Tetraruthenium Polyoxometalate as an Atom-Efficient Bifunctional Oxygen Evolution Reaction/Oxygen Reduction Reaction Catalyst and Its Application in Seawater Batteries

Boosting Oxygen Reduction Reaction Selectivity in Metal Nanoparticles with Polyoxometalates

The lack of selectivity toward the oxygen reduction reaction (ORR) in metal nanoparticles can be linked to the generation of intermediates. This constitutes a crucial constraint on the performance of specific electrochemical devices, such as fuel cells and metal-air batteries. To boost selectivity of metal nanoparticles, a novel methodology that harnesses the unique electrocatalytic properties of polyoxometalates (POM) to scavenge undesired intermediates of the ORR (such as HO2 -) promoting selectivity is proposed. It involves the covalent functionalization of metal nanoparticle's surface with an electrochemically active capping layer containing a new sulfur-functionalized vanadium-based POM (AuNP@POM). To demonstrate this approach, preformed thiolate Au(111) nanoparticles with a relatively poor ORR selectivity are chosen. The dispersion of AuNP@POM on the surface of carbon nanofibers (CNF) enhances oxygen diffusion, and therefore the ORR activity. The resulting electrocatalyst (AuNP@POM/CNF) exhibits superior stability against impurities like methanol and a higher pH tolerance range compared to the standard commercial Pt/C. The work demonstrates for the first time, the use of a POM-based electrochemically active capping layer to switch on the selectivity of poorly selective gold nanoparticles, offering a promising avenue for the preparation of electrocatalyst materials with improved selectivity, performance, and stability for ORR-based devices.

A hydrogen radical pathway for efficacious electrochemical nitrate reduction to ammonia over an Fe-polyoxometalate/Cu electrocatalyst

Electrochemical nitrate (NO3-) reduction to ammonia (NH3), which is a high value-added chemical or high-energy density carrier in many applications, could become a key process overcoming the disadvantages of the Haber-Bosch process; however, current electrocatalysts have severe drawbacks in terms of activity, selectivity, and stability. Here, we report the hydrogen radical (H*) pathway as a solution to overcome this challenge, as demonstrated by efficacious electrochemical NO3- reduction to NH3 over the Fe-polyoxometalate (Fe-POM)/Cu hybrid electrocatalyst. Fe-POM, composed of Preyssler anions ([NaP5W30O110]14-) and Fe cations, facilitates efficient H* generation via H2O + e- → H* + OH-, and H* transfer to the Cu sites of the Fe-POM/Cu catalyst enables selective NO3- reduction to NH3. Operando spectroelectrochemical spectra substantiate the occurrence of the H* pathway through direct observation of Fe redox related to H* generation and Cu redox related to NO3- binding. With the H* pathway, the Fe-POM/Cu electrodes exhibit high activity for NO3- reduction to NH3 with 1.44 mg cm-2 h-1 in a 500 ppm NO3-/1 M KOH solution at -0.2 V vs. RHE, which is about 36-fold higher than that of the pristine Cu electrocatalyst. Additionally, it attains high selectivity with a faradaic efficiency of up to 97.09% at -0.2 V vs. RHE while exhibiting high catalytic stability over cycles.

Bipolar Membrane Seawater Splitting for Hydrogen Production: A Review

The growing demand for clean energy has spurred the quest for sustainable alternatives to fossil fuels. Hydrogen has emerged as a promising candidate with its exceptional heating value and zero emissions upon combustion. However, conventional hydrogen production methods contribute to CO2 emissions, necessitating environmentally friendly alternatives. With its vast potential, seawater has garnered attention as a valuable resource for hydrogen production, especially in arid coastal regions with surplus renewable energy. Direct seawater electrolysis presents a viable option, although it faces challenges such as corrosion, competing reactions, and the presence of various impurities. To enhance the seawater electrolysis efficiency and overcome these challenges, researchers have turned to bipolar membranes (BPMs). These membranes create two distinct pH environments and selectively facilitate water dissociation by allowing the passage of protons and hydroxide ions, while acting as a barrier to cations and anions. Moreover, the presence of catalysts at the BPM junction or interface can further accelerate water dissociation. Alongside the thermodynamic potential, the efficiency of the system is significantly influenced by the water dissociation potential of BPMs. By exploiting these unique properties, BPMs offer a promising solution to improve the overall efficiency of seawater electrolysis processes. This paper reviews BPM electrolysis, including the water dissociation mechanism, recent advancements in BPM synthesis, and the challenges encountered in seawater electrolysis. Furthermore, it explores promising strategies to optimize the water dissociation reaction in BPMs, paving the way for sustainable hydrogen production from seawater.

