Rhodium Electrodes Research Articles

Study of Oxygen Reduction Reaction on Polycrystalline Rhodium in Acidic and Alkaline Media

This study examines the kinetics and mechanism of the oxygen reduction reaction (ORR) on a polycrystalline rhodium electrode (Rh(poly)) in acidic and alkaline media, using rotating disc electrode measurements. This study found that the ORR activity of the Rh(poly) electrode decreases in the order of 0.1 M NaOH > 0.1 M HClO4 > 0.05 M H2SO4 concerning the half-wave potentials. The Tafel slopes for ORR on Rh(poly) in the cathodic direction are 60 and 120 mV dec−1 at low and high overpotentials, respectively, in perchloric acid and alkaline solutions. However, strongly adsorbed sulfate anions hinder the ORR on Rh(poly) in sulfuric acid, leading to higher Tafel slopes. The highest ORR activity of Rh(poly) in an alkaline media suggests the promoting role of the specifically adsorbed OH− anions and RhOH. In all cases, ORR on Rh(poly) proceeds through the 4e-series reaction pathway.

Ionic conductivity of the molten systems (LiF–CaF2)eut–NdF3, (LiF–NaF)eut–NdF3, (NaF–CaF2)eut–NdF3 and (LiF–MgF2)eut–NdF3

AbstractExperimental measurements were made to assess the electrical conductivity as a function of temperature and NdF3 concentration (0–20 mol %) of molten systems of (LiF–CaF2)eut–NdF3, (LiF–NaF)eut–NdF3, (NaF–CaF2)eut–NdF3 and (LiF–MgF2)eut–NdF3. The experiment used an altering current impedance spectroscopy technique with a platinum–rhodium electrode positioned in a pyrolytic BN tube and graphite a crucible/counter electrode. The conductivity of all systems under study increased with rising temperatures and decreasing NdF3 concentrations. The Arrhenius equation and linear regression have both been used to describe the experimental data. The results of the ionic conductivity for the temperature 850 °C and NdF3 concentrations 0, 10 and 15 mol %, respectively, can be compared as follows: the conductivity of the molten system of (LiF–CaF2)eut–NdF3 was determined to be 6.10, 5.95 and 5.10 S.cm−1, the results for the system (LiF–NaF)eut–NdF3 were 6.16, 5.56 and 4.13 S.cm−1, the results for the system (NaF–CaF2)eut–NdF3 were 3.78, 3.56 and 2.32 S.cm−1, and finally, the results for the system (LiF–MgF2)eut–NdF3 were determined to be for the same temperature as 5.35, 4.79 and 4.14 S.cm−1, respectively.

Electrical conductivity of the molten systems of (LiF – CaF2)eut – NdF3 and (LiF – NaF)eut – NdF3

The electrical conductivity of molten systems of (LiF – CaF2)eut – NdF3 and (LiF – NaF)eut – NdF3 was experimentally determined as a function of temperature and concentration of NdF3 (0 – 20 mol %). A platinum–rhodium electrode positioning in pyrolytic BN tube, graphite crucible, and altering current impedance spectroscopy were used for the study. The conductivity of both measured systems increases with the temperature and decreases with the increasing content of NdF3. The experimental data have been described by the linear regression and by the Arrhenius equation. The electrical conductivity of the molten system of (LiF – CaF2)eut – 5 mol % NdF3 has been identified to be 5.56 S.cm−1 compares to the electrical conductivity of the molten system of (LiF – NaF)eut – 5 mol % NdF3, 5.52 S.cm−1 (800 °C). The electrical conductivity of (LiF – CaF2)eut – 20 mol % NdF3 at the temperature of 800 °C is 4.02 S.cm−1, and the electrical conductivity of (LiF – NaF)eut – 20 mol % NdF3 is at the same temperature 3.46 S.cm−1. The XRD patterns of the solidified samples of the investigated systems (LiF – CaF2)eut – NdF3 and (LiF – NaF)eut – NdF3 were after the conductivity measurements also determined.

