Metal-Based Electrocatalysts for Methanol Electro-Oxidation: Progress, Opportunities, and Challenges

Metal oxides carbon xerogel nanocomposite for methanol oxidation fuel cell

The primary requirement for electrode materials in direct methanol fuel cells (DMFC) is efficient electrocatalyst that exhibit high tolerance to methanol oxidation, excellent stability, and reasonable cost. The combination of distinct active materials with distinctive architectures may facilitate the attainment of this objective. The present study included the preparation of a Carbon Xerogel Doped with various metal oxides derived from Banana peels. The nanocomposites were thoroughly examined utilizing several characterization modalities including XRD, FTIR, and SEM. The electrocatalytic performance of Carbon xerogel doped with Iron (Fe3O4/CX), carbon xerogel doped with magnesium (MgO/CX), and carbon xerogel doped with Copper (CuO/CX) about the Methanol Oxidation Reaction (MOR) was investigated using electrochemical methods such as cyclic voltammetry, impedance spectroscopy, and chronoamperometry. The results showed that the Fe3O4/CX, MgO/CX, and CuO/CX are effective electrocatalysts with an onset potential of around 1.00 V and current densities of approximately 42.98 mA cm - 2, 28.2784 mA.cm - 2, and 6.60698 mA.cm - 2, respectively, in the optimized electrolyte for methanol oxidation. The stability of Fe3O4/CX, MgO/CX, and CuO/CX electrodes was examined using chronoamperometry and the Cyclic Stability method. The results revealed that the (Fe3O4/CX) electrode exhibited outstanding stability throughout the whole 60-minute chronoamperometry Technique and demonstrated great stability for 100 cycles in the Cyclic Stability technique. The remarkable electrochemical activity and stability may be attributed to the synergistic effect of Fe3O4/CX, which provided sufficient active sites for methanol electro-oxidation and reduced the equivalent series resistance, as shown by the electrochemical impedance spectroscopy analysis. This work used environmentally friendly materials, which presents a novel opportunity to enhance the efficiency of methanol oxidation via the utilization of affordable catalysts. This study of the theoretical technique methods for establishing the route of methanol decomposition, and systematizes their confirmation with experimental data, within the methodological framework.

Metal-Support Interactions in PdCu/NiZnP Nanohybrids Enhance Alcohol Electrooxidation

Developing high-performance catalysts for the alcohol electrooxidation reaction is of significant importance for the practical application of direct fuel cells. Herein, a supported catalyst consisting of well-dispersive PdCu nanoparticles (NPs) and ultrathin NiZnP nanosheets (NSs) is synthesized. The high-surface-area NiZnP NSs provide a platform for good dispersion of PdCu NPs, resulting in stable catalysts with a large number of exposed surface atoms. Compared with PdCu NPs and commercial Pd/C, the metal-support interactions contribute to the activity and durability improvement of the PdCu/NiZnP nanohybrids. Moreover, the NiZnP NSs promote the formation of OH species, thereby facilitating the removal of carbonaceous intermediates and ensuring the long-term stability of PdCu/NiZnP nanohybrids. This study provides deep insight into the supported catalysts and a comprehensive understanding of the metal-support interactions, offering great opportunities for the design of efficient catalysts for direct fuel cells.

2D Carbon-Anchored Platinum-Based Nanodot Arrays as Efficient Catalysts for Methanol Oxidation Reaction

Ultrafine Pt-based alloy nanoparticles supported on carbon substrates have attracted significant attention due to their catalytic potential. Nevertheless, ensuring the stability of these nanoparticles remains a critical challenge, impeding their broad application. In this work, novel nanodot arrays (NAs) are introduced where superfine alloy nanoparticles are uniformly implanted in a 2D carbon substrate and securely anchored. Electrochemical testing of the PtCo NAs demonstrates exceptional methanol oxidation reaction (MOR) activity, achieving 1.25 A mg-1. Moreover, the PtCo NAs exhibit outstanding stability throughout the testing period, underscoring the effectiveness of the anchoring mechanism. Comprehensive characterization and theoretical calculations reveal that the 2D carbon-anchored structure optimizes the electronic structure and coordination environment of Pt, restricts nanoparticle migration, and suppresses transition metal dissolution. This strategy represents a major advancement in addressing the stability limitations of ultrafine nanoparticles in catalytic applications and offers broader insights into the design of next-generation catalysts with enhanced durability and performance.

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.

