Recent progress in the direct ethanol fuel cell: development of new platinum–tin electrocatalysts

Titanium Oxycarbide as Platinum-Free Electrocatalyst for Ethanol Oxidation

The compound material titanium oxycarbide (TiOC) is found to be an effective electrocatalyst for the electrochemical oxidation of ethanol to CO2. The complete course of this reaction is one of the main challenges in direct ethanol fuel cells (DEFCs). While TiOC has previously been investigated as catalyst support material only, in this study we show that TiOC alone is able to oxidize ethanol to acetaldehyde without the need of expensive noble metal catalysts like Pt. It is suggested that this behavior is attributed to the presence of both undercoordinated sites, which allow ethanol to adsorb, and oxygenated sites, which facilitate the activation of water. This is a milestone in DEFC research and development and opens up innovative possibilities for the design of catalyst materials for intermediate temperature fuel cells.

Ethanol Electro-Oxidation on Catalysts with S-ZrO2-Decorated Graphene as Support in Fuel Cell Applications

Direct ethanol fuel cells (DEFCs) are considered the most suitable direct alcohol fuel cell (DAFC) in terms of safety and current density. The obstacle to DEFC commercialization is the low reaction kinetics of ethanol (C₂H₅OH) oxidation because of the poor performance of the electrocatalyst. In this study, for the first time, graphene nanoplates (GNPs) were coated with sulfated zirconium dioxide (ZrO₂) as adequate support for platinum (Pt) catalysts in DEFCs. A Pt/S-ZrO₂-GNP electrocatalyst was prepared by a new process, polyol synthesis, using microwave heating. Field emission scanning electron microscope (FESEM) imaging revealed well-dispersed platinum nanoparticles supported on the S-ZrO₂-GNP powder. Analysis of the Fourier transform infrared (FTIR) spectrometry confirmed that sulfate modified the surfaces of the sample. In X-ray diffraction (XRD), no effect of S-ZrO₂ on the crystallinity net in Pt was found. Pt/S-ZrO₂-GNP electrode outperformed those with unsulfated counterparts, primarily for the higher access with electron and proton, confirming sulfonating as a practical approach for increasing the performance, electrocatalytic activity, and carbon monoxide (CO) tolerance in an electrocatalyst. A considerable decrease in the voltage of the CO electrooxidation peak from 0.93 V for Pt/C to 0.76 V for the Pt/S-ZrO₂-GNP electrode demonstrates that the new material increases activity for CO electrooxidation. Moreover, the as-prepared Pt/S-ZrO₂-GNPs electrocatalyst exhibits high catalytic activity for the EOR in terms of electrochemical surface area with respect to Pt/ZrO₂-GNPs and Pt/C (199.1 vs. 95 and 67.2 cm².mg^-1 Pt), which may be attributed to structural changes caused by the high specific surface area of graphene nanoplates catalyst support and sulfonating effect as mentioned above. Moreover, EIS results showed that the Pt/S-ZrO₂-GNPs electrocatalyst has a lower charge transfer resistance than Pt/ ZrO₂-GNPs and Pt/C in the presence of ethanol demonstrating an increased ethanol oxidation activity and reaction kinetics by Pt/S-ZrO₂-GNPs.

Structure of an Ultrathin Oxide on Pt3 Sn(111) Solved by Machine Learning Enhanced Global Optimization

Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt₃ Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.

Structure of an Ultrathin Oxide on Pt3Sn(111) Solved by Machine Learning Enhanced Global Optimization

Determination of the atomic structure of solid surfaces typically depends on comparison of measured properties with simulations based on hypothesized structural models. For simple structures, the models may be guessed, but for more complex structures there is a need for reliable theory-based search algorithms. So far, such methods have been limited by the combinatorial complexity and computational expense of sufficiently accurate energy estimation for surfaces. However, the introduction of machine learning methods has the potential to change this radically. Here, we demonstrate how an evolutionary algorithm, utilizing machine learning for accelerated energy estimation and diverse population generation, can be used to solve an unknown surface structure-the (4×4) surface oxide on Pt3Sn(111)-based on limited experimental input. The algorithm is efficient and robust, and should be broadly applicable in surface studies, where it can replace manual, intuition based model generation.

