Chromium doped copper vanadate photoanodes for water splitting

Solar hydrogen obtained from photoelectrochemical water splitting offers a versatile approach towards the substitution of fossil fuels by decentralized and sustainable resources, like water and sun. In the present study we have investigated the Chromium doped Copper Vanadate (Cr:Cu3V2O8) as a candidate photoanode for photoelectrochemical water splitting. We have synthetized this material through a simple aqueous precipitation reaction, which easily allows compositional modifications. We have studied the effect of extrinsic doping with substitutional atoms like Chromium on the optical and photoelectrochemical properties. The main limiting factor for performance is related to the high bulk recombination, which is partially overcome by 0.75 at.% Chromium doping, with a five-fold enhancement of the charge separation efficiency at 1.23 V vs RHE. Despite this remarkable milestone, significant further improvement is needed for the technological exploitation of this material

Stability of 3D-porous Ni/Cu cathodes under real alkaline electrolyzer operating conditions and its effect on catalytic activity

Despite the development and synthesis of new electrode materials for hydrogen generation in alkaline water electrolyzers has been a research topic widely exploited in the last years, stability tests on the obtained cathodes have been restricted to long-term potentiostatic/galvanostatic experiments which do not fulfil the real operating conditions that take place in those devices. In this work, two different Service Life Tests have been designed and implemented, aiming at including particular conditions (i.e. inverse polarity and short-circuit) in the durability and catalytic activity of cathode characterization. For this purpose, Ni/Cu bilayered porous electrodes were prepared using different Ni electrodeposition times (15, 30 and 45 min) following a double template electrochemical method. It has been confirmed that the electrode with the lowest Ni content can be considered as a promising electrocatalyst for hydrogen production under industrial conditions because of its optimal activity and stability after the two sets of testing conditions. In particular, electrochemical studies demonstrated that an inversion in polarity can positively affect the electrode performance, as a consequence of the synergetic interaction between CuO/Cu(OH)(2) and beta-Ni(OH)(2) species formed at potentials below the oxygen evolution domain.The authors gratefully acknowledge financial support given by the Generalitat Valenciana (PROMETEO/2010/023) and Spanish Government (Ministerio de Ciencia e Innovation) for the postgraduate grants AP2007-01243 (Carlos Valero-Vidal) and AP2007-03737 (Isaac Herraiz-Cardona).Valero Vidal, C.; Herraiz Cardona, I.; Pérez-Herranz, V.; Igual Muñoz, AN. (2016). Stability of 3D-porous Ni/Cu cathodes under real alkaline electrolyzer operating conditions and its effect on catalytic activity. Applied Catalysis B: Environmental. 198:142-153. doi:10.1016/j.apcatb.2016.05.030S14215319

Electrochemical characterization of a NiCo/Zn cathode for hydrogen generation

[EN] Hydrogen is considered to be the most promising candidate as a future energy carrier. One of the most used technologies for the electrolytic hydrogen production is alkaline water electrolysis. However, due to the high energy requirements, the cost of hydrogen produced in such a way is high. In continuous search to improve this process using advanced electrocatalytic materials for the hydrogen evolution reaction (HER), high area NiCo/Zn electrodes were prepared on AISI 304 stainless steel substrates by electrodeposition. After preparing, the alloys were leached of to remove part of the zinc and generate a porous layer (type Raney electrodes). The presence of a thin Ni layer between the substrate and the Raney coating favour the adherence of the latter. The porous NiCo/Zn electrode was characterized by SEM, EDX, confocal laser microscopy, and electrochemical impedance spectroscopy. HER on this electrode was evaluated in 30 wt.% KOH solution by means of polarization curves, hydrogen discharge curves, and galvanostatic tests. Results show that the developed electrode presents a most efficient behaviour for HER when comparing with the smooth Ni cathode. The high electrode activity was mainly attributed to the high surface area of the developed electrode. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.Isaac Herraiz-Cardona is grateful to the Ministerio de Ciencia e Innovacion (Spain) for a postgraduate grant (Ref. AP2007-03737). This work was supported by Generalitat Valenciana (Project PROMETEO/2010/023).Herraiz-Cardona, I.; Ortega Navarro, EM.; Vázquez-Gómez, L.; Pérez-Herranz, V. (2011). Electrochemical characterization of a NiCo/Zn cathode for hydrogen generation. International Journal of Hydrogen Energy. 36(18):11578-11587. https://doi.org/10.1016/j.ijhydene.2011.06.067S1157811587361