Recent advancements and prospects in noble and non-noble electrocatalysts for materials methanol oxidation reactions

The direct methanol fuel cell (DMFC) represents a highly promising alternative power source for small electronics and automobiles due to its low operating temperatures, high efficiency, and energy density. The methanol oxidation process (MOR) constitutes a fundamental chemical reaction occurring at the positive electrode of a DMFC. Pt-based materials serve as widely utilized MOR electrocatalysts in DMFCs. Nevertheless, various challenges, such as sluggish reaction rates, high production costs primarily attributed to the expensive Pt-based catalyst, and the adverse effects of CO poisoning on the Pt catalysts, hinder the commercialization of DMFCs. Consequently, endeavors to identify an alternative catalyst to Pt-based catalysts that mitigate these drawbacks represent a critical focal point of DMFC research. In pursuit of this objective, researchers have developed diverse classes of MOR electrocatalysts, encompassing those derived from noble and non-noble metals. This review paper delves into the fundamental concept of MOR and its operational mechanisms, as well as the latest advancements in electrocatalysts derived from noble and non-noble metals, such as single-atom and molecule catalysts. Moreover, a comprehensive analysis of the constraints and prospects of MOR electrocatalysts, encompassing those based on noble metals and those based on non-noble metals, has been undertaken.

A cutting-edge electrochemical sensing platform for the simultaneous determination of the residues of antimicrobial drugs, rifampicin and norfloxacin, in water samples

BACKGROUND

The widespread use and abuse of antibiotics has resulted in the pollution of water sources with antibiotic residues, posing a threat to human health, the environment, and the economy. Therefore, a highly sensitive and selective method is required for their detection in water samples. Herein, advanced ultrasensitive electrochemical sensor platform was developed by integrating gold-silver alloy nanocoral clusters (Au-Ag-ANCCs) with functionalized multi-walled carbon nanotube-carbon paste electrode (f-MWCNT-CPE) and choline chloride (ChCl) nanocomposites for simultaneously determining the residues of antimicrobial drugs, rifampicin (RAMP) and norfloxacin (NFX), in water samples.

RESULTS

The developed sensor (Au-Ag-ANCCs/f-MWCNTs-CPE/ChCl) was extensively characterized using several analytical (UV-Vis, FT-IR, XRD, SEM, and EDX) and electrochemical (EIS, CV, and SWV) techniques. It exhibited outstanding performance in a wide linear range, from 14 pM to 115 μM for RAMP, and from 0.9 nM to 200 μM for NFX, with a limit of detection (LOD, 3σ/m, S/N = 3, n = 5) and a limit of quantification (LOQ, 10σ/m, S/N = 3, n = 5) values of 2.7 pM and 8.85 pM for RAMP, and 0.14 nM and 0.47 nM for NFX, respectively. The sensor also exhibited exceptional reproducibility, stability, and resistance to interference.

SIGNIFICANCE

The developed sensor was effectively utilized to determine RAMP and NFX residues in hospital wastewater, river, and tap water samples, yielding recoveries within the range of 96.8-103 % and relative standard deviations below 5 %. Generally, the proposed sensor demonstrated remarkable performance in detecting the target analytes, making it an ideal tool and the first of its kind for addressing global antibiotic residue pollutants in water sources.

Wet-Chemical Synthesis of Concave Hexoctahedral Pd and Pd@Pt Nanocrystals for Methanol Electrooxidation

Nanocrystals (NCs) exposed with high-index facets usually show enhanced electrocatalytic performances. However, it is a great challenge to persevere with high-index facets against their high surface energy during the synthesis. Herein, we successfully synthesize concave hexoctahedral (c-HOH) Pd NCs exposed with 48 high-index {741} facets using a facile one-pot wet-chemical protocol. Control experiments illustrate that l-ascorbic acid plays a critical role in the formation of the c-HOH morphology, acting as both reducing and capping agents. Moreover, we can extend the synthesis for fabricating c-HOH Pd@Pt core-shell NCs by simply introducing a Pt precursor into the reaction solution, attaining remarkably boosted electrocatalysis for methanol electrooxidation reaction (MOR). Integrating the merits of {741} facets, concave structure, and ligand and strain effect of the core-shell structure, c-HOH Pd4@Pt1 core-shell NCs showed an excellent MOR mass activity of 1.18 A mgPGM-1 or 3.60 A mgPt-1, which is 3.80 or 11.61 times higher than that of commercial Pt/C, respectively.

Mixed-Dimensional Partial Dealloyed PtCuBi/C as High-Performance Electrocatalysts for Methanol Oxidation with Enhanced CO Tolerance

Developing efficient electrocatalysts for methanol oxidation reaction (MOR) is crucial in advancing the commercialization of direct methanol fuel cells (DMFCs). Herein, carbon-supported 0D/2D PtCuBi/C (0D/2D PtCuBi/C) catalysts are fabricated through a solvothermal method, followed by a partial electrochemical dealloying process to form a novel mixed-dimensional electrochemically dealloyed PtCuBi/C (0D/2D D-PtCuBi/C) catalysts. Benefiting from distinctive mixed-dimensional structure and composition, the as-obtained 0D/2D D-PtCuBi/C catalysts possess abundant accessible active sites. The introduction of Cu as a water-activating element weakens the COads, and oxophilic metal Bi facilitates the OHads, thereby enhancing its tolerance to CO poisoning and promoting MOR activity. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure spectroscopy (XAFS) collectively reveal the electron transfer from Cu and Bi to Pt, the electron-enrichment effect induced by dealloying, and the strong interactions among Pt-M (Cu, Pt, and Bi) multi-active sites, which improve the tuning of the electronic structure and enhancement of electron transfer ability. Impressively, the optimized 0D/2D D-PtCuBi/C catalysts exhibit the superior mass activity (MA) of 17.68 A mgPt -1 for MOR, which is 14.86 times higher than that of commercial Pt/C. This study offers a proposed strategy for Pt-based alloy catalysts, enabling their use as efficient anodic materials in fuel cell applications.

