Search

The behaviour of CTAB adsorbed on polycrystalline gold electrodes has been studied using a combination of spectroelectrochemical methods. The results indicate that the formation of the layer is the consequence of the precipitation of the CTAB micelles on the electrode surface as bromide ions, which stabilize the micelles, are replaced by perchlorate anions. This process leads to the formation of CTA+ layers in which perchlorate ions are intercalated, in which the adlayer suffers a continuous rearrangement that leads to the formation of micro-dominions of different types of hydrogen-bonded water populations throughout the adlayer. After prolonged cycling, a stable situation is reached. Under these conditions, water molecules permeate through the adlayer toward the electrode surface at potentials positive of the potential of zero charge, due to the repulsion between the CTA+ layer and the positive charge of the electrode. This research was funded by Ministerio de Ciencia e Innovación (Spain) grant number PID2019-105653GB-I00), Generalitat Valenciana (Spain) grant number PROMETEO/2020/063. A.C. gratefully acknowledges the support of the University of Aberdeen.

An active and stable hydrogen evolution electrocatalyst composed of MoO2 surface nanorod arrays was prepared using high-temperature reductive annealing. Electrodes with an area of 78.5 cm2 were evaluated in a three-cell AEM electrolyser stack.

Stabilization of cathode catalysts in hydrogen-fueled proton-exchange membrane fuel cells (PEMFCs) is paramount to their widespread commercialization. Targeting that aim, Pt-Au alloy catalysts with various compositions (Pt

The concentration dependence of the activity for the oxidation of formic acid at pH 1.2, 2.4, and 3.9 has been studied on Pt(111), Pt(100), and stepped Pt[n(111) x (110)] electrodes. The results clearly demonstrate that, in this pH range, formate, and not formic acid, is the active species. To analyse the data, a kinetic model in which adsorbed monodentate formate is the active intermediate has been proposed. On Pt(111), a steady increase in the reaction rate with increasing pH and a reaction order of 1 for the bulk formate concentration was obtained, in agreement with the proposed model. On the other hand, the competition between adsorbed hydrogen and formate for the adsorption sites on Pt(100) cancels both the concentration and the pH dependence of the reaction rate. On this electrode, the reaction rate for the dehydrogenation of formic acid to COad was also studied and found to go through a maximum both at pH 1.2 and at pH 3.9, although the rate of dehydrogenation is slower and the maximum is broader at pH 3.9. The slower rate at the higher pH is consistent with the well-known fact that dehydrogenation of HCOOH is an acid-catalysed reaction. On Pt[n(111) x (110)] electrodes the behaviour observed is similar to that of Pt(111), with a reaction order of 1 for formate. The only significant difference with respect to the Pt(111) surface is the formation of COad due to the presence of (110) steps. This research was funded by Ministerio de Ciencia e Innovación (Spain) grant number PID2019-105653GB-I00), Generalitat Valenciana (Spain) grant number PROMETEO/2020/063. M.S. and A.C. gratefully acknowledge the support of the University of Aberdeen.

The electro-oxidation of glucose (0.01 M) in a neutral solution phosphate buffer solution – pH ~7 (0.1 M) is strongly sensitive to the surface structure. On surfaces containing (100) terraces domains the reaction occurs mainly into these sites due to their higher activity. Higher (111) step density into (100) terrace domains conducts the reaction through a less poisoned and energetic pathway because steps inhibit the cyclic carbonate formation. On surfaces containing (111) terrace sites, on large atom-wide terrace (𝑛 ≥ 9) the reactivity of the (100) steps seems to be mainly associated to the anticipation of the formation of glucuronic acid and favor its oxidation to glucaric acid and/or xylonic acid + CO, that are more energetic reactions. On the other hand, when the step density is increased, the catalytic activity decreases as result of adsorption of intermediates and/products generated during the reaction on (100) steps. Lower and medium (110) density steps favor the adsorption of glucose on surfaces containing (111) terraces symmetry, but independently of the terrace step symmetry, as the (110) step density is increased the electrocatalytic activity is reduced because the reaction takes place through a more poisoned pathway, with the formation of a greater amount of species that adsorb strongly on the (110) sites and block the surface. Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (grant no. PDE 233268/2014-6); MICINN - Spain (grant number PID2019-105653GB-100).

