Calculation of the Tafel slope and reaction order of the oxygen evolution reaction between pH 12 and pH 14 for the adsorbate mechanism

Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction, the mechanism of the OER on transition metal oxides remains controversial. This is in part owed to the ambiguity of electrochemical parameters of the mechanism such as the Tafel slope and reaction orders. We took the most commonly assumed adsorbate mechanism and calculated the Tafel slopes and reaction orders with respect to pH based on microkinetic analysis. We demonstrate that number of possible Tafel slopes strongly depends on a number of preceding steps and surface coverage. Furthermore, the Tafel slope becomes pH dependent when the coverage of intermediates changes with pH. These insights complicate the identification of a rate-limiting step by a single Tafel slope at a single pH. Yet, simulations of reaction orders complementary to Tafel slopes can solve some ambiguities to distinguish between possible rate-limiting steps. The most insightful information can be obtained from the low overpotential region of the Tafel plot. The simulations in this work provide clear guidelines to experimentalists for the identification of the limiting steps in the adsorbate mechanism using the observed values of the Tafel slope and reaction order in pH-dependent studies.

Kinetics of V5+/V4+ Redox Reaction – Butler-Volmer and Marcus Models

Kinetics of the V5+/V4+ redox reaction on Vulcan XC-72 modified glassy carbon disk electrode is investigated in a three-electrode configuration. Cyclic voltammograms of V5+/V4+ redox couple suggest that the overpotential range for the kinetic analysis is limited to ±300 mV, after excluding V4+/V3+ redox reaction at the negative overpotential and the oxygen evolution reaction at the positive overpotential. Therefore, the linear sweep-voltammograms (LSVs) are corrected for potential drop due to solution resistance (iRs), mass-transfer resistance, and most importantly, for the back reaction current. These corrections are imperative to estimate the Tafel slope in the limited range of overpotential for V5+/V4+ redox reaction. The charge-transfer coefficient (α) estimated from the Tafel slope deviates significantly from the expected value of 0.5 for the single electron-transfer reaction. Moreover, the instantaneous slope of the Tafel plot suggests that the α is overpotential dependent. Therefore, Marcus theory of electrochemical kinetics is applied to estimate the α. The reorganization energy (λ) calculated from the Arrhenius plots is in the range of values reported in the literature for the other redox couples.

The Synergetic Effect of MoSO2/Graphite Nanosheets as Highly Efficient for Electrochemical Water Splitting in Acidic Media

Metal oxide nanoarchitectures have a wide range of qualities that can be used to produce novel technologies in the field of renewable energy, such as energy conversion solar fuels and storage via the photovoltaic effect, and electrochemical water splitting. The approach for the synthesis of earth abundant metal oxide nanostructures is facile and cost effective and involves scalable methodologies for the development of functional devices. The composite material exhibits enhanced active edge sites for the potential HER. The electrochemical experiments revealed satisfactory results of electrocatalytic gas production HER. The composite sample produces a current density at 10 mAcm−2 an over potential of 345 mV and Tafel value of 60 mVdec−1 it exhibits at which predominantly ensures the swift charge transfer kinetics during HER. The sample 3 remains durable and stable for 30 hours. EIS shown value of 21.88 Ohms as charge transfer resistance which further strengthened HER and Tafel results. The sample 3 exhibits a 4.69 µFcm−2 capacitance double layer and surface area of 177.25 cm2 it further supports the unique productivity for HER activity. The small Tafel slope which relatively close to Pt shows a clear and high potential of as prepared MoS2/Graphite nanosheets composite material for the replacement of noble metals in the field of renewable energy. The tiny Tafel slope value suggests that the efficient hydrogen evolution reaction has a lot of promise. This developed method provides the alternative method for the development of other materials for the energy harvesting applications.

Electrocatalytic performance of NiNH2BDC MOF based composites with rGO for methanol oxidation reaction

Present work comprehensively investigated the electrochemical response of Nickel-2 Aminoterephthalic acid Metal–Organic Framework (NiNH2BDC) and its reduced graphitic carbon (rGO) based hybrids for methanol (CH3OH) oxidation reaction (MOR) in an alkaline environment. In a thorough analysis of a solvothermally synthesized Metal–Organic Frameworks (MOFs) and its reduced graphitic carbon-based hybrids, functional groups detection was performed by FTIR, the morphological study by SEM, crystal structure analysis via XRD, and elemental analysis through XPS while electrochemical testing was accomplished by Chronoamperometry (CA), Cyclic Voltametric method (CV), Electrochemically Active Surface Area (EASA), Tafel slope (b), Electron Impedance Spectroscopy (EIS), Mass Activity, and roughness factor. Among all the fabricated composites, NiNH2BDC MOF/5 wt% rGO hybrid by possessing an auspicious current density (j) of 267.7 mA/cm2 at 0.699 V (vs Hg/HgO), a Tafel slope value of 60.8 mV dec−1, EASA value of 15.7 cm2, and by exhibiting resistance of 13.26 Ω in a 3 M CH3OH/1 M NaOH solution displays grander electrocatalytic activity as compared to state-of-the-art platinum-based electrocatalysts.

