Methanol Oxidation Using Ternary Ordered Intermetallic Electrocatalysts: A DEMS Study

Electronegativity- induced cobalt-doped platinum hollow nanospheres with high CO tolerance for efficient methanol oxidation reaction

Although Platinum (Pt)-based alloys have garnered significant interest within the realm of direct methanol fuel cells (DMFCs), there still exists a notable dearth in the exploration of the catalytic behavior of the liquid fuels on well-defined active sites and unavoidable Pt poisoning because of the adsorbed CO species (COads). Here, we propose an electronegativity-induced electronic redistribution strategy to optimize the adsorption of crucial intermediates for the methanol oxidation reaction (MOR) by introducing the Co element to form the PtCo alloys. The optimal PtCo hollow nanospheres (HNSs) exhibit excellent high-quality activity of 3.27 A mgPt-1, which is 11.6 times and 13.1 times higher than that of Pt/C and pure Pt, respectively. The in-situ Fourier transform infrared reflection spectroscopy validates that electron redistribution could weak CO adsorption, and subsequently decrease the CO poisoning adjacent the Pt active sites. Theoretical simulations result show that the introduction of Co optimize surface electronic structure and reduce the d-band center of Pt, thus optimized the adsorption behavior of COads. This study not only employs a straightforward method for the preparation of Pt-based alloys but also delineates a pathway toward designing advanced active sites for MOR via electronegativity-induced electronic redistribution.

Modification Strategies for Development of 2D Material-Based Electrocatalysts for Alcohol Oxidation Reaction

2D materials, such as graphene, MXenes (metal carbides and nitrides), graphdiyne (GDY), layered double hydroxides, and black phosphorus, are widely used as electrocatalyst supports for alcohol oxidation reactions (AORs) owing to their large surface area and unique 2D charge transport channels. Furthermore, the development of highly efficient electrocatalysts for AORs via tuning the structure of 2D support materials has recently become a hot area. This article provides a critical review on modification strategies to develop 2D material-based electrocatalysts for AOR. First, the principles and influencing factors of electrocatalytic oxidation of alcohols (such as methanol and ethanol) are introduced. Second, surface molecular functionalization, heteroatom doping, and composite hybridization are deeply discussed as the modification strategies to improve 2D material catalyst supports for AORs. Finally, the challenges and perspectives of 2D material-based electrocatalysts for AORs are outlined. This review will promote further efforts in the development of electrocatalysts for AORs.

The Electrochemical Peroxydisulfate-Oxalate Autocatalytic Reaction

Aqueous solutions containing both the strong oxidant, peroxydisulfate (S2O82-), and the strong reductant, oxalate (C2O42-), are thermodynamically unstable due to the highly exothermic homogeneous redox reaction: S2O82- + C2O42- → 2 SO42- + 2 CO2 (ΔG0 = -490 kJ/mol). However, at room temperature, this reaction does not occur to a significant extent over the time scale of a day due to its inherently slow kinetics. We demonstrate that the S2O82-/C2O42- redox reaction occurs rapidly, once initiated by the Ru(NH3)62+-mediated 1e- reduction of S2O82- to form S2O83•-, which rapidly undergoes bond cleavage to form SO42- and the highly oxidizing radical SO4•-. Theoretically, the mediated electrochemical generation of a single molecule of S2O83•- can initiate an autocatalytic cycle that consumes both S2O82- and C2O42- in bulk solution. Several experimental demonstrations of S2O82-/C2O42- autocatalysis are presented. Differential electrochemical mass spectrometry measurements demonstrate that CO2 is generated in solution for at least 10 min following a 30-s initiation step. Quantitative bulk electrolysis of S2O82- in solutions containing excess C2O42- is initiated by electrogeneration of immeasurably small quantities of S2O83•-. Capture of CO2 as BaCO3 during electrolysis additionally confirms the autocatalytic generation of CO2. First-principles density functional theory calculations, ab initio molecular dynamics simulations, and finite difference simulations of cyclic voltammetric responses are presented that support and provide additional insights into the initiation and mechanism of S2O82-/C2O42- autocatalysis. Preliminary evidence indicates that autocatalysis also results in a chemical traveling reaction front that propagates into the solution normal to the planar electrode surface.

