Preparation and characterization of CoeRu/TiO2/MWCNTs electrocatalysts in PEM hydrogen electrolyzer

The subject of this study is preparation and characterization of hypoehyper d-electrocatalysts with reduced amount of precious metals aimed for water electrolysis. The studied electrocatalysts contain 10% mixed metallic phase (Co:Ru ¼ 1:1 wt., Co:Ru ¼ 4:1 wt. and Co:Ru:Pt ¼ 4:0.5:0.5 wt.), 18% TiO2 as a crystalline anatase deposited on multiwalled carbon nanotubes (MWCNTs). Previously, MWCNTs were activated in 28% nitric acid. As a reference electrocatalyst for hydrogen evolution reaction, corresponding electrocatalysts with pure Pt metallic phase and mixed CoPt (Co:Pt ¼ 1:1 wt.) metallic phase were prepared. Also, as a reference electrocatalyst for oxygen evolution reaction, electrocatalyst with pure Ru metallic phase was prepared. The prepared electrocatalysts were structurally characterized by means of XPS, XRD,TEM, SEM and FTIR analysis. Electrochemical characterization was performed by means of cyclic voltammetry and potentiodynamic method in the PEM hydrogen electrolyzer. The range of the catalytic activity for hydrogen evolution of studied electrocatalysts was the following: CoRuPt (4:0.5:0.5) > CoPt (1:1) > Pt > CoRu (1:1) > CoRu (4:1). The order of the catalytic activity for oxygen evolution was the following: CoRu (1:1) > Ru > CoRu (4:1) > Pt > CoRuPt (4:0.5:0.5) > CoPt (1:1)

Improvement of performances of complex non-platinum electrode materials for hydrogen evolution

Abstract Structural and electrochemical characteristics of hypo–hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied and modified in order to improve their performances. All studied materials were of general composition 10% Ni + 18% TiO2 +C. All materials were prepared of amorphous or crystalline TiO2, crystalline Ni or NiCo (10–20 nm) and Vulcan XC-72, by sol–gel procedure. Both, material’s intrinsic catalytic activity and surface area were affected by applied modifications. As a result, the electrocatalytic activity was improved, e.g. transformation of TiO2 into anatase form lowers the HER overpotential for 60mV. Introduction of MWCNTs was even more effective, lowering η for 120mV. Co addition to metallic phase lowers η for utmost 195mV. Combined modification of TiO2 and carbon substrate lowers η for 145mV, while the complete modification of all three catalyst’s components was the most effective with 230mV decrease of overpotential. © 2006 Elsevier Ltd. All rights reserved. Keywords: Hydrogen evolution; Hypo–hyper d-electrocatalysts; Non-platinum metal

Modifications for the improvement of catalyst materials for hydrogen evolution

Abstract: The structural and electrocatalytic characteristics of composite materials based on non-precious metals were studied. Precursors of metallic phase (Ni, Co or CoNi) and oxide phase (TiO2) were grafted on a carbon substrate (Vulcan XC-72) by the sol–gel procedure and thermally treated at 250 ºC. Ni and CoNi crystals of 10–20 nm were produced, in contrast the Co and TiO2 were amorphous. The dissimilar electronic character of the components gives rise to a significant electrocatalytic activity for the hydrogen evolution reaction (HER), even in the basic series of prepared materials. Further improvement of the catalysts was achieved by modification of all three components. Hence, Mo was added into the metallic phase, TiO2 was converted into the crystalline form and multiwall carbon nanotubes (MWCNTs) were used instead of carbon particles. The improvement, expressed in terms of the lowering the hydrogen evolution overpotential at 60 mA cm-2, was the most pronounced in the Ni-based systems grafted on MWCNTs (120 mV lower HER overpotential) compared to 60 mV in case of Ni-based systems grafted on crystalline TiO2 (TiO2 prepared at 450 ºC) and of Ni-based systems containing 25 at.% Mo. Nevertheless, even with the realized enhancement, of all the tested materials, the Co-based systems remained superior HER catalysts. Keywords: composite electrocatalysts, hydrogen evolution, electronic interaction, real surface area

Study of structural and electrochemical characteristics of Co-based hypo–hyper d-electrocatalysts for hydrogen evolution