Laser Irradiation Synthesis of AuPd Alloy with Decreased Alloying Degree for Efficient Ethanol Oxidation Reaction

The preparation of electrocatalysts with high performance for the ethanol oxidation reaction is vital for the large-scale commercialization of direct ethanol fuel cells. Here, we successfully synthesized a high-performance electrocatalyst of a AuPd alloy with a decreased alloying degree via pulsed laser irradiation in liquids. As indicated by the experimental results, the photochemical effect-induced surficial deposition of Pd atoms, combined with the photothermal effect-induced interdiffusion of Au and Pd atoms, resulted in the formation of AuPd alloys with a decreased alloying degree. Structural characterization reveals that L-AuPd exhibits a lower degree of alloying compared to C-AuPd prepared via the conventional co-reduction method. This distinct structure endows L-AuPd with outstanding catalytic activity and stability in EOR, achieving mass and specific activities as high as 16.01 A mgPd-1 and 20.69 mA cm-2, 9.1 and 5.2 times than that of the commercial Pd/C respectively. Furthermore, L-AuPd retains 90.1% of its initial mass activity after 300 cycles. This work offers guidance for laser-assisted fabrication of efficient Pd-based catalysts in EOR.

Poly (3, 4-propylenedioxythiophene)/Hollow carbon sphere composites supported Pt NPs to facilitate methanol oxidation reactions

Direct methanol fuel cells (DMFCs) are thought of as portable, sustainable, and non-polluting energy devices. The exploration of efficient and affordable catalysts for the methanol oxidation reaction (MOR) is significant for the industrial application of DMFCs. In this study, nitrogen-doped hollow carbon spheres (HCS) derived from polydopamine were proposed for the catalyst support for platinum nanoparticles (Pt NPs) for serving as the anode catalyst for DMFCs, and a composite support material was fabricated by in-situ oxidation of 3,4-ethylenedioxythiophene (ProDOT) with HCS to get core-shell structured poly(3,4-propylenedioxythiophene) (PProDOT)-embellished hollow carbon spheres (HCS) (PProDOT/HCS) for further improving the catalytic activity for supported catalyst. The results indicated that the platinum (Pt) on the surface of HCS was well dispersed, and the Pt became smaller and more evenly distributed with the introduction of PProDOT. Simultaneously, the Schottky junction formed between PProDOT and Pt NPs contributes to enhanced charge transfer and catalytic activity of the catalyst. Notably, the core-shell structure of the ternary catalyst, its excellent charge transfer capability, and the interaction between platinum and the support contribute to its high electrocatalytic activity. Electrochemical tests demonstrated that the PProDOT/HCS/Pt catalyst exhibited a mass activity of 1169.6 mA mg-1Pt for methanol oxidation in acidic electrolytes, surpassing the activity of the HCS/Pt catalyst (472.4 mA mg-1Pt) and commercial Pt/C (281.0 mA mg-1Pt).

An active Ni(OH)2/MnCO3 catalyst with efficient synergism for alkaline methanol oxidation

A novel Ni(OH)2/MnCO3 hybrid catalyst was developed for high-performing alkaline methanol electro-oxidation, which could well overcome the shortages of inactive MnCO3 and low intrinsic Ni(OH)2 due to the good synergistic catalysis effect from the Jahn-Teller distortion effect.

Recent progress of heterogeneous catalysts for transfer hydrogenation under the background of carbon neutrality

Under the background of carbon neutrality, the direct conversion of greenhouse CO2 to high value added fuels and chemicals is becoming an important and promising technology. Among them, the generation of liquid C1 products (formic acid and methanol) has made great progress; nevertheless, it encounters the problem of how to use it efficiently to solve the overcapacity issue. In this review, we suggest that the catalytic transfer hydrogenation using formic acid and methanol as the hydrogen sources is a critical and potential route for the substitution for the fossil fuel-derived H2 to generate essential bulk and fine chemicals. We mainly focus on summarizing the recent progress of heterogeneous catalysts in such reactions, including thermal- and photo-catalytic processes. Finally, we also propose some challenges and opportunities for this development.