In-Situ surface enhanced infrared absorption spectroscopy study of electrocatalytic oxidation of ethanol on Platinum/Gold surface

The behavior of ethanol oxidation reaction on composite electrodes prepared by deposition platinum on a gold surface (Pt/Au) were studied by cyclic voltammetry and surface enhanced infrared absorption spectroscopy (SEIRAS) analysis. The results show that the Pt electrode has high oxidation activity and significant poison behavior; on the contrary, the Au electrode demonstrates low activity without a poison peak. The SEIRAS analyses reveal that both carbon monoxide (CO) and carbon dioxide (CO₂) appear during anode sweeping, and the CO peak density decreases with increasing potential and finally is eliminated. During the cathodic scanning, the CO peak reappears, and the peak intensity increases with scanning cycles, demonstrating a high poison behavior and the C1 reaction route on Pt. On the Au electrode, CO₂ and CO peaks were not observed; instead, an acetic acid peak appeared, indicating a C2 reaction path. For the Pt/Au composited electrodes, the electrochemical activities of the electrodes, as well as their poison behavior, increased with the deposition amount of Pt. However, the intensities of the poison peaks are smaller than those of oxidation ones; therefore, a higher tolerance to the CO poison can be achieved. For the 2 m-Pt/Au composite electrode, the activity is close to that of pure Pt, but the poison tolerance is 3 times the value of Pt.

Local probe investigation of electrocatalytic activity

As the world energy crisis remains a long-term challenge, development and access to renewable energy sources are crucial for a sustainable modern society. Electrochemical energy conversion devices are a promising option for green energy supply, although the challenge associated with electrocatalysis have caused increasing complexity in the materials and systems, demanding further research and insights. In this field, scanning probe microscopy (SPM) represents a specific source of knowledge and understanding. Thus, our aim is to present recent findings on electrocatalysts for electrolysers and fuel cells, acquired mainly through scanning electrochemical microscopy (SECM) and other related scanning probe techniques. This review begins with an introduction to the principles of several SPM techniques and then proceeds to the research done on various energy-related reactions, by emphasizing the progress on non-noble electrocatalytic materials.

A Phosphorus-Doped Ag@Pd Catalyst for Enhanced CC Bond Cleavage during Ethanol Electrooxidation

Ethanol is preferred to be oxidized into CO₂ for the construction of a high-performance direct ethanol fuel cell since this complete ethanol oxidation reaction (EOR) transfers 12 electrons. However, this EOR is sluggish and has the low activity as well as poor selectivity. To promote such a favorable EOR, more exactly the cleavage selectivity of CC bonds in ethanol, phosphorus-doped silver-core-and-Pd-shell catalysts (denoted as Ag@PdP) are designed and synthesized. In the alkaline media, a Ag@Pd₂ P_0.2 catalyst is superior toward EOR into CO₂ . It exhibits seven times higher mass activity and six times higher selectivity than the benchmark Pd/C catalyst. As confirmed by means of density functional theory calculation and in situ Fourier-transform infrared spectroscopy, such high performance stems from an increased adsorption energy of OH radicals on the Pd active sites. Meanwhile, the tensile strain effect of a core-shell structure of this Ag@Pd₂ P_0.2 catalyst favors the formation of adsorbed CH₃ CO intermediate, the key species for the enhanced C-C cleavage into CO₂ , instead of acetate. The proposed way to design and synthesize such high-performance EOR catalysts will explore the practical applications of direct alkaline ethanol fuel cells.

Ethanol Electrooxidation on Rhodium-Lead Catalysts in Alkaline Media: High Mass Activity, Long-Term Durability, and Considerable CO2 Selectivity

Rhodium (Rh)-based catalysts may solve the long-standing inefficient oxidation of ethanol for direct ethanol fuel cells (DEFCs); however, the performance of ethanol oxidation reaction (EOR) on existing Rh-based catalysts are far limited. Herein, the Rh-Pb catalysts are synthesized by building Pb and Pb oxide around Rh nanodomain, which shows highly efficient splitting CC bond and facile further oxidation of as-generated C1 intermediates (CO_ad and CH_x fragments). It exhibits an ever-highest EOR peak mass activity of ≈2636 mA mg^-1 _Rh among Rh-based catalysts in alkaline media. Meanwhile, its anodic current remains ≈50% even after a 4 h durability test at 0.53 V versus RHE. As for the C1-pathway selectivity, in situ infrared adsorption spectral (IRAS) results demonstrate that it could significantly improve the production of CO₂ . More directly, the apparent faraday efficiency of EOR C1 pathway is estimated to be as high as 20% (at 0.53 V versus RHE). This Rh-Pb catalyst could hold great promise for developing the commercial DEFCs.

