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CO2 electroreduction has been considered a promising alternative to simultaneously reduce CO2 emissions and produce value-added products. Among others, the production of formic acid/formate is particularly attractive. Although promising results have already been obtained in the literature, one of the recent approaches to improve the process deals with the use of an alternative reaction at the anode instead of the traditional oxygen evolution reaction (OER). In this context, this work reports, for the first time, the study of the CO2 electroreduction to formate coupled with the electrooxidation of glycerol to high-added value products where both half-reactions operate in a continuous mode with a single pass of the reactants through the electrochemical cell. Interestingly, at the cathode, similar results to those previously reported were obtained, reaching formate concentrations of about 18 g·L-1 at a 200 mA·cm-2. In addition, at the anode, promising dihydroxyacetone productions of 196 µmol·m-2·s-1 were simultaneously achieved in the output stream of the anodic compartment. These findings represent a significant step forward for the development and application of the technology. The authors gratefully acknowledge financial support through projects PID2019–108136RB-C31, PID2019–108136RB-C32 and PID2020–112845RB-I00 (AEI/10.13039/501100011033).

Electrochemical reduction of carbon dioxide (CO2) into green fuels and valuable chemicals is an up-and-coming method of CO2 valorization. Formate/formic acid is one the most desirable product among the many other possible chemicals that can be generated from CO2. Herein, we report on a simple and tunable method to prepare Bi-based electrocatalyst. An affordable metal-organic framework (MOF) precursor TAL-33 has been utilized upon carbonization. This MOF was fabricated from a novel modular carbon-rich ligand 1H-benzo[d]imidazole-5,6-diol and bismuth chloride. Cyclic voltammetry and chronoamperometric measurements were performed to investigate the electrocatalytic activity and selectivity towards the formate. The most promising samples have shown high Faradaic efficiency and stability. The in-depth physical characterization of catalyst structure (XPS, XRD, SEM, and TEM) was performed to investigate the structure-activity relationships. Theoretical studies have been performed to confirm that the enhanced CO2 electroreduction to formate is linked to the presence of metallic bismuth sites. This research was supported by Estonian Research Council grant PSG250; EU through the European Regional Development Fund, Estonia (TK141, “Advanced materials and high-technology devices for energy recuperation systems” and TK143, “Molecular Cell Engineering”); Environmental Investment Center circular economy program (KIK18070). A.B, V. M. and J.S.G. acknowledge the financial support of the Ministerio de Ciencia e Innovación-FEDER (Spain) through the project PID2019-108136RB-C32 and Generalitat Valenciana (Project PROMETEO/2020/063). S.V. acknowledges the financial support of ERA Chair MATTER from the European Union’s Horizon 2020, Estonia research and innovation programme under grant agreement No 856705.

The lack of an artificial system that mimics elemental sulfur (S8) oxidation by microorganisms inhibits a deep mechanistic understanding of the sulfur cycle in the biosphere and the metabolism of sulfur-oxidising microorganisms. In this article, we present a biphasic system that mimics biochemical sulfur oxidation under ambient conditions using a liquid|liquid (L|L) electrochemical cell and gold nanoparticles (AuNPs) as an interfacial catalyst. The interface between two solvents of very different polarity is an ideal environment to oxidise S8, overcoming the incompatible solubilities of the hydrophobic reactants (O2 and S8) and hydrophilic products (H+, SO32–, SO42–, etc.). Furthermore, the interfacial AuNPs provide a catalytic surface onto which O2 and S8 can adsorb. Control over the driving force for the reaction is provided by polarising the L|L interface externally and tuning the Fermi level of the interfacial AuNPs by the adsorption of aqueous anions. Comparison of electrochemical measurements using a 4-electrode closed bipolar electrochemical cell and a L|L electrochemical cell confirmed that electron transfer reactions are possible between O2, gold and S8 in biphasic systems. M.D.S. acknowledges Science Foundation Ireland (SFI), under Grant No. 13/SIRG/2137 and the European Research Council through a Starting Grant (Agreement No. 716792). M.F.S.-H. acknowledges the “Universidad Nacional de Colombia” for allowing his sabbatical leave and the “Fundación Banco de la República” through the grant 4.562.

The development of clean, renewable, and affordable energy sources such as wind or sun has become a necessity for a sustainable energy society. Consequently, it is highly required the development of energy storage systems capable of efficiently storing the energy generated by these intermittent sources. In recent years, hydrogen-based technologies have emerged as a way of mitigating the environmental footprint left by the fossil fuels-based ones. In this paper we report new insights into the performance of an environmentally friendly Acid-Base Electrochemical Flow Battery (ABEFB), using an electrolyte consisting of high NaCl concentration. Energy is obtained from the neutralization of two acid and alkaline solutions through hydrogen evolution and oxidation reactions. Different parameters such as the nature of the cationic membrane and the electrolyte concentration have been systematically evaluated. Under optimal conditions, this battery can provide a maximum power density of about 73.5 W m-2 at 215 A m−2, a coulombic efficiency of around 80%, and a round-trip efficiency of 52%. Authors would like to thank Consellería de Educación, Investigación, Cultura y Deporte de la Generalitat Valenciana (GRISOLIAP/2017/188) for supporting this research.