We report a study using Pt(111) and Pt(100) electrodes of the role of adsorbed formate in both the direct and indirect pathways of the electrocatalytic oxidation of formic acid. Cyclic voltammetry at different concentrations of formic acid and different scan rates and pulsed voltammetry were used to obtain a deeper insight into the effect of formate coverage on the rate of the direct pathway. Pulsed voltammetry also provided information on the effect of the concentration of formic acid on the rate of the formation of adsorbed CO on Pt(100). At low to medium coverage, increasing formate coverage increases the rate of its direct oxidation, suggesting that decreasing the distance between neighboring bidentate-adsorbed formate favors its interconversion to and/or stabilizes monodentate formate (the reactive species). However, increasing the formate coverage beyond approximately 50% results in a decrease of the rate of the direct oxidation, probably because bidentate formate is too closely packed for its conversion to monodentate formate to be possible. Cyclic voltammetry at very high scan rates reveals the presence of an order–disorder phase transition within the bidentate formate adlayer on Pt(111) when the coverage approaches saturation. The dependence of the potential of the maximum rate of dehydration to adsorbed CO, and of the rate at the maximum, on the concentration of formic acid is in good agreement with predictions made for a mechanism, in which adsorbed CO is formed through the adsorption of formate followed by its reduction to adsorbed CO, thus confirming that monodentate-adsorbed formate is the last intermediate common to both the direct and indirect pathways. This work has been financially supported by the MINECO-FEDER (Spain) through project CTQ2016-76221-P. A.B. and A.C. gratefully acknowledge the support of the University of Aberdeen.

The adsorption and reactivity of acetonitrile (CH3CN) have been studied on Pt(111) and Pt(100) electrodes in 0.1 M HClO4 and 0.1 M NaOH solutions with CH3CN concentrations ranging from 10–3 to 1 M. Cyclic voltammetry results show that these processes are structure sensitive and that the hydrogen adsorption/desorption region is partially blocked on Pt(111) in acidic solutions while the inhibition is almost complete on Pt(100) in both acidic and alkaline media. However, for Pt(111), hydrogen adsorption is practically unaltered in the 0.1 M NaOH electrolyte. In situ infrared measurements and density functional theory calculations suggest that rehybridized adsorbed acetonitrile reacts with adsorbed hydroxyl species at high potentials forming a hydroxylated adsorbed species. The latter is bonded to the Pt surface by electrodonation and can be reduced to an intermediate in which the double C–N bond is tilted with respect to the metal surface. Lastly, oxygen reduction reaction (ORR) has been investigated by using the hanging meniscus rotating disk electrode configuration. Limiting current densities decrease more drastically for Pt(100) than for Pt(111) as acetonitrile concentration is increased because of the higher acetonitrile coverage for the former one. The onset potential for ORR is shifted to less positive values in acidic media because of a blocking effect of acetonitrile. In alkaline media, the onset potential for Pt(111) is slightly more positive for low concentrations of acetonitrile because oxide formation, which hinders oxygen reduction for the more positive potentials, is inhibited due to the presence of adsorbed acetonitrile species at low coverages. This work has been financially supported by MCINN-FEDER (Spain) through project CTQ2016-76221-P. V.B.-M. thankfully acknowledges MINECO, the award of a predoctoral grant (BES-2014-068176, project CTQ2013-44803-P) and a student stay grant (EEBB-I-16-11656).

In this work, in situ external infrared reflection absorption spectroscopy (IRRAS) is successfully employed for the detection of intermediate species in the oxygen reduction reaction (ORR) mechanism on a flat and well-defined Pt surface. Superoxide anion species (O2–) are detected on the Pt(111) surface in an O2-saturated solution with a NaF/HClO4 mixture with pH 5.5 by the observation of a O–O vibration band at ca. 1080 cm–1. The observation of O2– without the use of any other additional method of signal enhancement is possible because in these experimental conditions O2– is the main ORR-generated intermediate and its reactivity is limited in this pH. This leads to the accumulation of O2– near the Pt surface, facilitating its identification. This work has been financially supported by MCINN (FEDER) (Spain) through project PID2019-105653GB-100.

