The Vanadium Redox Reactions - Electrocatalysis versus Non-Electrocatalysis

Influence of Nitrogen Doping into Carbon on the Activation Barrier of ORR in Alkaline Medium: An Investigation Based on Eyring Analysis

The oxygen reduction reaction (ORR) is investigated on metal-free carbon (Vulcan XC-72) and nitrogen-doped (∼≤1%) carbon (N/C-900) in 0.1 M KOH. The product distribution (O₂ to OH^- and HO₂^-) as a function of overpotential (η) in the temperature range of 293-323 K is analyzed using a rotating ring-disk electrode (RRDE) assembly. The kinetic current due to reduction of O₂ to HO₂^- is estimated and used in the Eyring analysis to determine the change in enthalpy of activation (ΔH^#). It is shown that doping of carbon with nitrogen (even with ≤1 wt %) causes substantial increase in the number of active sites (almost 2-fold) and reduction in ΔH^# at any η. Moreover, ΔH^# is a stronger function of η on N/C-900 as compared to that on the carbon surface.

The impact of overpotential on the enthalpy of activation and pre-exponential factor of electrochemical redox reactions

The kinetics of the V⁵⁺/V⁴⁺ redox reaction is investigated in a three-electrode configuration on a Vulcan XC-72 modified glassy carbon rotating disk electrode at four different temperatures (25 to 40 °C, with 5 °C interval). The values of enthalpy of activation (ΔH^#) and pre-exponential factor (A_f) estimated using the Eyring equation are in the range of 0.25-0.53 eV (24-51 kJ mol^-1) and -1.3 to 5, respectively. The Eyring plots tend to diverge with overpotential, causing an increase in the values of the estimated ΔH^# and A_f. This is perhaps due to the retarding effect of the precipitates/adsorbates on the electrode surface. The investigation of the kinetics suggests that the V⁵⁺/V⁴⁺ redox reaction is electrocatalysed through an increase in the entropy of activation (ΔS^#).

Kinetics of Hydrogen Evolution Reactions in Acidic Media on Pt, Pd, and MoS2

Hydrogen evolution reaction (HER) are investigated on Pt, Pd, and MoS₂ in a 0.5 M H₂SO₄ electrolyte in a rotating disk electrode (RDE) configuration in the temperature range of 285-335 K. The reaction is temperature-sensitive on all of the three catalyst surfaces at their respective overpotential ranges. The kinetic parameters (activation enthalpy (ΔH^#), free energy of activation (ΔG^#), and pre-exponential factor (A_f)) toward HER are obtained from the Arrhenius and Eyring relations, and the overall kinetics on the catalyst surfaces is analyzed. ΔH^# for HER is a strong function of the overpotential in the case of both Pt and Pd. On the other hand, the trend in A_f suggests that the electrocatalysis of HER on MoS₂ originates from an increase in entropy factor, perhaps due to the solvent-dipole interaction at the interface. Such analysis is pivotal to the investigation of electrocatalysis of HER, especially on surfaces for which determination of active-site density is not established.

Holey aligned electrodes through in-situ ZIF-8-assisted-etching for high-performance aqueous redox flow batteries

Fabricating electrodes with large specific surface area (SSA) and high permeability has been the long-standing target in redox flow batteries (RFBs). In this work, we propose a novel ZIF-8-assisted etching approach to form holey fibers in the electrospinning process of aligned electrode structures. The etching approach allows the formation of holey fibers with small pores of ~50 nm, offering large active surface areas for redox reactions, while the aligned macrostructure with the holey fibers of 3-5 μm in diameter ensures a high permeability along the fiber direction. The application of the prepared electrodes to a vanadium redox flow battery (VRFB) enables an energy efficiency (EE) of 87.2% at the current density of 200 mA cm^-2, which is 13.3% higher than that with conventional electrospun carbon electrodes. Even at high current densities of 300 and 400 mA cm^-2, the battery still maintains energy efficiencies of 83.3% and 79.3%. More excitingly, the prepared electrode yields a high limiting current density of 4500 mA cm^-2 and a peak power density of 1.6 W cm^-2. It is anticipated that the present electrospinning method combining the ZIF-8-assisted etching approach with a way to form ordered fiber structures will allow even more high-performance electrodes for RFBs in the future.

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms

The fundamental aspects of quantum electrocatalysts are discussed together with the newly developed electrochemical kinetic isotope effect (EC-KIE) approach.