Recent development and applications of differential electrochemical mass spectrometry in emerging energy conversion and storage solutions

Electrochemical energy conversion and storage are playing an increasingly important role in shaping the sustainable future. Differential electrochemical mass spectrometry (DEMS) offers an operando and cost-effective tool to monitor the evolution of gaseous/volatile intermediates and products during these processes. It can deliver potential-, time-, mass- and space-resolved signals which facilitate the understanding of reaction kinetics. In this review, we show the latest developments and applications of DEMS in various energy-related electrochemical reactions from three distinct perspectives. (I) What is DEMS addresses the working principles and key components of DEMS, highlighting the new and distinct instrumental configurations for different applications. (II) How to use DEMS tackles practical matters including the electrochemical test protocols, quantification of both potential and mass signals, and error analysis. (III) Where to apply DEMS is the focus of this review, dealing with concrete examples and unique values of DEMS studies in both energy conversion applications (CO2 reduction, water electrolysis, carbon corrosion, N-related catalysis, electrosynthesis, fuel cells, photo-electrocatalysis and beyond) and energy storage applications (Li-ion batteries and beyond, metal-air batteries, supercapacitors and flow batteries). The recent development of DEMS-hyphenated techniques and the outlook of the DEMS technique are discussed at the end. As DEMS celebrates its 40th anniversary in 2024, we hope this review can offer electrochemistry researchers a comprehensive understanding of the latest developments of DEMS and will inspire them to tackle emerging scientific questions using DEMS.

Cascaded p-d Orbital Hybridization Interaction in Ultrathin High-Entropy Alloy Nanowires Boosts Complete Non-CO Pathway of Methanol Oxidation Reaction

Designing high efficiency platinum (Pt)-based catalysts for methanol oxidation reaction (MOR) with high "non-CO" pathway selectivity is strongly desired and remains a grand challenge. Herein, PtRuNiCoFeGaPbW HEA ultrathin nanowires (HEA-8 UNWs) are synthesized, featuring unique cascaded p-d orbital hybridization interaction by inducing dual p-block metals (Ga and Pb). In comparison with Pt/C, HEA-8 UNWs exhibit 15.0- and 4.2-times promotion of specific and mass activity for MOR. More importantly, electrochemical in situ FITR spectroscopy reveals that the production/adsorption of CO (CO*) intermediate is effectively avoided on HEA-8 UNWs, leading to the complete "non-CO" pathway for MOR. Theoretical calculations demonstrate the optimized electronic structure of HEA-8 UNWs can facilitates a lower energy barrier for the "non-CO" pathway in the MOR.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Gaining Deep Understanding of Electrochemical CO2 RR with In Situ/Operando Techniques

Electrocatalysis for CO2 conversion has been extensively studied to mitigate the energy shortage and environmental issues, which are gaining ever-increasing attention. However, the complicated CO2 reduction process and the dynamic evolution occurring on electrocatalyst surface make it hard to understand the catalytic mechanism. The development of advanced in situ/operando techniques intelligently coupled with electrochemical cells sheds light on the related study via capturing surface atomic rearrangement, tracing chemical state change of catalysts, monitoring the behavior of intermediates and products, and depicting microenvironment near the electrode surface. In this review, fundamentals of the state-of-the-art in situ/operando techniques are clarified first. Case studies on the in situ/operando techniques performed to probe the CO2 reduction reaction processes are then discussed in detail. Finally, conclusions and outlook on this field are presented.

Recent Advances on Transition-Metal-Based Layered Double Hydroxides Nanosheets for Electrocatalytic Energy Conversion

Transition-metal-based layered double hydroxides (TM-LDHs) nanosheets are promising electrocatalysts in the renewable electrochemical energy conversion system, which are regarded as alternatives to noble metal-based materials. In this review, recent advances on effective and facile strategies to rationally design TM-LDHs nanosheets as electrocatalysts, such as increasing the number of active sties, improving the utilization of active sites (atomic-scale catalysts), modulating the electron configurations, and controlling the lattice facets, are summarized and compared. Then, the utilization of these fabricated TM-LDHs nanosheets for oxygen evolution reaction, hydrogen evolution reaction, urea oxidation reaction, nitrogen reduction reaction, small molecule oxidations, and biomass derivatives upgrading is articulated through systematically discussing the corresponding fundamental design principles and reaction mechanism. Finally, the existing challenges in increasing the density of catalytically active sites and future prospects of TM-LDHs nanosheets-based electrocatalysts in each application are also commented.