The behavior of adsorbed CTAB on Au(111) electrodes has been studied using electrochemical and FTIR experiments in different aqueous solutions. The results show that the adsorbed layer is stable in acidic solutions in the whole potential range of study. The observed electrochemical and FTIR behavior is compatible with the formation of a membrane of CTA+ on the electrode surface with the polar amino groups in contact with the surface. When the electrode charge is negative, the polar groups are attracted to the surface, so that the capacitance of the electrode is smaller than that recorded for the unmodified Au(111) electrode. As the charge becomes positive, the membrane detaches from the surface and water molecules permeate through it, changing the capacitance of the electrode and giving rise to characteristic peaks in the voltammetric profile. At potentials higher than these peaks, the behavior of the electrode is comparable to that observed for the unmodified electrode. The stability of the membrane is facilitated by the incorporation of anions of the supporting electrolyte. Those anions remain on the membrane even when the electrode is transferred to a different solution, as the electrochemical behavior shows. Financial support from Ministerio de Ciencia e Innovación (Project PID2019-105653GB-100 ) and Generalitat Valenciana (Project PROMETEO/2020/063 ) is acknowledged.

Glucose is an important biomolecule that participates as a main source of energy in living organism reactions. Considering its biological importance, the electrochemical investigation about glucose electro-oxidation has attracted attention with the aim to develop non-enzymatic catalysts able to achieve the complete oxidation of glucose to be used as fuel in Direct Glucose Fuel Cells (DGFC). However, there are some challenges related to the oxidation of this relatively large, complex molecule, such as product distributions, adsorbed species, surface poisoning and structure sensitivity that must be understood. The use of well-defined surfaces, as single crystals, is a helpful tool to progress in this knowledge. In this context, the goal of this manuscript is the elucidation of the glucose electro-oxidation reaction in phosphate buffered solution (pH 7) on Pt(100) using cyclic voltammetry and in situ FTIR techniques at room temperature. It was observed that the reaction product distribution depends on glucose concentration. The maximum current density was reached with 2 × 10−2 mol dm−3 glucose (~2.04 mA cm−2) and then decreases with the increase of glucose concentration, because the reaction becomes dominated by the formation of strongly adsorbed species. In situ FITR experiments on Pt(100) show that the reaction proceeds through a complex mechanism, involving CO2, COL, COB, cyclic carbonate, γ-lactone, δ-lactone and carboxylic acids as intermediates and/or oxidation products. CO2 is generated mainly from COB and cyclic carbonate. Adsorbed CO seems to be the poisoning species responsible of the decrease of the catalytic activity on Pt(100), especially in the linear configuration, at higher glucose concentration. Gisele A. B. Mello thanks the post doctorate fellowship from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), of the Ministery of Science, Technology and Innovation of Brazil (grant no. PDE 233268/2014–6). Juan M. Feliu thanks MICINN-FEDER, Spain (project CTQ2016–76221.P).

In this perspective paper, the most recent advances in the understanding of the ORR (and the HPRR) on well-oriented Pt surfaces are summarized in order to provide a general vision of what is known to date and the next strategies that could be employed. This work was supported by the MINECO-FEDER (project CTQ2016-76221-P). Valentín Briega-Martos thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176, project CTQ2013-44803-P).

In this work, the oxygen reduction reaction (ORR) on tellurium-modified Pt(111) surfaces has been studied. Adsorption of Te adatoms on Pt(111) progressively shifts toward less positive values of both the ORR reaction onset and the half-wave potential in 0.1M HClO4 for 0 < θTe < 0.25. However, at θTe > 0.25, the ORR activity increases relative to the one at θTe < 0.25, but remains lower than that on clean Pt(111). Results were analyzed in light of simulations of kinetic currents as a function of θTe, calculated by employing a simple mean field model including both site blocking and electronic effects. Inside this framework, experimental data are best explained by considering that oxygenated Te species inhibit the ORR by either negatively modifying adsorption energies of reaction intermediates or combined site-blocking and electronic effects. A redox ORR catalysis due to redox properties of Te adatoms is discarded. Contrarily, in 0.05M H2SO4, a positive catalytic effect has been found, interpreted in terms of a competitive adsorption–desorption mechanism involving the replacement of adsorbed sulfate by Te adatoms. On the other hand, despite the strong site-blocking effect on Hads and OHads adsorption by Te adatoms, it appears that the reduced Te–Pt(111) adlayer does not inhibit the reaction, suggesting different active sites for Hads and OHads adsorption and for the rate-determining step of the ORR mechanism. The authors would like to thank Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP—Proc. 2019/04484-9), Brazil (A.M.G.-M.), and the Spanish MINECO through Project No. CTQ2016-76221-P (FEDER) (J.M.F.) for financial support.