The use of shape-controlled nanomaterials represents an elegant way to improve the performance of many electrochemical reactions. However, two main aspects must be still optimised: decreasing particle size and increasing their activity. This study shows that the deposition of Pd on different small (3-5 nm) supported Pt materials (displaying a preferential structure containing ∼40% of Pt{100} terraces) leads to a great enhancement in the electrocatalytic activity towards the formic acid electrooxidation reaction. After Pd decoration, the oxidation takes place at lower overpotentials and oxidation current densities are significantly higher than those observed with all the bare Pt/C samples. For the Pt/C 45% catalyst, the intrinsic activity after Pd deposition was more than three times of that without Pd, while the oxidation overpotential shifted about ∼200 mV to lower overpotentials. This work was supported by the Ministerio de Ciencia e Innovación-FEDER (Spain) Project PID2019-105653GB-100 and Generalitat Valenciana (Project PROMETEO/2020/063). R.M.A and R.M.T thank to the support from FAPESP, Procs. 2019/08051-0, 2017/15469-5 and 15/26308-7. H.E. thanks the Estonian Research Council (grant No PUTJD841).

Shape-controlled nanoparticles are advanced nanomaterials containing a preferential surface structure. The use of these shaped nanoparticles in electrocatalysis has improved the activity and selectivity of many relevant electrocatalytic reactions because most of them are well-known to be surface structure sensitive. Among these relevant electrocatalytic reactions, CO2 electroreduction is one of the most important and appears as an interesting strategy not only to mitigate CO2 emissions but also to valorize CO2 into value-added chemicals. In this respect, enormous efforts are being dedicated to developing new electrocatalytic materials with improved properties toward CO2 electroreduction in terms of activity, selectivity, and stability. In this contribution, we will summarize and discuss some of the most representative and relevant findings on the application of different shaped-controlled nanoparticles toward CO2 electroreduction. The results reported here point out the potentialities of these shaped nanomaterials.

The application of shape-controlled metal nanoparticles in electrocatalysis has improved significantly the activity, selectivity, and even the stability of many relevant electrocatalytic reactions. It is well accepted that, by controlling the shape of the nanoparticles, it is possible to provide nanoparticles with a preferential surface structure. However, to fully understand the capabilities of these nanomaterials, it is extremely relevant to correlate shape, surface structure, and electrocatalytic reactivity. Particularly, establishing the correlations between surface structure and reactivity is the key point to be studied and understood. Consequently, having tools to characterize the surface structure of these nanoparticles is of critical importance. In this short review, we discuss about the progress in the in situ characterization of the surface structure of shaped Pt, Au, and Pd nanoparticles by electrochemical probes. The results here included clearly demonstrate the potentialities of the electrochemical tools to gain detailed information of the surface structure of these shaped nanomaterials. This article was performed under Ministerio de Ciencia e Innovación (MICINN)-FEDER (Spain) Project CTQ2016-76221-P. J.S-.G. also acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02).

Electrochemical denitrification is a promising approach to remove NO3− towards its conversion to innocuous N2. In this work, the electrocatalytic performance of Bi, Sn and BixSny carbon-supported nanoparticles (∼10 nm) was evaluated for the selective NO3− electroreduction (NER) to N2. The different electrocatalysts were synthesised using a simple chemical method, physicochemically characterised by TEM-EDS and XPS and, subsequently, air-brushed onto a Toray carbon paper to build electrodes. These electrodes were characterised by SEM-EDS and cyclic voltammetry. Electrolyses were carried out in a slightly alkaline media to find the most active electrocatalyst towards the NO3− removal. Among them, Bi60Sn40/C showed the best results with more than 50% of selectivity to N2 after exhaustive electrolyses, regardless of the cathode potential or the electric charge passed. Other detected non-desired secondary products were N2O, NO2− and NH4+/NH3. The results obtained point out that the combination of Sn and Bi gives rise to higher NER activity and selectivity to N2 than pure Bi and pure Sn. Therefore, these electrocatalysts are not only promising but also cost-effective for future practical applications like electrochemical denitrification of wastewater. Nevertheless, further investigation is still needed in order to assess the optimum operational conditions. I. Sanjuán acknowledges the Ph.D. fellowship granted and the material and human resources provided by the University of Alicante and the company Aguas de Valencia, S.A (UAIND2016-02). JSG acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02).

The practical application of Pt-based technologies depends on the lowering in the costs and one way to face this challenge is by reducing the amount of Pt. Herein, we decorate Ag nanoparticles with ultra-small (0.8 %) quantities of Pt and test them for the electrooxidation of glycerol. We showed that the Pt atoms of our catalyst are one order of magnitude more active (in terms of mass activity) than Pt/C commercial catalyst. By performing in situ FTIR measurements, electrolysis experiments and HPLC analysis, we showed that both catalysts form mainly glycerate and lactate, but Pt/C is more prone to poisoning and to form carbonate. We suggest that this different behavior is attributed to the high dispersion of the Pt atoms on Ag nanoparticles. The lower probability of finding neighboring Pt atoms diminishes the formation of multiple bonded intermediates, which are precursors for the formation of carbonate and some poisoning intermediates. PSF thanks Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (grants: 2016/01365-0, 2017/11986-5, 2018/20952-0). CCL thanks Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. J.S-G. also acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02). We also thanks Prof. M.A. Zezzi and Luana F. da Costa for the ICP analysis. The authors also extend gratitude to CNPEM open-facilities (LME, and LNNano) and Synchrotron Light Laboratory, CNPEM/MCTIC under proposals 20180571 and 20171115.

