on these findings, we employed RuPOM to develop efficient SWBs. RuPOM significantly lowered the charging potential and increased the discharging potential of SWBs, which are related to OER and ORR, respectively. This study can provide insights for the development of POM-based electrocatalysts and their application in energy-storage and conversion devices.

A simple modification of electrodes with hydrogel can facilitate hydrogen evolution reactions by readily eliminating gas bubbles.

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO₄ modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy (EIS). In particular, we prepared BiVO₄ photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO₄ due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation, but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalates (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (~ 369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

We report the development of semiconducting melanin‐based organic/inorganic hybrid photoanodes for solar water oxidation. Synthetic melanin thin‐film incorporating polyoxometalate (POM) water oxidation catalysts (WOCs) are readily deposited on the surface of various n‐type inorganic semiconductors (e.g., Fe₂O₃, BiVO₄, and TiO₂) by electropolymerization. The deposition of melanin and POM hybrid (MP) thin‐film results in the remarkably improved performance of an underlying semiconductor photoanode for solar water oxidation with a significantly increased photocurrent density and decreased onset potential for water oxidation through the formation of a melanin‐based p–n heterojunction structure. We believe that this study can provide insights into the design and fabrication of various melanin‐based optoelectronic devices by utilizing its unique physicochemical properties.

We report a fully solution processable bias-free photoelectrochemical (PEC) cell for overall solar water splitting using a Cu₂O photocathode and a BiVO₄ photoanode. They were modified with catalytic multilayers (CMs) for hydrogen and oxygen evolution reactions (HER and OER), respectively, using simple and versatile layer-by-layer (LBL) assembly. The deposition of the respective CMs composed of cationic polyelectrolyte and anionic molecular polyoxometalate (POM) electrocatalysts resulted in remarkably improved performance in terms of both photocatalytic activity and stability. Detailed PEC analysis revealed that the CMs not only facilitated (1) charge transport to and (2) catalytic charge transfer at the photoelectrode/electrolyte interface, but also suppressed the deactivation of photoelectrodes, especially a labile Cu₂O photocathode, by the formation of a protective coating layer. As a result, we could successfully fabricate an efficient and stable PEC cell for unassisted solar water splitting without the use of toxic/hazardous chemicals and energetic processes, which have been inevitably employed. We believe that this study provides a simple and versatile platform for the design and fabrication of various forms of electrochemical PEC devices in an environmentally friendly manner.

Water splitting is considered the most attractive pursuit in the field of solar energy conversion. In this study, we report the synthesis and application of a supramolecular hybrid of carbon nanodot (CD) and cobalt polyoxometalate (Co-POM) to solar water oxidation. The self-assembly of the alginate-based CD and Co-POM led to the formation of a spherical hybrid of CD/Co-POM. Owing to the facile transfer of photogenerated holes from CD under visible light irradiation, the hybrid donor–acceptor type of CD/Co-POM enabled the rapid scavenging of holes and accumulation of a long-lived oxidation state of Co-POM for efficient solar water oxidation, outperforming conventional [Ru(bpy)₃]²⁺-based systems. We believe that this study offers new insights into the development of CD-based nanocomposites with various photocatalytic and optoelectronic applications.