Developing an understanding of structure–activity relationships and reaction mechanisms of catalytic processes is critical to the successful design of highly efficient catalysts. As a fundamental reaction in fuel cells, elucidation of the oxygen reduction reaction (ORR) mechanism at Pt(hkl) surfaces has remained a significant challenge for researchers. Here, we employ in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculation techniques to examine the ORR process at Pt(hkl) surfaces. Direct spectroscopic evidence for ORR intermediates indicates that, under acidic conditions, the pathway of ORR at Pt(111) occurs through the formation of HO2*, whereas at Pt(110) and Pt(100) it occurs via the generation of OH*. However, we propose that the pathway of the ORR under alkaline conditions at Pt(hkl) surfaces mainly occurs through the formation of O2−. Notably, these results demonstrate that the SERS technique offers an effective and reliable way for real-time investigation of catalytic processes at atomically flat surfaces not normally amenable to study with Raman spectroscopy. This work was supported by the NSFC (21522508, 21427813, 21521004, 21533006, 21621091 and 21775127), “111” Project (B17027), Natural Science Foundation of Guangdong Province (2016A030308012), the Fundamental Research Funds for the Central Universities (20720180037), and the Thousand Youth Talents Plan of China. Support from MINECO and Generalitat Valenciana (Spain), through projects CTQ2016–76221-P (AEI/FEDER, UE) and PROMETEOII/2014/013, respectively, is greatly acknowledged. V.B.-M. acknowledges MINECO for the award of a pre-doctoral grant (BES-2014–068176, project CTQ2013–44803-P).

The hydrogen peroxide reduction reaction (HPRR) on Pt(111) has been studied for the first time in acid to neutral pH values in the absence of anion specific adsorption using the HMRDE configuration. The onset for the reduction is ca. 0.95 V (RHE) for the Pt(111), irrespective of the pH value. At more negative potential values, the reduction is inhibited. It has been found that the diminution of the activity on Pt(111) occurs at the same potential value in the SHE scale for the different pH values. This indicates that this deactivation is not dependent on the hydrogen adsorption process, as has been previously suggested. However, it appears to be related to the interface water reorganization and the potential of zero free charge of the metal surface. In addition, studies in alkaline conditions suggest that the presence of adsorbed OH species promotes the total conversion of hydrogen peroxide to water. This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P).

The oxygen reduction reaction (ORR) at different pH values has been studied at platinum single crystal electrodes using the hanging meniscus rotating disk electrode (HMRDE) configuration. The use of NaF/HClO4 mixtures allows investigating the reaction up to pH = 6 in solutions with enough buffering capacity and in the absence of anion specific adsorption. The analysis of the currents shows that the kinetic current density measured at 0.85 V for the Pt(111) electrode follows a volcano curve with the maximum located around pH = 9. This maximum activity for pH = 9 can be related to the effects of the electrode charge and/or water structure in the ORR. On the other hand, the catalytic activity for the other basal planes shows a monotonic behavior with a small dependence of the activity with pH. For stepped surfaces with (111) terraces, the behavior with pH changes gets closer to that of the Pt(111) surface as the terrace length increases. Additionally, the ORR curves show a dependence of the limiting diffusion current with pH. It is observed that the limiting current density diminishes as the pH increases in a potential region where hydrogen peroxide is readily reduced. These results suggest the existence of a bifurcation point in the mechanism previous to peroxide formation, in which OOH• is proposed as the bifurcation intermediate. The reduction of OOH• requires proton addition and would be more difficult at neutral pH values, justifying the diminution of the limiting currents. This work has been financially supported by the MCINN-FEDER (Spain) and Generalitat Valenciana (Feder) through projects CTQ2016-76221-P and PROMETEOII/2014/013, respectively. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P).