Controlled Synthesis of Palladium Nanocubes as an Efficient Nanocatalyst for Suzuki-Miyaura Cross-Coupling and Reduction of p-Nitrophenol

Anisotropic nanocatalysts have attracted considerable attention in comparison to bulk/nanocatalysts for their enhanced activity and reactivity. The demand toward anisotropic palladium (Pd) nanostructures has increased rapidly in the field of catalysis. Pd is a well-known active catalyst for several carbon-carbon (C-C) cross-coupling reactions; among them, the Suzuki-Miyaura cross-coupling reaction is one of the most versatile and dominant methods for constructing the extraordinarily useful unsymmetrical biaryls and also for hydrogenation of organic contaminants like p-nitrophenol (p-NP). This paper provides a brief explanation about the controlled synthesis, characterization, and catalytic activity of well-defined palladium nanocubes (Pd NCs) prepared by a seed-mediated method. The synthesized monodispersed Pd NCs were characterized by spectroscopic and microscopic tools such as UV-visible, XRD, FESEM, HRTEM, and EDS analyses. Pd NCs proved as an efficient catalyst for Suzuki-Miyaura cross-coupling reactions and p-NP reduction. The catalyst shows enhanced activity, greater stability, and higher selectivity with remarkable recyclability up to 92% for five consecutive cycles. The catalytic performance of the synthesized Pd NCs was also studied in the reduction of the organic contaminant p-NP, which showed an excellent performance screening of 99% conversion in 6 min.

Highly Strained Au Nanoparticles for Improved Electrocatalysis of Ethanol Oxidation Reaction

Au is an ideal noble metal for use as an electrocatalyst for the ethanol oxidation reaction owing to its high performance-to-cost ratio. The catalyst usually exists as nanoparticles (NPs) for high surface area-to-volume ratio. In the present work, a nontraditional physical approach has been developed to fabricate ultrasmall and homogeneous single-crystalline Au NPs by ion bombardment in a precision ion polishing system. Transmission electron microscopy characterizations show that the Au NPs produced with 5 keV Ar⁺ are highly strained to form twinned crystals, which accumulate a large amount of surface energy, and this was found to be an underlying reason causing strong catalysis. Electrochemistry tests reveal that in alkaline medium the C1 pathway occurs much more preferentially with the strained Au NPs than the normal Au NPs. The surface area-to-volume ratio is no longer the only factor that affects the performance; instead, surface energy might play a more important role in enhancing the catalytic activities.

Ethanol Electrooxidation at Platinum-Rare Earth (RE = Ce, Sm, Ho, Dy) Binary Alloys

Proton exchange membrane fuel cells and direct alcohol fuel cells have been extensively studied over the last three decades or so. They have emerged as potential systems to power portable applications, providing clean energy, and offering good commercial viability. Ethanol is considered one of the most interesting fuels in this field. Herein, platinum-rare earth (Pt-RE) binary alloys (RE = Ce, Sm, Ho, Dy, nominal composition 50 at.% Pt) were produced and studied as anodes for ethanol oxidation reaction (EOR) in alkaline medium. A Pt-Dy alloy with nominal composition 40 at.% Pt was also tested. Their electrocatalytic performance was evaluated by voltammetric and chronoamperometric measurements in 2 M NaOH solution with different ethanol concentrations (0.2–0.8 M) in the 25–45 °C temperature range. Several EOR kinetic parameters were determined for the Pt-RE alloys, namely the charge transfer and diffusion coefficients, and the number of exchanged electrons. Charge transfer coefficients ranging from 0.60 to 0.69 and n values as high as 0.7 were obtained for the Pt0.5Sm0.5 electrode. The EOR reaction order at the Pt-RE alloys was found to vary between 0.4 and 0.9. The Pt-RE electrodes displayed superior performance for EOR than bare Pt, with Pt0.5Sm0.5 exhibiting the highest electrocatalytic activity. The improved electrocatalytic activity in all of the evaluated Pt-RE binary alloys suggests a strategy for the solution of the existing anode issues due to the structure-sensitive EOR.