Double-template fabrication of three-dimensional porous nickel electrodes for hydrogen evolution reaction

[EN] Three-dimensional (3D) porous nickel structures were fabricated via a double-template electrochemical deposition process. The construction of the foam structures was achieved by means of a hydrogen bubble dynamic template, prepared from Cu electrodeposition at high current densities. Subsequently, a Ni layer was electrodeposited on the Cu 3D template. During the nickel coating, the typical finger-like microstructure of the Cu foam becomes denser and changes to a cauliflower microstructure. The hydrogen evolution reaction (HER) on these macroporous Ni electrodes was evaluated in 30 wt.% KOH solution by means of polarization curves and electrochemical impedance spectroscopy (EIS). Results demonstrate greater apparent activity of the developed electrodes towards HER in comparison with commercial smooth Ni electrode. The 3D porous Ni electrocatalyst obtained from Cu templates synthesized at the lowest current density and the highest electrodeposition time yielded the best electrochemical activity for HER. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.Isaac Herraiz-Cardona is grateful to the Ministerio de Educacion (Spain) for a postgraduate grant (Ref. AP2007-03737). This work was supported by Generalitat Valenciana (PROMETEO/2010/023) and Universidad Politecnica de Valencia (PAID-06-10-2227).Herraiz-Cardona, I.; Ortega Navarro, EM.; Vázquez-Gómez, L.; Pérez-Herranz, V. (2012). Double-template fabrication of three-dimensional porous nickel electrodes for hydrogen evolution reaction. International Journal of Hydrogen Energy. 37(3):2147-2156. https://doi.org/10.1016/j.ijhydene.2011.09.155S2147215637

The HER in alkaline media on Pt-modified three-dimensional Ni cathodes

[EN] Electrodeposited porous Ni layers and commercial Ni foams were submitted to spontaneous deposition of Pt, achieved by immersing the Ni substrates in H2PtCl6 solutions, at open circuit, to produce Pt-modified 3D Ni electrodes. When using Ni foams, the immersion was prolonged until the whole amount of H2PtCl6 in the solution had reacted. Such an approach, which granted an easy control of the Pt loading, could not be used for Ni trodeposits, since they underwent significant corrosion. The true Pt surface area was determined by measuring, for each electrode, the hydrogen desorption charge according to methods described in the literature. The ratios between Pt surface area and Pt loading were higher for Ni foam electrodes than for porous Ni electrodeposits. Both kinds of Pt-modified Ni electrodes were used as cathodes for hydrogen evolution in 1 M KOH. Cathodes with Pt loading below 0.5 mg cm(-2) (referred to geometric surface area) evolved hydrogen at -100 mA cm(-2) with a -75 mV overpotential. The better activity of foam electrodes as compared to electrodeposits, especially at low Pt loading, was mainly due to their higher Pt surface area per unit Pt mass. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.The authors from IENI-CNR acknowledge the financial support of the Italian Ministry for Economic Development (MSE) - MSE-CNR Agreement on National Electrical System. I. Herraiz-Cardona is grateful to the Ministerio de Educacion of Spain for a post-graduate grant (Ref. AP2007-03737). The authors are indebted to Dr. Arianna Gambirasi, ICIS CNR, Padova, Italy for recording SEM images and to FILA INDUSTRIA CHIMICA SPA, San Martino di Lupari, Padova, Italy, owner of the Fei-ESem FEI Quanta 200 FEG instrument, for allowing its use for the research work described in this article.Fiameni, S.; Herraiz Cardona, I.; Musiani, M.; Pérez-Herranz, V.; Vázquez-Gómez, L.; Verlato, E. (2012). The HER in alkaline media on Pt-modified three-dimensional Ni cathodes. International Journal of Hydrogen Energy. 37(14):10507-10516. https://doi.org/10.1016/j.ijhydene.2012.04.100S1050710516371

