Mechanism Of Charge Transfer Process Research Articles

(Physical and Analytical Electrochemistry Division David C. Grahame Award) Vignettes of Charge Transfer Processes at Interfaces Inspired By D.C. Grahame and Previous ECS Grahame Awardees

I am very humbled to be recognized with the 2023 ECS David C. Grahame Award. My presentation will start with the acknowledgment that this award does not happen without all the efforts of students, post-doctoral fellows and colleagues over the past 30 years. Notably, the basis of all my graduate work was inspired by Grahame’s 1947 seminal work [1] on electric double layers at metal-solution interfaces. The study of “clean” pseudo-single crystalline silver electrodes enabled us to quantify and understand the electrochemical formation of stable monolayers of various anions and cations, e.g. [2,3]. In hindsight, also I recognize that these studies provided me with the fundamental underpinnings and motivations to study charge transfer processes in much more complicated semi-metal- and semiconductor-solution systems using a plethora of experimental methods during my academic career. As discussed long ago by Gerischer [4], the key distinction between metal-solution and semiconductor-solution interfaces is that the mechanisms of charge transfer processes can be divided into many groups, mainly with those with weak interactions and the reactants in the electrolyte and those with strong interactions. A more complicating factor with semiconductor-solution interfaces is that the charge-transfer process typically involves both reaction with the electrolyte and a change in the conductivity of the electrode, e.g. [5]. I will provide a series of vignettes (case studies) inspired by several previous ECS Grahame awardees of the distinctive features of different charge transfer processes and their relation to energy conversion, energy storage and chemical sensing applications.[1] Grahame, D. C. Chem. Rev. 1947, 41, 441.[2] Stevenson, K. J.; Gao, X.; Hatchett, D. W.; White, H. S. J. Electroanal. Chem. 1998, 447, 43.[3] White, H. S.; Peterson, J.; Cui, Q; Stevenson, K. J. J. Phys. Chem. B 1998, 102, 2930.[4] Gerischer, H. 1969, 18, 97.[5] Nikitina, V. A.; Vassiliev, S. Yu; Stevenson, K. J. Adv. Ener. Mater. 2020, 1903933.

Efficient Design for a High-Energy and High-Power Capability Hybrid Electric Power Device with Enhanced Electrochemical Interfaces.

Fabrication of novel electrode architectures with tailored electrochemical interfaces (EI) is an effective strategy for enhancing charge and mass transport processes within electrochemical devices. Here, we design and fabricate a well-hybrid electrode based on the coupling of polyaniline (PANI) nanowires and Pt-based electrocatalysts to manufacture a hybrid electric power device (HEPD) combining the advantages of supercapacitors and fuel cells. Because of the boosted charge transfer between PANI nanowires and Pt-based materials via enhanced EIs, the HEPD assembled with hybrid electrodes shows remarkable performance with a peak power density of 222 mW cm-2, a specific power of 3810 W kg-1, and a specific energy of 2100 Wh kg-1, normalized to the mass of membrane electrode assemblies. The in situ Raman spectra and extended electrochemical studies demonstrate the intrinsic mechanism of charge transfer processes within hybrid electrodes, shedding light on the alternative progress of electrochemical energy conversion systems and storage devices.

Synthesis of oxy-sulfide based nanocomposite catalyst for visible light-driven reduction of Cr(VI)

The oxy-sulfide based V2O5@(In,Ga)2(O,S)3 nanocomposite catalyst, at different weight percentages of V2O5, was successfully synthesized via a simplistic procedural route for the detoxification of hazardous Cr(VI). The two pure catalysts were intimately allied and used for visible light-driven reduction of hazardous Cr(VI). The nanocomposite catalysts were characterized to observe the effects of V2O5 on crystal phase, morphology, light absorption, catalytic activity, and electrical properties. Compared to all, 40% V2O5 loaded nanocomposite catalyst, designated as VOS-2, exhibited the best-reducing capability. It completely reduced toxic Cr(VI) at 2 min under visible light illumination. From the kinetics, it was found that the rate constant of the nanocomposite catalyst was improved by a factor of 3.6 compared to the host nanoflower catalyst. The plausible mechanism of charge transfer process across the interfacial region indicates the diminished recombination probability of photogenerated charge carriers. Therefore, the nanocomposite catalyst is promising for enhanced reduction of Cr(VI) in the Cr-based industrial activities, which is significantly relevant for environmental remediation.

A Method Developed for Getting Kinetic Information from Electrochemical Impedance Spectroscopy Measured for Anodized Ti in Fluoride

EIS has been acknowledged to be a powerful tool for investigating the kinetic mechanisms of charge transfer processes at electrode/solution interfaces. The method of using equivalent electrical circuits (EECs) has been most frequently used for interpreting measured EIS spectra. But the mechanistic information that this method can deliver is very limited. In this work, an alternative but innovative method, which combines the EECs and the mechanism analysis, is developed. With anodically polarized, oxide film covered titanium in NaF-containing perchloric acid solutions as an example, a detailed procedure is described and discussed for obtaining kinetic information (such as the rate constant, rate-determining step) from the results estimated by EECs. Two double-step mechanism models for anodized titanium are proposed and tested based on the strategy of the developing approach.

Electronic non-equilibrium charge transfer mechanisms in alternating photocurrents on semiconductor electrodes

The theoretical treatment of alternating photocurrents on semiconductor electrodes has been extended, taking into account electronic non-equilibrium charge transfer kinetics of chemical processes at the interface. Some basic mechanisms of charge transfer processes under illumination are investigated, including anodic irreversible one- to multistep redox processes, anodic or cathodic chemisorption, and superposition of chemisorption with irreversible redox processes. Computational examples are given for alternating photocurrent peaks due to n-III-V-semiconductor dissolution, and to light-driven chemisorption. As possibly interfering processes, electron or hole transport-controlled surface charging, and a non-equilibrium contribution to space charge capacity are considered, as well as the quantum-chemical and dynamical criteria for the distinction of charge transfer and surface charing. In the experimental field, some further alternating photocurrent data on n-GaP and n-GaAs are supplied, including the first phase-sensitive measurements; light-pulsed measurements are discussed briefly.

Charge transfer processes at semiconductor-electrolyte interfaces in connection with problems of catalysis

The mechanisms of charge transfer processes can be devided into two groups, those with weak interaction between the semiconductor surface and the reactants in the electrolyte and those with strong ones. The basic features of both mechanisms and their relation to catalytic processes are discussed. Interesting reactions which are closely related to photocatalysis can be initiated by illumination. Some examples are given and the mechanism of spectral photosensitization is analysed in connection with the fundamental aspects of such charge transfer processes.