Elucidating Hydrogen Oxidation/Evolution Kinetics in Base and Acid by Enhanced Activities at the Optimized Pt Shell Thickness on the Ru Core

Ring-shaped cavity anchor Pt to derive Pt/WO3-x heterointerfaces for efficient hydrogen evolution in acidic water and seawater

Developing novelplatinum (Pt)-based hydrogen evolution reaction (HER) catalysts with high activity and stability is significant for the ever-broader applications of hydrogen energy. However, achieving precise modulation of the ultrafine Pt nanoparticles coordination environment in conventional catalysts is challenging. In this work, we developed a unique "ring-shaped cavity induced" strategy to anchor the Ptx through the ring-shaped cavity of polyoxometalates (POMs) Na33H7P8W48O184 (denoted as P8W48). The NayPtx[P8W48O184] (PtxP8W48) was in-situ converted into abundant Pt/WO3-x heterostructure with Pt (∼2 nm) and highly depressed Pt-O-W heterointerfaces. Pt/WO3-x nanoparticles supported on highly conductive rGO exhibit superior HER activity. The overpotentials of the catalyst are only 2.8 mV and 4.7 mV at 10 mA·cm-2 in acidic water and seawater, far superior to commercial 20 % Pt/C catalyst. Additionally, the catalyst can be stabilized at a current density of 30 mA·cm-2 for 180 h. This study provides a feasible strategy for rational design of Pt-based catalysts for renewable energy applications.

Recent Advances in Revealing the Electrocatalytic Mechanism for Hydrogen Energy Conversion System

In light of the intensifying global energy crisis and the mounting demand for environmental protection, it is of vital importance to develop advanced hydrogen energy conversion systems. Electrolysis cells for hydrogen production and fuel cell devices for hydrogen utilization are indispensable in hydrogen energy conversion. As one of the electrolysis cells, water splitting involves two electrochemical reactions, hydrogen evolution reaction and oxygen evolution reaction. And oxygen reduction reaction coupled with hydrogen oxidation reaction, represent the core electrocatalytic reactions in fuel cell devices. However, the inherent complexity and the lack of a clear understanding of the structure-performance relationship of these electrocatalytic reactions, have posed significant challenges to the advancement of research in this field. In this work, the recent development in revealing the mechanism of electrocatalytic reactions in hydrogen energy conversion systems is reviewed, including in situ characterization and theoretical calculation. First, the working principles and applications of operando measurements in unveiling the reaction mechanism are systematically introduced. Then the application of theoretical calculations in the design of catalysts and the investigation of the reaction mechanism are discussed. Furthermore, the challenges and opportunities are also summarized and discussed for paving the development of hydrogen energy conversion systems.

Modelling the activity trend of the hydrogen oxidation reaction under constant potential conditions

A microkinetic model is constructed for the electrocatalytic alkaline hydrogen oxidation reaction based on grand canonical density functional theory calculations and linear relationships with the adsorption energies of hydrogen and hydroxide as descriptors. Using this model, the activity trend suitable for efficient catalyst screening has been identified.

Engineering active and robust alloy-based electrocatalyst by rapid Joule-heating toward ampere-level hydrogen evolution

Rational design of bimetallic alloy is an effective way to improve the electrocatalytic activity and stability of Mo-based cathode for ampere-level hydrogen evolution. However, it is still critical to realise desirable syntheses due to the wide reduction potentials between different metal elements and uncontrollable nucleation processes. Herein, we propose a rapid Joule heating method to effectively load RuMo alloy onto MoOx matrix. As-prepared catalyst exhibits excellent stability (2000 h @ 1000 mA cm-2) and ultralow overpotential (9 mV, 18 mV and 15 mV in 1 M KOH, 1 M PBS, 0.5 M H2SO4 solution, respectively) at 10 mA cm-2. Based on first-principle simulations and operando measurements, the impressive electrocatalytic stability and activity are investigated. And the role of rapid Joule heating method is highlighted and discussed in details. This study showcases rapid Joule heating as a feasible strategy to construct highly efficient alloy-based electrocatalysts.

Unraveling Stoichiometry Effect in Nickel-Tungsten Alloys for Efficient Hydrogen Oxidation Catalysis in Alkaline Electrolytes

Anion-exchange membrane fuel cells provide the possibility to use platinum group metal-free catalysts, but the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics and its source is still debated. Here, over nickel-tungsten (Ni-W) alloy catalysts, we show that the Ni : W ratio greatly governs the HOR performance in alkaline electrolyte. Experimental and theoretical studies unravel that alloying with W can tune the unpaired electrons in Ni, tailoring the potential of zero charge and the catalytic surface to favor hydroxyl adsorption (OHad). The OHad species coordinately interact with potassium (K+) ions, which break the K+ solvation sheath to leave free water molecules, yielding an improved connectivity of hydrogen-bond networks. Consequently, the optimal Ni17W3 alloy exhibits alkaline HOR activity superior to the state-of-the-art platinum on carbon (Pt/C) catalyst and operates steadily with negligible decay after 10,000 cycles. Our findings offer new understandings of alloyed HOR catalysts and will guide rational design of next-generation catalysts for fuel cells.