Constructing sulfur and oxygen super-coordinated main-group electrocatalysts for selective and cumulative H2O2 production

Direct electrosynthesis of hydrogen peroxide (H2O2) via the two-electron oxygen reduction reaction presents a burgeoning alternative to the conventional energy-intensive anthraquinone process for on-site applications. Nevertheless, its adoption is currently hindered by inferior H2O2 selectivity and diminished H2O2 yield induced by consecutive H2O2 reduction or Fenton reactions. Herein, guided by theoretical calculations, we endeavor to overcome this challenge by activating a main-group Pb single-atom catalyst via a local micro-environment engineering strategy employing a sulfur and oxygen super-coordinated structure. The main-group catalyst, synthesized using a carbon dot-assisted pyrolysis technique, displays an industrial current density reaching 400 mA cm-2 and elevated accumulated H2O2 concentrations (1358 mM) with remarkable Faradaic efficiencies. Both experimental results and theoretical simulations elucidate that S and O super-coordination directs a fraction of electrons from the main-group Pb sites to the coordinated oxygen atoms, consequently optimizing the *OOH binding energy and augmenting the 2e- oxygen reduction activity. This work unveils novel avenues for mitigating the production-depletion challenge in H2O2 electrosynthesis through the rational design of main-group catalysts.

Insights on the Roles of Nitrogen Configuration in Enhancing the Performance of Electrocatalytic Methanol Oxidation over Pt Nanoparticles

Stabilization of the Pt in N-doped carbon materials is an effective method to improve the performance of electrocatalytic methanol oxidation reaction (MOR). Nevertheless, the roles of different N configurations (pyridinic N, pyrrolic N, and graphitic N) toward the electrochemical performance of Pt-based catalysts remain unclear. Herein, Density Functional Theory calculations are adopted to elucidate the synergistic promotion of MOR by different N-configurations with Pt nanoparticles (NPs). Guided by the theoretical study, a series of MOR electrocatalysts with different ratios of pyridinic N and pyrrolic N (denoted as Pt/N-CNT-X (500, 600, 700, 800, and 900)) are designed and synthesized. Surprisingly, the electrocatalytic activity of Pt/N-CNT-600 with a suitable ratio of pyrrolic-N and pyridinic-N for MOR reaches 2394.7 mA mg-1 Pt and 5515.8 mA mg-1 Pt in acidic and alkaline media, respectively, which are superior to the Pt/CNTs, commercial Pt/C, and the ever-reported Pt-based electrocatalysts. The strong metal-support interaction induced by the N-doping is the crucial reason for the superior electrocatalytic performance. More importantly, the ability of pyrrolic-N and pyridinic-N in promoting the adsorption and oxidation of CH3 OH and the oxidation of CO* is substantiated for the first time in methanol oxidation. This work provides new insights on the design of efficient electrocatalysts for MOR.

Facile Synthesis of Multifunctional Ni(OH)2 -Supported Core-Shell Ni@Pd Nanocomposites for the Electro-Oxidation of Small Organic Molecules

In the domain of proton exchange membrane fuel cells (PEMFCs), the development of efficient and durable catalysts for the electro-oxidation of small organic molecules, especially of alcohols (methanol, ethanol, ethylene glycol, et al.) has always been a hot topic. A large number of related electrocatalysts with splendid performance have been designed and synthesized till now, while the preparation processes of most of them are demanding on experimental operations and conditions. Herein, we put forward a facile and handy method for the preparation of multifunctional Ni(OH)2 -supported core-shell Ni@Pd nanocomposites (Ni(OH)2 /Ni@Pd NCs) with the assistance of galvanic replacement reaction (GRR) at room temperature and ambient pressure. As expected, the Ni(OH)2 substrate can prevent the aggregation of core-shell (CS) Ni@Pd nanoparticles (NPs) and inhibit the formation of COads and further prevent Pd from being poisoned. The synergistic effect between CS Ni@Pd NPs and Ni(OH)2 substrate and the electronic effect between Pd shell and Ni core contribute to the outstanding electrocatalytic performance for methanol, ethanol, and ethylene glycol oxidation in alkaline condition. This study provides a succinct method for the design and preparation of efficient Pd-based electrocatalysts for alcohol electro-oxidation.

Dinitrogen Reduction Coupled with Methanol Oxidation for Low Overpotential Electrochemical NH3 Synthesis Over Cobalt Pyrophosphate as Bifunctional Catalyst

Electrochemical dinitrogen (N₂ ) reduction to ammonia (NH₃ ) coupled with methanol electro-oxidation is presented in the current work. Here, methanol oxidation reaction (MOR) is proposed as an alternative anode reaction to oxygen evolution reaction (OER) to accomplish electrons-induced reduction of N₂ to NH₃ at cathode and oxidation of methanol at anode in alkaline media thereby reducing the overall cell voltage for ammonia production. Cobalt pyrophosphate micro-flowers assembled by nanosheets are synthesized via a surfactant-assisted sonochemical approach. By virtue of structural and morphological advantages, the maximum Faradaic efficiency of 43.37% and NH₃ yield rate of 159.6 µg h^-1 mg_ca ^-1 is achieved at a potential of -0.2 V versus RHE. The proposed catalyst is shown to also exhibit a very high activity (100 mA mg^-1 at 1.48 V), durability (2 h) and production of value-added formic acid at anode (2.78 µmol h^-1 mg_cat ^-1 and F.E. of 59.2%). The overall NH₃ synthesis is achieved at a reduced cell voltage of 1.6 V (200 mV less than NRR-OER coupled NH₃ synthesis) when OER at anode is replaced with MOR and a high NH₃ yield rate of 95.2 µg h^-1 mg_cat ^-1 and HCOOH formation rate of 2.53 µmol h^-1 mg^-1 are witnessed under full-cell conditions.

