Carbon supported Pt–Co alloys as methanol-resistant oxygen-reduction electrocatalysts for direct methanol fuel cells

The electrocatalysis of the oxygen reduction reaction on carbon supported Pt and Pt–Co (Pt/C and Pt–Co/C) alloy electrocatalysts was investigated in sulphuric acid (both in the absence and in the presence of methanol) and in direct methanol fuel cells (DMFCs). In pure sulphuric acid Pt–Co/C alloys showed improved specific activity towards the oxygen reduction compared to pure platinum. In the methanol containing electrolyte a higher methanol tolerance of the binary electrocatalysts than Pt/C was observed. The onset potential for methanol oxidation at Pt–Co/C was shifted to more positive potentials. Accordingly, Pt–Co/C electrocatalyts showed an improved performance as cathode materials in DMFCs.     

Bimetallic Pt–Sn catalysts supported on activated carbon. II. CO oxidation

A commercial activated carbon (AC) was used as a catalyst support either in its original form or after two different oxidation treatments, namely air oxidation and HNO₃ oxidation, aiming at the enhancement of its textural and surface chemical characteristics. These properties were determined by N₂ adsorption and temperature programmed desorption (TPD), respectively. Monometallic Pt and bimetallic Pt–Sn catalysts were prepared on the AC supports. Impregnation was used in the preparation of the monometallic samples. For the bimetallic samples, coimpregnation and a sequential impregnation procedure, in which the Sn precursor is introduced prior to Pt, were used. The Pt load was kept fixed as 1 wt.% for all monometallic and bimetallic samples. Two different Sn loads, 0.25 and 0.50 wt.%, were used for the bimetallic samples in order to investigate the effects of Sn load on the catalytic properties. The catalyst samples were characterized by H₂ adsorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and structure insensitive benzene hydrogenation. The activities of all samples were measured in CO oxidation. The results indicate the strong effects of the surface chemistry of the AC supports, the Pt:Sn ratio, the preparation procedure and the reduction procedure, on the CO oxidation activities of the catalysts.     

Liquid phase methanol reforming with water over silica supported Pt–Ru catalysts

The mechanism of the liquid phase methanol reforming reaction over silica supported Pt–Ru catalyst was investigated by kinetic studies, employing a pyrex glass reactor with reflux condensers connected to a closed gas circulation system under ambient pressure. The rate of H₂ formation over Pt–Ru/SiO₂ catalysts was more than 20 times faster than that over Pt/SiO₂ catalysts with high selectivity for CO₂ (72.3%), indicating a marked addition effect of Ru. In the case of HCHO–H₂O reaction over Pt–Ru/SiO₂, the H₂ formation rate was five times larger than that in the CH₃OH–H₂O reaction but selectivity to CO₂ was only 4%. On the contrary, in the HCOOCH₃–H₂O and HCOOH–H₂O reactions, both high activity and selectivity were observed over Pt–Ru/SiO₂. These results clearly indicate that the CO₂ formation does not proceed via HCHO decomposition and following water gas shift reaction. We propose the following pathway for liquid phase methanol reforming reaction over Pt–Ru/SiO₂; a partly dehydrogenated methanol (CH₂OH^*) is the initial reaction intermediate, from which H₂ and CO₂ are formed through HCOOCH₃ and HCOOH as the successive reaction intermediates.     

Preparation and characterization of successively deposited Pt/Ru bimetallic electrocatalysts for the methanol oxidation

Two types of Pt/Ru electrocatalysts, which have different structural characteristics, were prepared with different synthetic routes. That is, Pt/Ru electrocatalysts were synthesized by the coreduction and successive deposition methods, respectively. The structural and catalytic properties of Pt/Ru electrocatalysts were characterized by XRD, TEM, voltammetry and chronoamperometry. From the XRD analysis, coreduced and successively deposited Pt/Ru electrocatalysts had an alloyed structure. TEM analyses showed that all the electrocatalysts had a highly dispersed state on the Vulcan XC-72R substrate. From the voltammetry, the coreduced electrocatalysts displayed higher catalytic activity than the successively deposited electrocatalysts for the electrooxidation of methanol. These results explain why coreduced catalysts are better able to dehydrogenate methanol and have a greater CO tolerance than the successively deposited ones. But chronoamperometry showed that successively deposited Pt/Ru electrocatalysts had stability similar to that of the coreduced ones. Although the successively deposited electrocatalysts showed lower catalytic activity than the coreduced ones, their enhanced catalytic activity was obtained by the successive deposition method in the comparison of methanol oxidation current density with pure platinum electrocatalyst.     