Immobilizing Ordered Oxophilic Indium Sites on Platinum Enabling Efficient Hydrogen Oxidation in Alkaline Electrolyte

Addressing the sluggish kinetics in the alkaline hydrogen oxidation reaction (HOR) is a pivotal yet challenging step toward the commercialization of anion-exchange membrane fuel cells (AEMFCs). Here, we have successfully immobilized indium (In) atoms in an orderly fashion into platinum (Pt) nanoparticles supported by reduced graphene oxide (denoted as O-Pt3In/rGO), significantly enhancing alkaline HOR kinetics. We have revealed that the ordered atomic matrix enables uniform and optimized hydrogen binding energy (HBE), hydroxyl binding energy (OHBE), and carbon monoxide binding energy (COBE) across the catalyst. With a mass activity of 2.3066 A mg-1 at an overpotential of 50 mV, over 10 times greater than that of Pt/C, the catalyst also demonstrates admirable CO resistance and stability. Importantly, the AEMFC implementing this catalyst as the anode catalyst has achieved an impressive power output compared to Pt/C. This work not only highlights the significance of constructing ordered oxophilic sites for alkaline HOR but also sheds light on the design of well-structured catalysts for energy conversion.

Multidimensional Electrochemistry Decodes the Operando Mechanism of Hydrogen Oxidation

Being an efficient approach to the utilization of hydrogen energy, the hydrogen oxidation reaction (HOR) is of particular significance in the current carbon-neutrality time. Yet the mechanistic picture of the HOR is still blurred, mostly because the elemental steps of this reaction are rapid and highly entangled, especially when deviating from the thermodynamic equilibrium state. Here we report a strategy for decoding the HOR mechanism under operando conditions. In addition to the wide-potential-range I-V curves obtained using gas diffusion electrodes, we have applied the AC impedance spectroscopy to provide independent and complementary kinetic information. Combining multidimensional data sources has enabled us to fit, in mathematical rigor, the core kinetic parameter set in a 5-D data space. The reaction rate of the three elemental steps (Tafel, Heyrovsky, and Volmer reactions), as a function of the overpotential, can thus be distilled individually. Such an undocumented kinetic picture unravels, in detail, how the HOR is controlled by the elemental steps on polarization. For instance, at low polarization region, the Heyrovsky reaction is relatively slow and can be ignored; but at high polarization region, the Heyrovsky reaction will surpass the Tafel reaction. Additionally, the Volmer reaction has been the fastest within overpotentials of interest. Our findings not only offer a better understanding of the HOR mechanism, but also lay the foundation for the development of improved hydrogen energy utilization systems.

Analyzing the Active Site and Predicting the Overall Activity of Alloy Catalysts

As one of the potential catalysts, disordered solid solution alloys can offer a wealth of catalytic sites. However, accurately evaluating their activity localization structure and overall activity from each individual site remains a formidable challenge. Herein, an approach based on density functional theory and machine learning was used to obtain a large number of sites of the Pt-Ru alloy as the model multisite catalyst for the hydrogen evolution reaction. Subsequently, a series of statistical approaches were employed to unveil the relationship between the geometric structure and overall activity. Based on the radial frequency distribution of metal elements and the distribution of ΔGH, we have identified the surface and subsurface sites occupied by Pt and Ru, respectively, as the most active sites. Particularly, the concept of equivalent site ratio predicts that the overall activity is highest when the Ru content is 20-30%. Furthermore, a series of Pt-Ru alloys were synthesized to validate the proposed theory. This provides crucial insights into understanding the origin of catalytic activity in alloys and thus will better guide the rational development of targeted multisite catalysts.

Sub-3 nm Pt@Ru toward Outstanding Hydrogen Oxidation Reaction Performance in Alkaline Media

Anion-exchange membrane fuel cells (AEMFCs) are promising alternative hydrogen conversion devices. However, the sluggish kinetics of the hydrogen oxidation reaction in alkaline media hinders further development of AEMFCs. As a synthesis method commonly used to prepare disordered PtRu alloys, the impregnation process is ingeniously designed herein to synthesize sub-3 nm Pt@Ru core-shell nanoparticles by sequentially reducing Pt and Ru at different annealing temperatures. This method avoids complex procedures and synthesis conditions for organic synthesis systems, and the atomic structure evolution of the synthesized core-shell nanoparticles can be tracked. The synthesized Pt@Ru electrocatalyst shows an ultrasmall average size of ∼2.5 nm and thereby a large electrochemical surface area (ECSA) of 166.66 m2 gPt+Ru-1. Exchange current densities (j0) normalized to the mass (Pt + Ru) and ECSA of this electrocatalyst are 8.0 and 5.8 times as high as those of commercial Pt/C, respectively. To the best of our knowledge, the achieved mass-normalized j0 measured by rotating disk electrodes is the highest reported so far. The membrane electrode assembly test of the Pt@Ru electrocatalyst shows a peak power density of 1.78 W cm-2 (0.152 mgPt+Ru cmanode-2), which is higher than that of commercial PtRu/C (1.62 W cm-2, 0.211 mgPt+Ru cmanode-2). The improvement of the intrinsic activity can be attributed to the electron transfer from the Ru shell to the Pt core, and the ultrafine particles further enhance the mass activity. This work reveals the feasibility of using simple impregnation to synthesize fine core-shell nanocatalysts and the importance of investigating the atomic structure of PtRu nanoparticles and other disordered alloys.

