Rh Electrodes Research Articles

Disordering of Rh(111) single crystalline electrode surface in O2 saturated acid

The Rh(111) electrode shows superior oxygen reduction reaction (ORR) activity to Ir(111), contradicting the predicted trend based on the volcano plot. To unveil the origin for this, we investigate the interfacial structures of Rh(111) single crystalline electrode in 0.1 M HClO4 and 0.5 M H2SO4 solutions using cyclic voltammetry and potential step chronoamperometry under a hanging meniscus rotating disk electrode configuration. Our results reveal that i) in N2 saturated solution even at a potential below 0.20 V, oxygen-containing species adsorbates can be formed on the Rh(111) surface probably generated by oxidation of water; ii) Even under mild conditions, multiple cycles and current transients in O2 saturated solution at potentials where ORR taking place can cause significant surface disordering; iii) The disordering of the Rh(111) electrode induced by ORR exhibits similar base cyclic voltammograms to those of vicinal Rh(hkl) such as Rh(553) and Rh(332) etc., depending on the potentials applied during ORR, which, in turn, enhances the ORR activity in both 0.1 M HClO4 and 0.5 M H2SO4. These new findings provide valuable insights into the structure-performance relationship of ORR.

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.

Hydrogen and deuterium adsorptions and their equilibrium isotope effect on tungsten in alkaline solutions

The equilibrium isotope effect (EIE) of hydrogen isotopes (H and D) on the W electrode in alkaline aqueous (H2O) and heavy water (D2O) solutions was empirically studied based on the relevant electrochemical adsorption isotherms, which were determined using the phase-shift method and electrochemical impedance spectra. For a fractional surface coverage range of 0.2 < θ < 0.8, the potential differences (ΔE) for hydrogen isotope adsorptions on the W electrode were 70 mV, which were much lower than those on the Ni, Ti, Pd, and Rh electrodes. Furthermore, the value of EIE on the W electrode was 13.4, significantly higher than that on the Ti and most Pt-group metal (Pt, Ir, Pt–Ir alloy, and Rh) electrodes. The W electrode can therefore be considered as a potential electrocatalyst for optimizing the hydrogen and deuterium evolution reactions (HER and DER) and EIE of hydrogen isotopes, particularly in water electrolysis and the hydrogen isotope effect.

Atomically Adjustable Rhodium Catalyst Synthesis with Outstanding Mass Activity via Surface‐Limited Cation Exchange

Rh has been widely studied as a catalyst for the promising hydrazine oxidation reaction that can replace oxygen evolution reactions for boosting hydrogen production from hydrazine‐containing wastewater. Despite Rh being expensive, only a few studies have examined its electrocatalytic mass activity. Herein, surface‐limited cation exchange and electrochemical activation processes are designed to remarkably enhance the mass activity of Rh. Rh atoms were readily replaced at the Ni sites on the surface of NiOOH electrodes by cation exchange, and the resulting RhOOH compounds were activated by the electrochemical reduction process. The cation exchange‐derived Rh catalysts exhibited particle sizes not exceeding 2 nm without agglomeration, indicating a decrease in the number of inactive inner Rh atoms. Consequently, an improved mass activity of 30 A mgRh−1 was achieved at 0.4 V versus reversible hydrogen electrode. Furthermore, the two‐electrode system employing the same CE‐derived Rh electrodes achieved overall hydrazine splitting over 36 h at a stable low voltage. The proposed surface‐limited CE process is an effective method for reducing inactive atoms of expensive noble metal catalysts.

A Study of Cu‐Rh Electrodeposition**

AbstractThis manuscript reports the simultaneous electrodeposition of Cu and Rh from an aqueous nitrate solution. The early stages of nucleation and growth of the bimetallic layer were explored using techniques such as cyclic voltammetry and current transients. Non‐dimensional Scharifker‐Hills graphs showed the occurrence of diffusion‐controlled three‐dimensional nucleation and growth best described by the Volmer‐Weber mechanism. Additionally, different ratios of Cu−Rh electrodes were synthesized by varying the potential of deposition and the Rh content in the deposition bath. Characterization techniques including electron microscopy, x‐ray diffraction, and x‐ray photoelectron spectroscopy were employed to investigate the chemical composition and structure of the deposits. The results showed a higher amount of Cu2+species in the layer than Cu+/Cu0when a more negative potential was applied, and when Rh was present in high amounts in the deposition bath. The final morphology of the obtained material proved to be dependent on the deposition potentials and the Cu : Rh content, showing interdependency between the metals.