Co-modification of Ni-based type Raney electrodeposits for hydrogen evolution reaction in alkaline media

[EN] In this work, high performance Ni and Ni-Co porous electrodes are prepared using the Raney strategy by galvanic co-deposition for hydrogen evolution reaction (HER) in alkaline solution (KOH 30 wt.%). The incorporation of Co into the Raney Ni matrix causes a surface morphology modification, from cracked to "cauliflower-like", which dominates the superficial structure of the Co-richest obtained material. The evaluation of these electrodes as H-2-evolving cathodes is done through pseudo-steady-state polarization curves and electrochemical impedance spectroscopy (EIS). Ni Raney electrode (without Co) manifests the highest apparent catalytic activity per unit of geometric surface area, which is attributed to the higher surface roughness factor, determined by EIS. HER on the investigated electrocatalysts proceeds via the Volmer-Heyrovsky mechanism, with Heyrovsky as the rate-determining step (rds). From the kinetic parameters it is derived that Co presence, in a composition range of 5-22 at.%, increases the intrinsic catalytic activity of the developed cathodes per unit of true surface area, as a consequence of the synergism between the properties of Ni and of Co. Nevertheless, this improvement does not compensate the lower surface roughness factor, originated by the surface morphology modification as the Co content increases, reporting lower apparent catalytic activities. (C) 2013 Elsevier B.V. All rights reserved.Isaac Herraiz-Cardona is grateful to Fundacion Iberdrola for the economical support by means of the Project: "Desarrollo y caracterizacion de materiales electrodicos porosos estables para la produccion de hidrogeno a partir de la electrolisis alcalina del agua" - II Convocatoria de Ayudas a la Investigacion en Energia y Medio Ambiente. The authors would like to thank the financial support from Generalitat Valenciana (PROMETEO/2010/023) and Universidad Politecnica de Valencia (PAID-06-10-2227).Herraiz Cardona, I.; González Buch, C.; Valero Vidal, C.; Ortega Navarro, EM.; Pérez-Herranz, V. (2013). Co-modification of Ni-based type Raney electrodeposits for hydrogen evolution reaction in alkaline media. Journal of Power Sources. 240:698-704. https://doi.org/10.1016/j.jpowsour.2013.05.041S69870424

Cobalt Hexacyanoferrate on BiVO4 Photoanodes for Robust Water Splitting

The efficient integration of photoactive and catalytic materials is key to promoting photoelectrochemical water splitting as a sustainable energy technology built on solar power. Here, we report highly stable water splitting photoanodes from BiVO4 photoactive cores decorated with CoFe Prussian blue-type electrocatalysts (CoFe-PB). This combination decreases the onset potential of BiVO4 by,similar to 0.8 V (down to 0.3 V vs reversible hydrogen electrode (RHE)) and increases the photovoltage by 0.45 V. The presence of the catalyst also leads to a remarkable 6-fold enhancement of the photocurrent at 1.23 V versus RHE, while keeping the light-harvesting ability of BiVO4. Structural and mechanistic studies indicate that CoFe-PB effectively acts as a true catalyst on BiVO4. This mechanism, stemming from the adequate alignment of the energy levels, as showed by density functional theory calculations, allows CoFe-PB to outperform all previous catalyst/BiVO4 junctions and, in addition, leads to noteworthy long-term stability. A bare 10-15% decrease in photocurrent was observed after more than 50 h of operation under light irradiation

Synthesis and characterization of Au-modified macroporous Ni electrocatalysts for alkaline water electrolysis