PtNi/PtIn-Skin Fishbone-Like Nanowires Boost Alkaline Hydrogen Oxidation Catalysis

The development of high-performance platinum (Pt)-based electrocatalysts for the hydrogen oxidation reaction (HOR) is highly desirable for hydrogen fuel cells, but it is limited by the sluggish kinetics and severe carbon monoxide (CO) poisoning in alkaline medium. Herein, we explore a class of facet-selected Pt-nickel-indium fishbone-like nanowires (PtNiIn FNWs) featuring high-index facets (HIFs) of Pt3In skin as efficient alkaline HOR catalysts. Impressively, the optimized Pt66Ni6In28 FNWs show the highest mass and specific activities of 4.02 A mgPt-1 and 6.56 mA cm-2, 2.0/2.1 and 13.9/15.6 times larger than those of commercial PtRu/C and commercial Pt/C, respectively, along with a competitive CO-tolerance ability. Specifically, they exhibit only 6.0% current density decay after 10000 s of operation and 25.7% activity loss after 2000 s in the presence of 1000 ppm of CO. Moreover, an isotope experiment and density functional theory (DFT) calculations further prove that the unique structure and synergy among Pt, Ni, and In endow these Pt66Ni6In28 FNWs with an optimized hydrogen binding energy (HBE) and an advantageous hydroxide binding energy (OHBE), giving them excellent alkaline HOR properties. The combined construction of surface-skin and HIFs in PtNiIn FNWs will offer an available method to realize the potential applications of advanced non-PtRu-based catalysts in fuel cells and beyond.

A Ni-doped Mo2C/NCF composite for efficient electrocatalytic hydrogen evolution

Ni-Mo₂C nano catalysts dispersed on N-doped carbon flowers: a composite with nitrogen-containing carbon flowers carrying nickel-modified molybdenum carbide exhibits enhanced HER catalytic activity in alkaline electrolyte.

Hydroxyl-Binding Energy-Induced Kinetic Gap Narrowing between Acidic and Alkaline Hydrogen Oxidation Reaction on Intermetallic Ru3 Sn7 Catalyst

Developing highly efficient catalysts toward alkaline hydrogen oxidation reaction (HOR) and narrowing the kinetic gap between acidic and alkaline electrolytes are of great importance for the practical application of alkaline exchange membrane fuel cell . Herein, ordered Ru₃ Sn₇ /C intermetallic compound has been developed for the HOR under alkaline and acidic conditions. The authors demonstrate that the ordered intermetallic Ru₃ Sn₇ /C shows much enhanced HOR activity, stability, and CO-tolerance compared with its disordered RuSn solid solution alloy counterpart. More importantly, the authors find that the kinetic gap of HOR between acidic and alkaline media is significantly narrowed in the as-synthesized intermetallic Ru₃ Sn₇ /C catalysts. Combined experiment results and theoretical calculations, the authors understand that promoted hydroxyl-binding energy on Ru₃ Sn₇ /C derived from the intermetallic-induced strong electron interaction is responsible for the accelerated alkaline HOR performance and narrowed kinetic gap.

Fast and Durable Alkaline Hydrogen Oxidation Reaction at the Electron-Deficient Ruthenium-Ruthenium Oxide Interface

The slow hydrogen oxidation reaction (HOR) kinetics under alkaline conditions remain a critical challenge for the practical application of alkaline exchange membrane fuel cells. Herein, Ru/RuO₂ in-plane heterostructures are designed with abundant active Ru-RuO₂ interface domains as efficient electrocatalysts for the HOR in alkaline media. The experimental and theoretical results demonstrate that interfacial Ru and RuO₂ domains at Ru-RuO₂ interfaces are the optimal H and OH adsorption sites, respectively, endowing the well-defined Ru(100)/RuO₂ (200) interface as the preferential region for fast alkaline hydrogen electrocatalysis. More importantly, the metallic Ru domains become electron deficient due to the strong interaction with RuO₂ domains and show substantially improved inoxidizability, which is vital to maintain durable HOR electrocatalytic activity. The optimal Ru/RuO₂ heterostructured electrocatalyst exhibits impressive alkaline HOR activity with an exchange current density of 8.86 mA cm^-2 and decent durability. The exceptional electrocatalytic performance of Ru/RuO₂ in-plane heterostructure can be attributed to the robust and multifunctional Ru-RuO₂ interfaces endowed by the unique metal-metal oxide domains.