CO2 and Formate Pathway of Methanol Electrooxidation at Rhodium Electrodes in Alkaline Media: An In Situ Electrochemical Attenuated Total Refection Surface-Enhanced Infrared Absorption Spectroscopy and Infrared Reflection Absorption Spectroscopy Investigation.

Rh catalysts exhibit unexpected high activity for the methanol oxidation reaction (MOR) in alkaline conditions, making them potential anodic catalysts for direct methanol fuel cells (DMFCs). Nevertheless, the MOR mechanism on Rh electrodes has not been clarified thus far, which impedes the development of high-efficiency Rh-based MOR catalysts. To investigate it, a combination of in situ electrochemical techniques called attenuated total refection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and infrared reflection absorption spectroscopy (IRAS) is used. Cyclic voltammograms of MOR at Rh electrodes show considerable activity in alkaline media rather than acidic media, although the real-time ATR-SEIRA spectral results demonstrate that methanol can rarely self-decompose on Rh at open-circuit conditions. Meanwhile, in combination of ATR-SEIRAS and IRAS results, CO2 and formate are thought to be MOR products, suggesting a dual-pathway mechanism ("CO2 pathway" and "formate pathway"). Specifically, COad species, which are the major intermediates in the CO2 pathway, can produce at lower potentials and be oxidized into CO2 at a potential of 0.5-0.75 V. Concurrently, the formate can be produced from 0.5 V and diffuse into the bulk electrolyte to become one of the MOR products, while the further electrochemical conversion of formate to CO2 is essentially negligible. More directly, the apparent selectivity (r) of the CO2 pathway is estimated to reach ca. 0.63 at 0.9 V, confirming the potential-dependent selectivity of MOR at Rh surfaces. This study might provide fresh insights into the design and fabrication of effective Rh-based catalysts for MOR.

Effect of Rh Coverage on CO-Adsorption and -Stripping Behaviors of Rhodium-Adlayer-Modified Platinum Electrodes

Pt/Rh bimetal catalysts are often used in various reactions such as alcohol oxidation and hydrogenation. For comparison, their catalytic activity is often standardized by the real surface area or electrochemical surface area. However, the conventional method of using the electric charge for hydrogen desorption is difficult to apply to Pt/Rh bimetal catalysts because the potential regions of hydrogen desorption for Pt and Rh overlap. In this study, Rh-adlayer-modified Pt (Rhx/Pt) electrodes with different Rh coverages were prepared by underpotential depositions of Cu adatoms at different potentials followed by galvanic replacement of Cu with rhodium, and CO stripping was applied to determine the Rh coverages of these electrodes. No CO that had been adsorbed on a Pt electrode was removed by potentiostatic CO oxidation at 0.75 V vs reversible hydrogen electrode for 15 s in an Ar-saturated 0.5 M sulfuric acid solution at 5 °C, but 85% of CO on an Rh electrode was removed. The Rh coverages for the Rhx/Pt electrodes corrected on the basis of this result were in good agreement with the corresponding Rh coverages estimated from the electric charge for the stripping of the Cu adlayer. Moreover, the Rhx/Pt electrodes exhibited noteworthy CO-stripping and CO-adsorption behaviors. A single CO-stripping cyclic voltammetry peak was observed regardless of Rh coverage, and shifted toward lower potential as the Rh coverage was increased. This result resembled the trend of initiation potentials of chemisorbed oxygen formation, suggesting a bifunctional mechanism. In infrared reflectance–absorption spectra of the CO-adsorbed Rhx/Pt electrodes, a single asymmetric band assigned to “atop” CO was observed irrespective of the Rh coverage, and shifted to lower wavenumbers as the Rh coverage was increased. The d-band center estimated from the valence level spectra of the Rhx/Pt electrodes, which reflected the Pt 5d electronic structure, shifted downward as the Rh coverage was increased.