Integration of multiple advantages into one catalyst: non-CO pathway of methanol oxidation electrocatalysis on surface Ir-modulated PtFeIr jagged nanowires

Developing highly active methanol oxidation electrocatalysts with superior anti-CO poisoning capability remains a grand challenge. Herein, a simple strategy was employed to prepare distinctive PtFeIr jagged nanowires with Ir located at the shell and Pt/Fe located at the core. The Pt₆₄Fe₂₀Ir₁₆ jagged nanowire possesses an optimal mass activity of 2.13 A mg_Pt^-1 and specific activity of 4.25 mA cm^-2, giving the catalyst a great edge over PtFe jagged nanowire (1.63 A mg_Pt^-1 and 3.75 mA cm^-2) and Pt/C (0.38 A mg_Pt^-1 and 0.76 mA cm^-2). The in-situ Fourier transform infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS) unravel the origin of extraordinary CO tolerance in terms of key reaction intermediates in the non-CO pathway. Density functional theory (DFT) calculations add to the body of evidence that the surface Ir incorporation transforms the selectivity from CO pathway to non-CO pathway. Meanwhile, the presence of Ir serves to optimize surface electronic structure with weakened CO binding strength. We believe this work will advance the understanding of methanol oxidation catalytic mechanism and provide some insight into structural design of efficient electrocatalysts.

Operando studies reveal active Cu nanograins for CO2 electroreduction

Carbon dioxide electroreduction facilitates the sustainable synthesis of fuels and chemicals¹. Although Cu enables CO₂-to-multicarbon product (C₂₊) conversion, the nature of the active sites under operating conditions remains elusive². Importantly, identifying active sites of high-performance Cu nanocatalysts necessitates nanoscale, time-resolved operando techniques^3-5. Here, we present a comprehensive investigation of the structural dynamics during the life cycle of Cu nanocatalysts. A 7 nm Cu nanoparticle ensemble evolves into metallic Cu nanograins during electrolysis before complete oxidation to single-crystal Cu₂O nanocubes following post-electrolysis air exposure. Operando analytical and four-dimensional electrochemical liquid-cell scanning transmission electron microscopy shows the presence of metallic Cu nanograins under CO₂ reduction conditions. Correlated high-energy-resolution time-resolved X-ray spectroscopy suggests that metallic Cu, rich in nanograin boundaries, supports undercoordinated active sites for C-C coupling. Quantitative structure-activity correlation shows that a higher fraction of metallic Cu nanograins leads to higher C₂₊ selectivity. A 7 nm Cu nanoparticle ensemble, with a unity fraction of active Cu nanograins, exhibits sixfold higher C₂₊ selectivity than the 18 nm counterpart with one-third of active Cu nanograins. The correlation of multimodal operando techniques serves as a powerful platform to advance our fundamental understanding of the complex structural evolution of nanocatalysts under electrochemical conditions.

Toward Electrocatalytic Methanol Oxidation Reaction: Longstanding Debates and Emerging Catalysts

The study of direct methanol fuel cells (DMFCs) has lasted around 70 years, since the first investigation in the early 1950s. Though enormous effort has been devoted in this field, it is still far from commercialization. The methanol oxidation reaction (MOR), as a semi-reaction of DMFCs, is the bottleneck reaction that restricts the overall performance of DMFCs. To date, there has been intense debate on the complex six-electron reaction, but barely any reviews have systematically discussed this topic. To this end, the controversies and progress regarding the electrocatalytic mechanisms, performance evaluations as well as the design science toward MOR electrocatalysts are summarized. This review also provides a comprehensive introduction on the recent development of emerging MOR electrocatalysts with a focus on the innovation of the alloy, core-shell structure, heterostructure, and single-atom catalysts. Finally, perspectives on the future outlook toward study of the mechanisms and design of electrocatalysts are provided.