Activating RuOCo Interaction on the a-Co(OH)2 @Ru Interface for Accelerating the Volmer Step of Alkaline Hydrogen Evolution

The state-of-the-art active hydrogen evolution reaction (HER) catalysts in acid electrolytes generally lose considerable catalytic performance in alkaline electrolytes mainly due to the additional water dissociation step. Designing composite materials is an effective strategy to accelerate alkaline water electrolysis by optimizing the electronic structure of materials. Here, different phases of Co(OH)₂ -supported Ru clusters (α/β-Co(OH)₂ @Ru) are prepared for enabling a highly efficient electrocatalytic HER performance in alkaline solution. The prepared α-Co(OH)₂ nanosheets facilitate the loading of uniform and high-density Ru clusters and the formed highly active RuOCo bonds at the interface. The synergistic interaction endows the hybrid catalyst with low overpotential of 33 mV at 10 mA cm^-2 . Moreover, the homemade anion exchange membrane water electrolysis cell based on α-Co(OH)₂ @Ru affords a cell voltage of 2 V to drive a current density of 270 mA cm^-2 and performs stably during continuous operation for over 100 h. Density functional theory calculations demonstrate that active RuOCo bonds in α-Co(OH)₂ @Ru optimize the energy barriers for H₂ O dissociation and OH^- desorption to facilitate the Volmer reaction step. This work offers a strategy for designing interfacial chemical bonds for high electrocatalytic activity.

Modulating Hydrogen Adsorption via Charge Transfer at the Semiconductor-Metal Heterointerface for Highly Efficient Hydrogen Evolution Catalysis

Designing and synthesizing highly efficient and stable electrocatalysts for hydrogen evolution reaction (HER) is important for realizing the hydrogen economy. Tuning the electronic structure of the electrocatalysts is essential to achieve optimal HER activity, and interfacial engineering is an effective strategy to induce electron transfer in a heterostructure interface to optimize HER kinetics. In this study, ultrafine RhP₂ /Rh nanoparticles are synthesized with a well-defined semiconductor-metal heterointerface embedded in N,P co-doped graphene (RhP₂ /Rh@NPG) via a one-step pyrolysis. RhP₂ /Rh@NPG exhibits outstanding HER performances under all pH conditions. Electrochemical characterization and first principles density functional theory calculations reveal that the RhP₂ /Rh heterointerface induces electron transfer from metallic Rh to semiconductive RhP₂ , which increases the electron density on the Rh atoms in RhP₂ and weakens the hydrogen adsorption on RhP₂ , thereby accelerating the HER kinetics. Moreover, the interfacial electron transfer activates the dual-site synergistic effect of Rh and P of RhP₂ in neutral and alkaline environments, thereby promoting reorganization of interfacial water molecules for faster HER kinetics.

Electroshock synthesis of a bifunctional nonprecious multi-element alloy for alkaline hydrogen oxidation and evolution

The design and production of active, durable, and nonprecious electrocatalysts toward alkaline hydrogen oxidation and evolution reactions (HOR/HER) are extremely appealing for the implementation of hydrogen economy, but remain challenging. Here, we report a facile electric shock synthesis of an efficient, stable, and inexpensive NiCoCuMoW multi-element alloy on Ni foam (NiCoCuMoW) as a bifunctional electrocatalyst for both HOR and HER. For the HOR, the current density of NiCoCuMoW could reach ∼11.2 mA cm-2 when the overpotential is 100 mV, higher than that for commercial Pt/C (∼7.2 mA cm-2) and control alloy samples with less elements, along with superior CO tolerance. Moreover, for the HER, the overpotential at 10 mA cm-2 for NiCoCuMoW is only 21 mV, along with a Tafel slope of low to 63.7 mV dec-1, rivaling the commercial Pt/C as well (35 mV and 109.7 mV dec-1). Density functional theory calculations indicate that alloying Ni, Co, Cu, Mo, and W can tune the electronic structure of individual metals and provide multiple active sites to optimize the hydrogen and hydroxyl intermediates adsorption, collaboratively resulting in enhanced electrocatalytic activity.

Enhanced Performance of Sn@Pt Core-Shell Nanocatalysts Supported on Two Different Carbon Structures for the Hydrogen Oxidation Reaction in Acid Media

Sn@Pt core-shell nanocatalysts, supported on Vulcan XC-72 and home-developed nitrogen-doped graphene (Sn@Pt/C and Sn@Pt/NG, respectively), were evaluated for the hydrogen oxidation reaction (HOR) in acid electrolyte. The nanocatalysts were synthesized by the bromide anion exchange (BAE) method. TEM characterization confirmed the nanosize nature of Sn@Pt/C and Sn@Pt/NG, with an average particle size of 2.1 and 2.3 nm, respectively. Sn@Pt/C delivered a similar mass limiting current density (jl, m) of the HOR compared to Sn@Pt/NG, which was higher than those of Pt/C and Pt/NG (ca. 2 and 2.3-fold increase, respectively). Moreover, the Sn@Pt/C and Sn@Pt/NG core-shell nanocatalysts demonstrated a higher specific activity related to Pt/C and Pt/NG. Mass and specific Tafel slopes further demonstrated the improved catalytic activity of Sn@Pt/C for the HOR, followed by Sn@Pt/NG. The application of the nanocatalysts was proposed for polymer electrolyte membrane fuel cells (PEMFC).