Real-time electrochemical ATR-SEIRAS investigation of CO adsorption and oxidation on Rh electrode in 0.1 M NaOH and 0.1 M H2SO4

Rh gets close attention recently since it might improve C1 pathway efficiency toward ethanol oxidation reaction. Considering COad species are the vital intermediates for the C1 pathway, thus we fundamentally investigate the CO adsorption and oxidation on Rh surface. The results show that the formation of COad layer on Rh is more sluggish in alkaline media, may be due to the high OH− concentration induced bigger electronegativity of Rh electrode. This local environment change also leads to the onset potential of COML oxidation in 0.1M NaOH negatively shift about 200mV (at ca. 0.43V) with respect to that in 0.1M H2SO4. The oxidation of COML on Rh in alkaline media might be divided into two potential sections. One may follow the Eley-Rideal mechanism at 0.45–0.55V with OH− ions directly attracting the surface weakly adsorbed COL species; the other is the oxidation of COML species through the Langmuir-Hinshelwood mechanism. This work may provide some fundamental supporting for fabricating highly efficient ethanol oxidation catalysts.

The Effect of Polarization and Reaction Mixture on the Rh/YSZ Oxidation State During Ethylene Oxidation Studied by Near Ambient Pressure XPS

In this study, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied to investigate an electrochemical cell consisting of a rhodium thin film catalyst supported on an yttria-stabilized zirconia (YSZ) solid electrolyte under various ethylene-oxygen reaction mixtures. The aim of the study is twofold: first to show how the surface oxidation state of the Rh catalyst is correlated with the reactants feed composition and the temperature, and second, to reveal the effect of the anodic polarization on the stability of Rh oxides and the implications on the electrochemical promotion of catalysis. It is clearly shown that even under reducing conditions part of the Rh electrode remains oxidized at temperatures up to 250 °C. Reduction of the oxide can take place by increasing the temperature under C2H4 excess, something which is not happening under oxidizing reaction mixtures. Moreover, anodic polarization, i.e. oxygen ion supply to the surface, facilitates reduction of oxidized Rh electrodes over a broad range of ethylene–oxygen reaction mixtures. Remarkably, under mildly reducing conditions a stable ultrathin Rh surface oxide film forms over metallic Rh. This surface Rh oxide film (RhOx) is associated to higher cell currents, counterintuitive to the case of bulk Rh oxides (Rh2O3).

Synergistic electrocatalytic effect of Pd and Rh nanoislands co-deposited on Au(poly) on HER in alkaline solution

Hydrogen evolution reaction (HER) was explored on tri-metallic Rh@Pd/Au(poly) and bimetallic Pd/Au(poly) and Rh/Au(poly) electrodes in alkaline solution. Electrodes were prepared by the spontaneous deposition of either Pd or Rh and by co-deposition of both Rh and Pd on polycrystalline gold electrode. Characterization of modified electrodes was performed by cyclic voltammetry in alkaline solution, while additional information about phase analysis, chemical composition and surface morphology of tri-metallic Rh@Pd/Au(poly) electrode were acquired using X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and atomic force microscopy. Investigations of HER catalysis have shown that the activity of tri-metallic Rh@Pd/Au(poly) electrode exceeds the activities of both Pd/Au(poly) and Rh/Au(poly) electrodes, meaning that the synergism between co-deposited Pd and Rh islands is achieved. Moreover, Rh@Pd/Au(poly) electrode showed significant approach to the activities of bulk Pd and Rh electrodes for HER, especially when taking into account low activity of bare Au substrate. Synergistic effect of co-deposited Pd and Rh islands is a consequence of the strong electronic interaction between three metals in a close contact, which promotes the activity for HER by lowering the adsorption energy of Hads intermediate.

Enhancing the light-extraction efficiency of AlGaN deep-ultraviolet light-emitting diodes using highly reflective Ni/Mg and Rh as p-type electrodes

Improving the light-extraction efficiency (LEE) is a major issue for the development of deep-ultraviolet (DUV) light-emitting diodes (LEDs). For this improvement, we introduced a transparent p-AlGaN contact layer and a reflective p-type electrode. In this work, we investigated the improvements obtained by replacing conventional Ni/Au p-type electrodes with highly reflective Ni/Mg and Rh electrodes. The external quantum efficiencies (EQEs) of 279 nm DUV LEDs were increased from 4.2 to 6.6% and from 3.4 to 4.5% by introducing Ni/Mg and Rh p-type electrodes, respectively. The LEE enhancement factors for the Ni/Mg and Rh electrodes were 1.6 and 1.4, respectively. These results are explained by the fact that the measured reflectances of the Ni/Mg and Rh electrodes were approximately 80 and 55%, respectively. Moreover, it was concluded that a passivation layer is required for Ni/Mg electrodes to prevent the degradation of the LED properties by the oxidation of Mg.

