Electrochemical catalysis of hydrogen evolution reaction on modified surfaces of gold, platinium and palladium

Elektrokataliza reakcije izdvajanja vodonika ispitana je u kiseloj i alkalnojsredini na bimetalnim sistemima Pd/Au(111), Rh/Au(111), Pd/Pt(poly), Rh/Pt(poly) iRh/Pd(poly). Kao tehnika pripreme bimetalnih elektroda odabrana je spontanadepozicija, čime je omogućeno da se pri različitim vremenima dobijaju strukture sarazličitim pokrivenostima. Sam proces spontane depozicije je praćen promenompotencijala otvorenog kola, čime je omogućen adekvatan izbor vremena depozicije naosnovu brzine deponovanja i dostizanja saturacione pokrivenosti supstrata.Strukturna (površinska) karakterizacija modifikovanih i osnovnih elektrodaizvedena je ex situ tehnikom mikroskopije atomskih sila. Istovremenim snimanjemtopografskih i faznih slika, omogućen je uvid u različite karakteristike površinebimetalnih elektroda, kao što su dimenzije deponovanih ostrva, preferencijalnadepozicija na određenim površinskim mestima, eventualno prisustvo nečistoća ipokrivenost supstrata depozitom. U svim slučajevima primećen je ostrvski rast depozita,kao i pokrivenosti ispod jednog monosloja.Klasične elektrohemijske tehnike ciklične i linearne voltametrije su poslužile zaelektrohemijsku karakterizaciju, ispitivanje katalitičkih svojstava bimetalnih sistema ipredlaganje mogućih reakcionih mehanizama za izdvajanje vodonika. Dobijeni rezultatisu pokazali da svi bimetalni sistemi pokazuju bolju aktivnost od čistog suptrata, u baremjednom elektrolitu. Sistem Pd/Au(111) je pokazao značajno poboljšanje aktivnosti uodnosu na monokristal zlata, dok je sistem Rh/Au(111) pokazao još veću aktivnost zaizdvajanje vodonika, koja je veoma bliska aktivnosti platine. Sistemi Pd/Pt(poly) iRh/Pt(poly) su aktivniji od Pt(poly) u alkalnom elektrolitu, što je veoma značajnoimajući u vidu da je Pt najaktivniji metal za reakciju izdvajanja vodonika. SistemRh/Pd(poly) je takođe pokazao poboljšana katalitička svojstva od supstrata, posebno ualkalnoj sredini. Unapređena katalitička aktivnost ispitanih nanostruktura je u svimslučajevima objašnjena geometrijskim i elektronskim efektom supstrata na depozit, usled kojeg bimetalne strukture poseduju drugačija elektrokatalitička svojstva odkonstitutivnih metala.Electrocatalysis of the hydrogen evolution reaction was examined on bimetallicelectrodes Pd/Au(111), Rh/Au(111), Pd/Pt(poly), Rh/Pt(poly) and Rh/Pd(poly) in bothacid and alkaline electrolytes. Preparation of the bimetallic electrodes with differentcoverage was carried out by the spontaneous deposition during different times. Thespontaneous deposition process was monitored by chronopotentiometric measurementof open circuit potential, which enabled adequate choice of the deposition times basedon the deposition rate and the achievement of the saturation coverage.Surface characterization of the modified bimetallic electrodes, as well as basesubstrate electrodes, were performed ex situ by atomic force microscopy.Simultaneously recorded topographic and phase images provided insight into differentcharacteristics of the bimetallic surfaces, such as the dimensions of the depositedislands, occurrence of the preferential deposition along specific surface sites and thesubstrate coverage with deposited islands. In all cases, atomic force microscopyrevealed that the spontaneous deposition follows the island growth with submonolayercoverage.Classical electrochemical techniques were used for electrochemicalcharacterization and exploration of the catalytic properties for hydrogen evolution, andproposition of the possible reaction mechanisms. Results have shown that all bimetallicelectrodes exhibited higher activities than bare substrate, in at least one electrolyte. Inthe case of Au(111) substrate, Pd/Au(111) electrodes have shown high activity forhydrogen evolution, while even higher activity, which is close to the activity ofplatinum, was obtained for Rh/Au(111) electrodes. Bimetallic electrodes Pd/Pt(poly)and Rh/Pt(poly) are more active than bare Pt(poly) in alkaline media, which is veryimportant due to the fact that Pt is the most active metal for hydrogen evolution.Rh/Pd(poly) electrodes have shown enhanced catalytic properties for this reaction withrespect to Pd(poly), which are more pronounced in alkaline media. The improved catalytic activity of the hereby explored bimetallic electrodes wasinterpreted by the means of geometric and electronic effects of the substrates, which areresponsible for different electrocatalytic properties with respect to both constitutivemetals