Low Platinum-Loaded Molybdenum Co-catalyst for the Hydrogen Evolution Reaction in Alkaline and Acidic Media

Developing an efficient catalytic system for electrolysis with reduced platinum (Pt) loading while maintaining performance comparable to bulk platinum metal is important to decrease costs and improve scalability of the hydrogen fuel economy. Here we report the performance of a novel sputter-deposited molybdenum (Mo) thin film with an extremely low co-loading of Pt, where Pt atoms were dispersed on Mo (Pt_d-Mo) as an electrocatalyst for the hydrogen evolution reaction (HER) in either alkaline or acidic media. The Pt_d-Mo electrocatalyst presents similar catalytic activity to bulk Pt in alkaline media, while the performance is only slightly decreased in acidic media. Differential electrochemical mass spectrometry (DEMS) results confirm that the Pt_d-Mo electrocatalyst produced hydrogen at a rate comparable with that of a pristine Pt sample at the same potential. A comparison with Pt-loaded degenerately doped p-type doped silicon (Pt_d-Si) suggests that Mo and Pt work synergistically to boost the performance of Pt_d-Mo catalysts. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) before and after 1000 cycles of continuous operation confirm the significant durability of the Pt_d-Mo performance. Overall, the Pt_d-Mo electrocatalyst, with comparable HER activity to bulk Pt despite an ultra-low Pt loading, could be a strong candidate for hydrogen production in either acidic or basic conditions.

Manipulating the Water Dissociation Electrocatalytic Sites of Bimetallic Nickel-Based Alloys for Highly Efficient Alkaline Hydrogen Evolution

Transition-metal alloys are currently drawing increasing attention as promising electrocatalysts for the alkaline hydrogen evolution reaction (HER). However, traditional density-functional-theory-derived d-band theory fails to describe the hydrogen adsorption energy (ΔG_H ) on hollow sites. Herein, by studying the ΔG_H for a series of Ni-M (M=Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, W) bimetallic alloys, an improved d-band center was provided and a potential NiCu electrocatalyst with a near-optimal ΔG_H was discovered. Moreover, oxygen atoms were introduced into Ni-M (O-NiM) to balance the adsorption/desorption of hydroxyl species. The tailored electrocatalytic sites for water dissociation can synergistically accelerate the multi-step alkaline HER. The prepared O-NiCu shows the optimum HER activity with a low overpotential of 23 mV at 10 mA cm^-2 . This work not only broadens the applicability of d-band theory, but also provides crucial understanding for designing efficient HER electrocatalysts.

In Situ Probe of the Hydrogen Oxidation Reaction Intermediates on PtRu a Bimetallic Catalyst Surface by Core-Shell Nanoparticle-Enhanced Raman Spectroscopy

In situ monitoring of the evolution of intermediates and catalysts during hydrogen oxidation reaction (HOR) processes and elucidating the reaction mechanism are crucial in catalysis and energy science. However, spectroscopic information on trace intermediates on catalyst surfaces is challenging to obtain due to the complexity of interfacial environments and lack of in situ techniques. Herein, core-shell nanoparticle-enhanced Raman spectroscopy was employed to probe alkaline HOR processes on representative PtRu surfaces. Direct spectroscopic evidence of an OH_ad intermediate and RuO_x (Ru(+3)/Ru(+4)) surface oxides is simultaneously obtained, verifying that Ru doping onto Pt promotes OH_ad adsorption on the RuO_x surface to react with H_ad adsorption on the Pt surface to form H₂O. In situ Raman, XPS, and DFT results reveal that RuO_x coverage tunes the electronic structure of PtRuO_x to optimize the adsorption energy of OH_ad on catalyst surfaces, leading to an improvement in HOR activity. Our findings provide mechanistic guidelines for the rational design of HOR catalysts with high activity.

Ordered micropattern arrays fabricated by lung-derived dECM hydrogels for chemotherapeutic drug screening

AIMS

This study aims to evaluate ECM-coated micropattern arrays derived from decellularization of native porcine lungs as a novel three-dimensional cell culture platform.

METHODS

ECM derived from decellularization of native porcine lungs was exploited to prepare hydrogels. Then, dECM-coated micropattern arrays were fabricated at four different diameters (50, 100, 150 and 200 ​μm) using polydimethylsiloxane (PDMS). Two lung cancer cell lines, A549 and H1299, were tested on a dECM-coated micropattern array as a novel culture platform for cell adhesion, distribution, proliferation, viability, phenotype expression, and drug screening to evaluate the cytotoxicity of paclitaxel, doxorubicin and cisplatin.

