Structural effects on voltammograms of the low index planes of palladium and Pd(S)-[n(100)×(111)] surfaces in alkaline solution

Electrochemical Behavior and Shape Evolution of Structured Pd Nanoparticles in Alkaline Media─Influence of Electrochemically Absorbed Hydrogen

We report on the electrochemical behavior and shape evolution of Pd nanocubes (Pd NCs) and Pd nanooctahedrons (Pd NOs) with an average size of 9.8 and 6.9 nm, respectively, in aqueous alkaline medium in the potential range of the underpotential deposition of H (UPD H) and H absorption. While the Pd NCs and Pd NOs remain stable in the potential region of the UPD H, H absorption and desorption of absorbed H (Habs) induce structural changes to the Pd NPs, as indicated by the results of electrochemical measurements and identical location-transmission electron microscopy (IL-TEM) analyses. Because both Pd NCs and Pd NOs are known to be stable in the potential region of H absorption and Habs desorption in acidic medium and maintain their structure, the irreversible structural changes are attributed to their interfacial interaction with the aqueous alkaline medium. In the alkaline medium, the nanoparticle surface/electrolyte interfacial structure plays an essential role in the mechanism of Habs desorption that is observed at higher potentials than that in the acidic medium. Hydrogen desorption is substantially hindered due to the structure of the water network adjacent to the Pd nanoparticles or the interaction between hydrated cations and adsorbed OH on the nanoparticle surface, resulting in the trapping of a small amount of H (incomplete Habs desorption). It is proposed that H trapping and associated structural strain lead to the deformation of the Pd nanoparticles and the loss of their initial structure.

Subsurface Hydride Formation Leads to Slow Surface Adsorption Processes on a Pd(111) Single-Crystal Electrode in Acidic Electrolytes

Palladium is one of the most important catalysts due to its widespread use in heterogeneous catalysis and electrochemistry. However, an understanding of the electrochemical processes and interfacial phenomena at Pd single-crystal electrodes/electrolytes is still scarce. In this work, the electrochemical behavior of the Pd(111) electrode was studied by the combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in different acidic electrolytes, namely, sulfuric acid, perchlorate acid, methane sulfonic acid, and hydrofluoric acid. An analysis of CV profiles shows the strong adsorption of all anions at low electrode potential, partially overlapping with underpotential deposited hydrogen (UPD-H), leading to the appearance of a pair of sharp peaks in what would be considered the "hydrogen region". All anions studied (HSO4-, ClO4-, CH3SO3-, and F-) adsorb specifically and interact with (or effectively block) the surface-adsorbed hydroxyl phase formed on the Pd(111) terrace at higher potentials. Strikingly, the scan rate-dependent results show that the process of anion adsorption and desorption is a kinetically rather slow step. EIS measurements show that the exact mechanism of this slow anion ad/desorption process actually stems from (sub)surface phenomena: the direct hydrogen insertion into Pd lattice (hydrogen subsurface absorption) commences from ca. 0.40 V and leads to the formation of (subsurface) Pd hydrides (PdHx). We argue that the subsurface hydrogen phase significantly alters the work function and thereby the kinetics of the anion adsorption and desorption processes, leading to irreversible peaks in the voltammetry. This precise understanding is important in guiding further fundamental work on Pd single crystals and will be crucial to advancing the eventual design of optimized Pd electrocatalysts.

Oxidative Stability Matters: A Case Study of Pallidum Hydride Nanosheets for Alkaline Fuel Cells

Pd-based electrocatalysts are considered to be a promising alternative to Pt in anion-exchange membrane fuel cells (AEMFCs), although major challenges remain. Most of the Pd-based electrocatalysts developed for the sluggish oxygen reduction reaction (ORR) have been exclusively evaluated by rotating disk electrode (RDE) voltammetry at room temperature, rather than in membrane electrode assemblies (MEAs), making it challenging to apply them in practical fuel cells. We have developed a series of carbon-supported novel PdH_x nanosheets (PdH_x NS), which displayed outstanding ORR performance in room-temperature RDE tests. Specifically, a sample synthesized at 190 °C displayed a mass activity of 0.67 A mg^-1 and a specific activity of 1.07 mA cm^-2 at 0.95 V vs RHE, representing the highest reported value among Pd-based ORR electrocatalysts in alkaline media and higher than Pt-based catalysts reported in the literature. Furthermore, we employed PdH_x NS and commercial Pd/C as model catalysts to systematically study the effects of temperature on their ORR activity in RDE measurements and subsequently evaluated their performance in MEA testing. Our observations indicate/demonstrate how oxidative stability affected the ORR performance of Pd-based electrocatalysts, which provided some critical insights into future ORR catalyst development for alkaline fuel cell applications.