Ethanol Oxidation on Pd/Au(111) Bimetallic Surfaces in Alkaline Solution

Catalytic properties of Pd/Au(111) nanostructures obtained by spontaneous palladium deposition using PdSO4 and PdCl2 salts were examined for the oxidation of ethanol in alkaline media. Atomic force microscopy has shown that counter anions in Pd salts are responsible for the differences in surface topography. In both cases the oxidation of ethanol is characterized by the rise of a new peak at approximately the same potential for the same coverage, indicating similar electronic modification of Pd islands by the Au(111) substrate. Pd/Au(111) nanostructures obtained using PdCl2 salt have shown higher ethanol oxidation current densities, which can be ascribed to the surface structure consisting of thinner and smoother Pd deposit providing more convenient sites for the adsorption of ethanol and its subsequent oxidation

Oxygen Reduction on Polycrystalline Pt and Au Electrodes in Perchloric Acid Solution in the Presence of Acetonitrile

Oxygen reduction reaction (ORR) was studied on polycrystalline Pt and Au electrodes in 0.1 M HClO(4) solutions containing various amounts of acetonitrile (AcN). The state of the electrode surface was characterized by cyclic voltammetry in oxygen free electrolytes, while ORR studies were performed on polycrystalline Pt and Au rotating disc electrodes by a linear sweep voltammetry in oxygen saturated electrolytes. Acetonitrile is chemisorbed on Pt over a wide potential range inhibiting both H adsorption and oxide formation. The extent of AcN chemisorption depends on its concentration in the solution. Initial potential of oxygen reduction is shifted negatively, while the kinetics of ORR is increasingly hindered with the increase of AcN concentration. Inhibiting effect of acetonitrile on ORR is pronounced on both Pt(poly) and Au(poly). Complete inhibition of ORR in the potential range of AcN chemisorptions is achieved for 0.1 M HClO(4) solution containing 1 M AcN on Au(poly) and 3 M AcN on Pt(poly)

Hydrogen evolution reaction on bimetallic Ir/Pt(poly) electrodes in alkaline solution

Hydrogen evolution reaction (HER) was studied in alkaline solution on Pt(poly) electrode modified by spontaneously deposited Ir nanoislands. Comprehensive insight into the characteristics of the bimetallic Ir/Pt(poly) catalysts was obtained by a combination of Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FESEM), X-ray Photoelectron Spectroscopy (XPS) and classical electrochemical techniques. HER investigations have shown that the presence of spontaneously deposited Ir enhances the activity of bare Pt(poly) in alkaline solution. This was attributed to the heterogeneity of the active surface sites and to the electronic interaction between two metals in close contact which together facilitated the adsorption of the H intermediate species.This is peer-reviewed version of the article: Svetlana Štrbac, Milutin Smiljanić, Thomas Wakelin, Jelena Potočnik, Zlatko Rakočević, Hydrogen evolution reaction on bimetallic Ir/Pt(poly) electrodes in alkaline solution, Electrochimica Acta (2019), [https://dx.doi.org/10.1016/j.electacta.2019.03.100][http://cer.ihtm.bg.ac.rs/handle/123456789/2951

Ethanol Oxidation on Pd/Au(111) Bimetallic Surfaces in Alkaline Solution

Catalytic properties of Pd/Au(111) nanostructures obtained by spontaneous palladium deposition using PdSO4 and PdCl2 salts were examined for the oxidation of ethanol in alkaline media. Atomic force microscopy has shown that counter anions in Pd salts are responsible for the differences in surface topography. In both cases the oxidation of ethanol is characterized by the rise of a new peak at approximately the same potential for the same coverage, indicating similar electronic modification of Pd islands by the Au(111) substrate. Pd/Au(111) nanostructures obtained using PdCl2 salt have shown higher ethanol oxidation current densities, which can be ascribed to the surface structure consisting of thinner and smoother Pd deposit providing more convenient sites for the adsorption of ethanol and its subsequent oxidation

Low-Loaded Pt Nanoparticles Supported on Electrochemically Exfoliated Graphene as a Sustainable Catalyst for Electrochemical Ethanol Oxidation