Electrochemical Evolution of Ru-Based Polyoxometalates into Si,W-Codoped RuOx for Acidic Overall Water Splitting

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 RuOx -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 RuOx 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 RuOx 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.

Tunable Ordered Nanostructured Phases by Co-assembly of Amphiphilic Polyoxometalates and Pluronic Block Copolymers

The assembly of polyoxometalate (POM) metal-oxygen clusters into ordered nanostructures is attracting a growing interest for catalytic and sensing applications. However, assembly of ordered nanostructured POMs from solution can be impaired by aggregation, and the structural diversity is poorly understood. Here, we present a time-resolved small-angle X-ray scattering (SAXS) study of the co-assembly in aqueous solutions of amphiphilic organo-functionalized Wells-Dawson-type POMs with a Pluronic block copolymer over a wide concentration range in levitating droplets. SAXS analysis revealed the formation and subsequent transformation with increasing concentration of large vesicles, a lamellar phase, a mixture of two cubic phases that evolved into one dominating cubic phase, and eventually a hexagonal phase formed at concentrations above 110 mM. The structural versatility of co-assembled amphiphilic POMs and Pluronic block copolymers was supported by dissipative particle dynamics simulations and cryo-TEM.

Sacrificial Catalyst of Carbothermal-Shock-Synthesized 1T-MoS2 Layers for Ultralong-Lifespan Seawater Battery

A Pt-nanoparticle-decorated 1T-MoS₂ layer is designed as a sacrificial electrocatalyst by carbothermal shock (CTS) treatment to improve the energy efficiency and lifespan of seawater batteries. The phase transition of MoS₂ crystals from 2H to metallic 1T─induced by the simple but potent CTS treatment─improves the oxygen-reduction-reaction (ORR) activity in seawater catholyte. In particular, the MoS₂-based sacrificial catalyst effectively decreases the overpotential during charging via edge oxidation of MoS₂, enhancing the cycling stability of the seawater battery. Furthermore, Pt nanoparticles are deposited onto CTS-MoS₂ via an additional CTS treatment. The resulting specimen exhibits a significantly low charge/discharge potential gap of Δ0.39 V, high power density of 6.56 mW cm^-2, and remarkable cycling stability up to ∼200 cycles (∼800 h). Thus, the novel strategy reported herein for the preparation of Pt-decorated 1T-MoS₂ by CTS treatment could facilitate the development of efficient bifunctional electrocatalysts for fabricating seawater batteries with long service life.

Carbazole-based bis-imidazole ligand-involved synthesis of inorganic–organic hybrid polyoxometalates as electrochemical sensors for detecting bromate and efficient catalysts for selective oxidation of thioether