In this work, the application of Cu oxides/ZnO-based electrocatalytic surfaces for the continuous and selective gas-phase electroreduction of CO2 to ethylene in a filter-press type electrochemical cell is studied. The prepared catalytic materials are characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Then, the Cu oxides/ZnO-based gas diffusion electrodes are electrochemically characterized by cyclic voltammetry and Tafel plot analyses. The ethylene formation rate and Faradaic efficiency are as high as 487.9 μmol m−2s−1 and 91.1% when a current density of 7.5 mAcm−2 (-2.5 V vs. Ag/AgCl) is applied to the system, with an ethylene/methane production ratio of 139, showing a better performance than previous electrocatalytic systems for the production of ethylene from CO2 conversion. Consequently, the use of Cu oxides/ZnO-based electrocatalysts for gas-phase CO2 reduction is a step forward in the production of C2 products, such as ethylene. The authors gratefully acknowledge the financial sources from the Spanish Ministry of Economy and Competitiveness (MINECO), through the projects CTQ2016-76231-C2-1-R (AEI/FEDER, UE) and CTQ2016-76231-C2-2-R (AEI/FEDER, UE). Besides, I.M.-G. and J.A. would also like to thank the MINECO for the postdoctoral period of the predoctoral research contract (BES-2014-070081) and Ramón y Cajal programme (RYC-2015-17080), respectively. J.S-G. also acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02).

In this communication we report a voltammetric study of the adsorption–desorption of cytosine (C) and methylcytosine (mC) on well-defined gold (Au) electrodes. The voltammetric measurements clearly indicate that these processes are extremely sensitive to the Au surface structure and in particular to the presence of (111) surface domains. Interestingly, on Au(111) surfaces, a linear correlation between the C and mC concentrations (logarithm scale) and the peak potential of the main voltammetric feature is found. In addition, in the simultaneous presence of both molecules, mC governs the electrochemical response, which has allowed its accurate quantification in C-mC mixtures. In situ FTIR spectroscopic measurements have been carried out to deepen on this mC electrochemical sensitivity. This research may contribute to the future development of an electrochemical sensor for the determination of the degree of methylation in DNA. This work has been financially supported by the MINECO through projects CTQ2013-48280-C3-3-R, CTQ2013-44083-P and by the Generalitat Valenciana through project PROMETEOII/2014/013.

Electrocatalytic reduction of CO2 has been taken into consideration as a fascinating option to store energy from intermittent renewable sources in the form of chemical value-added products. Among the different value-added products, formic acid or formate is particularly attractive since it can be used as a fuel for low-temperature fuel-cells and as a renewable hydrogen carrier. Very recently, a rapidly increasing number of studies have revealed Bi as a promising electrocatalytic material for the CO2 electroreduction to formate, but the performance of Bi electrodes operating in a continuous mode and high current density (j) has been hardly investigated yet. Thus, this work aims at studying the CO2 electroreduction to formate working in a continuous mode in a filter-press-reactor at a j up to 300 mA·cm−2 using Bi electrodes. Bismuth Gas Diffusion Electrodes (Bi-GDEs) were fabricated from carbon-supported Bismuth-nanoparticles. The influence of j and the electrolyte flow/area ratio in the performance of the Bi-GDEs towards formate were evaluated. Working at j of 300 mA·cm−2, a concentration of 5.2 g formate·L−1 with a faradaic efficiency (FE) and rate of 70% and 11 mmol·m−2·s−1, respectively were achieved. Lowering the j to 90 mA·cm−2, formate concentrations of up to 7.5 g·L−1 could be obtained with an excellent FE of 90%. Interestingly, the highest concentration of formate obtained was 18 g·L−1, but at expenses of an important decrease in FE. Although the results of this study are interesting and promising, further research is required to increase formate concentration for a future implementation at industrial scale. The authors of this work would like to show their gratitude to the financial support from the MINECO, through the projects CTQ2016-76231-C2-1-R and CTQ2016-76231-C2-2-R (AEI/FEDER, UE). Jose Solla-Gullón also acknowledges the financial support from VITC of the University of Alicante (UTALENTO16-02).

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 use of adatom decorated shape-controlled metal nanoparticles in electrocatalysis, and particularly for energy conversion reactions, has made important contributions to the development of better electrocatalysts. In this short review, we highlight some of the most relevant findings and discuss about future challenges. J.S-G and P.S.F. acknowledges financial support from FAPESP (Process numbers 2017/09780-0 and 2016/01365-0). J.S-G. also acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UATALENTO16-02). M.B.C.S acknowledges CAPES for his fellowship.

The electrochemical valorisation of captured CO2 is an attractive option to obtain value-added products, and at the same time, to chemically store energy from intermittent renewable sources. Among the different products, formic acid/formate is particularly interesting since it is one of the most promising materials for hydrogen storage and candidate fuel for low-temperature fuel cells. In this work, a process for CO2 electroreduction to formate is studied on a continuous filter-press cell using an innovative electrode: Sn Catalyst Coated Membrane Electrodes (Sn-CCMEs) - comparing with previous approaches based on Sn Gas Diffusion Electrodes (Sn-GDEs), using the same synthesised tin nanoparticles (Sn NPs) and operating conditions. The Sn-CCME is prepared by depositing Sn NPs directly over a Nafion 117 membrane, and it allows working with a gaseous CO2 flow humidified with water as the input of the electrochemical cell, avoiding the use of the liquid catholyte. Sn-CCME operates at lower current densities (45 mA cm-2) than previous Sn-GDEs (200 mA cm-2), which resulted in lower rates of formate production. However, the proposed Sn-CCME, allowed achieving even higher formate concentrations with an energy consumption 50% lower than with the Sn-GDEs. The influence of key variables such as temperature and water input flow on the performance of the process using Sn-CCMEs was also analysed in a controlled experimental set-up specifically designed and built for this goal. Increasing the temperature of the gaseous stream did not improve the performance. The best results were obtained at ambient conditions of temperature (20 °C) and with the amount of water in the CO2 stream at 0.5 g h-1, giving the highest formate concentration (19.2 g L-1) with a Faradaic efficiency close to 50% and an energy consumption of 244 kW h kmol-1. More research is still required to further improve CCME configuration in order to increase formate rate and efficiency without increasing energy consumption. This work was conducted under the framework of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO), projects CTQ2016-76231-C2-1-R (AEI/FEDER, UE) and CTQ2016-76231-C2-2-R (AEI/FEDER, UE). JSG acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante (UTALENTO16-02).