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.

Chemical Surface Grafting of Pt Nanocatalysts for Reconciling Methanol Tolerance with Methanol Oxidation Activity

A conceptual challenge toward more versatile direct methanol fuel cells (DMFCs) is the design of a single material electrocatalyst with high activity and durability for both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). This requires to conciliate methanol tolerance not to hinder ORR at the cathode with a good MOR activity at the anode. This is especially incompatible with Pt materials. We tackled this challenge by deriving a supramolecular concept where surface-grafted molecular ligands regulate the Pt-catalyst reactivity. ORR and MOR activities of newly reported Pt-calix[4]arenes nanocatalysts (Pt CF 3 ${{_{{\rm CF}{_{3}}}}}$ NPs/C) are compared to commercial benchmark PtNPs/C. Pt CF 3 ${{_{{\rm CF}{_{3}}}}}$ NPs/C exhibit a remarkable methanol tolerance without losing the MOR reactivity along with outstanding durability and chemical stability. Beyond designing single-catalyst material, operable in DMFC cathodic and anodic compartments, the results highlight a promising strategy for tuning interfacial properties.

Hierarchical Pt-In Nanowires for Efficient Methanol Oxidation Electrocatalysis

Direct methanol fuel cells (DMFC) have attracted increasing research interest recently; however, their output performance is severely hindered by the sluggish kinetics of the methanol oxidation reaction (MOR) at the anode. Herein, unique hierarchical Pt-In NWs with uneven surface and abundant high-index facets are developed as efficient MOR electrocatalysts in acidic electrolytes. The developed hierarchical Pt₈₉In₁₁ NWs exhibit high MOR mass activity and specific activity of 1.42 A mg_Pt^-1 and 6.2 mA cm^-2, which are 5.2 and 14.4 times those of Pt/C, respectively, outperforming most of the reported MORs. In chronoamperometry tests, the hierarchical Pt₈₉In₁₁ NWs demonstrate a longer half-life time than Pt/C, suggesting the better CO tolerance of Pt₈₉In₁₁ NWs. After stability, the MOR activity can be recovered by cycling. XPS, CV measurement and CO stripping voltammetry measurements demonstrate that the outstanding catalytic activity may be attributed to the facile removal of CO due to the presence of In site-adsorbing hydroxyl species.

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.

The Role of Bismuth in Suppressing the CO Poisoning in Alkaline Methanol Electrooxidation: Switching the Reaction from the CO to Formate Pathway

While tuning the electronic structure of Pt can thermodynamically alleviate CO poisoning in direct methanol fuel cells, the impact of interactions between intermediates on the reaction pathway is seldom studied. Herein, we contrive a PtBi model catalyst and realize a complete inhibition of the CO pathway and concurrent enhancement of the formate pathway in the alkaline methanol electrooxidation. The key role of Bi is enriching OH adsorbates (OH_ad) on the catalyst surface. The competitive adsorption of CO adsorbates (CO_ad) and OH_ad at Pt sites, complementing the thermodynamic contribution from alloying Bi with Pt, switches the intermediate from CO_ad to formate that circumvents CO poisoning. Hence, 8% Bi brings an approximately 6-fold increase in activity compared to pure Pt nanoparticles. This notion can be generalized to modify commercially available Pt/C catalysts by a microwave-assisted method, offering opportunities for the design and practical production of CO-tolerance electrocatalysts in an industrial setting.

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).

One-Pot Synthesis of Pt Nanobowls Assembled from Ultrafine Nanoparticles for Methanol Oxidation Reaction

Simultaneously engineering a bowl-like and ultrafine nano-size structure offers an attractive route to not only increase the utilization efficiency of noble metals, the specific surface areas and the availability of active sites, but also boost the structural robustness and long-term stability. However, a great challenge remains in terms of the methods of synthesis. Herein, we report a facile one-pot hydrothermal method for the preparation of hollow porous Pt nanobowls (NBs) assembled from ultrafine particles. N,N'-methylenebisacrylamide (MBAA) acts as a structure-directing agent that forms a self-template with Pt ions and drives the nucleation and assembly of Pt metals, resulting in the fabrication of Pt NBs from ultrafine particles. By virtue of their unique structure and morphology, the optimized Pt NBs exhibited enhanced electrocatalytic methanol oxidation reaction (MOR) activity with 3.1-fold greater mass activity and 2.6-fold greater specific activities compared with those of commercial Pt black catalysts, as well as excellent stability and anti-poisoning ability.

