Competitive Oxygen Evolution in Acid Electrolyte Catalyzed at Technologically Relevant Electrodes Painted with Nanoscale RuO2

Regulation of Electron and Mass Transport Pathways in Efficient and Stable Low-Loading PEM Water Electrolyzers

Improving the utilization of iridium in proton exchange membrane (PEM) water electrolyzer is critical in reducing their cost for future development. Titanium dioxide (TiO2) has notable electrochemical stability at high operating potential and has been developed as a promising support of iridium-based OER nano-catalysts. However, limited by insufficient conductivity, the iridium content on TiO2 support catalysts is normally above 50 wt.%. Herein, support is provided for iridium on conductivity-enhanced TiO2 for low-iridium-loading PEMWE, successfully reducing the iridium content to 28 wt.% by the regulation of electron transport pathway. A new ionomer distribution strategy is then applied to the Ir@Pt@TiO2 catalyst layer to release the iridium sites and regulate the local mass transport pathways in the anode. This work reveals that the catalyst-ionomer interface played an important role in activity and stability in the anode of PEMWE. Building a thin and uniform ionomer distribution on supports with iridium exposure can result in continuous proton and electron transport pathways, promoting bubble escape, and exposing more effective active sites during reaction situations. This work provides a novel perspective for future research on the catalyst-ionomer interface and mass transport in PEMWEs.

Increased Readiness for Water Splitting: NiO-Induced Weakening of Bonds in Water Molecules as Possible Cause of Ultra-Low Oxygen Evolution Potential

The development of non-precious metal-based electrodes that actively and stably support the oxygen evolution reaction (OER) in water electrolysis systems remains a challenge, especially at low pH levels. The recently published study has conclusively shown that the addition of haematite to H2SO4 is a highly effective method of significantly reducing oxygen evolution overpotential and extending anode life. The far superior result is achieved by concentrating oxygen evolution centres on the oxide particles rather than on the electrode. However, unsatisfactory Faradaic efficiencies of the OER and hydrogen evolution reaction (HER) parts as well as the required high haematite load impede applicability and upscaling of this process. Here it is shown that the same performance is achieved with three times less metal oxide powder if NiO/H2SO4 suspensions are used along with stainless steel anodes. The reason for the enormous improvement in OER performance by adding NiO to the electrolyte is the weakening of the intramolecular O─H bond in the water molecules, which is under the direct influence of the nickel oxide suspended in the electrolyte. The manipulation of bonds in water molecules to increase the tendency of the water to split is a ground-breaking development, as shown in this first example.

Fe-Incorporated Ni/MoO2 Hollow Heterostructure Nanorod Arrays for High-Efficiency Overall Water Splitting in Alkaline and Seawater Media

Developing high-efficiency and cost-effective bifunctional catalysts for water electrolysis is fascinating but still remains challenging. Thus, diverse strategies have been utilized to boost the activity toward oxygen/hydrogen evolution reactions (OER/HER) for water splitting. Among them, composition and structure engineering as an effective strategy has received extensive attention. Here, by means of a self-sacrificing template strategy and simultaneous regulation of the composition and structure, Fe-incorporated Ni/MoO₂ heterostructural (NiFe/Fe-MoO₂ ) hollow nanorod arrays are designed and constructed. Benefiting from abundant catalytic active sites, high intrinsic activity, and fast reaction kinetics, NiFe/Fe-MoO₂ exhibits superior OER (η₂₀  = 213 and 219 mV) and Pt-like HER activity (η₁₀  = 34 and 38 mV), respectively, in 1 m KOH and alkaline seawater media. This results in attractive prospects in alkaline water and seawater electrolysis with only voltages of 1.48 and 1.51 V, and 1.69 and 1.73 V to achieve current densities of 10 and 100 mA cm^-2 , respectively, superior to the Pt/C and RuO₂ pair as a benchmark. Undoubtedly, this work provides a beneficial approach to the design and construction of noble-metal-free bifunctional catalysts toward efficient hydrogen production from alkaline water and seawater electrolysis.

Bifunctional WC-Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media

We report the strong catalyst-support interaction in WC-supported RuO₂ nanoparticles (RuO₂ -WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η₁₀ of 347 mV and a mass activity of 1430 A g_Ru ^-1 , eight-fold higher than that of commercial RuO₂ (176 A g_Ru ^-1 ). Theoretical calculations demonstrate that the strong catalyst-support interaction between RuO₂ and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO₂ -WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm^-2 . The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

Recent Progress on Transition Metal Based Layered Double Hydroxides Tailored for Oxygen Electrode Reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), namely, so-called oxygen electrode reactions, are two fundamental half-cell reactions in the energy storage and conversion devices, e.g., zinc–air batteries and fuel cells. However, the oxygen electrode reactions suffer from sluggish kinetics, large overpotential and complicated reaction paths, and thus require efficient and stable electrocatalysts. Transition-metal-based layered double hydroxides (LDHs) and their derivatives have displayed excellent catalytic performance, suggesting a major contribution to accelerate electrochemical reactions. The rational regulation of electronic structure, defects, and coordination environment of active sites via various functionalized strategies, including tuning the chemical composition, structural architecture, and topotactic transformation process of LDHs precursors, has a great influence on the resulting electrocatalytic behavior. In addition, an in-depth understanding of the structural performance and chemical-composition-performance relationships of LDHs-based electrocatalysts can promote further rational design and optimization of high-performance electrocatalysts. Finally, prospects for the design of efficient and stable LDHs-based materials, for mass-production and large-scale application in practice, are discussed.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Plasma-engineered bifunctional cobalt-metal organic framework derivatives for high-performance complete water electrolysis