RESULTS

The ECM derived from decellularization of native porcine lungs supported cell adhesion, distribution, viability and proliferation better than collagen I and Matrigel as the coated matrix on the surface. Moreover, the optimal diameter of the micropattern arrays was 100-150 ​μm, as determined by measuring the morphology, viability, proliferation and phenotype of the cancer cell spheroids. Cell spheroids of A549 and H1299 on dECM-coated micropattern arrays showed chemoresistance to anticancer drugs compared to that of the monolayer. The different distributions of HIF-1α, MCL-1 (in the center) and Ki-67 and MRP2 (in the periphery) of the spheroids demonstrated the good establishment of basal-lateral polarity and explained the chemoresistance phenomenon of spheroids.

CONCLUSIONS

This novel three-dimensional cell culture platform is stable and reliable for anticancer drug testing. Drug screening in dECM-coated micropattern arrays provides a powerful alternative to existing methods for drug testing and metabolic profiling in the drug discovery process.

Nano-Sized PtRu/C Electrocatalyst With Separated Phases and High Dispersion Improves Electrochemical Performance of Hydrogen Oxidation Reaction

Alloys and core-shell nanoparticles have recently received enormous attention which opened up new avenues for highly active catalysts. Despite considerable advances in this field, the majority of proposed approaches suffer from either complicated procedures or unstable structures, severely hindering their practical applications. Here, we successfully synthesized alloy electrocatalyst with separated phases, PtRu alloy nanoparticles robustly supported by carbon matrix (PtRu/C), using a convenient two-step solvothermal method. The constructed PtRu/C at different NaOH contents (0–1.25 mmol) were compared and electrochemical activity were evaluated by the hydrogen oxidation reaction (HOR). In contrast, the homogeneous distribution and minimum average size of Ru and Pt nanoparticles on carbon, appeared at approximately 4 nm, proving that PtRu/C-0.75 possessed abundant accessible active sites. The catalytic activities and the reaction mechanism were studied via electrochemical techniques. PtRu/C-0.75 has excellent activity due to its unique electronic structure and efficient charge transfer, with the largest j₀ value of 3.68 mA cm⁻² in the HOR.

Electrocatalytic Hydrogen Oxidation in Alkaline Media: From Mechanistic Insights to Catalyst Design

With the potential to circumvent the need for scarce and cost-prohibitive platinum-based catalysts in proton-exchange membrane fuel cells, anion-exchange membrane fuel cells (AEMFCs) are emerging as alternative technologies with zero carbon emission. Numerous noble metal-free catalysts have been developed with excellent catalytic performance for cathodic oxygen reduction reaction in AEMFCs. However, the anodic catalysts for hydrogen oxidation reaction (HOR) still rely on noble metal materials. Since the kinetics of HOR in alkaline media is 2-3 orders of magnitude lower than that in acidic media, it is a major challenge to either improve the performance of noble metal catalysts or to develop high-performance noble metal-free catalysts. Additionally, the mechanisms of alkaline HOR are not yet clear and still under debate, further hampering the design of electrocatalysts. Against this backdrop, this review starts with the prevailing theories for alkaline HOR on the basis of diverse activity descriptors, i.e., hydrogen binding energy theory and bifunctional theory. The design principles and recent advances of HOR catalysts employing the aforementioned theories are then summarized. Next, the strategies and recent progress in improving the antioxidation capability of HOR catalysts, a thorny issue which has not received sufficient attention, are discussed. Moreover, the significance of correlating computational models with real catalyst structure and the electrode/electrolyte interface is further emphasized. Lastly, the remaining controversies about the alkaline HOR mechanisms as well as the challenges and possible research directions in this field are presented.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Catalyst overcoating engineering towards high-performance electrocatalysis

Clean and sustainable energy needs the development of advanced heterogeneous catalysts as they are of vital importance for electrochemical transformation reactions in renewable energy conversion and storage devices. Advances in nanoscience and material chemistry have afforded great opportunities for the design and optimization of nanostructured electrocatalysts with high efficiency and practical durability. In this review article, we specifically emphasize the synthetic methodologies for the versatile surface overcoating engineering reported to date for optimal electrocatalysts. We discuss the recent progress in the development of surface overcoating-derived electrocatalysts potentially applied in polymer electrolyte fuel cells and water electrolyzers by correlating catalyst intrinsic structures with electrocatalytic properties. Finally, we present the opportunities and perspectives of surface overcoating engineering for the design of advanced (electro)catalysts and their deep exploitation in a broad scope of applications.

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

The rapid progress of proton exchange membrane fuel cells (PEMFCs) and alkaline exchange membrane fuel cells (AMFCs) has boosted the hydrogen economy concept via diverse energy applications in the past decades. For a holistic understanding of the development status of PEMFCs and AMFCs, recent advancements in electrocatalyst design and catalyst layer optimization, along with cell performance in terms of activity and durability in PEMFCs and AMFCs, are summarized here. The activity, stability, and fuel cell performance of different types of electrocatalysts for both oxygen reduction reaction and hydrogen oxidation reaction are discussed and compared. Research directions on the further development of active, stable, and low-cost electrocatalysts to meet the ultimate commercialization of PEMFCs and AMFCs are also discussed.