Ultrathin rhodium nanosheet-gold nanowire nanocomposites for alkaline methanol oxidation reaction

Electrostatically assembled ultrathin rhodium nanosheet-gold nanowire nanocomposites (Rh-Au CNSs) were used as an advanced electrocatalyst for the methanol oxidation reaction, which revealed a mass activity of 355 mA mg_Rh^-1 at 0.607 V potential, much higher than single metal Rh nanosheets (273 mA mg_Rh^-1) and commercial Rh nanoparticles (165 mA mg_Rh^-1).

Electrocatalytic Oxygen Reduction to Produce Hydrogen Peroxide: Rational Design from Single-Atom Catalysts to Devices

Electrocatalytic production of hydrogen peroxide (H2O2) via the 2e- transfer route of the oxygen reduction reaction (ORR) offers a promising alternative to the energy-intensive anthraquinone process, which dominates current industrial-scale production of H2O2. The availability of cost-effective electrocatalysts exhibiting high activity, selectivity, and stability is imperative for the practical deployment of this process. Single-atom catalysts (SACs) featuring the characteristics of both homogeneous and heterogeneous catalysts are particularly well suited for H2O2 synthesis and thus, have been intensively investigated in the last few years. Herein, we present an in-depth review of the current trends for designing SACs for H2O2 production via the 2e- ORR route. We start from the electronic and geometric structures of SACs. Then, strategies for regulating these isolated metal sites and their coordination environments are presented in detail, since these fundamentally determine electrocatalytic performance. Subsequently, correlations between electronic structures and electrocatalytic performance of the materials are discussed. Furthermore, the factors that potentially impact the performance of SACs in H2O2 production are summarized. Finally, the challenges and opportunities for rational design of more targeted H2O2-producing SACs are highlighted. We hope this review will present the latest developments in this area and shed light on the design of advanced materials for electrochemical energy conversion.

Graphical abstract

Hierarchical Design in Nanoporous Metals

Hierarchically porous metals possess intriguing high accessibility of matter molecules and unique continuous metallic frameworks, as well as a high level of exposed active atoms. High rates of diffusion and fast energy transfer have been important and challenging goals of hierarchical design and porosity control with nanostructured metals. This review aims to summarize recent important progress toward the development of hierarchically porous metals, with special emphasis on synthetic strategies, hierarchical design in structure-function and corresponding applications. The current challenges and future prospects in this field are also discussed.

Passive Small Direct Alcohol Fuel Cells for Low-Power Portable Applications: Assessment Based on Innovative Increments since 2018

Passive small direct alcohol fuel cells (PS-DAFCs) are compact, standalone devices capable of electrochemically converting the chemical energy in the fuel/alcohol into electricity, with low pollutant emissions and high energy density. Thus, PS-DAFCs are extremely attractive as sustainable/green off-grid low-power sources (milliwatts to watts), considered as alternatives to batteries for small/portable electric and electronic devices. PS-DAFCs benefit from long life operation and low cost, assuring an efficient and stable supply of inherent non-polluting electricity. This review aims to assess innovations on PS-DAFC technology, as well as discuss the challenges and R&D needs covered on practical examples reported in the scientific literature, since 2018. Hence, this compilation intends to be a guidance tool to researchers, in order to help PS-DAFCs overcome the barriers to a broad market introduction and consequently become prime renewable energy converters and autonomous micropower generators. Only by translating research discoveries into the scale-up and commercialization process of the technology can the best balance between the economic and technical issues such as efficiency, reliability, and durability be achieved. In turn, this will certainly play a crucial role in determining how PS-DAFCs can meet pressing sustainable energy needs.

Defect-Rich, Highly Porous PtAg Nanoflowers with Superior Anti-Poisoning Ability for Efficient Methanol Oxidation Reaction

The design of efficient and sustainable Pt-based catalysts is the key to the development of direct methanol fuel cells. However, most Pt-based catalysts still exhibit disadvantages including unsatisfied catalytic activity and serious CO poisoning in the methanol oxidation reaction (MOR). Herein, highly porous PtAg nanoflowers (NFs) with rich defects are synthesized by using liquid reduction combining chemical etching. It is demonstrated that the proportion of precursors determines the inhomogeneity of alloy elements, and the strong corrosiveness of nitric acid to silver leads to the eventual porous flower-like structure. Impressively, the optimal etched Pt₁ Ag₂ NFs have the mixed defects of surface steps, dislocations, and bulk holes, and their mass activity (1136 mA mg_Pt^-1 ) is 2.6 times higher than that of commercial Pt/C catalysts, while the ratio of forward and backward peak current density (I_f /I_b ) can reach 3.2, exhibiting an excellent anti-poisoning ability. Density functional theory calculations further verify their high anti-poison properties from both an adsorption and an oxidation perspective of CO intermediate. The introduction of Ag makes it easier for CO to be oxidized and removed. This study provides a facile approach to prepare rich defects and porous alloy with excellent MOR performance and superior anti-poisoning ability.