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.

Developments of the Electroactive Materials for Non-Enzymatic Glucose Sensing and Their Mechanisms

A comprehensive review of the electroactive materials for non-enzymatic glucose sensing and sensing devices has been performed in this work. A general introduction for glucose sensing, a facile electrochemical technique for glucose detection, and explanations of fundamental mechanisms for the electro-oxidation of glucose via the electrochemical technique are conducted. The glucose sensing materials are classified into five major systems: (1) mono-metallic materials, (2) bi-metallic materials, (3) metallic-oxide compounds, (4) metallic-hydroxide materials, and (5) metal-metal derivatives. The performances of various systems within this decade have been compared and explained in terms of sensitivity, linear regime, the limit of detection (LOD), and detection potentials. Some promising materials and practicable methodologies for the further developments of glucose sensors have been proposed. Firstly, the atomic deposition of alloys is expected to enhance the selectivity, which is considered to be lacking in non-enzymatic glucose sensing. Secondly, by using the modification of the hydrophilicity of the metallic-oxides, a promoted current response from the electro-oxidation of glucose is expected. Lastly, by taking the advantage of the redistribution phenomenon of the oxide particles, the usage of the noble metals is foreseen to be reduced.

Revealing Facet Effects of Palladium Nanocrystals on Electrochemical Biosensing

Noble-metal nanocrystals (NCs) are functional segments of biosensing platforms, but their sensitivity and facet effects are still challenging. Conventional synthesis using surfactants to direct crystal growth unfortunately causes adsorbate-surface hindrance, which not only reduces sensing responses but also leads to misunderstanding on facet-dependence. Herein, we utilize electrochemical CO displacement to remove residual surfactants from facet-engineered Pd NCs, and further investigate the structure-activity relationship on specific facets, for example, {100} in cubes, {111} in octahedrons, and {110} in rhombic dodecahedrons. Along with the remarkably boosted response, facet dependence is obvious for H₂O₂ sensing after surface cleaning. The Pd{100} shows high sensitivity, low detection limit, and wide applicable concentration range, superior to the {110} and {111}. This can be theoretically interpreted by the befitting *OH binding on {100} and thereby the facilitated H₂O₂ reduction kinetics. The outstanding selectivity to H₂O₂ ensures the high efficiency of Pd NCs to measure intracellular H₂O₂ and recognize different types of cancer cells. Moreover, facet effects are also evidenced in glucose detection, highlighting that this work can provide guidelines to design efficient sensing platforms.

Ethanol Electrooxidation Catalyzed by Tungsten Core@Palladium Shell Nanoparticles

Bimetallic nanostructures represent effective electrocatalysts toward a number of important reactions. In the present study, carbon-supported palladium-tungsten alloy nanoparticles with a quasi-tungsten core@palladium shell structure (W@Pd/C) were synthesized by a galvanic replacement reaction of amorphous tungsten nanoparticles with Pd(II) at different temperatures (0, 25, and 50 °C), and exhibited apparent electrocatalytic activity toward ethanol oxidation reaction (EOR). When the sample was prepared at 0 °C, large amorphous tungsten nanoparticles were etched off and much smaller W@Pd nanoparticles were formed and dispersed rather evenly on the carbon surface whereas at higher reaction temperatures (25 and 50 °C), the W@Pd nanoparticles became agglomerated. The structures of the obtained samples were characterized by a range of experimental tools, including (scanning) transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical methods. Among the series, the W@Pd/C sample prepared at 0 °C was observed to exhibit the best EOR performance, with a mass activity (9535.5 mA mg_Pd^-1) over three times better than that of commercial Pd/C and markedly enhanced stability.