The study of the oxygen reduction reaction (ORR) at high-index Pt(

Interfacial charge has a decisive influence on the reactivity of the most common electrochemical reactions for practical applications. In this work, the concepts of surface charge and the different types of potential of zero charge are revised on model electrodes. The different methodologies for the determination of the potential of zero charge are reviewed. After that, a thorough examination of the effect of surface charge on different electrochemical reactions of interest is carried out. The acid–base dissociation constants on the surface or the kinetics of relevant electrocatalytic reactions such as nitrate and peroxodisulfate reduction, formic acid oxidation, oxygen and hydrogen peroxide reduction or hydrogen evolution is affected by the interfacial charge. Therefore, it is crucial to understand the surface charge characteristics of the electrode materials to obtain better electrocatalytic activity. This work was supported by the MCINN-FEDER (Spain) through Project CTQ2016-76221-P. VB-M thankfully acknowledges MINECO for the award of a predoctoral grant (BES-2014-068176, Project CTQ2013-44803-P).

The formic acid oxidation reaction has been studied on Pt(111), Pt(100) and Pt nanoparticles with preferential (111) surface structure in 0.1 M HClO4 in the presence of different concentrations of acetonitrile. An electrocatalytic enhancement towards the formic acid oxidation has been observed under these conditions, and it is proposed that this enhancement is due to two different effects of the adsorbed acetonitrile species: a third-body effect which hinders the formation of CO and a promoting effect of the direct oxidation of formic acid. This promoting effect is structure sensitive. The different enhancement between Pt(111) and Pt(100) indicates that the ratio of free Pt sites and sites occupied by adsorbed acetonitrile is important in order to have the better electrocatalytic activity. On-line mass spectrometry (OLEMS) measurements confirmed the preference for the direct oxidation of formic acid to CO2, which is almost complete below 0.3 V vs. RHE for Pt(111). Finally, chronoamperometric studies confirmed the lower poisoning rate in the presence of acetonitrile but they also pointed out a competition of formed CO for the Pt sites occupied by acetonitrile species. This work constitutes an example of electrocatalytic enhancement by using an organic molecule for surface modification, which is not as common as using metallic adlayers. This work has been financially supported by the MINECO-FEDER (Spain) through project CTQ2016-76221-P. V. B-M thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176, project CTQ2013-44803-P) and a student stay grant (EEBB-I-16-11656). S-G. acknowledges financial support from Vicerrectorado de Investigación y Transferencia de Conocimiento of the University of Alicante (UATALENTO16-02).

The study of the oxygen reduction reaction (ORR) at high-index Pt(hkl) single crystal surfaces has received considerable interest due to their well-ordered, typical atomic structures and superior catalytic activities. However, it is difficult to obtain direct spectral evidence of ORR intermediates during reaction processes, especially at high-index Pt(hkl) surfaces. Herein, in situ Raman spectroscopy has been employed to investigate ORR processes at high-index Pt(hkl) surfaces containing the [011̅] crystal zone—i.e., Pt(211) and Pt(311). Through control and isotope substitution experiments, in situ spectroscopic evidence of OH and OOH intermediates at Pt(211) and Pt(311) surfaces was successfully obtained. After detailed analysis based on the Raman spectra and theoretical simulation, it was deduced that the difference in adsorption of OOH at high-index surfaces has a significant effect on the ORR activity. This research illuminates and deepens the understanding of the ORR mechanism on high-index Pt(hkl) surfaces and provides theoretical guidance for the rational design of high activity ORR catalysts. This work was financially supported by the National Natural Science Foundation of China (21902137, 21925404, 21775127, and 21427813), the Fundamental Research Funds for the Central Universities (20720190044), the China Postdoctoral Science Foundation (2019M652250), and the China Postdoctoral Innovation Talent Support Program (BX20190184). Support from MINECO through project CTQ2016-76221-P (AEI/FEDER, UE) is greatly acknowledged.