Study of the Formic Acid Electrooxidation on Rhodium on Steady State Using a Flow Cell: Potential Dependence of the CO Coverage

The HCOOH electrooxidation reaction (FAO) was experimentally studied on rhodium electrode using a flow cell in a 0.5-M HCOOH + 0.5-M H2SO4 solution. It was obtained the steady-state current-potential curve by chronoamperometry, which demonstrated that the FAO is verified in the potential region 0.45 < E/V < 0.70. The amount of adsorbed CO on the electrode surface as a function of potential was quantified by stripping voltammetry after changing to a 0.50-M H2SO4 solution at closed electric circuit. It was established that the spontaneous dissociative adsorption of HCOOH on rhodium produces irreversibly adsorbed CO only for potential values E ≤ 0.40 V, and therefore, the FAO takes place in the absence of COad. These results can explain the current-potential profiles obtained when the FAO is studied by cyclic voltammetry at different values of the potential scan limits, where the behaviour is strongly influenced by the inhibition of reaction sites due to the adsorbed CO. A kinetic mechanism for the FAO on rhodium is proposed on the basis of the results obtained, which are also in agreement with recently published spectroscopic measurements.

Oxygen Passivation of Rhodium and Platinum with Direct Current Polarization in 12 m Hydrochloric Acid Solution

It was found that the surfaces of rhodium and platinum electrodes with polarization by direct current in 12 M hydrochloric acid solution are completely covered with oxygen atoms at a potential of +0.68 V (relative to the reference hydrogen electrode), and its increase rate is 10–90 mV/s.

Impact of Intermittent Operation on the Lifetime and Performance of a PEM Water Electrolyzer

In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen.1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm-2 geo) and low (0.1 Acm-2 geo) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrOx 2, the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrOx is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes.3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2).4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrOx.5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrOx. Interestingly, the polarization curve recorded directly after IrOx reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re-precipitation occurs. In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM-WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM-WE durability. In addition, PEM-WE load cycles to OCV may serve as an accelerated aging test. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013, 38, 4901-4934.Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017, Abstr. # I01F-1648.Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974, 49, 217-226.Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 220, 83-100.Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014, 161, F876-F882. Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min-1 H2O (anode)/ 50 mL min-1 H2 (cathode) for an MEA with ~1.6 mgIr cm-²MEA anode and ~0.3 mgPt cm-²MEA cathode loading using a Nafion® 212 (~50 µm) membrane Figure 1

Investigation of Electrochemical Properties of Elemental Sulfur Dissolved in Alkaline Solution on Rhodium Electrode

In this article, the electrochemical properties of elemental sulfur dissolved in alkaline solution has been studied for the first time using the method of removing the anodic and anodic-cathodic potentiodynamic polarization curves on rhodium electrodes. The influences of different parameters for oxidation process of elemental sulfur, namely sulfur component in electrolyte concentration, scan rate and temperature were investigated. The resulting polarogram presented in oxidation reaction of polysulfide ions to elemental sulfur process. This shows that polysulfide-ions in electrolyte at cathodic polarization was restored to monosulfide, and monosulfide –ions oxidized to elemental sulfur. The effective activation energy was calculated which equals 13,67 Kj/mol, indicating the oxidation reaction of sulfur occurred in diffusion mode.

Kinetics of the hydrogen oxidation reaction on nanostructured rhodium electrodes in alkaline solution

The hydrogen oxidation reaction was studied on a nanostructured rhodium electrode at different rotation rates in alkaline solution. The electrode was prepared via sputtering on a glassy carbon disc support and it was characterized by atomic force microscopy and cyclic voltammetry. The real surface area was evaluated by CO stripping voltammetry. Experimental current density (j) – overpotential (η) curves of the hydrogen oxidation reaction were obtained in the range −0.015 ≤ η/V ≤ 0.40 at different rotation rates (900 ≤ ω/rpm ≤ 4900). The resulting curves were correlated by kinetic expressions derived from the Tafel-Heyrovsky-Volmer mechanism with a Frumkin type adsorption of the reaction intermediate and the kinetic parameters were evaluated. It was verified that over this overpotential region the reaction in alkaline solution proceeds mainly through the Tafel-Volmer route. These results were compared with those previously obtained in acid solutions.

Enhanced electrocatalytic oxidation of isoniazid at electrochemically modified rhodium electrode for biological and pharmaceutical analysis

A simple and sensitive electrochemical method has been proposed for the determination of isoniazid (INZ). For the first time, rhodium (Rh) modified glassy carbon electrode (GCE) has been employed for the determination of INZ by linear sweep voltammetry technique (LSV). Compared with the unmodified electrode, the proposed Rh modified electrode provides strong electrocatalytic activity toward INZ with significant enhancement in the anodic peak current. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) results reveal the morphology of Rh particles. With the advantages of wide linearity (70–1300μM), good sensitivity (0.139μAμM−1cm−2) and low detection limit (13μM), this proposed sensor holds great potential for the determination of INZ in real samples. The practicality of the proposed electrode for the detection of INZ in human urine and blood plasma samples has been successfully demonstrated using LSV technique. Through the determination of INZ in commercially available pharmaceutical tablets, the practical applicability of the proposed method has been validated. The recovery results are found to be in good agreement with the labeled amounts of INZ in tablets, thus showing its great potential for use in clinical and pharmaceutical analysis.