The state of Tin on Pt–Sn/Nb2O5 catalysts

The effect of tin addition on niobia supported catalysts was studied and compared to the properties of alumina supported bimetallic Pt–Sn catalysts. The catalyst surfaces were probed by methylcyclopentane conversion, showing that both the presence of Sn and the reduction of the support caused a decrease in hydrogenolysis activity, favoring the ring enlargement reaction. The thermodynamics of reduction of these systems, evaluated by following the reduction step (temperature programmed reduction — TPR) with a differential scanning calorimeter (DSC), and irreversible H₂ and CO uptakes, allowed to conclude that a Pt–Sn alloy is formed on niobia supported catalysts.     

Pt45Ru45M10/C (M = Fe, Co, and Ni) catalysts for methanol electro-oxidation

Pt₄₅Ru₄₅M₁₀/C (M = Fe, Co, and Ni) catalysts were synthesized and physical and electrochemical properties were analyzed by XRD, TEM, CO stripping and methanol electro-oxidation activity measurement. Among these catalysts, the Pt₄₅Ru₄₅Fe₁₀/C catalyst exhibited the highest mass activity of 2.6 A/g catal. while those of the Pt₄₅Ru₄₅Co₁₀/C and Pt₄₅Ru₄₅Ni₁₀/C catalysts were 2.2 and 2.5 A/g catal., respectively. In the case of specific activity, the catalysts exhibited much higher activities of 110 (130%), 120 (140%) and 150 (170%) mA/m² for the Fe, Co and Ni incorporated catalysts, respectively, than 88 mA/m² of a commercial PtRu/C catalyst.     

The use of the heteropoly acids, H3PMo12O40 and H3PW12O40, for the enhanced electrochemical oxidation of methanol for direct methanol fuel cells

Polarization and electrochemical impedance spectroscopy experiments were performed on a direct methanol fuel cell (DMFC) incorporating the heteropoly acids (HPAs) phosphomolybdic acid, H₃PMo₁₂O₄₀, (HPMo) or phosphotungstic acid, H₃PW₁₂O₄₀, (HPW) in the anode Pt/C catalyst layer. Both HPW-Pt and HPMo-Pt showed higher performance than the Pt control at 30 psig of backpressure and at ambient pressure. Anodic polarizations were also performed, and Tafel slopes were extracted from the data between 0.25 V and 0.5 V. At 30 psig, Tafel slopes of 133 mV/dec, 146 mV/dec, and 161 mV/dec were found for HPW-Pt, HPMo-Pt and the Pt control, respectively. At 0 psig, the Tafel slopes were 172 mV/dec, 178 mV/dec, and 188 mV/dec for HPW-Pt, HPMo-Pt and the Pt control. An equivalent circuit model, which incorporated constant phase elements (CPEs), was used to model the impedance data. From the impedance model it was found that the incorporation of HPAs into the catalyst layer resulted in a reduction in the resistances to charge transfer. This shows that these two heteropoly acids do act as co-catalysts with platinum for methanol electrooxidation.     