Electrocatalytic Oxygen Evolution Reaction in Acidic Conditions: Recent Progress and Perspectives

The electrochemical oxygen evolution reaction (OER) is an important half-cell reaction in many renewable energy conversion and storage technologies, including electrolyzers, nitrogen fixation, CO₂ reduction, metal-air batteries, and regenerative fuel cells. Among them, proton exchange membrane (PEM)-based devices exhibit a series of advantages, such as excellent proton conductivity, high durability, and good mechanical strength, and have attracted global interest as a green energy device for transport and stationary sectors. Nevertheless, with a view to rapid commercialization, it is urgent to develop highly active and acid-stable OER catalysts for PEM-based devices. In this Review, based on the recent advances in theoretical calculation and in situ/operando characterization, the OER mechanism in acidic conditions is first discussed in detail. Subsequently, recent advances in the development of several types of acid-stable OER catalysts, including noble metals, non-noble metals, and even metal-free OER materials, are systematically summarized. Finally, the current key issues and future challenges for materials used as acidic OER catalysis are identified and potential future directions are proposed.

Enhanced Electrocatalytic Oxidation of Formate via Introducing Surface Reactive Oxygen Species to a CeO2 Substrate

Direct formate fuel cells (DFFCs) as promising energy technologies have been applied for portable and wearable devices. However, for the formate oxidation reaction (FOR), the deficiency of catalysts has prevented DFFCs from practical applications. Herein, we prepared a Pd-loaded CeO₂ catalyst through a simple steam treatment at 400 °C to enhance the catalytic FOR performance. In comparison with the counterpart of Pd/CeO₂ without stream treatment, the as-prepared Pd/CeO₂-ST catalyst has a lower onset potential of 381 mV and a lower peak potential of 0.64 V with a higher peak current of 10.62 mA cm^-2. The experimental results show that the enhanced FOR properties of Pd/CeO₂-ST are ascribed to the introduction of surface reactive oxygen species to the CeO₂ substrate, which substantially promotes the desorption of adsorbed hydrogen (H*) intermediates. Density functional theory (DFT) calculations reveal that on the surface of CeO₂, the abundant oxygen vacancies boost the OH* adsorption ability and accelerate the kinetics of the potential-limiting step. This work not only proposes a new strategy for enhancing the activity of FOR catalysts but also highlights the understanding of the FOR mechanism in alkaline media for DFFC applications.

Proton Exchange Membrane Fuel Cells (PEMFCs): Advances and Challenges

The study of the electrochemical catalyst conversion of renewable electricity and carbon oxides into chemical fuels attracts a great deal of attention by different researchers. The main role of this process is in mitigating the worldwide energy crisis through a closed technological carbon cycle, where chemical fuels, such as hydrogen, are stored and reconverted to electricity via electrochemical reaction processes in fuel cells. The scientific community focuses its efforts on the development of high-performance polymeric membranes together with nanomaterials with high catalytic activity and stability in order to reduce the platinum group metal applied as a cathode to build stacks of proton exchange membrane fuel cells (PEMFCs) to work at low and moderate temperatures. The design of new conductive membranes and nanoparticles (NPs) whose morphology directly affects their catalytic properties is of utmost importance. Nanoparticle morphologies, like cubes, octahedrons, icosahedrons, bipyramids, plates, and polyhedrons, among others, are widely studied for catalysis applications. The recent progress around the high catalytic activity has focused on the stabilizing agents and their potential impact on nanomaterial synthesis to induce changes in the morphology of NPs.

Supramolecular Anchoring Strategy for Facile Production of Ruthenium Nanoparticles Embedded in N-Doped Mesoporous Carbon Nanospheres for Efficient Hydrogen Generation

Because of the favorable mass transport and increased available active sites, the rational design and preparation of porous carbon structures are essential but still challenging. Herein, a novel and facile supramolecular anchoring strategy was developed to achieve the embedding of ruthenium (Ru) nanoparticles in N-doped mesoporous carbon nanospheres through pyrolyzing the precursor formed by coordination assembly between metal ions and zinc gluconate (G(Zn)). Featuring rich hydroxyl groups, the G(Zn) can effectively chelate Ru³⁺ via metal-oxygen bonds to form 3D supramolecular nanospheres, and meanwhile, mesopores in carbon nanospheres were expanded after subsequent pyrolysis thanks to the volatilization of zincic species at high temperature. As a demonstration, the best-performing catalyst displayed extraordinary activity for the hydrogen evolution reaction (HER) with a small overpotential of 43 mV versus reversible hydrogen electrode (vs RHE) at 10 mA/cm² and a Tafel slope of 39 mV/dec, which was superior to that of commercial Pt/C in alkaline medium. Theoretical calculations revealed that the catalytic activity was significantly promoted by the strong electronic coupling between Ru nanoparticles and N-doped porous carbon, which increased the electron transfer capability and facilitated the adsorption and dissociation of H₂O to realize an efficient HER.