Effect of the Random Defects Generated on the Surface of Pt(111) on the Electro-oxidation of Ethanol: An Electrochemical Study

In the present work, the Pt(111) surface was disordered by controlling the density of {110}- and {100}-type defects. The cyclic voltammogram (CV) of a disordered surface in acid media consists of three contributions within the hydrogen adsorption/desorption region: one from the well-ordered Pt(111) symmetry and the other two transformed from the {111}-symmetry with contributions of {110}- and {100}-type surface defects. The ethanol oxidation reaction (EOR) was studied on these disordered surfaces. Electrochemical studies were performed in 0.1 M HClO₄ +0.1 M ethanol using cyclic voltammetry and chronoamperometry. Changes in current densities associated to the specific potentials at which each oxidation peak appears suggest that different surface domains of disordered platinum oxidize ethanol independently. Additionally, as the surface-defect density increases, the EOR is catalysed better. This tendency is directly observed from the CV parameters because the onset and peak potentials are shifted to less positive values and accompanied by increases in the oxidation-peak current on disordered surfaces. Similarly, the CO oxidation striping confirmed this same tendency. Chronoamperometric experiments showed two opposite behaviors at short oxidation times (0.1 s). The EOR was quickly catalyzed on the most disordered surface, Pt(111)-16, and was then rapidly deactivated. These results provide fundamental information on the EOR, which contributes to the atomic-level understanding of real catalysts.

Nanocatalysts for Electrocatalytic Oxidation of Ethanol

The use of ethanol as a fuel in direct alcohol fuel cells depends not only on its ease of production from renewable sources, but also on overcoming the challenges of storage and transportation. In an ethanol-based fuel cell, highly active electrocatalysts are required to break the C-C bond in ethanol for its complete oxidation at lower overpotentials, with the aim of increasing the cell performance, ethanol conversion rates, and fuel efficiency. In recent decades, the development of wet-chemistry methods has stimulated research into catalyst design, reactivity tailoring, and mechanistic investigations, and thus, created great opportunities to achieve efficient oxidation of ethanol. In this Minireview, the nanomaterials tested as electrocatalysts for the ethanol oxidation reaction in acid or alkaline environments are summarized. The focus is mainly on nanomaterials synthesized by using wet-chemistry methods, with particular attention on the relationship between the chemical and physical characteristics of the catalysts, for example, catalyst composition, morphology, structure, degree of alloying, presence of oxides or supports, and their activity for ethanol electro-oxidation. As potential alternatives to noble metals, non-noble-metal catalysts for ethanol oxidation are also briefly reviewed. Insights into further enhancing the catalytic performance through the design of efficient electrocatalysts are also provided.

Pt-Based Catalysts for Electrochemical Oxidation of Ethanol

Despite its attractive features as a power source for direct alcohol fuel cells, utilization of ethanol is still hampered by both fundamental and technical challenges. The rationale behind the slow and incomplete ethanol oxidation reaction (EOR) with low selectivity towards CO₂ on most Pt-based catalysts is still far from being understood, and a number of practical problems need to be addressed before an efficient and low-cost catalyst is designed. Some recent achievements towards solving these problems are presented. Pt film electrodes and Pt monolayer (Pt_ML) electrodes on various single crystal substrates showed that EOR follows the partial oxidation pathway without C-C bond cleavage, with acetic acid and acetaldehyde as the final products. The role of the substrate lattice on the catalytic properties of Pt_ML was proven by the choice of appropriate M(111) structure (M = Pd, Ir, Rh, Ru and Au) showing enhanced kinetics when Pt_ML is under tensile strain on Au(111) electrode. Nanostructured electrocatalysts containing Pt-Rh solid solution on SnO₂ and Pt monolayer on non-noble metals are shown, optimized, and characterized by in situ methods. Electrochemical, in situ Fourier transform infrared (FTIR) and X-ray absorption spectroscopy (XAS) techniques highlighted the effect of Rh in facilitating C-C bond splitting in the ternary PtRh/SnO₂ catalyst. In situ FTIR proved quantitatively the enhancement in the total oxidation pathway to CO₂, and in situ XAS confirmed that Pt and Rh form a solid solution that remains in metallic form through a wide range of potentials due to the presence of SnO₂. Combination of these findings with density functional theory calculations revealed the EOR reaction pathway and the role of each constituent of the ternary PtRh/SnO₂ catalyst. The optimal Pt:Rh:Sn atomic ratio was found by the two in situ techniques. Attempts to replace Rh with cost-effective alternatives for commercially viable catalysts has shown that Ir can also split the C-C bond in ethanol, but the performance of optimized Pt-Rh-SnO₂ is still higher than that of the Pt-Ir-SnO₂ catalyst.