Metal-organic framework (MOF) derivatives are among the most promising catalysts for the hydrogen evolution reaction (HER) for clean hydrogen energy production. Herein, we report the in situ synthesized MOF-derived CoPO hollow polyhedron nanostructures by simultaneous high temperature annealing and Ar-N₂ radio frequency plasma treatment in the presence of a P precursor and subsequent oxygen incorporation from open air at lower temperature. The optimum Ar-N₂ gas flow rates are used to precisely tune the P/O ratio, cut Co bonds within the MOFs and reconnect Co with P. Consequently, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance are enhanced. Meanwhile, the filling of P elements can effectively change the electronic structure around the catalyst to ensure the uniform distribution of catalytically active sites. The resultant CoPO hollow nanocages with large specific surface areas show excellent bifunctional electrocatalytic activity towards both HER and OER with a low overpotential of 105 and 275 mV and a small Tafel slope of 48 and 52 mV dec^-1, respectively. Our results open a new avenue for precise plasma-assisted engineering of MOF-derived hybrid hetero-structured electrocatalysts with rich oxygen vacancies and P dopants to simultaneously boost both half reactions in water electrolysis.

A bifunctional nanoporous Ni-Co-Se electrocatalyst with a superaerophobic surface for water and hydrazine oxidation

The sluggish kinetics of the oxygen evolution reaction (OER) has severely hindered the energetic convenience of water splitting. Thus, developing a highly efficient catalyst for the OER and replacing the OER with hydrazine oxidation (HzOR) are effective strategies for water electrolysis to achieve sustainable hydrogen production. Herein, bifunctional nanosheet arrays Ni0.6Co0.4Se with a porous structure were fabricated on Ni foam (NF) by the bubble dynamic template method during electrodeposition. Compared with CoSe2 and NiSe2, Ni0.6Co0.4Se exhibits excellent electrocatalytic performance for both the OER and HzOR. A low overpotential of only 249 mV is required to drive 10 mA cm-2, and a retention rate of nearly 100% after 24 h at 10 mA cm-2 is observed for Ni0.6Co0.4Se towards the OER. By substituting the OER by HzOR, an extremely high current density of 300 mA cm-2 at 0.4 V vs. RHE and a retention rate of 86.8% at 200 mA cm-2 after 12 h can be achieved. Interestingly, the mechanistic reason for the enhanced catalytic ability of Ni0.6Co0.4Se was studied, which is associated with the synergistic effects of Ni and Co, large ECSA, high electrical conductivity and most importantly the superaerophobic nature induced by the porous structure of Ni0.6Co0.4Se. The non-noble metal bifunctional electrocatalyst demonstrates a promising potential for application in both the OER and HzOR.

Coupling Interface Constructions of MoS2 /Fe5 Ni4 S8 Heterostructures for Efficient Electrochemical Water Splitting

Water splitting is considered as a pollution-free and efficient solution to produce hydrogen energy. Low-cost and efficient electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are needed. Recently, chemical vapor deposition is used as an effective approach to gain high-quality MoS₂ nanosheets (NSs), which possess excellent performance for water splitting comparable to platinum. Herein, MoS₂ NSs grown vertically on FeNi substrates are obtained with in situ growth of Fe₅ Ni₄ S₈ (FNS) at the interface during the synthesis of MoS₂ . The synthesized MoS₂ /FNS/FeNi foam exhibits only 120 mV at 10 mA cm^-2 for HER and exceptionally low overpotential of 204 mV to attain the same current density for OER. Density functional theory calculations further reveal that the constructed coupling interface between MoS₂ and FNS facilitates the absorption of H atoms and OH groups, consequently enhancing the performances of HER and OER. Such impressive performances herald that the unique structure provides an approach for designing advanced electrocatalysts.

Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting

Layered double hydroxide (LDH)-based materials have attracted widespread attention in various applications due to their unique layered structure with high specific surface area and unique electron distribution, resulting in a good electrocatalytic performance. Moreover, the existence of multiple metal cations invests a flexible tunability in the host layers; the unique intercalation characteristics lead to flexible ion exchange and exfoliation. Thus, their electrocatalytic performance can be tuned by regulating the morphology, composition, intercalation ion, and exfoliation. However, the poor conductivity limits their electrocatalytic performance, which therefore has motivated researchers to combine them with conductive materials to improve their electrocatalytic performance. Another factor hampering their electrocatalytic activity is their large lateral size and the bulk thickness of LDHs. Introducing defects and tuning electronic structure in LDH-based materials are considered to be effective strategies to increase the number of active sites and enhance their intrinsic activity. Given the unique advantages of LDH-based materials, their derivatives have been also used as advanced electrocatalysts for water splitting. Here, recent progress on LDHs and their derivatives as advanced electrocatalysts for water splitting is summarized, current strategies for their designing are proposed, and significant challenges and perspectives of LDHs are discussed.

Steel-based electrocatalysts for efficient and durable oxygen evolution in acidic media

Upon electro-oxidation in LiOH, Ni42 alloy was rendered in an OER electrocatalyst that efficiently and durably works in the acidic regime.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)₆]^3-/4-) and notoriously slow ([Fe(H₂O)₆]^3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)₆]^3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO₂. For the classically outer-sphere [Fe(H₂O)₆]^3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.