To understand how the CO oxidation reaction proceeds on nanoparticles, which have complex surface structures, the behavior of the nanoparticles has to be related to that of single crystal electrodes with a well-defined surface structure. However, the direct extrapolation of the results is not possible because significant differences in the behavior between both type of surfaces are observed. In single crystal electrodes in both acidic and alkaline media, the reaction initiates on defects on the CO adlayer. These defects can be already present on the surface, as surface defects or steps, or generated during the formation of the CO adlayer. In the case of steps, the oxidation starts on the lower part of the step. The only difference between the reaction behavior between acidic and alkaline solutions is the lower mobility of the CO in alkaline solutions, which generates adlayers with higher number of defects and give rise to multiple stripping peaks in stepped surfaces. Using these results, the differences of the behavior between single crystal electrodes and nanoparticles can be rationalized. In spite of the fact that nanoparticles have small ordered domains, the presence of sites in which the reaction is initiated, equivalent to the site in the lower part of the step, is almost negligible, and thus, the oxidation reaction takes place at higher potential values than in stepped surfaces with similar domain size. Also the effect of nanoparticle agglomeration in the oxidation has been rationalized. This work has been financially supported by the MINECO-FEDER (projects CTQ 2013-44083-P and CTQ2013-48280-C3-3-R) and Generalitat Valenciana (project PROMETEO/2014/013). M.J.S. Farias acknowledges financial support from the CAPES – Brasil (PNPD).

The use of shape-controlled metal nanoparticles has produced not only a clear enhancement in the electrocatalytic activity of different reactions of interest but also a better understanding of the effect of the surface structure on nanoscaled materials. However, it is well-accepted that a correct understanding of the correlations between shape/surface structure and electrochemical reactivity indispensably requires the use of clean surfaces. In this regard, and considering that most of the synthetic methodologies available in the literature for the preparation of these shaped metal nanoparticles employ capping agents, the development of effective surface cleaning methodologies able to remove such capping agents from the surface of the corresponding nanoparticles, becomes an extremely important prerequisite to subsequently evaluate their electrocatalytic properties for any reaction of interest. Consequently, in this contribution, we summarize the most relevant advances about surface cleaning procedures applied to different shaped metal nanoparticles for electrocatalytic purposes. It is worth mentioning that this work will only include contributions in which the surface cleanness of the samples is specifically evaluated using well-established electrochemical tools. This work has been financially supported by the MINECO of Spain through project CTQ2013-48280-C3-3-R. JSG acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante.

Electrochemical reduction of CO2 has been pointed out as an interesting strategy to convert CO2 into useful chemicals. In addition, coupling CO2 electroreduction with renewable energies would allow storing electricity from intermittent renewable sources such as wind or solar power. In this work, an easy and fast method is adapted for the synthesis of pure and carbon supported Sn nanoparticles. The resulting nanoparticles have been characterized by transmission electron microscopy and their electrocatalytic properties towards CO2 reduction evaluated by cyclic voltammetry. Carbon supported Sn nanoparticles have been subsequently used to prepare Gas Diffusion Electrodes (Sn/C-GDEs). The electrodes have been characterized by scanning electron microscopy and also by cyclic voltammetry. Finally, the electrodes were tested on a continuous and single pass CO2 electroreduction filter-press type cell system in aqueous solution, to obtain formate at ambient pressure and temperature. These Sn/C-GDEs allow working at high current densities with low catholyte flow. Thus, for instance, at 150 mA cm−2, a 70% Faradaic Efficiency (FE) was obtained with a formate concentration of 2.5 g L−1. Interestingly, by increasing the current density to 200 mA cm−2 and decreasing the flow rate, a concentration over 16 g L−1 was reached. Despite the high concentrations obtained, further research is still required to keep high FE operating at high current densities. This work was conducted under the framework of the Spanish Ministry of Economy and Competitiveness projects CTQ2013-48280-C3-1-R and CTQ2013-48280-C3-3-R. Andrés Del Castillo also acknowledges the research grant from University of Cantabria, co-financed by the Regional Government of Cantabria.

Palladium-based catalysts for electrochemical reduction of oxygen have received increasing attention as potential replacement for platinum-based materials in the fuel cells. This Review summarises the research conducted with nanostructured palladium catalysts, including thin nanostructured films and Pd nanoparticles on various carbon and non-carbon supports. The mechanism of oxygen reduction on palladium is described and the effect of the particle size and shape on the electrocatalytic activity is emphasised. The role of the support material and additives on the oxygen reduction activity of Pd nanoparticles is also discussed. The electrocatalytic activity of Pd-based catalysts is evaluated in terms of specific activity and mass activity. The application of supported Pd nanoparticles as cathode catalysts for low-temperature fuel cells is highlighted. Some insights into the remaining challenges and directions for further development of Pd-based oxygen reduction electrocatalysts are provided. This work was financially supported by institutional research funding (IUT20-16) of the Estonian Ministry of Education and Research. We would like to acknowledge the financial support by the EU through the European Regional Development Fund (TK141 “Advanced materials and high-technology devices for energy recuperation systems”).

The effect of interfacial pH during the surface cleaning of shape-selected PtNi nanoparticles was investigated. High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) elemental mapping techniques were used to analyze the morphology and composition of the particles at the nanoscale. The particles show similar atomic compositions for both treated samples but different elemental distribution on the surface of the nanooctahedra. X-ray photoelectron spectroscopy (XPS) analysis confirmed different surface compositions and the presence of different oxidation states species at the outer part of the nanoparticles. In addition, we compare characteristic voltammetric profiles of these nanocatalysts when immersed in three different aqueous supporting electrolytes (H2SO4, HClO4 and NaOH). The behavior of the bimetallic nanoparticles towards adsorbed CO oxidation has been analyzed and compared with that observed after surface disordering of the same catalysts. The electrocatalytic activity of these nanoparticles has been also tested for the electroreduction of oxygen showing high specific and mass activity and better catalytic performance than pure Pt shaped nanoparticles. The different treatments applied to the surface of the nanocatalysts have led to remarkably different catalytic responses, pointing out the outstanding importance of the control of the surface of the alloyed shape-selected nanoparticles after their synthesis and before their use as electrocatalysts. This work was carried out under financial support of MICINN (Project no. CTQ2013-44083-P). M.H. and M.G. thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the grant HE7192/1-1. P.S. acknowledges partial financial support by the German Research Foundation (DFG) through Grants STR 596/5-1 and STR 596/4-1.

Formic acid and ethanol oxidations on spherical platinum nanoparticles dispersed on carbon with different loadings have been studied. The increasing loading of the catalyst leads to a lower diffusion flux of reactants in the internal parts of the catalyst layer, resulting in a lower apparent activity. In some cases, as in ethanol oxidation, it may also affect the diffusion of the products. As a practical consequence, the structure of the supporting layer and the catalyst loading should be optimized so that the maximum catalyst utilization is obtained. Finally, these diffusion effects may mask some important catalytic activity increase of the nanoparticles. In the case of formic acid, a significant increase in the activity is obtained for very small nanoparticles. Keywords: Formic acid oxidation, Ethanol oxidation, Platinum nanoparticles, Size effects

The electrochemical reduction of oxygen on cubic Pd nanoparticles (PdNPs) was studied in acid solution using the rotating disk electrode (RDE) method. The electrocatalytic properties of Pd nanocubes were compared with those of spherical PdNPs and bulk Pd. The morphology of PdNPs was examined by transmission electron microscopy (TEM). For electrochemical experiments, PdNPs were deposited on a gold support and oxidation of pre-adsorbed CO on Pd was employed for cleaning the surface of the nanoparticles. The Pd nanocubes displayed superior electrocatalytic activity towards the oxygen reduction reaction (ORR), exhibiting a threefold increase in the specific activity in comparison with spherical PdNPs and bulk palladium. Four-electron reduction of oxygen to water was observed on the Pd catalysts studied. Keywords: Oxygen reduction, Palladium nanocubes, Electrocatalysis

The application of shape-controlled Pt nanoparticles is rapidly increasing mainly because surface reactivity is, for many cases, structure sensitive. Shape control is generally obtained by using capping agents which finally cover the surface. However, for many applications these adsorbed molecules have to be effectively removed. For this purpose UV/ozone has been recently proposed. However, although this methodology was shown to be effective for cleaning, its effects over the catalyst surface structure are still unclear. In this paper the effect of UV/ozone cleaning treatment on uncapped/clean shaped-controlled Pt nanoparticles is evaluated. The results demonstrate that this procedure strongly perturbs the surface structure of the nanoparticles, significantly modifying their catalytic properties, but without altering their size and shape. Keywords: Shape-controlled, Nanoparticles, Platinum, Ozone treatment, Electrocatalysis

Carbon nanofibers synthesized via the thermo catalytic decomposition of methane were investigated for the first time as an electrocatalyst support in PEMFC cathodes. Their textural and physical properties make them a highly efficient catalyst support for cathodic oxygen reduction in low temperature PEMFC. Tests performed in MEAs showed that Pt supported on carbon nanofibers exhibited an enhancement of ca. 94% in power density at 0.600 V, in comparison with a commercial catalyst supported on conventional carbon black, Pt/Vulcan XC-72R. Keywords: Carbon nanofibers, Catalyst support, Platinum, Electrocatalyst, Oxygen reduction reaction, PEMFC

In this paper, formic acid electrooxidation on ethylidyne modified Pt nanoparticles is reported. The formation as well as the stability electrochemical range of the ethylidyne adlayers was studied by surface enhanced Raman spectroscopy (SERS) and cyclic voltammetry. The presence of adsorbed ethylidyne on platinum nanoparticles improved their electrocatalytic activity towards formic acid oxidation, which could be attributed to an instabilization of the carbon monoxide poisonous species as evidenced by SERS. The use of in situ spectroscopic measurements with electrocatalysts similar to those applied in practice is highlighted. Keywords: Pt nanoparticles, SERS, Electrocatalysis, Formic acid electrooxidation

Ethanol oxidation on different shape-controlled platinum nanoparticles at different pHs was studied using electrochemical, Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and, especially, Differential Electrochemical Mass Spectrometry (DEMS) techniques, the latter giving interesting quantitative information about the products of ethanol oxidation. Two Pt nanoparticle samples were used for this purpose: (100) and (111) preferentially oriented Pt nanoparticles. The results are in agreement with previous findings that the preferred decomposition product depends on surface structure, with COads formation on (100) domains and acetaldehyde/acetic acid formation on (111) domains. However, new information has been obtained about the changes in CHx and CO formation at lower potentials when the pH is changed, showing that CHx formation is favored against the decrease in CO adsorption on (100) domains. At higher potentials, complete oxidation to CO2 occurs from both CHx and CO fragments. In (111) Pt nanoparticles, the splitting of Csingle bondC bond is hindered, favoring acetaldehyde and acetate formation even in 0.5 M H2SO4. C1 fragments become even less when the pH increases, being nearly negligible in the highest pH studied. This work has been financially supported by the MCINN-FEDER (Spain) and Generalitat Valenciana through projects CTQ 2013-44083-P and PROMETEO/2014/013, respectively.

The development of electrochemical processes for the conversion of CO2 into value-added products allows innovative carbon capture & utilization (CCU) instead of carbon capture & storage (CCS). In addition, coupling this conversion with renewable energy sources would make it possible to chemically store electricity from these intermittent renewable sources. The electroreduction of CO2 to formate in aqueous solution has been performed using Sn particles deposited over a carbon support. The effect of the particle size and Sn metal loading has been evaluated using cyclic voltammetry and chronoamperometry. The selected electrode has been tested on an experimental filter-press type cell system for continuous and single pass CO2 electroreduction to obtain formate as main product at ambient pressure and temperature. Experimental results show that using electrodes with 0.75 mg Sn cm−2, 150 nm Sn particles, and working at a current density of 90 mA cm−2, it is possible to achieve rates of formate production over 3.2 mmol m−2 s−1 and faradaic efficiencies around 70% for 90 min of continuous operation. These experimental conditions allow formate concentrations of about 1.5 g L−1 to be obtained on a continuous mode and with a single pass of catholyte through the cell. This work was conducted under the framework of the Spanish Ministry of Science and Innovation Projects ENE2010-14828 and Spanish Ministry of Economy Competitiveness CTQ2013-48280-C3-1-R and CTQ2013-48280-C3-3-R. Andrés Del Castillo also acknowledges the research grant from University of Cantabria, co-financed by the Regional Government of Cantabria.

The influence of the electrolyte pH in the formic acid oxidation reaction on a polyoriented Pt electrode in the presence of different anions, including sulfate, perchlorate, phosphate and chloride, in different concentrations has been investigated, using cyclic voltammetric measurements. The curves of the peak currents in the negative scan direction vs. the pH in pure sulfate and perchlorate solutions are very similar. For these solutions, the maximum oxidation currents increase steadily with increasing pH up to pH ≈ 5, followed by a plateau until pH 10. This suggests that the reaction proceeds via a similar reaction mechanism, in which the concentration of HCOO− anions in solution plays a key role. For phosphate or chloride containing solutions, in contrast, the maximum oxidation currents show a bell-shaped pH-oxidation current correlation, whose exact shape depends on the anion concentration. We suggest that in these cases the pH-current relation is modified by competing specific adsorption of anions which act as site blocking spectator species. These results will be discussed in relation with compatible mechanistic proposals. This work has been financially supported by the MICINN (Spain) (projects CTQ2013-44083-P and CTQ2013-48280-C3-3-R) and Generalitat Valenciana (project PROMETEOII/2014/013, FEDER), as well as by the Deutsche Forschungsgemeinschaft via the project BE 1201 / 17-1.

Platinum-based catalysts are the best electrocatalysts for oxygen reduction reaction (ORR) in fuel cells.1 However, because of the high price and limited supply of Pt, numerous researches have focused on finding a way to reduce its amount in the catalysts. One way to decrease Pt loading is to replace it partially by other metals. Pd is a promising substitute due to the similar properties to the Pt (same group of the periodic table, similar atomic size and crystalline structure).2 In this work, PdPt alloy nanocubes with different metal ratios were synthesized in the presence of polyvinylpyrrolidone.3 The surface morphology of the PdPt samples was characterized by transmission electron microscopy (TEM). TEM images showed that PdPt nanoparticles were cubic-shaped and the average size of the cubes was about 8–10 nm. TEM image for sample Pd54Pt46 is shown in Fig. 1a. Electrochemical measurements were performed to explore the electrocatalytic properties of the PdPt alloy nanocubes toward the ORR in 0.5 M H2SO4 using the rotating disk electrode (RDE) method. CO stripping and cyclic voltammetry (CV) were used for cleaning and characterizing the surface of catalysts. The CV responses showed improvement in surface cleanness after CO oxidation compared with those CVs initially measured. Comparison of polarization curves received from RDE measurements at 1900 rpm are presented in Fig. 1b. The half-wave potentials (E 1/2) for O2 reduction on PdPt alloys were found to be higher than that of Pd nanocubes and were comparable with that of Pt nanocubes. The onset potential for Pd36Pt64 was the highest. The RDE data were analyzed using the Koutecky-Levich (K-L) equation and from the slopes the number of transferred electrons (n) was found. The value of n was close to four for all the catalysts studied. For better comparison of the kinetic parameters of all the electrodes the Tafel plots were constructed from the RDE data (Fig. 1c). Two regions with distinct slope values were observed. In the low overpotential region the slope value was close to -60 mV dec‒ 1 and at high overpotentials the Tafel slope value was over -130 mV dec‒ 1. All the alloyed catalysts showed enhanced electrocatalytic activity for ORR as compared to the monometallic cubic Pd nanoparticles. From the alloyed catalysts Pd36Pt64 exhibited the highest specific activity, which was only slightly lower than that of cubic Pt nanoparticles. Work is in progress to study the electroreduction of oxygen on PdPt alloy nanocubes in 0.1 M KOH solution using the RDE method. Single-waved polarization curves with well-defined current plateaus were observed. The E 1/2 values for PdPt alloys in alkaline solution were comparable. The K-L analysis showed that O2 reduction on these alloys proceeds via 4-electron pathway in 0.1 M KOH. The Tafel analysis revealed that the mechanism of O2 reduction on PdPt alloy nanocubes in alkaline media is similar to that in acid media. The PdPt alloy nanocubes exhibit good electrocatalytic activity for the electroreduction of oxygen in both electrolytes. This indicates a great promise of these materials as cathode catalysts for low-temperature fuel cells. References 1. H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, Appl. Catal. B: Environ., 56, 9 (2005). 2. E. Antolini, Energy Environ. Sci., 2, 915 (2009). 3. K. Jukk, N. Kongi, K. Tammeveski, J. Solla-Gullón, and J. M. Feliu, Electrochem. Commun., 56, 11 (2015). Figure 1