Two-Dimensional Layered NiLiP2S6 Crystals as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting

The quest of earth-abundant bifunctional electrocatalysts for highly efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential for clean and renewable energy systems. Herein, directed by the experimental analysis, we demonstrate layered nickel lithium phosphosulfide (NiLiP2S6) crystal as a highly efficient water-splitting catalyst in alkaline media. With strained lattice due to stacked layers as observed by TEM and electronic structure analysis performed by XPS showed mixed Ni2+,3+ oxidation states induced by addition of Li as a cation, NiLiP2S6 displays excellent OER (current density of 10 mA cm–2 showed an overpotential of 303 mV vs. RHE and a Tafel slope of 114 mV dec–1) and HER activity (current density of −10 mA cm–2 showed an overpotential of 184 mV vs. RHE and a Tafel slope of 94.5 mV dec–1). Finally, an alkaline media was employed to demonstrate the overall water splitting using NiLiP2S6 as both the anode and the cathode, which attained a 50 mA cm−2 current density at 1.68 V. This bimetallic phosphosulfide, together with long-term stability and enhanced intrinsic activity, shows enormous potential in water splitting applications.

Polythionine Coated on Au/Co3O4 Enhances the Performance for Hydrogen Evolution Reaction

Developing a general, green and effective strategy to improve the hydrogen evolution reaction (HER) electrocatalyst is urgently needed but challenging. Herein, we report a novel conducting polymer/metal/metal oxide composite as HER catalyst prepared by a two-step strategy, in which the Au/Co3O4 is synthesized by a one-pot hydrothermal method, subsequently, cyclic voltammetry is employed to electropolymerize polythionine on the Au/Co3O4. It is demonstrated that the HER electrocatalytic activity of Au/Co3O4 is effectively improved by the coating polythionine layer. The optimal polythionine/Au/Co3O4 displays an excellent electrocatalytic ability towards HER with an overpotential of 168[Formula: see text]mV at [Formula: see text] and low Tafel slope of 79[Formula: see text]mV[Formula: see text]dec[Formula: see text] in 0.5[Formula: see text]M H2SO4, which is greatly superior to those of the Au/Co3O4 (an overpotential of 300[Formula: see text]mV at [Formula: see text] and tafel slope of [Formula: see text]. Interestingly, the HER performance of N-doped reduced graphene (N-rGO) can also be significantly boosted by the coating polythionine layers, indicating the multifunctionality of polythionine in the enhancement of HER performance of nano electrocatalysts.

Electrocatalytic Performance of NiNH2BDC MOF Based Composites with rGO For Methanol Oxidation Reaction

Present work comprehensively investigated the electrochemical response of Nickel-2 Aminoterephthalic acid metal organic framework (NiNH2BDC) and its reduced graphitic carbon (rGO) based hybrids for methanol (CH3OH) oxidation reaction (MOR) in an alkaline environment. In thorough analysis of solvothermally synthesized Metal-organic frameworks (MOFs) and its reduced graphitic carbon based hybrids, functional groups detection was performed by FTIR, morphological study by SEM, crystal structure analysis via XRD, and elemental analysis through XPS while electrochemical testing was accomplished by chronoamperometry (CA), cyclic voltametric method (CV), electrochemically active surface area (EASA), Tafel slope (b), Electron impedance spectroscopy (EIS), mass activity, and roughness factor. Among all the fabricated composites, NiNH2BDC MOF/5 wt % rGO hybrid by possessing auspicious current density (j) of 267.7 mA / cm2 at 0.699 V (vs Hg/HgO), a Tafel slope value of 60.8 mV.dec-1, EASA value of 15.7 cm2, and by exhibiting resistance of 13.26 Ω in a 3 M CH3OH / 1M NaOH solution displays grander electrocatalytic activity as compared to state-of-the-art platinum-based electrocatalysts.

A Supramolecular Porous Organic Cage Platform Promotes Electrochemical Hydrogen Evolution from Water Catalyzed by Cobalt Porphyrins

We report a supramolecular porous organic cage platform composed of cobalt porphyrins for catalyzing the electrochemical hydrogen evolution reaction (HER) from water at neutral pH. Owing to its permanent porosity, the supramolecular structure yields a catalyst film with a 5-fold increase in the number of electrochemically active cobalt atoms and an improvement in Tafel slope from 170 mV/decade to 119 mV/decade compared to a planar cobalt porphyrin analog, reaching activities over 19,000 turnovers for HER over a 24-hour period with 100% Faradaic efficiency.

A Supramolecular Porous Organic Cage Platform Promotes Electrochemical Hydrogen Evolution from Water Catalyzed by Cobalt Porphyrins

We report a supramolecular porous organic cage platform composed of cobalt porphyrins for catalyzing the electrochemical hydrogen evolution reaction (HER) from water at neutral pH. Owing to its permanent porosity, the supramolecular structure yields a catalyst film with a 5-fold increase in the number of electrochemically active cobalt atoms and an improvement in Tafel slope from 170 mV/decade to 119 mV/decade compared to a planar cobalt porphyrin analog, reaching activities over 19,000 turnovers for HER over a 24-hour period with 100% Faradaic efficiency.