Abstract Structural and electrochemical characteristics of hypo–hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied. The basic catalytic material was prepared of 10% amorphous Co (grain size <2 nm), 18% amorphous TiO2 and Vulcan XC-72, by sol–gel procedure. A number of modifications were applied aimed at improving the materials performances: (i) TiO2 was transformed into anatase by heating at 480 ◦C for 1 h, (ii) multiwalled carbon nanotubes (MWCNT) were used as a catalyst support instead of Vulcan XC-72 and (iii) Mo was added to Co phase in a quantity of 25 at.% (Mo:Co = 1:3). Both, material’s intrinsic catalytic activity and surface area were affected by these modifications. As a result, the electrocatalytic activity for hydrogen evolution was improved, e.g. transformation of TiO2 into anatase form lowers the HER overpotential (η) for 15mV at 60mAcm−2. Introduction of MWCNTs lowered η for 30mV, while addition of Mo to metallic phase for 40mV. The complete modification of all three catalyst’s components (10% MoCo3 + 18% anatase + MWCNTs) was the most effective with 60mV decrease of overpotential. Characterization was made by XRD, SEM, IR and XPS methods. Surface area was measured by means of cyclic voltammetry. © 2007 Elsevier Ltd. All rights reserved. Keywords: Hydrogen evolution; Hypo–hyper d-electrocatalysis; Co-based electrocatalyst

Co-Magneli phases electrocatalysts for hydrogen/oxygen evolution

The subject of this work is the use of non-stoichiometric titanium oxides e Magneli phases as support material of Co-based electrocatalysts aimed for hydrogen/oxygen evolution reaction. Commercial micro-scaled Ebonex (Altraverda, UK) was mechanically treated for 4, 8, 12, 16 and 20 h and further Co metallic phase was grafted by sol-gel method. Morphology of Co/Ebonex electrocatalysts was observed by means of TEM and SEM microscopy, while electrochemical behavior by means of cyclic voltammetry and steadystate galvanostatic method. As the duration of mechanical treatment increases, the size of Magneli phases decreases, and consequently catalytic activity of the corresponding electrocatalysts increases. Structural characteristics of the electrocatalysts deposited on Ebonex treated for 16 and 20 h are very similar. Also, these electrocatalysts show similar electrocatalytic activity for both hydrogen and oxygen evolution reaction. So, optimal duration of mechanical treatment of Magneli phases is in the range of 16e20 h. Catalytic activity for hydrogen evolution of the studied electrocatalysts is inferior related to the corresponding catalysts deposited on carbonaceous support materials such as activated multiwalled carbon nanotubes or Vulcan XC-72 þ TiO2 (anatase). This inferiority is due to lower real surface area of the Magneli phases. Catalytic behavior for oxygen evolution achieves its maximal value even at the catalyst deposited on Ebonex treated for 12 h and it is very promising related to the similar electrocatalytic system such as CoPt/Ebonex

Effect of activation/purification of multiwalled carbon nanotubes (MWCNTs)on the activity of non-platinum based hypo-hyper d-electrocatalysts for hydrogen evolution

Commercial multiwalled carbon nanotubes (MWCNTs) were used as a catalyst support for non-platinum hypo-hyper d-electrocatalysts. In order to improve the performance of these catalysts, activation/ purification of MWCNTs in acid medium (HNO3) was carried out. The physical and surface changes of MWCNTs were investigated by DTA/TGA analysis, Raman spectroscopy, and cyclic voltammetry. Structural changes of the electrocatalysts were observed by infrared spectroscopy and SEM. Their use as a support for electrocatalysts for hydrogen evolution was demonstrated, and shown to be more suitable compared to the traditional carbon support material � carbon black (Vulcan XC-72). The electrocatalysts consisted of 10% Co + 18% TiO2 + MWCNTs. Activation/purification removes the amorphous carbon phase in theMWCNTs. As a result of both shortening and opening of carbon nanotubes, better dispersion of metallic particles (the active catalytic centers) was achieved. Thus, trans-particle and inter-particle porosity of the electrocatalytic material was improved, implying increase of catalytic activity for hydrogen evolution

Effect of non-stationary current regimes on the morphology of silver electrodeposits