Effect of reduction conditions on electrocatalytic activity of a ternary PtNiCr/C catalyst for methanol electro-oxidation

The effect of reduction conditions on a Pt₂₈Ni₃₆Cr₃₆/C catalyst was investigated by using two different reduction methods: hydrogen reduction and NaBH₄ reduction. In hydrogen reduced catalysts, dissolution of metallic Ni and Cr was observed during cyclic voltammetry (CV) tests, and a larger amount of Ni and Cr was dissolved when reduced at higher temperatures. For methanol electro-oxidation, the highest specific current density of 1.70 A m⁻² at 600 s of the chronoamperometry tests was observed in the catalyst reduced at 300 °C, which was ∼24 times that of a Pt/C catalyst (0.0685 A m⁻²). In NaBH₄ reduced catalysts, formation of an amorphous phase and a more Pt-rich surface was observed in X-ray diffraction and CV results, respectively, with increasing amounts of NaBH₄. When reduced by 50 times of the stoichiometric amount of NaBH₄, the PtNiCr/C catalyst (PtNiCr-50t) showed a current density of 34.1 A g_{noble metal}⁻¹, which was 81% higher than the 18.8 A g_{noble metal}⁻¹ value of a PtRu/C catalyst at 600 s of the chronoamperometry tests. After 13 h of chronoamperometry testing, the activity of the PtNiCr-50t (15.0 A g_{noble metal}⁻¹) was 110% higher than the PtRu/C catalyst (7.15 A g_{noble metal}⁻¹). The PtNiCr/C catalyst shows promise as a Ru-free methanol oxidation catalyst.     

Porous-microelectrode study on Pt/C catalysts for methanol electrooxidation

We have developed a porous-microelectrode (PME) to investigate the electroactivity of catalyst particles for proton exchange membrane fuel cells. The cavity at the tip of the PME was filled with Pt/C catalysts prepared by impregnation method. Cyclic voltammograms (CVs) recorded in 1 N H₂SO₄ aqueous solution revealed that the active area of the stacked catalysts exist not only at the surface but also inside of the stack. For methanol electrooxidation, 30 wt.% Pt/C exhibited the highest electroactivity, whereas the 50 wt.% Pt/C showed extremely small current. The small current is considered as a result of a small active-surface area. Methanol oxidation peak potential shifted toward cathodic direction as Pt-loading decreased, which agrees well with the Pt-oxide formation potential. The activation energy for methanol oxidation was assessed to be 44±3 kJ mol⁻¹ for all Pt/C catalysts and Pt-disc electrode.     

Novel system of electro-catalysts for methanol oxidation based on platinum and organic metal complexes

Novel system of electro-catalysts was developed for methanol oxidation reaction (MOR) in direct methanol fuel cells. Mixture of platinum tetraammine complex with cobalt quinolyldiamine complex with various mixing ratio was supported on graphite powder, and heat treated at 400-1000 °C in argon atmosphere. Powder thus obtained was put on a graphite disk electrode, and tested for electrochemical MOR in acid media. Although the cobalt complex itself showed almost no catalytic ability for MOR, it enhanced the activity of Pt more than several 10-fold when it was mixed with Pt. MOR performance was best exploited at about equal mixing ratio of platinum and cobalt complexes. Compared with platinum-ruthenium alloy catalysts, the new catalysts exhibited promising catalytic ability. The present investigation revealed good potentiality of organic complex catalysts in combination with metal catalysts for MOR, which opens the way to synthesize and develop a new class of electro-catalysts of low cost through wide range of molecular designing.     

Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells

Pt/TiO_x/C nanocomposites have been synthesized by depositing hydrated titanium oxide on carbon-supported Pt (Pt/C), reducing H₂PtCl₆ with sodium formate on carbon-supported hydrated titanium oxide (TiO₂/C), and simultaneously depositing hydrated titanium oxide and reducing H₂PtCl₆ with formate on carbon support, followed by heat treatment at 500 and 900 °C in 90% Ar-10% H₂ mixture. The catalytic activity for oxygen reduction was evaluated in half cells with sulfuric acid electrolyte and in single direct methanol fuel cells (DMFC). Tolerance to methanol was studied with half cells containing sulfuric acid mixed with methanol. Charge transfer resistance and electrochemical active surface area of the Pt/TiO_x/C catalysts were studied with impedance and cyclic voltammetry measurements. Both the synthesis methods and heat treatments influence the catalytic activity, and some of the Pt/TiO_x/C composites exhibit higher catalytic activity than Pt/C. The Pt/TiO_x/C catalysts also exhibit better methanol tolerance than Pt/C. The mechanism for the enhanced catalytic activity of Pt/TiO_x/C is discussed.     