High-Entropy Alloys for Electrocatalysis: Design, Characterization, and Applications

High-entropy alloys (HEAs) are expected to function well as electrocatalytic materials, owing to their widely adjustable composition and unique physical and chemical properties. Recently, HEA catalysts are extensively studied in the field of electrocatalysis; this motivated the authors to investigate the relationship between the structure and composition of HEAs and their electrocatalytic performance. In this review, the latest advances in HEA electrocatalysts are systematically summarized, with special focus on nitrogen fixation, the carbon cycle, water splitting, and fuel cells; in addition, by combining this with the characterization and analysis of HEA microstructures, rational design strategies for optimizing HEA electrocatalysts, including controllable preparation, component regulation, strain engineering, defect engineering, and theoretical prediction are proposed. Moreover, the existing issues and future trends of HEAs are predicted, which will help further develop these high-entropy materials.

l-Lysine derived fabrication of Cu@Ni core–satellite nanoassemblies as efficient non-Pt catalysts for the methanol oxidation reaction

With assistance of l-lysine, Cu@Ni core–satellite nanoassemblies were fabricated, which could serve as efficient non-Pt electrocatalysts for the methanol oxidation reaction due to both the component effects and structural features.

Energy-saving H2 Generation Coupled with Oxidative Alcohol Refining over Bimetallic Phosphide Ni2 P-CoP Junction Bifunctional Electrocatalysts

The realization of large-scale H₂ production from electrocatalytic water splitting is severely impeded by the kinetically sluggish and economically less viable anodic oxygen evolution reaction. Here, an efficient strategy was established for the concurrent H₂ production and oxidative alcohols refining into value-added formate by utilizing self-supported Ni₂ P-CoP bifunctional electrocatalysts. Benefiting from high intrinsic activity, abundant active sites, and synergistic promoting effects of bimetallic phosphides, the constructed two-electrode electrolyzer required a cell voltage of around 1.3 V to achieve 10 mA cm^-2 , which is more than 200 mV lower than that of pure water splitting. Moreover, simultaneous productions of H₂ with near-unity conversion efficiency and formate at high faradaic efficiencies of 99.8 and 89.6 % oxidatively produced from methanol and glycerol, respectively, were achieved with excellent durability. This work presents a general and economic approach toward the fabrication of cost-effective electrocatalysts for energy-efficient and profitable large-scale renewable energy integration.

Engineering PtCu nanoparticles for a highly efficient methanol electro-oxidation reaction

Achieving a highly efficient and durable methanol electro-oxidation catalyst in acid media is critical for the practical utilization of direct methanol fuel cells (DMFCs) at the commercial scale. Herein, we report a facile and effective one-pot strategy for the synthesis of carbon-supported PtCu alloy nanoparticles (PtCu NPs) with a Pt-rich surface, small particle size and uniform dispersion. The as-prepared PtCu NPs with the optimal alloy composition (Pt₂Cu) exhibit a significantly improved electrochemical methanol oxidation reaction performance in terms of a high activity, superior CO tolerance and remarkable durability, in contrast to those of commercial Pt/C catalysts in acid media. Particularly, the Pt₂Cu/C catalyst exerts a 4.5 times enhancement in the mass activity and a larger I_f/I_b value compared to those of commercial Pt/C (Pt/C_comm). The enhanced catalytic activities can be ascribed to the high utilization of Pt and the high index facets of the surface. Also, the addition of Cu downshifts the d-band center of Pt and improves the CO tolerance during the methanol oxidation reaction process. This work provides an efficient strategy for designing desired Pt-based alloys for various catalytic reactions.

Intermetallic Compounds: Liquid-Phase Synthesis and Electrocatalytic Applications

Characterized by long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

Ultrasonically Surface-Activated Nickel Foam as a Highly Efficient Monolith Electrode for the Catalytic Oxidation of Methanol to Formate

Most of the current electrocatalysts for the methanol oxidation reaction are precious group metals such as Pt, Pd, and Ru. However, their use is limited due to their high cost, scarcity, and issues with carbon monoxide poisoning. We developed a simple method to prepare a nickel foam (NF)-based monolith electrode with a NiO nanosheet array structure as an efficient electrocatalyst toward the oxidation of methanol to produce formate. By a simple ultrasonic acid treatment and air oxidation at room temperature, an inert NF was converted to NiO/NF as a catalytically active electrode due to the uniform NiO nanosheet array that was rapidly formed on the surface of NiO/NF. In alkaline electrolytes containing methanol, the as-prepared NiO/NF catalysts exhibited a lower methanol oxidation reaction (MOR) potential of +1.53 V vs RHE at 100 mA cm^-2 compared to that of inert NF samples. The difference in potentials between the E_MOR and the E_OER at that current density was found to be 280 mV, indicating that methanol oxidation occurred at lower potentials as compared to the oxygen evolution reaction (OER). We also observed that the NiO/NF could also efficiently catalyze the oxidation of CO without being poisoned by it. NiO/NF retained close to 100% of its initial activity after 20,000 s of methanol oxidation tests at high current densities above 200 mA cm^-2. Because of the simple synthesis method and the enhanced catalytic performance and stability of NiO/NF, this allows methanol to be used as an OER masking agent for the energy-efficient generation of value-added products such as formic acid and hydrogen.