Securing ever-increasing energy demands while reducing resilience on fossil fuels is a major task of modern society. Fuel cells are devices in which the chemical energy of various fuels can be converted into clean electricity. Direct ethanol fuel cells (DEFC) are increasingly popular for their eco-friendliness and significantly easier liquid fuel manipulation compared to hydrogen-fed fuel cells. Carbon-supported Pt nanoparticles are considered reference catalysts for fuel oxidation in DEFCs. Several challenges hinder DEFC commercialization: high Pt-loading, Pt poisoning by CO intermediates, and the instability of the Pt and carbon supports. This work demonstrates an efficient electrocatalyst for ethanol oxidation reaction (EOR) composed of Pt nanoparticles supported on electrochemically exfoliated graphene (Pt/el-rGO). Graphene was obtained through anodic electrochemical exfoliation using graphitic tape as the anode, while Pt nanoparticles were synthesized using chemical reduction with formic acid. As-obtained Pt/el-rGO with only 7.5 wt.% Pt was characterized using TEM, SEM, and XPS. Pt/el-rGO exhibited notably higher EOR catalytic activity in an alkaline electrolyte than the Pt/C benchmark. This enhancement can be linked with the functional groups present on the graphene support, which facilitate ethanol dehydrogenation as the first step in the EOR mechanism and thus enhance reaction kinetics on Pt-active sites

Electrochemical Stability and Degradation of Commercial Pd/C Catalyst in Acidic Media

Palladium has attracted significant attention as a catalyst or co-catalyst for many electrochemical reactions in energy conversion devices. We have studied electrochemical stability of a commercial Pd/C sample in an acidic electrolyte by exposing it to an accelerated stress test (AST) to mimic potential spikes in fuel cells and electrolyzers during start/stop events. AST consisted of extensive rapid potential cycling (5000 cycles, 1 V/s) in two potential regions, namely AST1 was performed between 0.4 and 1.4 VRHE, while AST2 was performed between 0.05 and 1.4 VRHE. Degradation of Pd/C was monitored by the changes in Pd electrochemical surface area, while the hydrogen evolution reaction (HER) was used as a test reaction to observe the corresponding impact of the degradation on the activity of Pd/C. Significant Pd/C degradation and HER activity loss were observed in both potential regions. Coupling of the electrochemical flow cell with an inductively coupled plasma mass spectrometry device showed substantial Pd dissolution during both ASTs. Identical location scanning electron microscopy revealed that Pd dissolution is followed by redeposition during both ASTs, resulting in particle size growth. Particle size growth was seen as especially dramatic in the case of AST2, when particularly large Pd nanostructures were obtained on top of the catalyst layer. According to the results presented in this work, (in)stability of Pd/C and other Pd-based nanocatalysts should be studied systematically as it may present a key factor limiting their application in energy conversion devices

Oxygen Reduction on Au(100)-like Polycrystalline Gold Electrode in Alkaline Solution

Catalytic properties of polycrystalline gold, Au(poly), were examined for the oxygen reduction reaction in alkaline solution using the rotating disc electrode technique. On electrochemically prepared Au(poly), oxygen reduction proceeds partly through 4e-reaction pathway resembling the activity of bare Au(100) in alkaline solution. Electrochemical behavior of such Au(100)-like polycrystalline gold electrode was compared with bare Au(100) surface, as well as with stepped Au(210)=Au[2(100)x(110)] and Au(533)=Au[4(111)x(100)] surfaces. It is shown that polycrystalline gold electrode behaved in a similar manner as stepped Au[n(111)x(100)] surfaces, meaning that the enrichment in (111)x(100) steps, rather than in (100) orientation is responsible for a partial 4e-reaction pathway in alkaline solution

Enhanced Activity of Polycrystalline Palladium Decorated by Ru Nanoislands for Hydrogen Evolution in Alkaline Medium

Hydrogen evolution reaction (HER) was studied on polycrystalline Pd, Pd(poly), decorated by spontaneously deposited Ru nanoislands below full coverage. Surface features of as-prepared bimetallic Ru/Pd(poly) electrodes were explored by Field Emission Scanning Electron Microscopy with Energy Dispersive X-ray Spectrometer. Electrochemical properties and hydrogen evolution activities of obtained electrodes were investigated in an alkaline electrolyte by Cyclic and Linear Sweep Voltammetry, respectively. It was found that the activities of bimetallic Ru/Pd(poly) electrodes for HER significantly exceeded the activity of bare Pd(poly) and approached the activity of Pt, which is the most active material for this reaction. This enhancement was explained by the favorable influence of the electronic interaction between Pd substrate and Ru nanoislands on the adsorption of the reactive H species

Oxygen reduction on polycrystalline Au modified by nanosized Pd islands

Oxygen reduction reaction was studied on polycrystalline gold, Au(poly), modified by nanosized palladium islands. Linear sweep voltammetry measurements were performed using rotating Au-disk electrode in oxygen saturated 0.05 M H2SO4 solution. Morphology of obtained Pd/Au(poly) electrodes was characterized by tapping-mode atomic force microscopy, after each deposition from Pd containing solution. Only homogeneous distribution of deposited Pd islands nonuniform in size is observed. Active surface area of the deposited Pd was estimated from cyclic voltammetry profiles. Obtained Pd/Au(poly) surfaces have shown a significant catalytic activity towards oxygen reduction reaction which increases with the increase of the active surface area