Measurement of the Protonic and Electronic Conductivities of PEM Water Electrolyzer Electrodes

Reducing anode catalyst layer proton- and electron-transport resistances in polymer electrolyte membrane water electrolyzers is critical to improving its performance and maximizing catalyst utilization at high current density. A hydrogen pump technique is adapted to measure the protonic conductivity of IrO_x-based catalyst layers. The protonic resistance of the catalyst layer is obtained by subtracting the protonic resistance of an assembly of two NRE211 membranes hot-pressed together from an assembly of two NRE211 membranes with an IrO_x intermediate layer. The through-plane and in-plane electronic conductivities were also measured using two- and four-probe methods, respectively. Using these techniques, the protonic and electronic conductivities of the IrO_x catalyst layers with varying Nafion loading were measured. The results show that the limiting charge-transport phenomena in the IrO_x catalyst layer can be either proton or electron transport, depending on the ionomer loading in the catalyst layer. These results are validated by numerical simulation, as well as by comparison to the high-frequency resistance of an electrolyzer with the same layer.

Construction of a C@MoS2@C sandwiched heterostructure for accelerating the pH-universal hydrogen evolution reaction

Herein a facile and versatile hydrothermal method has been developed to construct a polypyrrole-derived carbon nanotube (PCN), MoS2 nanosheets and a carbon shell integrated sandwich-like heterostructure (PCN@MoS2@C). This heterostructure shows excellent performance in the hydrogen evolution reaction (HER) over a wide pH range. The results indicate that the porous carbon shell coated heterostructure provides MoS2 nanosheets with sufficient conductivity, increased number of active sites, and strong structural stability, and thus boosts its HER performance.

A Novel Heterostructure Based on RuMo Nanoalloys and N-doped Carbon as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec^-1 ), and a high turnover frequency (3.57 H₂ s^-1 ) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon-metal heterostructures for highly efficient electrocatalysis.

3D Hydrangea Macrophylla-like Nickel-Vanadium Metal-Organic Frameworks Formed by Self-Assembly of Ultrathin 2D Nanosheets for Overall Water Splitting

The development of highly efficient and low-cost bifunctional noble metal-free electrocatalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is an effective strategy for improving efficiency. Herein, novel three-dimensional (3D) bimetallic metal-organic frameworks containing Ni and V with adjustable stoichiometry were synthesized on nickel foam successfully. Notably, Ni2V-MOFs@NF only require rather low overpotentials of 244 and 89 mV for the OER and HER, respectively, and expedites overall water splitting with 1.55 V at 10 mA cm^-2 with robust durability during the 80 h test. The high efficiency of the novel obtained electrocatalysts should be attributed to the particular morphological design of the two-dimensional (2D) ultrathin nanosheets self-assembling into a 3D nanoflower and the electronic structure regulation resulting from the synergetic interaction between nickel and vanadium. Subsequent theoretical calculations reveal the following conclusions: (I) the exceptional electronic conductivity of Ni2V-MOFs shows enhanced optimization as a result of electronic structure reconstruction, (II) the energy barrier reduction of the rate-limiting step is responsible for the enhanced dynamics of Ni2V-MOFs for the OER, and (III) the facilitation of the adsorption of H⁺ and H₂O plays a key role in progressing the HER catalytic activity of Ni2V-MOFs.

Random alloy and intermetallic nanocatalysts in fuel cell reactions

Fuel cells that use small organic molecules or hydrogen as the anode fuel can power clean electric vehicles. From an experimental perspective, the possible fuel cells' electrocatalytic reaction mechanisms are obtained through in situ electrochemical spectroscopy techniques and density functional theory calculations, providing theoretical guidance for further development of novel nanocatalysts. As advanced nanocatalysts for fuel cells' electrochemical reactions, alloy nanomaterials have greatly improved electrocatalytic activity and stability and have attracted widespread attention. Enhanced electrocatalytic performance of alloy nanocatalysts could be closely related to the synergistic effects, such as electronic and strain effects. Depending on the arrangement of atoms, alloys can be classified into random alloy and intermetallic compounds (ordered structure). Intermetallic compounds generally have lower heats of formation and stronger heteroatomic bonding strength relative to the random alloy, resulting in high chemical and structural stability in either full pH solutions or electrochemical tests. Here, we summarize the latest advances and the structure-function relationship of noble metal alloy nanocatalysts, among which Pt-based catalysts are the main ones, as well as comprehensively understand why they significantly affect the electrocatalytic performance of fuel cells. Novel alloy nanocatalysts with a robust three-phase interface to achieve efficient charge and mass transfer can obtain desirable activity and stability in the electrochemical workstation tests, and is expected to acquire a higher power density on fuel cell test systems with harsh test conditions.