Co supported on N and S dual-doped reduced graphene oxide as highly active oxygen-reduction catalyst for direct ethanol fuel cells

Oxygen reduction reaction (ORR) is one of the key features for the efficient functioning of several energy conversion devices such as fuel cells, appearing the necessity of development of new low-cost catalyst materials. Heteroatom-doped carbon materials have attracted attention in this field due to its physicochemical and electronic properties. In this work, a nitrogen and sulfur doped material with anchored Co3O4 nanoparticles (Co/SN-rGO) is proposed as cathode catalyst for direct ethanol fuel cells (DEFCs) and results are compared with different doped graphene nanomaterials (GMs). The effect of the heteroatoms and cobalt oxide nanoparticles in the final efficiency was studied. Synthesized materials were characterized and the activity of Co/SN-rGO and GMs for the ORR was studied. Co/SN-rGO presents high ORR performance in terms of onset potential (Eonset), 0.86 V (vs RHE) and half-wave potential (E1/2) 0.72 V (vs RHE). Tafel analysis shows 60 mV dec-1 at low overpotential for potential dependent ORR mechanism. Besides, when Co/SN-rGO performance is evaluated in a DEFC using a fuel cell test station, main results indicate higher catalytic activity, stability, and ethanol tolerance of Co/SN-rGO in comparison to a carbon-supported Pt catalystThis work has been developed in the framework of the projects PID2020-117586RB-100, PID2020-112594RB-C33, PID2020- 116712RBC21 funded by MCIN/AEI/10.13039/501100011033, and ProID2021010098 funded by the Gobierno de Canarias (FEDER). S. Fajardo acknowledge the MCIN for the pre-doctoral grant (PRE2018- 085718). The authors thank SEGAI-ULL for the collaboratio

MnO2-modified ZIF-67 supported on doped reduced graphene oxide as highly active catalyst for the oxygen reduction reaction

In this investigation, ZIF-67 supported on non-doped reduced graphene oxide (rGO), nitrogen- and sulphurdoped reduced graphene oxide (SN-rGO), and manganese-modified ZIF-67/SN-rGO are synthetized and compared as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline conditions. XPS, SEM, XRD and Raman spectroscopy techniques are used to physiochemically characterize the materials. Cyclic and linear sweep voltammetry unveil kinetic parameters, including rate constants, Tafel slopes, and exchange current density, providing insights into the reaction mechanism. Nitrogen and sulphur doping of graphene reduces hydrogen peroxide production, lowering overpotential and enhancing cathodic currents. Manganese modification further enhances performance, matching cathodic currents with Pt/C and achieving a potential of 0.85 V vs RHE at mA⋅cm demonstrating superior efficiency. The study highlights the impact of graphene doping and manganese incorporation on ORR activity, contributing to a comprehensive understanding of these electrocatalytic systemsThis work has been supported by the Ministerio de Ciencia e Innovación (MCIN) under projects PCI2020-112249, PID2020- 116712RB-C21 and PID2020-117586RB-I00 funded by MCIN/AEI/ 10.13039/501100011033. S. Fajardo acknowledges the MCIN for the pre-doctoral grant (PRE2018-085718). The author thanks SEGAI for its collaboration during the physical characterization of the studied material

Chemical physics insight of PPy-based modified ion exchange membranes: a fundamental approach

Four commercially available, cost-effective ion exchange membranes (two cationic and two anionic exchange membranes, CEMs and AEMs, respectively) were modified to mitigate crossover phenomena of the redox active species typically observed in Aqueous Organic Redox Flow Batteries (AORFB) systems. The modification strategy was carried out using a pyrrole(Py)-based polymer which successfully reduced the permeation of two redox active organic molecules, a viologen derivative (named BP7 throughout this study) and TEMPOL, by an order of magnitude. Additionally, modified membranes showed not significant changes in ion conductivity, with negligible effect on the electrical conductivity of the membranes at a given conditions. The morphology, physicochemical, mechanical, and electrochemical properties of the membranes were determined to evaluate the impact of these modifications. AEMs modified in this manner were found to have optimal properties, showing an increase in ion exchange capacity while maintaining excellent mechanical stability and unaltered permselectivity. Additionally, the diffusion boundary layer of these AEMs was slightly extended, which suggests a greater double layer stability for ion exchange processes than in the case of CEMs. Our work shows that these modified membranes could be an appealing approach for AORFB applicationsThis work has been funded by the European Union under the HIGREEW project, Affordable High-performance Green Redox Flow batteries (Grant agreement no. 875613). H2020: LC-BAT-4-2019875613

