In situ Spectroscopic Insight into the Origin of the Enhanced Performance of Bimetallic Nanocatalysts towards the Oxygen Reduction Reaction (ORR)

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Revealing Interfacial Reactions on Pt Electrodes in Ionic Liquids by In Situ Fourier-Transform Infrared Spectroscopy

In situ monitoring of the electrolyte/electrode interfacial processes, such as the oxygen reduction reaction (ORR), is crucial for the design of electrolytes for fuel cells. In this study, we investigate the electrochemical behavior of platinum electrodes in protic ionic liquids (PILs) by means of in situ Fourier-transform infrared spectroscopy coupled with cyclic voltammetry. The result provides direct evidence of the change of water at the Pt electrode surface due to Pt oxide formation and reduction. A decrease in the interfacial water was observed in the spectra upon the formation of the Pt oxide. Conversely, the local water concentration at the electrode surface increases if the Pt oxide is reduced and the ORR takes place. At the same time, more cations replace anions on the electrode. The ORR kinetics in the [TFSI]-based PILs is slower than in the [TfO]-based ones, which could result from a blockage of catalytic sites by the adsorbed [TFSI] anions. It suggests that reducing the anion adsorption on the platinum surface could provide an opportunity to enhance the ORR activity.

Microporous 3D-Structured Hierarchically Entangled Graphene-Supported Pt3Co Alloy Catalyst for PEMFC Application with Process-Friendly Features

To improve the oxygen reduction reaction (ORR) performance in a proton-exchange membrane fuel cell (PEMFC) cathode with respect to mass activity and durability, a suitable electrocatalyst design strategy is essentially needed. Here, we have prepared a sub-three nm-sized platinum (Pt)-cobalt (Co) alloy (Pt₃Co)-supported N-doped microporous 3D graphene (Pt₃Co/pNEGF) by using the polyol synthesis method. A microwave-assisted synthesis method was employed to prepare the catalyst based on the 3D porous carbon support with a large pore volume and dense micro-/mesoporous surfaces. The ORR performance of Pt₃Co/pNEGF closely matches with the state-of-the-art commercial Pt/C catalyst in 0.1 M HClO₄, with a small overpotential of 10 mV. The 3D microporous structure of the N-doped graphene significantly improves the mass transport of the reactant and thus the overall ORR performance. As a result of the lower loading of Pt in Pt₃Co/pNEGF as compared to that in Pt/C, the alloy catalyst achieved 1.5 times higher mass activity than Pt/C. After 10,000 cycles, the difference in the electrochemically active surface area (ECSA) and half-wave potential (E_1/2) of Pt₃Co/pNEGF is found to be 5 m² g_Pt^-1 (ΔECSA) and 24 mV (ΔE_1/2), whereas, for Pt/C, these values are 9 m² g_Pt^-1 and 32 mV, respectively. Finally, in a realistic perspective, single-cell testing of a membrane electrode assembly (MEA) was made by sandwiching the Pt₃Co/pNEGF-coated gas diffusion layers as the cathode displayed a maximum power density of 800 mW cm^-2 under H₂-O₂ feed conditions with a clear indication of helping the system in the mass-transfer region (i.e., the high current dragging condition). The nature of the I-V polarization shows a progressively lower slope in this region of the polarization plot compared to a similar system made from its Pt/C counterpart and a significantly improved performance throughout the polarization region in the case of the system made from the Pt₃Co/NEGF catalyst (without the microwave treatment) counterpart. These results validate the better process friendliness of Pt₃Co/pNEGF as a PEMFC electrode-specific catalyst owing to its unique texture with 3D architecture and well-defined porosity with better structural endurance.

Boosting Reactive Oxygen Species Generation Using Inter-Facet Edge Rich WO3 Arrays for Photoelectrochemical Conversion

Water oxidation reaction leaves room to be improved in the development of various solar fuel productions, because of the kinetically sluggish 4-electron transfer process of oxygen evolution reaction. In this work, we realize reactive oxygen species (ROS), H₂ O₂ and OH⋅, formations by water oxidation with total Faraday efficiencies of more than 90 % by using inter-facet edge (IFE) rich WO₃ arrays in an electrolyte containing CO₃ ^2- . Our results demonstrate that the IFE favors the adsorption of CO₃ ^2- while reducing the adsorption energy of OH⋅, as well as suppresses surface hole accumulation by direct 1-electron and indirect 2-electron transfer pathways. Finally, we present selective oxidation of benzyl alcohol by in situ using the formed OH⋅, which delivers a benzaldehyde production rate of ≈768 μmol h^-1 with near 100 % selectivity. This work offers a promising approach to tune or control the oxidation reaction in an aqueous solar fuel system towards high efficiency and value-added product.