[EN] Platinum is a very effective electrode for the hydrogen evolution and oxidation reactions (HER/HOR) in acidic media. However, the activity for the HOR on platinum falls two orders of magnitude from acidic to alkaline media, which has not been completely understood yet. Here, we provide an explanation for that. Both the HER and the HOR were investigated on the three basal planes of platinum in a pH range near neutral pH conditions in buffered solutions in the absence of anion specific adsorption for guaranteeing the protons availability. Whereas changes in the pH from acid to neutral values produced negligible effects on the HER, the HOR was found to be pH sensitive, even under near neutral pH conditions. From these results, it can be consistently reasoned that the drastic fall in the activity of the HOR on platinum from acidic to alkaline media is an effect of the charge on the electrode, which is more negative as the pH increases. With the aid of density functional theory calculations, kinetic arguments explaining the unfavorable effect that negative charge on the electrode has on the HOR are provided., This work was supported by the MCINN-FEDER (Spain) through Project CTQ2016-76221-P. V.B.-M. thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176, Project CTQ2013-44803-P).

The hydrogen peroxide reduction reaction (HPRR) is investigated on Pt(111) as well as on Pt(S)[(n-1)(111) × (110)] and Pt(S)[n(111) × (100)] stepped surfaces in 0.1 M HClO4 by means of voltammetric studies using the hanging meniscus rotating disk electrode (HMRDE) configuration. Results point out that there is a direct correlation between the potential value (Einhibition) at which the inhibition of the HPRR at low potential starts and the local potential of maximum entropy (pme) for the terraces. In addition, there is also a correspondence between the potential of the peak (Epeak) that appears at E < 0.3 V vs. RHE for stepped surfaces and the local pme for the steps. Additional experiments for stepped surfaces from acidic to neutral pH confirm this correlation since Einhibition shifts 0.059 V per pH units towards more positive potentials, which is the same observed behavior for the pme from laser-induced temperature-jump experiments. However, adsorbed OH can influence this trend when the pme values are near the region of adsorption of these species. The effect of surface charge on the structure of interfacial water can also influence the current inhibition as inferred from measurements in alkaline media. Finally, ORR measurements and the differences observed with the HPRR results in the same conditions suggest that the formation of H2O2 intermediate is less favored as the pH is increased, and therefore a previous bifurcation point in the mechanism should exist. This work has been financially supported by the MICINN-FEDER (project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176).

Bromide adsorption on Pt(111) is investigated by means of cyclic voltammetry and CO displacement experiments at different pH values. In acidic pH, bromide adsorption is strongly overlapped with hydrogen desorption process. However, as the pH increases, hydrogen adsorption process displaces toward negative potentials while bromide adsorption remains nearly in the same potential region. In consequence, both processes decouple at higher pH values. The structural transition from Pt(111)-(1×1) to Pt(111)(3×3)-4Br is pH independent, in the SHE scale, and not observed for pH > 9.1. Values of pztc are extracted from the combination of voltammetric and CO displaced charges. An alternative approach to obtain charge curves is based on the coincidence of the curves at the structural transition characteristic of the bromide adlayer completion. Pztc values obtained from different approaches with and without bromide are compared, and their dependence on pH discussed. A thermodynamic analysis is carried out to obtain hydrogen Gibbs excess and charge number from the Esin Markov analysis. This work has been financially supported by the MCINN-FEDER (Spain) through Project CTQ2016-76221-P. V.B.-M. thankfully acknowledges to MINECO the award of a predoctoral grant (BES-2014-068176, Project CTQ2013-44803-P). G.A.B.M. expresses thanks for the postdoctorate fellowship from CNPq (Grant No. PDE 233268/2014-6).

Oxygen reduction reaction (ORR) is studied on Pt(111) in the presence of different concentrations of bromide anions at different pH values ranging from very acidic to neutral solutions. While adsorbed bromide inhibits the ORR, the strength of the inhibition decreases when the pH is increased. This is a consequence of the lower relative adsorption energy of bromide at higher pH values, caused by the lower absolute applied potential. This is reflected in a shift of the onset of the ORR (as measured with the hanging meniscus rotating disc electrode, HMRDE) to higher values as the pH is increased. HMRDE measurements reveal that the limiting current density (jlim) coincides with the theoretical value for two electrons only at very acidic solutions. However, when pH is increased, jlim tends toward the value for a four electrons reaction. From pH > 3 jlim coincides both in the presence and in the absence of bromide despite the specific anion adsorption. Experiments in solutions with different ionic strength and hydrogen peroxide reduction measurements suggest that the formation of a reaction intermediate different from H2O2 is favored at neutral pH values. This work has been financially supported by the MCINN-FEDER (Spain) through project CTQ2016-76221-P. VBM thankfully acknowledges to MINECO the award of a pre-doctoral grant (BES-2014-068176, project CTQ2013-44803-P). GABM thanks the post-doctorate fellowship from CNPq (grant no. PDE 233268/2014-6).