Electrochemical promotion of carbon supported Pt, Rh and Pd catalysts for H2 oxidation in aqueous alkaline media

AbstractBACKGROUNDThe effect of electrochemical promotion of catalysis (EPOC) for the case of H2 oxidation reaction over commercial M (M=Pt, Rh, Pd)/Vulcan XC72 (ETEK) electrodes immersed in alkaline media was explored in this study. Various reactant mixtures (H2 and O2) fed simultaneously through a glass frit to the anodic electrode which was immersed in the electrolyte. Both positive and negative currents were applied between anode and cathode while all the reactants were followed by mass spectrometry.RESULTSIn the case of Pt electrode, it was found that application of small positive currents resulted in a reversible enhancement of both oxygen and hydrogen consumption rate, higher than the expected Faradaic one (I/2F). Pd and Rh electrodes exhibited lower catalytic activity at ambient temperature while positive current applications induced an acceleration of the catalytic reaction. Interestingly, modification of the catalytic activity of Pd and Rh electrodes was found to be irreversible even under negative current applications.CONCLUSIONCommercially available, carbon supported metal (Pt, Rh, Pd) catalysts, can be electrochemically promoted during hydrogen oxidation reaction in alkaline media. The rate enhancement could be explained by modification of the work function of the catalytic surface (thus the strength of the reactant's adsorption bond) during potential or current application as well as modification of the catalyst surface composition. © 2018 Society of Chemical Industry

Properties of Pd–Ru–Rh electrodeposits studied by electrochemical, structural and spectroscopic methods

Abstract Pd–Ru–Rh electrodeposits were obtained potentiostatically from aqueous baths containing different concentrations of Pd, Ru and Rh ions. The formation of Pd–Ru–Rh alloys was confirmed electrochemically through the examination of sample properties in the acidic solution with the use of cyclic voltammetry. The alloy formation was also proved with the use of energy dispersive X-ray spectroscopy and X-ray diffraction. XRD and EDX measurements have certified the homogeneity of a single bulk alloy phase. The XRD results show that the lattice parameter of the Pd–Ru–Rh alloy is smaller than that of Pd, i.e. Pd, Ru and Rh form contracted alloys. This is also mirrored in the course of hydrogen absorption curves (H/M vs. E), with the α–β phase transition region shifted into lower potentials. Pd–Ru–Rh alloys with ca. 98.5% Pd in the bulk absorb by ca. 11% more hydrogen than pure Pd. In the process of electrochemical aging, the surface and bulk of Pd–Ru–Rh electrodes become enriched with Pd due to the preferential electrochemical dissolution of Ru and Rh.

Improved carbonate formation from ethanol oxidation on nickel supported Pt–Rh electrode in alkaline medium at room temperature

In search for catalysts capable to oxidize small alcohols completely to CO32− in alkali, a few nickel supported platinum-rhodium (Pt–Rh) binary electrodes of varying mutual composition have been constructed by galvanostatic co-deposition of Pt–Rh on Ni support from suitable precursors’ solutions. The co-deposited particles are found to lie in the nano-size domain by electron microscopy. Catalyst with approximately 35 atom% of Rh shows the highest electrocatalytic activity among the various compositions of the binary Ni/PtRh catalyst system studied for anodic oxidation of ethanol. Ex-situ FTIR studies show that molar ratio of the two products CO32− and CH3COO− in sequence, is increased on increasing the content of Rh in the binary deposit. Extended studies of ethanol, acetaldehyde and acetate at low concentrations reveal that electrochemically produced M−OH is responsible for chemical oxidations of organic fuels and related intermediates. The results lead to develop a proposed mechanism for the reaction.

ATR-SEIRAS study of CO adsorption and oxidation on Rh modified Au(111-25 nm) film electrodes in 0.1 M H2SO4

Rh modified Au(111-25nm) electrodes, prepared by electron beam evaporation and galvanostatic deposition, were employed to study adsorption and electro-oxidation of CO on Rh in 0.1M sulfuric acid solution by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The results of ATR-SEIRAS experiments were compared with those obtained by infrared reflection absorption spectroscopy on three low-index Rh single crystal surfaces. The Rh film deposited on Au(111-25nm) electrode consists of 3D clusters forming a highly stepped [n(111)×(111)]-like surface with narrow (111) terraces. When CO was dosed at the hydrogen adsorption potential region, CO adsorbed in both atop (COL) and bridge (COB) configurations, as well as coadsorbed water species, were detected on the Rh film electrode. A partial interconversion of spectroscopic bands due to the CO displacement from bridge to atop sites was found during the anodic potential scan, revealing that there is a potential-dependent preference of CO adsorption sites on Rh surfaces. Our data indicate that CO oxidation on Rh electrode surface in acidic media involves coadsorbed water and follows the nucleation and growth model of a Langmuir-Hinshelwood type reaction.