Enhanced Methanol Electro-Oxidation Activity of Nanoclustered Gold

Size-selected 3 nm gas-phase Au clusters dispersed by cluster beam deposition (CBD) on a conducting fluorine-doped tin oxide template show strong enhancement in mass activity for the methanol electro-oxidation (MEO) reaction compared to previously reported nanostructured gold electrodes. Density functional theory-based modeling on the corresponding Au clusters guided by experiments attributes this high MEO activity to the high density of exposed under-coordinated Au atoms at their faceted surface. In the description of the activity trends, vertices and edges are the most active sites due to their favorable CO and OH adsorption energies. The faceted structures occurring in this size range, partly preserved upon deposition, may also prevent destructive restructuring during the oxidation-reduction cycle. These results highlight the benefits of using CBD in fine-tuning material properties on the nanoscale and designing high-performance fuel cell electrodes with less material usage.

Acid-Etched Co3O4 Nanoparticles on Nickel Foam: The Highly Reactive (311) Facet and Enriched Defects for Boosting Methanol Oxidation Electrocatalysis

The confirmation and regulation of active sites are particularly critical for the design of methanol oxidation reaction (MOR) catalysts. Here, an acid etching method for facet control combined with defect construction was utilized to synthesize Co₃O₄ nanoparticles on nickel foam for preferentially exposing the (311) facet with enriched oxygen vacancies (V_O). The acid-leached oxides exhibited superior MOR activity with a mass activity of 710.94 mA mg^-1 and an area-specific activity of 3.390 mA cm^-2 as a result of (i) abundant active sites for MOR promoted by V_O along with the highly active (311) facet being exposed and (ii) phase purification-reduced adsorption energy (E_ads) of methanol molecules. Ex situ X-ray photoelectron spectroscopy proved that highly active CoOOH obtained via the activation of plentiful Co²⁺ effectively improved the MOR. Density functional theory calculations confirmed that the selective exposed (311) facet has the lowest E_ads for CH₃OH molecules. This work puts forward acid etching as the facet modification and defect engineer for nanostructured non-noble catalysts, which is expected to result in superior electrochemical performance required for advanced alkaline direct methanol fuel cells.

Nickel Iron Diselenide for Highly Efficient and Selective Electrocatalytic Conversion of Methanol to Formate

The electro-oxidation of methanol to formate is an interesting example of the potential use of renewable energies to add value to a biosourced chemical commodity. Additionally, methanol electro-oxidation can replace the sluggish oxygen evolution reaction when coupled to hydrogen evolution or to the electroreduction of other biomass-derived intermediates. But the cost-effective realization of these reaction schemes requires the development of efficient and low-cost electrocatalysts. Here, a noble metal-free catalyst, Ni_{1- x} Fe_x Se₂ nanorods, with a high potential for an efficient and selective methanol conversion to formate is demonstrated. At its optimum composition, Ni_0.75 Fe_0.25 Se₂ , this diselenide is able to produce 0.47 mmol cm^-2  h^-1 of formate at 50 mA cm^-2 with a Faradaic conversion efficiency of 99%. Additionally, this noble-metal-free catalyst is able to continuously work for over 50 000 s with a minimal loss of efficiency, delivering initial current densities above 50 mA cm^-2 and 2.2 A mg^-1 in a 1.0 m KOH electrolyte with 1.0 m methanol at 1.5 V versus reversible hydrogen electrode. This work demonstrates the highly efficient and selective methanol-to-formate conversion on Ni-based noble-metal-free catalysts, and more importantly it shows a very promising example to exploit the electrocatalytic conversion of biomass-derived chemicals.

Convenient synthesis of polymetallic metal–organic gels for efficient methanol electro-oxidation

Novel Ni-based AlNiCu-MOG and AB&AlNiCu-MOG composite materials were successfully fabricated, which exhibited superior MOR activities with a current density of 17.1 and 33.24 mA cm⁻², respectively.

Bismuth Oxyhydroxide-Pt Inverse Interface for Enhanced Methanol Electrooxidation Performance

Developing efficient Pt-based electrocatalysts for the methanol oxidation reaction (MOR) is of pivotal importance for large-scale application of direct methanol fuel cells (DMFCs), but Pt suffers from severe deactivation brought by the carbonaceous intermediates such as CO. Here, we demonstrate the formation of a bismuth oxyhydroxide (BiO_x(OH)_y)-Pt inverse interface via electrochemical reconstruction for enhanced methanol oxidation. By combining density functional theory calculations, X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy, and electrochemical characterizations, we reveal that the BiO_x(OH)_y-Pt inverse interface can induce the electron deficiency of neighboring Pt; this would result in weakened CO adsorption and strengthened OH adsorption, thereby facilitating the removal of the poisonous intermediates and ensuring the high activity and good stability of Pt₂Bi sample. This work provides a comprehensive understanding of the inverse interface structure and deep insight into the active sites for MOR, offering great opportunities for rational fabrication of efficient electrocatalysts for DMFCs.