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/abs/10.1021/acscatal.6b03043, In order to produce hydrogen peroxide in small-scale electrochemical plants, selective catalysts for the oxygen reduction reaction (ORR) toward the desired species are required. Here, we report about the synthesis, characterization, ORR electrochemical behavior, and reaction mechanism of an aza-fused Ï€-conjugated microporous polymer, which presents high selectivity toward hydrogen peroxide. It was synthesized by polycondensation of 1,2,4,5-benzenetetramine tetrahydrochloride and triquinoyl octahydrate. A cobalt-modified version of the material was also prepared by a simple postsynthesis treatment with a Co(II) salt. The characterization of the material is consistent with the formation of a conductive robust porous covalent laminar polyaza structure. The ORR properties of these catalysts were investigated using rotating disk and rotating disk-ring arrangements. The results indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. Density functional theory calculations provide key elements to understand the reaction mechanism. It is found that, at the relevant potential for the reaction, half of the nitrogen atoms of the material would be hydrogenated. This hydrogenation process would destabilize some carbon atoms in the lattice and would provide segregated charge. On the destabilized carbon atoms, molecular oxygen would be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state, which would have the characteristics of a superoxide species, would be only slightly stable, promoting the formation of hydrogen peroxide, This work has been financially supported by the MCINNFEDER (projects CTQ 2016-76221-P, MAT2013-46753-C2-1-P, and MAT2014-52305-P) and Generalitat Valenciana (project PROMETEO/2014/013)

In this communication, we will present recent results on the oxygen reduction reaction (OOR) on nitrogen doped carbons. DFT calculations on nitrogen doped graphene systems will be used to understand the key elements that activate the reaction on these materials. The study is centered on the monodentate chemisorption of molecular oxygen as the first step of the process. It will be shown that carbon atoms are the active sites provided that several conditions are fulfilled. First, water should be included in the models, since it stabilizes the adsorption of the oxygen molecule with the characteristics of a superoxide species on the carbon atoms, which are the active sites. This carbon atom should be able to transition from a sp2 hybridization state to sp3 upon adsorption of the oxygen molecule to have a favorable interaction energy with the oxygen molecule. This transition is favored destabilized carbon atoms, that is, those neighboring a nitrogen dopant. Second, a significant charge transfer to this site should occur, so that charge can be concentrated on the adsorbed oxygen molecule. This charge transfer requires the presence of additional nitrogen dopants, which are charge donors, even when they are in distant positions. When the nitrogen dopant is located in an edge of the graphene structure, a clearly favorable chemisorbed state for molecular oxygen was found when nitrogen-dopant is hydrogenated and located at an armchair edge. The chemisorbed state is further favored by additional available charge from other nitrogen. By contrast, the chemisorbed state of oxygen is much less favorable when the hydrogenated pyridinic nitrogen-dopants are located at zigzag edges. To demonstrate the applicability of this approach, the ORR was experimentally studied on an aza-fused π-conjugated microporous polymer, which has a very well characterized structure. The experimental results using a rotating disk configuration indicate that hydrogen peroxide is almost exclusively produced at very low overpotentials on these materials. The DFT calculations indicate half of the nitrogen atoms of the material are hydrogenated at the potentials at which the reaction takes place. This hydrogenation process destabilizes some carbon atoms in the lattice and also provide a source of charge. On these carbon atoms, molecular oxygen could be chemisorbed with the aid of charge transferred from the hydrogenated nitrogen atoms and solvation effects, as in the previous cases. Due to the low destabilization of the carbon sites, the resulting molecular oxygen chemisorbed state is only slightly stable, promoting the formation of hydrogen peroxide.

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates. This work has been financially supported by the MICINN (Spain) (project 2013-44083-P) and Generalitat Valenciana (project PROMETEOII/2014/013).