Alternative platinum electrocatalyst supporter with micro/nanostructured polyaniline for direct methanol fuel cell applications

In this study, a series of micro/nanostructured polyanilines were synthesized and their morphology-dependent electrochemical properties for acting as a catalyst supporter for direct methanol fuel cell (DMFC) applications were investigated. These micro/nanostructures include submicron spheres, hollow microspheres, nanotubes, and nanofibers. Among the four micro/nanostructures, polyaniline nanofibers (PANF) manifest their superiority in high electrochemical active surface. Accordingly, PANF is adopted as the catalyst supporter thereafter. To couple with the use of the alternative catalyst supporter, this study also investigates the effect of reductant type on morphology and electrocatalytic properties of the PANF-supported Pt particles through a chemical reduction reaction. TEM images indicate that formic acid as a reductant results in well-dispersed Pt particles on the PANF surface. On the other hand, aggregations of Pt are observable when NaBH₄ is selected as a reductant. Moreover, the methanol oxidation current density measured with the Pt/PANF electrode being prepared by using formic acid is double that by using NaBH₄. Compared with Pt/XC-72, the Pt/PANF electrode possesses higher electrocatalytic activity and exhibits double power density. Moreover, Pt/PANF is superior to Pt/XC-72 in the aspect of operation stability based on a continuous discharge for 5 h.     

Effect of heat treatment on the performance of TiO2-Pt/CNT catalysts for methanol electro-oxidation

TiO₂-Pt/CNT catalysts before and after heat treatment were prepared. Their catalytic activities for methanol and CO electro-oxidation were studied in detail. The results showed that the proper amount of hydrous TiO₂ in TiO₂-Pt/CNTs (e.g. heated at 200 °C for 2 h) was favorable for enhancing the catalytic activity of Pt/CNTs, which provided evidence for bi-functional mechanism. The studies on the catalysts with different TiO₂/Pt molar ratio displayed that the optimum molar ratio varied with the increase of heat treatment temperature. It was found that the optimum molar ratio of TiO₂/Pt was at 1:2 for the catalysts without heat treatment and was at 1:1 for the catalysts by heat treatment at 500 °C. This fact was ascribed to the difference in compact degree between TiO₂ and Pt/CNTs before and after heat treatment. Considering the influence of heating temperature, it was found that TiO₂-Pt/CNT catalyst heated at 200 °C for 2 h had better catalytic activity for methanol oxidation.     

Pt-Co/C nanoparticles as electrocatalysts for oxygen reduction in H2SO4 and H2SO4/CH3OH electrolytes

The oxygen reduction reaction (ORR) was studied on carbon dispersed Pt and Pt-Co alloyed nanocatalysts with high contents of Co in H₂SO₄ and H₂SO₄/CH₃OH solutions. The characterization techniques considered were transmission electron microscopy (TEM), X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical activity for the ORR was evaluated from steady state polarization measurements, which were carried out in an ultra thin layer rotating disk electrode. The results showed that with the increase of Co content, the nanoparticle size distributions become sharper and the mean particle diameters become smaller. XRD indicated low degree of alloy formation but significant phase segregation of Co was observed only for Pt-Co/C 1:3 and 1:5 (Pt:Co atomic ratios). The electrochemical measurements indicated that the four-electrons mechanism is mainly followed for the ORR on all materials and the electrocatalytic activities per gram of Pt is higher for the catalysts with higher Co contents. This was explained based on the XANES results which evidenced a decrease of the coverage of oxygenated Pt adsorbates due to the presence of Co. In the methanol-containing electrolyte, the Pt-Co/C 1:5 catalyst showed the highest performance. This was attributed to its low activity for the methanol oxidation due to the smaller probability for presenting three Pt neighboring Pt active sites.     