Plasmonic Photoelectrochemical Coupling Reactions of para-Aminobenzoic Acid on Nanostructured Gold Electrodes

Surface plasmon resonance (SPR) bridges photonics and photoelectrochemistry by providing an effective interaction between absorption and confinement of light to surface electrons of plasmonic metal nanostructures (PMNs). SPR enhances the Raman intensity enormously in surface-enhanced Raman spectroscopy (SERS) and leads to the plasmon-mediated chemical reaction on the surface of nanostructured metal electrodes. To observe variations in chemical reactivity and selectivity, we studied the SPR photoelectrochemical reactions of para-aminobenzoic acid (PABA) on nanostructured gold electrodes. The head-to-tail coupling product "4-[(4-imino-2,5-cyclohexadien-1-ylidene)amino]benzoic acid (ICBA)" and the head-to-head coupling product p,p'-azodibenzoate (ADBA) were obtained from PABA adsorbed on PMN-modified gold electrodes. In particular, under acidic and neutral conditions, ICBA was obtained as the main product, and ADBA was obtained as the minor product. At the same time, under basic conditions, ADBA was obtained as the major product, and ICBA was obtained as the minor product. We have also provided sufficient evidence for the oxidation of the tail-to-tail coupling reaction product that occurred in a nonaqueous medium rather than in an aqueous medium. The above finding was validated by the cyclic voltammetry, SERS, and theoretical calculation results of possible reaction intermediates, namely, 4-aminophenlylenediamine, 4-hydroxyphenlylenediamine, and benzidine. The theoretical adsorption model and experimental results indicated that PABA has been adsorbed as para-aminobenzoate on the gold cluster in a bidentate configuration. This work offers a new view toward the modulation of selective surface catalytic coupling reactions on PMN, which benefits the hot carrier transfer efficiency at photoelectrochemical interfaces.

Shell isolated nanoparticle enhanced Raman spectroscopy for mechanistic investigation of electrochemical reactions

Electrochemical conversion of abundant resources, such as carbon dioxide, water, nitrogen, and nitrate, is a remarkable strategy for replacing fossil fuel-based processes and achieving a sustainable energy future. Designing an efficient and selective electrocatalysis system for electrochemical conversion reactions remains a challenge due to a lack of understanding of the reaction mechanism. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a promising strategy for experimentally unraveling a reaction pathway and rate-limiting step by detecting intermediate species and catalytically active sites that occur during the reaction regardless of substrate. In this review, we introduce the SHINERS principle and its historical developments. Furthermore, we discuss recent SHINERS applications and developments for investigating intermediate species involved in a variety of electrocatalytic reactions.

Zn, Co, and Fe Tridoped N-C Core-Shell Nanocages as the High-Efficiency Oxygen Reduction Reaction Electrocatalyst in Zinc-Air Batteries

Transition metal-nitrogen-carbon (TM-N-C) nanomaterials are promising platinum-based substitutes for the oxygen reduction reaction (ORR). However, large-scale commercial production of high-efficiency, durable TM-N-C catalysts remains a formidable challenge. In this work, a facile ″ZIF-on-ZIF″ strategy is first adopted to design ZIF-8@ZIF-67 core-shell polyhedral nanocages, and then, ferrocene (Fc) is added to form ZIF-8@ZIF-67@Fc double-layer encapsulating polyhedral nanocages. Finally, Zn, Co, and Fe tridoped N-C nanocages (ZnCoFe-N-C) as the high-efficiency ORR electrocatalyst are prepared through high-temperature annealing. Benefiting from the trimetal, nitrogen and carbon species bond to each other to form highly efficient active sites, and the material exhibits outstanding performance in 0.1 M KOH, onset potential and half-wave potential of up to 0.95 and 0.878 V (vs RHE), respectively, and long-term durability and methanol tolerance. Furthermore, when utilizing as a zinc-air battery (ZAB) air electrode, it exhibits wonderful indicators, reflected in an open circuit voltage of 1.525 V, power density of 350.2 mW cm^-2, and specific capacity of 794.7 mAh g_zn^-1, which outperforms the benchmark Pt/C catalyst. This work provides a facile and effective strategy to obtain a highly efficient and stable TM-N-C electrocatalyst for the ORR in ZABs.

Spectroscopic Verification of Adsorbed Hydroxy Intermediates in the Bifunctional Mechanism of the Hydrogen Oxidation Reaction

Elucidating hydrogen oxidation reaction (HOR) mechanisms in alkaline conditions is vital for understanding and improving the efficiency of anion-exchange-membrane fuel cells. However, uncertainty remains around the alkaline HOR mechanism owing to a lack of direct in situ evidence of intermediates. In this study, in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and DFT were used to study HOR processes on PtNi alloy and Pt surfaces, respectively. Spectroscopic evidence indicates that adsorbed hydroxy species (OH_ad ) were directly involved in HOR processes in alkaline conditions on the PtNi alloy surface. However, OH_ad species were not observed on the Pt surface during the HOR. We show that Ni doping promoted hydroxy adsorption on the platinum-alloy catalytic surface, improving the HOR activity. DFT calculations also suggest that the free energy was decreased by hydroxy adsorption. Consequently, tuning OH adsorption by designing bifunctional catalysts is an efficient method for promoting HOR activity.

Strong coupling between magnetic resonance and propagating surface plasmons at visible light frequencies

Light-matter interactions in nanostructures have shown great potential in physics, chemistry, surface science, materials science, and nanophotonics. Herein, for the first time, the feasibility of strong coupling between plasmon-induced magnetic resonant and propagating surface plasmonic modes at visible light frequencies is theoretically demonstrated. Taking advantage of the strong coupling between these modes allowed for a narrow-linewidth hybrid mode with a huge electromagnetic field enhancement to be acquired. This work can serve as a promising guide for designing a platform with strong coupling based on magnetic resonance at visible and even ultraviolet light frequencies and also offers an avenue for further exploration of strong light-matter interactions at the nanoscale.