PtCuRu Nanoflowers with Ru-Rich Edge for Efficient Fuel-Cell Electrocatalysis

Enhancing the catalytic activity of Pt-based alloy by a rational structural design is the key to addressing the sluggish kinetics of direct alcohol fuel cells. Herein, a facile one-pot method is reported to synthesize PtCuRu nanoflowers (NFs). The synergetic effect among Pt, Cu, and Ru can lower the d-band center of Pt, regulate the morphology, generate Ru-rich edge, and allow the exposure of more high index facets. The optimized Pt_0.68 Cu_0.18 Ru_0.14 NFs exhibit outstanding electrocatalytic performances and excellent anti-poisoning abilities. The specific activities for the methanol oxidation reaction (MOR) (7.65 mA cm^-2 ) and ethanol oxidation reaction (EOR) (7.90 mA cm^-2 ) are 6.0 and 7.1 times higher than commercial Pt/C, respectively. The CO stripping experiment and the chronoamperometric (5000 s) demonstrate the superior anti-poisoning property and durability performance. Density functional theory calculations confirm that high metallization degree leads to the decrease of d-band center, the promotion of oxidation of CO, and improvement of the inherent activity and anti-poisoning ability. A Ru-rich edge exposes abundant high index facets to accelerate the reaction kinetics of rate-determining steps by decreasing the energy barrier for forming *HCOOH (MOR) and CC bond breaking (EOR).

Intercepting Hydrogen Evolution with Hydrogen-Atom Transfer: Electron-Initiated Hydrofunctionalization of Alkenes

Hydrogen-atom transfer mediated by earth-abundant transition-metal hydrides (M-Hs) has emerged as a powerful tool in organic synthesis. Current methods to generate M-Hs most frequently rely on oxidatively initiated hydride transfer. Herein, we report a reductive approach to generate Co-H, which allows for canonical hydrogen evolution reactions to be intercepted by hydrogen-atom transfer to an alkene. Electroanalytical and spectroscopic studies provided mechanistic insights into the formation and reactivity of Co-H, which enabled the development of two new alkene hydrofunctionalization reactions.

Disentangling Electronic and Geometric Effects in Electrocatalysis through Substitution in Isostructural Intermetallic Compounds

Efficient development of catalytic materials requires knowledge of the decisive parameters defining the catalytic properties. In multicomponent metallic catalysts, these are categorized as electronic and geometric effects, yet they are strongly interrelated. A systematic disentanglement can be achieved by fixing one parameter while altering the other, which becomes possible through the substitution in isostructural intermetallic compounds. This approach enables the evaluation of electronic or geometric contributions both individually and combined. Herein, this is achieved by substitution of indium (three valence electrons) with tin (four valence electrons) in the series In_1-xSn_xPd₂, which allows for a systematic variation of the total number of electrons per unit cell with only a minor variation of the unit cell parameters and thus the evaluation of the electronic effect. Geometric effects were evaluated by substitution of indium with gallium in the Ga_1-xIn_xPd₂ series, which allows for a systematic variation of the interatomic distances while maintaining the same number of valence electrons per unit cell and close atomic coordinates. By substituting gallium with tin in the Ga_1-xSn_xPd₂ series, both effects are combined and addressed simultaneously. The activity enhancement of the methanol oxidation reaction on the Ga_1-xSn_xPd₂ series is attributed to the synergy of the combined effects.

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.

Tunable Assembly of Protein Enables Fabrication of Platinum Nanostructures with Different Catalytic Activity

Proteins are promising biofunctional units for the construction of nanomaterials (NMs) due to their abundant binding sites, intriguing self-assembly properties, and mild NM synthetic conditions. Tobacco mosaic virus coat protein (TMVCP) is a protein capable of self-assembly into distinct morphologies depending on the solution pH and ionic strength. Herein, we report the use of TMVCP as a building block to organize nanosized platinum into discrete nanorings and isolated nanoparticles by varying the solution pH to modulate the protein assembly state. Compared with a commercial Pt/C catalyst, the TMVCP-templated platinum materials exhibited significant promotion of the catalytic activity and stability toward methanol electrooxidation in both neutral and alkaline conditions. The enhanced catalytic performance is likely facilitated by the protein support. Additionally, Pt nanorings outperformed isolated nanoparticles, although they are both synthesized on TMVCP templates. This could be due to the higher mechanical stability of the protein disk structure and possible cooperative effects between adjacent nanoparticles in the ring with narrow interparticle spacing.