Considering the potential application on preparing electrode and catalyst materials of inorganic–organic hybrid polyoxometalates, a bis-imidazole ligand with carbazole as a connector, 3,6-di(1H-imidazol-1-yl)-9H-carbazole (L), was used for preparing inorganic–organic hybrid polyoxometalates. As a result, three complexes formulated by [NiL₂(Mo₂O₇)] (1), [Cu(H₂O)₂(HL)₂ (β-Mo₈O₂₆)]·H₂O (2) and [Ni₂(H₂O)₄L₂ (CrMo₆(OH)₅O₁₉)]·6H₂O (3) were obtained successfully. Structural analysis indicated that the different polyoxoanions and metal ions showed important influences on the formation of structures. In the presence of Ni²⁺ ions and heptamolybdate, a 2D network constructed from Ni²⁺ ions and L ligands was formed in complex 1, in which the [Mo₄O₁₄]⁴⁻ polyoxoanions were encapsulated. But the use of Cu²⁺ ions led to a 1D chain of complex 2, which was composed of [β-Mo₈O₂₆]⁴⁻ polyoxoanions and mononuclear {CuL₂} units. By utilizing [CrMo₆(OH)₅O₁₉]⁴⁻ as the inorganic building block, complex 3 showed a 2D (4, 4)-connected layer. Complexes 1–3 could be employed as electrode materials for sensing bromate with the limits of detection of 0.315 μM for 1, 0.098 μM for 2 and 0.551 μM for 3. Moreover, these complexes showed efficient catalytic activity for the selective oxidation of thioethers.

Three inorganic–organic hybrid polyoxometalates were prepared using a bis-imidazole ligand featuring carbazole as a connector, exhibiting not only diverse structures, but also good electrochemical sensing activities for bromate, as well as efficient catalytic performances for oxidation of thioether.

Molecular design of heterogeneous electrocatalysts using tannic acid-derived metal-phenolic networks

Electrochemistry could play a critical role in the transition to a more sustainable society by enabling the carbon-neutral production and use of various chemicals as well as efficient use of renewable energy resources. A prerequisite for the practical application of various electrochemical energy conversion and storage technologies is the development of efficient and robust electrocatalysts. Recently, molecularly designed heterogeneous catalysts have drawn great attention because they combine the advantages of both heterogeneous solid and homogeneous molecular catalysts. In particular, recently emerged metal-phenolic networks (MPNs) show promise as electrocatalysts for various electrochemical reactions owing to their unique features. They can be easily synthesized under mild conditions, making them eco-friendly, form uniform and conformal thin films on various kinds of substrates, accommodate various metal ions in a single-atom manner, and have excellent charge-transfer ability. In this minireview, we summarize the development of various MPN-based electrocatalysts for diverse electrochemical reactions, such as the hydrogen evolution reaction, the oxygen evolution reaction, the CO₂ reduction reaction, and the N₂ reduction reaction. We believe that this article provides insight into molecularly designable heterogeneous electrocatalysts based on MPNs and guidelines for broadening the applications of MPNs as electrocatalysts.

Electrochemically Deposited Amorphous Cobalt-Nickel-Doped Copper Oxide as an Efficient Electrocatalyst toward Water Oxidation Reaction

Production of hydrogen through water splitting is one of the green and the most practical solutions to cope with the energy crisis and greenhouse effect. However, oxygen evolution reaction (OER) being a sluggish step, the use of precious metal-based catalysts is the main impediment toward the viability of water splitting. In this work, amorphous copper oxide and doped binary- and ternary-metal oxides (containing CoII, NiII, and CuII) have been prepared on the surface of fluorine-doped tin oxide by a facile electrodeposition route followed by thermal treatment. The fabricated electrodes have been employed as efficient binder-free OER electrocatalysts possessing a high electrochemical surface area due to their amorphous nature. The cobalt-nickel-doped copper oxide (ternary-metal oxide)-based electrode showed promising OER activity with a high current density of 100 mA cm-2 at 1.65 V versus RHE that escalates to 313 mA cm-2 at 1.76 V in alkaline media at pH 14. The high activity of the ternary-metal oxide-based electrode was further supported by a smaller semicircle in the Nyquist plot. Furthermore, all metal-oxide-based electrodes offered high stability when tested for continuous production of oxygen for 50 h. This work highlights the synthesis of efficient and cost-effective amorphous metal-based oxide catalysts to execute electrocatalytic OER employing an electrodeposition approach.

Different Anderson-type polyoxometalate-based metal–organic complexes exhibiting –OH group-directed structures and electrochemical sensing performance

Four new Anderson-type polyoxometalate (POM)-based metal–organic complexes were hydrothermally synthesized and the –OH groups in POMs have significant effects on their structures and electrochemical performance.