Electrocatalytic oxidation of ethanol and ethylene glycol on cubic, octahedral and rhombic dodecahedral palladium nanocrystals

Ethanol and ethylene glycol electrocatalytic oxidation on Pd cubic, octahedral and rhombic dodecahedral nanocrystals in alkaline media was systematically investigated.

A Pd nanocatalyst supported on multifaceted mesoporous silica with enhanced activity and stability for glycerol electrooxidation

The current density of glycerol electrooxidation on Pd/SiO₂reached twice the values found for Pd/C. Mesosporous silica support works as a trap that confines the reactant.

Facet effects of palladium nanocrystals for oxygen reduction in ionic liquids and for sensing applications

Palladium nanocrystals enclosed by {100} and {110} crystal facets, were successfully synthesized through an aqueous one-pot synthesis method. A new thermal annealing approach was developed for fabricating these palladium nanocrystals as a working electrode on a gas permeable membrane to study the facet effects of the oxygen reduction process in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpy][NTf2]). Results were compared with the same processes at a conventional platinum electrode. Our study shows that the structural difference between the two facets of Pd nanocrystals has little effect on the oxygen reduction process but significantly affects the oxidation process of the superoxide. It is found that the Pd{110}/IL interface can better stabilize superoxide radicals revealed by a more positive oxidation potential compared to that of Pd{100}. In addition, the analytical characteristic of utilizing both palladium nanocrystals as electrodes for oxygen sensing is comparable with a polycrystal platinum oxygen sensor, in which Pd{110} presents the best sensitivity and lowest detection limit. Our results demonstrate the facet-dependence of oxygen reduction in an ionic liquid medium and provide the fundamental information needed to guide the applications of palladium nanocrystals in electrochemical gas sensor and fuel cell research.

Infrared spectroelectrochemical study of dissociation and oxidation of methanol at a palladium electrode in alkaline solution

The dissociative adsorption and electrooxidation of CH(3)OH at a Pd electrode in alkaline solution are investigated by using in situ infrared spectroscopy with both internal and external reflection modes. The former (ATR-SEIRAS) has a higher sensitivity of detecting surface species, and the latter (IRAS) can easily detect dissolved species trapped in a thin-layer-structured electrolyte. Real-time ATR-SEIRAS measurement indicates that CH(3)OH dissociates to CO(ad) species at a Pd electrode accompanied by a "dip" at open circuit potential, whereas deuterium-replaced CH(3)OH doesn't, suggesting that the breaking of the C-H bond is the rate-limiting step for the dissociative adsorption of CH(3)OH. Potential-dependent ATR-SEIRAS and IRAS measurements indicate that CH(3)OH is electrooxidized to formate and/or (bi)carbonate, the relative concentrations of which depend on the potential applied. Specifically, at potentials negative of ca. -0.15 V (vs Ag/AgCl), formate is the predominant product and (bi)carbonate (or CO(2) in the thin-layer structure of IRAS) is more favorable at potentials from -0.15 to 0.10 V. Further oxidation of the CO(ad) intermediate species arising from CH(3)OH dissociation is involved in forming (bi)carbonate at potentials above -0.15 V. Although the partial transformation from interfacial formate to (bi)carbonate may be justified, no bridge-bonded formate species can be detected over the potential range under investigation.

Low-coordination sites in oxygen-reduction electrocatalysis: their roles and methods for removal

Low-coordination sites, including edges, kinks, and defects, play an important role in oxygen-reduction electrocatalysis. Their role was studied experimentally and theoretically for various Pt surfaces. However, the roughness effect on similar-sized nanoparticles that could elucidate the role of low-coordination sites has attracted much less attention, with no studies on Pd nanoparticles. Here, using Br- adsorption/desorption, we introduce an effective approach to reduce surface roughness, yielding Pd nanoparticles with smoother surfaces and an increased number of (111)-oriented facets. The resulting nanoparticles have a slightly contracted structure and narrow size distribution. Pt monolayer catalysts that contain such nanoparticles as the cores showed a 1.5-fold enhancement in specific and Pt mass activities for the oxygen reduction reaction compared with untreated ones. Furthermore, a dramatic increase in durability was observed with bromide-treated Pd(3)Co cores. These results demonstrate a simple approach to preparing nanoparticles with smooth surfaces and confirm the adverse effect of low-coordination sites on the kinetics of the oxygen-reduction reaction.