Trimetallic synergy in dendritic intermetallic PtSnBi nanoalloys for promoting electrocatalytic alcohol oxidation

Developing effective and robust novel electrocatalysts for direct alcohol fuel cells has been gaining much attention. However, the widely used Pt catalyst suffers from limitations including the sluggish kinetics, severe CO poisoning, and catalyst lost caused by aggregation and Ostwald ripening during alcohol oxidation reaction. Herein, dendritic intermetallic PtSnBi nanoalloys were synthesized via a facile hydrothermal approach with high electrocatalytic performance and enhanced CO resistance for methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) owing to the synergism of the chosen three elements and unique three-dimensional morphology. Specifically, the PtSnBi nanoalloys display 4.6 and 6.7 times higher of mass activity (7.02 A mg^-1_Pt) and specific activity (16.65 mA cm^-2) toward MOR than those of commercial Pt/C, respectively. The mass activity of PtSnBi nanoalloys still retains 75.7% of the initial value after 800 cycles of stability test, superior to Pt/C (38.0%). The dual-functional effect of Sn, optimized electronic structure by the ligand effect, and unique atomic arrangement are responsible for the enhanced MOR activity and stability of PtSnBi nanoalloys. Furthermore, the PtSnBi nanoalloys with highlighted anti-CO poisoning capacity also improve the electrocatalytic performance toward EOR, indicating their great promise as broad energy electrocatalysts.

Effect of Anode Material on Electrochemical Oxidation of Low Molecular Weight Alcohols-A Review

The growing climate crisis inspires one of the greatest challenges of the 21st century-developing novel power sources. One of the concepts that offer clean, non-fossil electricity production is fuel cells, especially when the role of fuel is played by simple organic molecules, such as low molecular weight alcohols. The greatest drawback of this technology is the lack of electrocatalytic materials that would enhance reaction kinetics and good stability under process conditions. Currently, electrodes for direct alcohol fuel cells (DAFCs) are mainly based on platinum, which not only provides a poor reaction rate but also readily deactivates because of poisoning by reaction products. Because of these disadvantages, many researchers have focused on developing novel electrode materials with electrocatalytic properties towards the oxidation of simple alcohols, such as methanol, ethanol, ethylene glycol or propanol. This paper presents the development of electrode materials and addresses future challenges that still need to be overcome before direct alcohol fuel cells can be commercialized.

Synergistic Bimetallic Metallic Organic Framework-Derived Pt-Co Oxygen Reduction Electrocatalysts

The rational design of Pt-based electrocatalysts is of paramount importance for the commercialization of proton exchange membrane fuel cells (PEMFCs). Pt-Co alloys and nitrogen-doped carbons have been shown to be effective in enhancing the kinetics of the oxygen reduction reaction (ORR). Herein, we reported on two kinds of Pt-Co electrocatalysts, PtCo ordered intermetallic and PtCo₂ disordered alloys, supported on bimetallic MOF-derived N-doped carbon. The synergistic interaction between Pt-Co nanoparticles and Co-N-C enhanced the overall ORR activity and maintained the integrity of both structures and their electrochemical properties during long-term stability testing. The optimal activity for both PtCo and PtCo₂ occurred after 20 000 potential cycles. The enhanced performance of PtCo was ascribed to the formation of a two-atomic-layer Pt-rich shell and the lattice strain caused by the core-shell PtCo@Pt structure. The increased activity of PtCo₂ was ascribed to the formation of large, spongy, and small solid nanoparticles during electrochemical dealloying and thus the exposure of more Pt sites on the surface. The strategy described herein advances our understanding of the structure-activity relationship in electrocatalysis and sheds light on the future development of more active and durable ORR electrocatalysts.