Characterization and kinetic study of a nanostructured rhodium electrode for the hydrogen oxidation reaction

The hydrogen oxidation reaction was studied on a nanostructured rhodium electrode at different rotation rates. The electrode was prepared via sputtering on a glassy carbon disc support and it was characterized by X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and cyclic voltammetry, which allowed verifying the nanostructured morphology and the absence of any phase other than metallic rhodium. The real surface area was evaluated by CO stripping voltammetry. Experimental current density (j) – overpotential (η) curves of the hydrogen oxidation reaction were obtained in the range −0.015 V ≤ η ≤ 0.25 V at different rotation rates in sulphuric acid solution. They were correlated by kinetic expressions derived from the Tafel–Heyrovsky–Volmer mechanism and thus the kinetic parameters were evaluated. It was verified that over this overpotential region the reaction proceeds through the simultaneous occurrence of the Tafel–Volmer and the Heyrovsky–Volmer route.

Zero charge potentials and work function of rhodium

The data on the behavior of rhodium and platinum electroplates in sulfate solutions acquired by the method of potentiometric titration under isoelectric conditions are presented. These results are compared with the data obtained earlier by the same method for rhodium and platinum in chloride media. The potentials of zero free charge (pzfc) and zero total charge (pztc) and their dependence on the solution pH are determined. The reasons for the pH-induced changes in the dependences of the free surface charge on the potential are analyzed. It is demonstrated that the measurements on electrodes of disperse materials make it possible to assess the reliability of certain procedures used in processing the data found by the CO-displacement method and also in the determination of pztc and pzfc of single-crystal faces and other well characterized surfaces of platinum-group metals. The data on pztc and potentials of zero displaced charges (pzdc) for disperse and polycrystalline rhodium electrodes, rhodium single crystal faces and Rh adlayers on platinum crystal faces are summarized. The correlation between the potentials of zero charge (pzc) and the electron work function is considered. Actually, both pzfc and pztc can be used in building such correlations; however, preference should be given to the former. It is concluded that together with platinum and palladium, rhodium pertains to materials with the high interfacial term, which is probably due to the strong chemisorption of water on its surface.

Degradation of polycrystalline rhodium and rhodium nanoparticles

The electrochemical stability of polycrystalline rhodium and rhodium nanoparticles is quantitatively investigated in non-complexing sulfate electrolyte under potential cycling conditions. In situ measurements of the active surface area are complemented by discrete elemental analysis of the electrolyte solution. Rhodium electrodes are not stable and dissolve upon potential excursions into the oxide region above approximately +0.5 VRHE; the higher the positive potential limit, the higher the mass loss rate per cycle. Interestingly, the normalized catalyst mass loss is independent of the initial catalyst loading under identical conditions. The dissolution of Rh and the concomitant structural changes have to be considered in any further electrocatalytic study of this kind of electrodes.

Core–shell nanoparticle based SERS from hydrogen adsorbed on a rhodium(111) electrode

We present the first in situ surface Raman spectra of hydrogen on rhodium under electrochemical conditions using gold-core rhodium-shell (Au@Rh) nanoparticles for SERS or gold-core silica-shell (Au@SiO(2)) nanoparticles for SHINERS. The advantage of SHINERS lies in the versatility to study single crystal surfaces such as the H-Rh(111) system.

Electrocatalytic reduction of nitrite on a polycrystalline rhodium electrode

This paper addresses the mechanism of the electrochemical reduction of HNO 2 and NO 2 - on polycrystalline rhodium. Intermediates and/or reaction products were detected by means of various (combined) techniques: rotating ring-disk voltammetry (for NH 2OH detection), online electrochemical mass spectrometry (for volatile products) and transfer experiments (for NO ads). In acidic media, HNO 2 depletion due to homogeneous-phase reactions generates dissolved NO: the latter species can be adsorbed at Rh and is reduced to N 2O when 0.3 < E < 0.5 V (vs. RHE), while HNO 2 is reduced in a diffusion-limited wave to mainly NH 3 at potentials preceding hydrogen evolution. In alkaline media, the predominant product for the reduction of NO 2 - when E < 0.2 V is still NH 3, which can poison the electrode via dehydrogenation to NH x ,ads species. For more positive potentials, reduction still occurs via NO ads and stops at NH 2OH for E > 0.3 V. The behavior of Rh is compared to Pt and explained in terms of general properties of these metals. A mechanistic scheme including NO, HNO 2 and NO 2 - is discussed.