Au nanoparticles (Au-NPs) were successfully synthesized and incorporated into the surface of a macroporous Ni electrode fabricated via galvanostatic electrodeposition at high current densities in order to produce hydrogen by means of alkaline water electrolysis. The developed electrodes were morphologically characterized by means of confocal laser scanning and field emission scanning electron microscopes. The electrocatalytic behaviour towards the hydrogen evolution reaction was studied by Tafel polarization curves and electrochemical impedance spectroscopy. It was clear that enlarging the real surface area of an electrode its catalytic activity was greatly enhanced. This improvement was further increased when Au-NPs were added to the macroporous Ni surface. In this case, the improvement was not only caused by enlarging the surface area but also by an improvement in the intrinsic catalytic activity of the alloy, as it was shown by the exchange current densities values, calculated from the real surface area.The authors acknowledge the support of Generalitat Valenciana (PROMETEO/2010/023) and Universidad Politecnica de Valencia (PAID-06-10-2227).González Buch, C.; Herraiz Cardona, I.; Ortega Navarro, EM.; Mestre, S.; Pérez Herranz, V. (2016). Synthesis and characterization of Au-modified macroporous Ni electrocatalysts for alkaline water electrolysis. International Journal of Hydrogen Energy. 41(2):764-772. https://doi.org/10.1016/j.ijhydene.2015.10.142S76477241

Energy Efficiency Improvement of Alkaline Water Electrolysis by using 3D Ni Cathodes Fabricated via a Double-Template Electrochemical Process

[EN] Alkaline water electrolysis is one of the easiest methods for hydrogen production, offering the advantage of simplicity. Moreover, it represents an environmentally friendly technology for production of high purity hydrogen. Nevertheless, the elevated production costs due to low conversion efficiency and electrical power expenses can be named as the main drawbacks of electrochemical hydrogen production. This work is focused on the development and characterization of 3D porous Ni cathodes for alkaline electrolyzers. The electrodes were synthesized by nickel electrodeposition on copper foams obtained from hydrogen bubbles dynamic templates (double-template electrochemical process). The developed electrodes were characterized by SEM, confocal laser scanning microscopy, and EDX. The electrocatalytic performance of the developed electrodes for hydrogen evolution reaction (HER) was evaluated in 30 wt.% KOH solution by using hydrogen discharge curves and galvanostatic tests. Results show that the use of the developed electrodes as cathodes in electrolysis systems makes possible an energy saving of ca. 25% in conditions at which industrial alkaline water electrolysis is carried out, in comparison with the smooth commercial Ni electrodes.I. Herraiz-Cardona is grateful to Fundación Iberdrola for the financial support. This work was supported by Generalitat Valenciana (Project PROMETEO/2010/023) and Universitat Politècnica de València (PAID-06-10-2227).Herraiz Cardona, I.; González Buch, C.; Ortega Navarro, EM.; Garcia-Anton, J.; Pérez-Herranz, V. (2013). Energy Efficiency Improvement of Alkaline Water Electrolysis by using 3D Ni Cathodes Fabricated via a Double-Template Electrochemical Process. Chemical Engineering Transactions. 32:451-456. https://doi.org/10.3303/CET1332076S4514563

Development of Ni-Mo, Ni-W and Ni-Co Macroporous Materials for Hydrogen Evolution Reaction

[EN] The hydrogen evolution reaction (HER) is the cathodic process in many important electrochemical technologies. According to the Brewer intermetallyc bonding theory, molybdenum and tungsten were alloyed with nickel (hypo-hyper-d- electronic transition metal) in order to increase the intrinsic catalytic activity for HER. Cobalt was also combined with nickel to evaluate the effect of this metal in the intrinsic catalytic activity. In all the cases, electrodeposition at very high current densities provided macroporous materials of quasy-cylindrical pores due to the fact that the metallic deposition takes place simultaneously to the gas bubbling at high current densities. Morphology of the developed electrodes was characterized by confocal laser scanning microscopy, and HER on these electrodes was evaluated in 30 wt.% KOH solution by means of steady-state polarization curves and electrochemical impedance spectroscopy (EIS). Results show that the developed materials manifest higher apparent catalytic activity than that reported for a smooth commercial polycrystalline nickel electrode.This work was supported by Generalitat Valenciana (Project PROMETEO/2010/023) and Universitat Politècnica de València (PAID-06-10-2227). I. Herraiz-Cardona is grateful to Fundación Iberdrola for the financial support.González Buch, C.; Herraiz Cardona, I.; Ortega Navarro, EM.; Garcia-Anton, J.; Pérez-Herranz, V. (2013). Development of Ni-Mo, Ni-W and Ni-Co Macroporous Materials for Hydrogen Evolution Reaction. Chemical Engineering Transactions. 32:865-870. https://doi.org/10.3303/CET1332145S8658703