Scale-dependent roughening kinetics in vapor deposited gold

The roughening kinetics of gold deposits grown from vapor was studied by scanning tunneling microscopy. The dynamic scaling yielded the following growth exponents α(I)=0.90±0.06 and β(I)=0.25±0.06 for Ls>ds, and α(II)=0.37±0.05 and β(II)=0.45±0.06 for Ls>ds, where Ls is the scan length and ds is the average diameter of columns. The scaling properties of the domain-dependent-surface roughness exponents allowed us to give the rationale for experimental data on the fractal behavior of thin metal films.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

STM study of fractal scaling in evaporated gold films

Fractal characterization of vacuum-evaporated gold films on glass substrates with thicknesses (h) comprised between 30 and 850 nm is made from STM data by using the dynamic scaling approach for surface growth. The rms roughness (ξ) and the scan length (L) obey a ξ versus Lα relationship with α depending on h. For h ⪰ 500 nm and L >ds, the averag e column diameter, α≌13 in agreement with the predictions of ballistic deposition models without restructing. For L s, α ≌ 0.9 approaching a euclidean value. The STM method is verified through its application to several computer-generated surfaces, leading to a good agreement with the theoretical values.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Self-affine fractal vapour-deposited gold surfaces characterization by scanning tunnelling microscopy

The morphological evolution of the surfaces of gold deposits grown from the vapour on smooth glass under nonequilibrium conditions and incident angle near substrate normal is studied at the nanometer level by scanning tunnelling microscopy. For an average film thickness equal to or greater than 500 nm, the interface thickness (ξ) reaches a steady state. Under these conditions, ξ depends on the scan length (L) as ξ ∝ Lα with α = 0.35 ± 0.05 for L > ds, where ds is the columnar size, and α = 0.89 ± 0.05 for L s. These results indicate that the growing surface spontaneously reaches a steady state and it can be described as a self-affine fractal. The value of α for L > ds agrees with the prediction of ballistic deposition models without restructuring, whereas that for L s exceeds the prediction of ballistic models including restructuring.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Methods of fractal analysis applied to STM imaging

Vapor deposited gold films have been characterized by applying methods of fractal analysis to scanning tunneling microscopy (STM) images. Results from the use of five different methods are in agreement within the limitation of each method. Those methods are suitable to characterize rough surfaces at the nanometer level provided that a large number of images is considered.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

A comparative study of electrodeposited and vapour deposited gold films : Fractal surface characterization through scanning tunnelling microscopy

The surfaces of Au deposits grown under non-equilibrium conditions from either the electroreduction of Au oxide or from the vapour have been analysed as fractals by measuring the perimeter (P) and the area (A) of intergranular voids. The values of P and A were determined from scanning tunnelling microscopy (STM) topographic imaging of the deposit surfaces. A frsctal behaviour P ∝ 4 D/2 was found with D = 1.5 ± 0.1 and D = 1.7 ± 0.1 for the electrodeposited and vapour deposited Au iilms, respectively. These figures remain constant for film thicknesses between 100 and 1000 mn. The value of D, the fractal dimension of the surfaces, is 2.5 ± 0.1 for the Au electrodeposits, and 2.7 + 0.1 for the Au vapour deposited films. The former value is consistent with either a diffusion or an electric field controlled growth model, whereas the latter is in agreement with a ballistic growth model.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Scanning-tunneling-microscopy study on the growth mode of vapor-deposited gold films

The growth of gold deposits on smooth glass from the vapor phase at 30 nm−1 s −1, 298 K, and incident angle near the substrate normal covering the 30—1000 nm average film thickness (h¯) range is investigated through scanning tunneling microscopy (STM) complemented with oxygen-adatom electrosorption measurements. The STM images of the deposits reveal a columnar structure resulting from a mechanism involving shadowing and surface diffusion. Quantitative data are obtained directly from STM images. The height distribution N(h) of the interface obeys an N(h)∝ e −kh relationship. For h¯ 500 nm it reaches a steady state. Under the latter condition, ξ depends on the STM scan length (S) as ξ∝ Sα with a close to 1/3. These results indicate that the growth process of the gold deposits results in compact nonfractal structures with self-affine fractal surfaces, as predicted by ballistic deposition models. However, the latter fail to describe some aspects of the morphology and evolution of thin vapor-deposited gold films on this substrate.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta

Self-affine fractal electrodeposited gold surfaces: Characterization by scanning tunneling microscopy

The morphological evolution of columnar gold electrodeposits grown at 100 nm s −1 by electroreducing a gold oxide layer on a gold cathode has been studied at a nanometer level by scanning tunneling microscopy. The interface thickness (ξ) depends on the scan length (L) as ξ∝ L α with α=0.49±0.07 for L > ds, where ds is the average top columnar size, and α=0.90±0.07 for L s. These results prove that the growing surface can be described as a self-affine fractal for length scales greater than the columnar size. Conversely, the columnar surface approaches the behavior of an Euclidean surface.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta