• Energy Research

AbstractDye-sensitized solar cells (DSSCs) rely heavily on the counter electrode for their performance, which is responsible for collecting and transferring electrons generated at the photoanode. While platinum (Pt) has traditionally been used as a counter-electrode material, its cost, limited availability, and environmental concerns make it an unsuitable option for large-scale implementation. Iron–nitrogen––carbon (Fe–N–C) catalysts are receiving increasing attention due to their high catalytic activity and low cost. This study aims to investigate the performance of Fe–N–C materials as counter electrodes in DSSCs and assess their potential as a sustainable alternative to currently used platinum. Two different Fe–N–C-based materials have been synthesized using different carbon and nitrogen sources, and their electrochemical behavior has been assessed using current–voltage curves and impedance spectroscopy. The catalyst comprised a higher amount of iron and nitrogen shows higher efficiency and lower charge-transfer resistance due to improved iodide reaction kinetics and proper stability under potential cycling. However, this catalyst shows lower stability under a passive ageing procedure, which requires further clarification. Results provide new insights into the performance of Fe–N–C-based materials in DSSCs and aid in the further development of this promising technology.

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

Carbon xerogels represent nowadays an outstanding alternative to carbon blacks for the preparation of efficient fuel cell electrocatalysts, due to their easily tunable and well developed mesoporous structure. To further improve both activity and durability of Pt/C catalysts, the introduction of heteroatoms (such as O, N, S, P, B, etc.) in the structure of carbon materials has been proposed. In the present work, highly mesoporous carbon xerogels (CXGs) have been subjected to a sulfurization process with elemental sulfur. The insertion of S into the carbon matrix does not compromise their mesoporous structure. Pt catalysts supported on sulfurized carbon xerogels show enhanced catalytic activity towards both the methanol electro-oxidation and the oxygen electro-reduction reactions, exceeding not only the performance of the catalyst supported on the bare xerogel, but also of the catalyst supported on a commercial carbon black. The sulfurization treatment is also effective in improving the resistance of Pt/CXG catalysts towards corrosion phenomena occurring at the fuel cell cathode. The authors wish to thank the Spanish Ministry of Economy and Competitiveness (Secretaría de Estado de I+D+I) and FEDER for financial support under the project CTQ2011-28913-C02-01. Peer reviewed

9 páginas.- 1 tabla.- 18 figuras. Several graphite-based electrodes are investigated for vanadium flow battery applications. These materials are characterized both as-received and after chemical or electrochemical treatments in order to vary the content of oxygen functional groups on the electrode surface. The surface properties of the samples are investigated by X-ray photoelectron spectroscopy. Electrochemical performance is evaluated by cyclic voltammetry and electrochemical impedance spectroscopy measurements in a three electrode half-cell. The chemical treatment with HNO3 causes a cleaning of the electrode surface from adsorbed oxygen species or labile bonded functional groups in highly graphitic samples. Whereas, carbonaceous materials characterized by smaller graphitic domains or a higher degree of amorphous carbon show an increase of oxygen functional groups upon chemical and electrochemical pre-treatments. In both cases, an increase of oxygen species content on the surface above 5% causes a decrease of electrochemical performance for the redox battery determined by an increase of ohmic and charge transfer resistance Authors from CNR-ITAE acknowledge the financial support from “Ministero dello Sviluppo Economico – Accordo di Programma MSE-CNR per la Ricerca del Sistema elettrico Nazionale”. Peer reviewed

The low oxidation kinetics of alcohols and the need for expensive platinum group metals are still some of the main drawbacks for the commercialization of energy efficient direct alcohol fuel cells. In this work, we investigate the influence of nitrogen doping of ordered mesoporous carbon (CMK) as support on the electrochemical activity of PtRu nanoparticles. Nitrogen doping procedures involve the utilization of pyrrole as both nitrogen and carbon precursor by means of a templating method using mesoporous silica. This method allows obtaining carbon supports with up to 14 wt. % nitrogen, with an effective introduction of pyridinic, pyrrolic and quaternary nitrogen. PtRu nanoparticles were deposited by sodium formate reduction method. The presence of nitrogen mainly influences the Pt:Ru atomic ratio at the near surface, passing from 50:50 on the bare (un-doped) CMK to 70:30 for the N-doped CMK catalyst. The electroactivity towards the methanol oxidation reaction (MOR) was evaluated in acid and alkaline electrolytes. The presence of nitrogen in the support favors a faster oxidation of methanol due to the enrichment of Pt at the near surface together with an increase of the intrinsic activity of PtRu nanoparticles.

In recent decades, significant efforts have been focused on the direct electrochemical oxidation of alcohol and hydrocarbon fuels. Organic liquid fuels are characterized by high energy density and the electromotive force associated with their electrochemical combustion to CO2 is comparable to that of hydrogen combustion to water. Among the liquid organic fuels, methanol and ethanol have promising characteristics in terms of reactivity at low temperatures, storage and handling. Compared with ethanol, methanol has the significant advantage of faster reaction kinetics and higher selectivity to CO2 formation for the electrochemical oxidation process. Highly dispersed carbon-supported bimetallic PtRu and PtSn alloy catalysts are widely recognized among the most performing anode formulations for these processes. Alloying Pt with Ru and Sn promotes oxidation of methanol and ethanol by the adsorption of OH species at considerably lower overpotentials and, thus, favoring the occurrence of a bifunctional mechanism. Furthermore, the electronic effect caused by a second metal on the neighboring Pt atoms affects the strength of CO adsorption on the catalyst surface. This causes a decrease of the coverage of poisoning CO intermediate species. Catalysts characterized by a high degree of alloying and metallic behavior on the surface appear to be very active towards methanol oxidation. However, beside the alloyed catalysts, noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx, TiOx and VOx) can be suitable promoters for methanol and ethanol oxidation in acidic environment. An effective use of such oxide promoters in combination with the alloy catalysts can provide a multifunctional catalytic system. Recent studies carried out in our laboratory have shown that IrOx can give rise to a significant promoting effect, even larger than that of RuOx, both in the case of methanol and ethanol oxidation. Whereas, the electrocatalytic enhancement produced by the valve metal oxides is generally lower than IrOx and RuOx but well evident. These results are interpreted in terms of the different water displacement mechanisms for the various oxides and the related effects on adsorbed CO removal. The effect of temperature is also discussed with reference to the coverage of adsorbed methanolic residues or change in selectivity in the case of ethanol electro-oxidation. For the ethanol oxidation process, anode catalyst selectivity towards CO2, acetic acid and acetaldehyde reaction products is discussed in relation to the alloy and oxide content in the catalyst. In particular, SnOx species on the surface of Sn-rich Pt-Sn-based electrocatalyst appear to assist the further oxidation of ethanolic adsorbates, leading to larger yields of acetic acid and CO2. In addition to the enhancement of reaction rates, there is an effect of the promoter on the stability of the bimetallic alloy as evidenced by accelerated stress tests. All these evidences seem to indicate that a multifunctional catalyst may represent a valid route to enhance performance and reliability of methanol and ethanol electro-oxidation processes at low temperature.

Developing efficient, durable, and low cost catalysts based on earth-abundant elements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. We report herein a simple one-pot procedure for the synthesis of non-precious metals N-doped graphene composites employing urea as nitrogen source, and their application as bifunctional catalysts for both the ORR and OER in alkaline environment. In this study, the effects of the addition of Ti and Co on the structure and performance of the N-doped graphene composites are investigated. The incorporation of Ti leads to a composite with both anatase and rutile TiO2 crystalline phases as well as Ti3+ species stabilized upon hybridization with N-doped reduced graphene oxide. The ORR onset potential for the Ti-based composite is 0.85 V (vs. RHE) and the number of electrons transferred is 3.5, showing superior stability than Pt/C after accelerated potential cycling in alkaline solution. However, this composite shows low activity and stability for the OER. On the other hand, the composite consisting of metallic Co and Co3O4 nanocrystals grown on N-doped reduced graphene oxide exhibits the best performance as bifunctional catalyst, with ORR and OER onset potentials of 0.95 V and 1.51 V (vs. RHE), respectively, and a number of electrons transferred of 3.6 (ORR). The results reveal the presence of important structural features such as metallic Co as the predominant crystalline component, amorphous Co3O4 phase and the coordination of Co-N-doped graphene. All of them seem to be fundamental for the high activity and stability towards ORR and OER. Authors acknowledge financial support given by Spanish Ministry of Economy and Competitiveness (MINECO) through projects ENE2014-52158-C2-1-R and 2-R (co-founded by FEDER). J. M. Luque and G. Lemes also thank MINECO and Aragon Government, respectively, for their Ph.D. grants. Peer reviewed