CO monolayer oxidation on platinum nanoparticles with different shapes (semi-spherical, cubic and tetrahedral-hexagonal nanoparticles) has been studied. We found a very clear dependence between bidimensional (111) and (100) ordered domains present on the surface of the nanoparticles and their CO electrocatalytic activity. The results reported in this work demonstrate the importance of controlling the intrinsic structural properties of the Pt nanoparticles, in terms of number and nature of the surface active sites, to understand their electrocatalytic properties. Thus, adsorbed CO oxidation has been revealed as an extremely structure sensitive reaction useful to establish correlations with the surface properties of platinum nanoparticles. Keywords: Carbon monoxide oxidation, Nanoparticles, Platinum, Surface structure

Evidence of the participation of adsorbed azide anion (N3-) in the electrooxidation of ammonia in alkaline medium on nanostructured Pt electrodes has been found by using surface-enhanced Raman spectroscopy. This finding is discussed in terms of the Gerischer–Mauerer’s mechanism for ammonia oxidation. The formation of azide anion would take place through the recombination of NH3 and N2H4,ad (and maybe other N2Hx (x = 2–4) species). The results support the Gerischer–Mauerer’s mechanism in which partially dehydrogenated ammonia adsorbates are believed to play a role as active intermediates in the selective oxidation of ammonia. Keywords: Ammonia oxidation, Azide, SERS, Nanoparticles, Platinum

A shape-dependent electrocatalysis for the ammonia oxidation reaction is reported. Pt nanoparticles with different (100) preferentially oriented surface structures were synthesised. Cyclic voltammetry of the Pt nanoparticles is presented and the “so-called” hydrogen adsorption/desorption process is used as structure sensitive reaction to obtain qualitative information about the surface structure of the nanoparticles. While Pt nanoparticles prepared in microemulsion show a voltammetric profile very similar to that characteristic of a polyoriented surface, colloidal Pt nanoparticles appear to be (100) preferentially oriented. The electrochemical activity of these nanoparticles for ammonia oxidation in basic medium was studied. The results show that the anodic current density is very sensitive to the existence of Pt(100) domains. Thus oxidation current densities seven times higher can be obtained among the different nanoparticles, showing the importance of controlling the surface structure of the nanoparticles for the development of an active electrocatalyst for ammonia oxidation. Keywords: Nanoparticles, Ammonia oxidation, Shape-dependent, Electrocatalysis

In this study, we examine the performance of Cu2O and Cu2O/ZnO surfaces in a filter-press electrochemical cell for the continuous electroreduction of CO2 into methanol. The electrodes are prepared by airbrushing the metal particles onto a porous carbon paper and then are electrochemically characterized by cyclic voltammetry analyses. Particular emphasis is placed on evaluating and comparing the methanol production and Faradaic efficiencies at different loadings of Cu2O particles (0.5, 1 and 1.8 mg cm−2), Cu2O/ZnO weight ratios (1:0.5, 1:1 and 1:2) and electrolyte flow rates (1, 2 and 3 ml min−1 cm−2). The electrodes including ZnO in their catalytic surface were stable after 5 h, in contrast with Cu2O-deposited carbon papers that present strong deactivation with time. The maximum methanol formation rate and Faradaic efficiency for Cu2O/ZnO (1:1)-based electrodes, at an applied potential of −1.3 V vs. Ag/AgCl, were r = 3.17 × 10−5 mol m−2 s−1 and FE = 17.7 %, respectively. Consequently, the use of Cu2O–ZnO mixtures may be of application for the continuous electrochemical formation of methanol, although further research is still required in order to develop highly active, selective and stable catalysts the electroreduction of CO2 to methanol. The authors gratefully acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO), under the projects CTQ2013-48280-C3-1-R, CTQ2013-48280-C3-3-R and Juan de la Cierva program (JCI-2012-12073).