Rhodium-Doped Barium Titanate Perovskite as a Stable p-Type Semiconductor Photocatalyst for Hydrogen Evolution under Visible Light

Rhodium-doped barium titanate (BaTiO3:Rh) powder was prepared by the polymerized complex (PC) method, and the photocatalytic activity for H2 evolution from water was examined. BaTiO3 is a wide-gap n-type semiconductor having a band gap of 3.0 eV. Doping Rh species into the lattice of BaTiO3 resulted in the formation of new absorption bands in visible light region. Upon visible light (λ > 420 nm), BaTiO3:Rh modified with nanoparticulate Pt as a water reduction promoter was capable of producing H2 from water containing an electron donor such as methanol and iodide. The best material prepared by the PC method exhibited higher activity than that made by a conventional solid-state reaction method. Visible-light-driven Z-scheme water splitting was also accomplished using Pt/BaTiO3:Rh as a building block for H2 evolution in combination with PtOx-loaded WO3 as an O2 evolution photocatalyst in the presence of an IO3(-)/I(-) shuttle redox mediator. Photoelectrochemical analysis indicated that a porous BaTiO3:Rh electrode exhibited cathodic photoresponse due to water reduction in a neutral aqueous Na2SO4 solution upon visible light.

Glucose biosensor based on a platinum electrode modified with rhodium nanoparticles and with glucose oxidase immobilized on gold nanoparticles

We have developed an enzymatic glucose biosensor that is based on a flat platinum electrode which was covered with electrophoretically deposited rhodium (Rh) nanoparticles and then sintered to form a large surface area. The biosensor was obtained by depositing glucose oxidase (GOx), Nafion, and gold nanoparticles (AuNPs) on the Rh electrode. The electrical potential and the fractions of Nafion and GOx were optimized. The resulting biosensor has a very high sensitivity (68.1 μA mM−1 cm−2) and good linearity in the range from 0.05 to 15 mM (r = 0.989). The limit of detection is as low as 0.03 mM (at an SNR of 3). The glucose biosensor also is quite selective and is not interfered by electroactive substances including ascorbic acid, uric acid and acetaminophen. The lifespan is up to 90 days. It was applied to the determination of glucose in blood serum, and the results compare very well with those obtained with a clinical analyzer.

PH dependence of the electroreduction of nitrate on Rh and Pt polycrystalline electrodes

From a study of the electrocatalytic reduction of nitrate on Pt and Rh electrodes over a wide pH range, HNO3 is suggested as the only reducible species in nitrate reduction on Pt, whereas both HNO3 and the nitrate anion are reducible on Rh. Rh is the more active catalyst of the two because it can activate nitrate even if no protons are available in solution. This is an important insight into the development of more effective nitrate reduction catalysts.

Unusually High Activity of Pt Islands on Rh(1 1 1) Electrodes for Ethanol Oxidation

Financial support by the MICINN (Spain) (project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER) is acknowledged. V. Del Colle thanks the Universidad de Alicante by the financial support during his stay as invited researcher.

Quantification of organic acids using voltammetric tongues

Recently, electronic tongues (ET) have appeared as an excellent alternative to traditional techniques for the evaluation of food quality and processes. ET systems are based on arrays of low selectivity sensors that are simultaneously sensitive to several components in a measured sample (cross-sensitivity). The aim of this study was to determine the ability of an ET based on pulse voltammetry to quantify organic acids (ascorbic, citric and malic acids) in simple (SS) and binary solutions (BS) using different electrodes. The most significant electrodes for ascorbic acid prediction were Ni and Ag for SS, and Ag and Ir for BS where positive pulses were more suitable than negative ones. The prediction of citric and malic acids in SS and BS were suitable using Ir, Rh, Pt, Ag and Cu electrodes, using both positive and negative pulses.