Abstract: This work is concerned with the use of reverse current regimes in order to form small-grained and compact silver deposits during the electrorefining process. Several parameters were varied, i.e., i) anodic overpotential,ii) cathodic vs. anodic time ratio and iii) duration of the anodic pulse. After optimization of these parameters, phosphate ions were added and the electrolyte was stirred. The effects of a rise of the anodic overpotential on the grain sizes of the silver deposit and compactness were studied. Prolongation of the anodic time had a similar influence but with a decrease in current efficiency. An increase of the cathodic vs. anodic time ratio caused an enlargement of the grains and a decrease in the compactness of the deposit. Optimal morphological characteristics were obtained when 3-4 PO were added and the electrolyte was stirred. Keywords: silver; reverse current; electrorefining; nitrate solution; electrodeposition

Effect of carbon nanotubes support in improving the performance of mixed electrocatalysts for hydrogen evolution

The effect of using multiwalled carbon nanotubes (MWCNTs) vs. traditional carbon materials (as e.g., Vulcan XC-72) as supports for mixed non-platinum catalysts for hydrogen evolution was studied. Intrinsic changes in catalyst’s structure, surface and activity for hydrogen evolution were registered. It was found that MWCNTs significantly improve the activity of the catalysts as a result of (i) increase of the real surface area of the catalyst, (ii) improving the electrical conductivity of the electrode, (iii) better dispersion of active catalytic centers over the electrode surface and (iv) geometric nature of the nanotubes. This effect is most pronounced in the case of Ni-based catalyst, where the overpotential for hydrogen evolution was lowered by as much as 85 mV at a current density of 60 mA·cm-2 in alkaline electrolyte. The corresponding lowering of overpotential in the Co-based system was 35 mV. Key words: hydrogen evolution; mixed electrocatalysts; carbon black; multiwalled carbon nanotubes (MWCNTs

New nano-structured and interactive supported composite electrocatalysts for hydrogen evolution with partially replaced platinum loading

This work is concerned with preparation and characterization of nano-structured composite electrocatalytic material for hydrogen evolution based on CoPt hyper d-metallic phase and anatase (TiO2) hypo d-phase, both deposited on multiwalled carbon nanotubes (MWCNTs) as a carbon substrate. The main goal is partially or completely to replace Pt as the electrocatalytic material. Four electrocatalytic systems were prepared with common composition 10% Me þ 18% TiO2 þ MWCNTs, where Me ¼ Co, CoPt (4:1, wt. ratio), CoPt (1:1,wt. ratio) and Pt. The structural changes and their influence on electrocatalytic activity were studied by means of XRD, TEM, SEM and FTIR. The electrocatalytic activity was assessed in aqueous alkaline and polymer acidic electrolytes by means of steady-state galvanostatic method. It was found that Co strongly affects the platinum particle size. The addition of Co reduces platinum particle’s size from 11 nm (in pure Pt metallic system) to 4 nm (in both systems 4:1 and 1:1), i.e. almost by 3 times. The corresponding increase of the surface area and the number of the active catalytic centres improves the efficiency, despite the fact that the amount of used platinum was decreased up to 5 times. The catalyst based on CoPt (1:1) performed the best, while the activity of the pure platinum and CoPt (4:1) systems were very close. Generally, the studied electrocatalysts have shown good and stable performances for hydrogen evolution in PEM electrochemical cell. The influence of the hydrogen electrodes under investigation on the water electrolysis efficiency at current density of 0.3 A cm�2 was assessed, using previous data oxygen evolution on IrOx electrode. Related to the performances of commercial Pt (ELAT) electrode, when hydrogen electrodes with the prepared mixed electrocatalysts were used, the water electrolysis efficiency was only 5% lower for CoPt (1:1), nearly 10% lower for CoPt (4:1) and 13% lower in the case of pure Co-based electrocatalyst

Sophisticated electrocatalysts for economical production of hydrogen, Part B: Elekctrochemical characterization

Electrochemical characterization of composite hypo-hyper d - electrocatalysts for hydrogen evolution reaction (HER) was performed using cyclic voltammetry and steady - state galvanostatic method. Cyclic voltammograms are without any pronounced peaks corresponding to some surface processes, typically for porous electrodes