The adsorption of 4-aminobenzenethiol (4-ABT) on Ag and Pt nanoparticles is studied by spectroelectrochemical means (cyclic voltammetry, Surface-Enhanced Raman Spectroscopy (SERS) and Surface-Enhanced Infrared Reflection Absorption Spectroscopy (SEIRAS). Similar SERS spectra are obtained when 4-ABT is adsorbed on platinum and silver nanostructured substrates. In addition, unless a low power density of the laser is used, these spectra show, both under open circuit conditions and when applying electrode potentials above -0.50 V, bands not observed in the normal Raman spectrum of 4-ABT. These bands disappear when the electrode potential is shifted to more negative values. Conversely, the SEIRA spectra of 4-ABT adsorbed on Ag do not show any significant change with the electrode potential, which indicates that there are not new species electrochemically formed in the range of potentials considered, which include some anodic and cathodic processes as shown in the corresponding cyclic voltammograms. In this regard, SERS measurements put in evidence the time dependence of the spectra obtained at potentials above -0.50 V just after switching on the laser suggesting the formation of new species, probably dimercaptoazobenzene (4-4’-DMAB), formed by a photochemical process of the adsorbed 4-ABT. Financial support from Ministerio de Ciencia e Innovación (projects CTQ2010-16271, CTQ2009-13142, and Fondos Feder), Generalitat Valenciana (Prometeo/2009/045 and ACOMP/2011/200), and University of Alicante is greatly acknowledged.

Silver–platinum core–shell (Ag@Pt) nanoparticles have been synthesized using various methods. In the case of Ag@Pt nanoparticles synthesized by the standard method based on the galvanic replacement reaction between Ag seeds and PtCl42−, transmission electron microscopy micrographs revealed well visible core–shell structure. However, electrochemical experiments showed that relatively large amount of silver can be easily stripped off from such nanoparticles. Significant improvement on the integrity of the deposited platinum shell can be achieved when nanoparticles are synthesized by the seeded growth reaction including reduction of PtCl42− with ascorbic acid at room temperature. To obtain pinhole-free platinum layers (where Ag oxidation is not observed) relatively large amount of platinum must be deposited. For example, to cover 11 nm Ag seeds, the number of moles of platinum in the formed Ag@Pt nanoparticles must be at least equal to the number of moles of Ag. It was also found that a similar seeded growth reaction may be used to form pinhole-free Au@Pt nanoparticles. The electrochemical behaviour of those two systems (Ag@Pt and Au@Pt nanoparticles) towards CO stripping was rather different. While the CO-stripping on Au@Pt occurred at typical potentials and without a significant reconstruction of the original surface, CO stripping voltammograms on Ag@Pt were very unusual and exhibited both exceptionally strong binding of CO to the surface, and such a reconstruction of the surface that silver atoms were no longer compactly covered by platinum. This project was financed from the funds of the National Science Centre (Poland) allocated on the basis of the decision number DEC-2011/01/B/ST4/00581 and co-financed by the European Regional Development Fund within the Innovative Economy Operational Programme 2007–2013 No POIG.02.01-00-14-032/08. S.W. is thankful for the support from the Foundation for Polish Science MPD Programme co-financed by the EU European Regional Development Fund.

In this work, the behavior of the CO electro-oxidation reaction on shape-controlled Pt nanoparticles in alkaline medium was examined in order to understand the effect of the surface structure on this reaction. A series of experiments using Pt nanoparticles of different surface structures/shapes was used and the results obtained were compared with the previous knowledge gained from stepped platinum single crystal electrodes. Independently of the preferential orientation of the nanoparticles, the CO oxidation voltammetry exhibits two main peaks: one at ca. 0.56–0.59 V and the second one at 0.66–0.67 V, being the intensity of the peaks dependent on the shape of the nanoparticle. These two peaks have been assigned to the oxidation of CO on the (1 1 1) terraces and on the rest of the sites, respectively. The appearance of two differentiated peaks reveals that these (1 1 1) terraces and the rest of the sites on the nanoparticle surface behave independently of the presence of the other type of sites, that is, they are not connected. The results are discussed considering the effects of the surface mobility of CO and of the OH adsorption properties on the different sites in the oxidation peaks. Farias, M.J.S. would like to thanks CNPq, Brazil, for financial support for his postdoctoral stay at Universidad de Alicante. This work has been financially supported by the MICINN (Spain) (project CTQ2010-16271) and Generalitat Valenciana (project PROMETEO/2009/045, FEDER).

Carbon nanotubes have been proposed as advanced metal catalyst support for electrocatalysis. In this paper, Pt nanoparticles supported on single-walled carbon nanotubes (SWCNTs)-Pt, were prepared using a solid-state reaction between the SWCNTs and two different Pt precursors, bis(dibenzylideneacetone)platinum [Pt(DBA)2] or tri(dibenzylideneacetone)platinum [Pt(DBA)3]. TEM images of the samples show Pt nanoparticles with a particle size around 2.5 nm with a high degree of dispersion on the SWCNTs. A detailed electrochemical characterization of the surface of the samples including irreversibly adsorbed adatoms of Bi and Ge as probe reactions has been carried out. It has been stated that SWCNTs-Pt samples subjected to the classical electrochemical activation induce a serious sintering of the Pt nanoparticles., This work has been performed in the framework of projects BQU2003-03877 and BQU2003-4029 from MCyT (Spain), project MAT2002-04630-C02-01 from MEC (Spain) and projects GRUPOS03/126 and GRUPOS03/208 from GV (Spain).