Enhanced activity of PtSn/C anodic electrocatalyst prepared by formic acid reduction for direct ethanol fuel cells

The Pt₃Sn/C catalyst with high electrochemical activity was synthesized under optimizing preparation conditions. The surface of carbon support pretreated by strong acid contains many O-H and CO groups, which will increase the active sites of PtSn/C catalysts. The catalyst structure was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and temperature programmed reduction (TPR). The co-reduction of Pt⁴⁺ and Sn²⁺ ions causes Sn to enter Pt crystal lattice to form PtSn alloy whose surface, however, contains tin oxides with Sn⁴⁺ and Sn²⁺ valences, which can promote the ethanol oxidation. The crystallinity of PtSn decreases with the reduction of the atomic ratio of Pt:Sn. By prolonging the reaction time of formic acid, the forward anodic peak current of ethanol oxidation reaches 16.2 mA on the Pt₃Sn/C catalyst with 0.025 mg Pt loading.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Characterization of alloy electrocatalysts for direct oxidation of methanol: new methods

The most active catalysts known for the direct electrochemical oxidation of methanol have a bimetallic or multimetallic composition, which are usually used in practice with the metals dispersed on an inert, electronically conductive support. One of the limitations to understanding the behavior of such catalysts has been the absence of methods for the systematic characterization of these complex materials, eg whether the metals are present in alloy phases, the particle size and surface area of the alloy phases, the surface composition, etc. The purpose of this paper is to review recent developments in characterization methodologies, both in situ and ex situ, and to show how these may be applied to multimetallic electrocatalysts.     

Synthesis of sulfated ZrO2/MWCNT composites as new supports of Pt catalysts for direct methanol fuel cell application

Sulfated zirconia supported on multi-walled carbon nanotubes as new supports of Pt catalyst (Pt–S-ZrO₂/MWCNT) was synthesized with aims to enhance electron and proton conductivity and also catalytic activity of Pt electrocatalyst in terms of larger concentrations of ionizable OH groups on surfaces. Fourier transform infrared spectroscopy analysis shows that the sample surfaces were modified with sulfate. Transmission electron microscopy results show that the Pt and sulfated ZrO₂ particles dispersed relatively uniformly on the surface of the multi-walled carbon nanotube. X-ray diffraction shows that S-ZrO₂ and Pt coexist in the Pt–S-ZrO₂/MWCNT composites and S-ZrO₂ has no effect on the crystalline lattice of Pt. Pt–S-ZrO₂/MWCNT catalyst was evaluated in terms of the electrochemical activity for methanol electro-oxidation using cyclic voltammetry, steady-state polarization experiments and electrochemical impedance spectroscopy technique at room temperature. Pt–S-ZrO₂/MWCNT catalyst show higher catalytic activity for methanol electro-oxidation compared with Pt catalyst on non-sulfated ZrO₂/MWCNT support and commercial Pt/C (E-TEK).     

MXene supported rhodium nanocrystals for efficient electrocatalysts towards methanol oxidation

Since conventional Pt/carbon catalysts usually suffer from CO poisoning as well as carbon corrosion issues during the methanol oxidation reaction, it is essential to explore high-efficiency Pt-alternative electrocatalysts supported by a robust matrix in the direct methanol fuel cells. Herein, we report a convenient low-temperature approach to the controllable fabrication of well-dispersive Rh nanocrystals in situ grown on Ti₃C₂T_x MXene nanosheets. The ultrathin lamellar MXene structure reveals unique superiorities on the construction of advanced Rh-based hybrid catalysts, which can not only provide a large number of efficient anchoring sites for immobilizing small-sized Rh nanocrystals with abundant exposed catalytic crystal planes, but also enable direct electronic interaction with Rh for strong synergistic effects and facilitate the fast charge transportation during the catalytic process. As a consequence, the resulting Rh/Ti₃C₂T_x hybrid exhibits prominent electrocatalytic properties towards methanol oxidation reaction, such as a large electrochemical active surface area of 71.6 m² g^?1, a high mass activity of 600.2 mA mg^?1, and good long-term stability, all of which are much better than those of conventional carbon-supported Rh as well as Pt/C and Pd/C catalysts.