ChemInform Abstract: Enhanced Oxygen Evolution on an Electrodeposited and Activated Rhodium Electrode in Alkaline Solution.

Abstract Hydrous oxide growth on rhodium electrodeposited on titanium was investigated with respect to electrocatalytic activity and stability of the coating in the oxygen evolution reaction from 1 mol dm −3 NaOH solution. An enhancement of the charge storage capacity by a factor of about four was achieved by square-wave pulsing between − 1.05 V and 0.45 V vs. SCE. The grown oxide exhibited, depending on the potential, a rate of oxygen evolution enhanced by about one order of magnitude accompanied by a decrease of the Tafel slope from 95 mV / decade to 65 mV / decade. Both untreated and activated coatings were stable within 6 h of anodic galvanostatic polarization at 0.04 A cm −2 .

Electrode specific information from voltammetric monitoring of biogas production

A sensor employing pulse voltammetry monitored the liquid phase of a biogas reactor during 32 days of gas production. An electrode array consisting of stainless steel, platinum and rhodium electrodes generated current responses for a sequence of voltage pulses. Plots of individual current responses against time indicated the electrochemical changes occurring in the broth from the perspective of each electrode. The responses from stainless steel had a pronounced diurnal oscillation which followed the daily introduction and consumption of substrate. The current responses for platinum were in a narrow range whereas those for rhodium exhibited several minima. A disturbance in the reactor caused by omission of substrate led to decreases in both gas production and current responses for all the electrodes. Multivariate data evaluation of all the current responses by principal component analysis indicated the daily fluctuations for concentrations of ions and redox active compounds in the broth.

Anomalous effect of the presence of methanol on the oxygen electroreduction rate on rhodium electrode in sulfuric acid solution

By the example of Rh electrodes, it was observed for the first time that, in certain cases, the presence of methanol can lead to an increase in the oxygen electroreduction rate.

The kinetics of the electrochemical reduction of perchlorate ions on rhodium

The reduction of ClO 4 − ions in acid media was studied at smooth polycrystalline rhodium electrodes by voltammetry, chronoamperometry and impedance spectroscopy. On the basis of the experimental results, a new mechanism for the electrocatalytic reduction of ClO 4 − ions has been proposed. According to this reaction scheme, the reduction process is starting with the adsorption of perchlorate ions at free active sites on the metal surface. It is also assumed that the whole process starting from perchlorate ion occurs on the surface and the role of desorption of intermediates can be neglected. On the basis of the kinetic model an expression has been derived for the impedance of the electrode, which fits the experimental data quite well.

Impedance spectroscopic investigation of a Rh/YSZ catalyst under polarization

Electrochemical impedance spectra at 450–600 °C and $${P_{{\rm{O}}_{\rm{2}} } = 0.5}$$ kPa of a rhodium catalyst interfaced with yttria-stabilized-zirconia (Rh/YSZ) were compared with a model based on the mechanism of electrochemical promotion. In the proposed equivalent electric circuit, existence of an “effective” double layer at the gas-exposed catalyst surface and its potential-controlled modification via diffusion of oxygen ions between the O2− conducting solid electrolyte support (YSZ) and the catalyst are represented by two additional elements: adsorption capacitance and Warburg impedance. Under positive polarization, the adsorption capacitance increases dramatically indicating reinforcement of the “effective” double layer at the catalyst/gas interface, in agreement with the observation known from electrochemical promotion practice that positive polarization of a rhodium electrode leads to rhodium oxide reduction, hence, to dramatic increase in catalytic reaction rate.

A flow injection voltammetric electronic tongue applied to paper mill industrial waters

A voltammetric electronic tongue with automated operation based on the flow injection (FIA) technique was applied to the characterization of wastewaters coming from the paper mill industry. A metallic multielectrode array – formed by platinum, gold and rhodium electrodes – was employed as the detection system, while the measurements were based on large amplitude pulse voltammetry (LAPV). LAPV consisted in scans of pulses from to 0 to 1.8V at 0.2V steps. Five current values were recorded for each pulse, so a set of 300 current values (three electrodes×20 pulses×five values) was recorded for each sample. Samples were first discriminated using Principal Component Analysis (PCA), while Artificial Neural Networks were used for the characterization and prediction of chemical oxygen demand, conductivity and pH. The system may be used for the quick identification and monitoring of the quality of used waters in these industrial facilities.