Carbon chain growth by formyl coupling over the Cu/γ-AlOOH(001) surface in syngas conversion

Carbon chain growth in syngas-to-ethanol conversion over the Cu/γ-AlOOH(001) surface is mainly ascribed to formyl coupling.

Status and prospects in higher alcohols synthesis from syngas

Higher alcohols are important compounds with widespread applications in the chemical, pharmaceutical and energy sectors. Currently, they are mainly produced by sugar fermentation (ethanol and isobutanol) or hydration of petroleum-derived alkenes (heavier alcohols), but their direct synthesis from syngas (CO + H₂) would comprise a more environmentally-friendly, versatile and economical alternative. Research efforts in this reaction, initiated in the 1930s, have fluctuated along with the oil price and have considerably increased in the last decade due to the interest to exploit shale gas and renewable resources to obtain the gaseous feedstock. Nevertheless, no catalytic system reported to date has performed sufficiently well to justify an industrial implementation. Since the design of an efficient catalyst would strongly benefit from the establishment of synthesis-structure-function relationships and a deeper understanding of the reaction mechanism, this review comprehensively overviews syngas-based higher alcohols synthesis in three main sections, highlighting the advances recently made and the challenges that remain open and stimulate upcoming research activities. The first part critically summarises the formulations and methods applied in the preparation of the four main classes of materials, i.e., Rh-based, Mo-based, modified Fischer-Tropsch and modified methanol synthesis catalysts. The second overviews the molecular-level insights derived from microkinetic and theoretical studies, drawing links to the mechanisms of Fischer-Tropsch and methanol syntheses. Finally, concepts proposed to improve the efficiency of reactors and separation units as well as to utilise CO₂ and recycle side-products in the process are described in the third section.

Defects and Interfaces on PtPb Nanoplates Boost Fuel Cell Electrocatalysis

Nanostructured Pt is the most efficient single-metal catalyst for fuel cell technology. Great efforts have been devoted to optimizing the Pt-based alloy nanocrystals with desired structure, composition, and shape for boosting the electrocatalytic activity. However, these well-known controls still show the limited ability in maximizing the Pt utilization efficiency for achieving more efficient fuel cell catalysis. Herein, a new strategy for maximizing the fuel cell catalysis by controlling/tuning the defects and interfaces of PtPb nanoplates using ion irradiation technique is reported. The defects and interfaces on PtPb nanoplates, controlled by the fluence of incident C⁺ ions, make them exhibit the volcano-like electrocatalytic activity for methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), and oxygen reduction reaction (ORR) as a function of ion irradiation fluence. The optimized PtPb nanoplates with the mixed structure of dislocations, subgrain boundaries, and small amorphous domains are the most active for MOR, EOR, and ORR. They can also maintain high catalytic stability in acid solution. This work highlights the impact and significance of inducing/controlling the defects and interfaces on Pt-based nanocrystals toward maximizing the catalytic performance by advanced ion irradiation strategy.

Ethanol Oxidation Reaction on Tandem Pt/Rh/SnOx Catalyst

To elucidate the atomic arrangement of a Pt-Rh-Sn ternary catalyst with a high catalytic activity for ethanol oxidation reaction (EOR) and high CO2 selectivity, we prepared a tandem Pt/Rh/SnOx, in which a Rh adlayer was deposited on a Pt substrate (Rh coverage: 0.28), followed by depositing several layers of SnOx only on the Rh surface (Sn coverage: 0.07). For reference, Sn was randomly deposited on the Rh-modified Pt (Pt/Rh) electrode whose Rh and Sn coverages were 0.22 and 0.36 (random Pt/Rh/SnOx). X-ray photoelectron spectroscopy demonstrated that Pt and Rh were metallic, and Sn was largely oxidized. Both Pt/Rh/SnOx electrodes were less positive in onset potential of EOR current density and higher in EOR current density than Pt and Rh/Pt electrodes. In situ infrared reflection-absorption spectroscopy demonstrated that the tandem Pt/Rh/SnOx electrode did not produce acetic acid, but produced CO2 in contrast to the random Pt/Rh/SnOx, suggesting that a tandem arrangement of Pt, Rh and SnOx, in which the Pt and SnOx sites were separated by the Rh sites, was effective for selective CO2 production. In the electrostatic electrolysis at 0.5 V vs. RHE, the tandem Pt/Rh/SnOx electrode exhibited higher EOR current density than the Pt and Pt/Rh electrodes after 1.5 h.