Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

Synthesis, characterization and electrocatalytic behaviour of non-alloyed PtCr methanol tolerant nanoelectrocatalysts for the oxygen reduction reaction (ORR)

In order to point out the effect of the second metal in platinum-based catalysts, a synthesis method by colloidal route derived from that of Bönnemann was used to prepare non-alloyed Pt_1−xCr_x/C electrocatalysts active towards the oxygen reduction reaction (ORR). The non-alloyed character of the catalysts was showed by XRD analysis. The Pt/Cr electrocatalyst having an nominal atomic ratio, as determined by EDX then corresponding to bulk composition and not surface composition, close to (0.8:0.2) showed higher activity for ORR in methanol-free oxygen saturated electrolyte, whereas the catalyst having an atomic ratio of (0.7:0.3) displayed higher activity for ORR at low overpotentials in saturated oxygen electrolyte containing 0.1 M methanol.Correlation of XRD and electrochemical results allows us to point out the effect of electronic interactions in catalyst activity towards ORR. It was also shown that adding chromium to platinum does not alter the reaction mechanism of oxygen reduction, and that in presence of low methanol concentration, the ORR occurs via the four-electron process according to the same mechanism as in methanol-free solution.     

Nitrogen-doped carbon coated palygorskite as an efficient electrocatalyst support for oxygen reduction reaction

An electrocatalyst support, nitrogen-doped graphitic layer (CN_x) coated palygorskite (PLS) (donated as PLS@CN_x), is synthesized by carbonizing the polypyrrole (PPy) coated PLS and is explored for the first time as a cathode electrocatalyst support in proton exchange membrane fuel cell. The structural and chemical properties of the PLS@CN_x are investigated by Fourier-Transform infrared spectrometer, thermogravimetric analysis, X-ray diffraction and transmission electron microscopy. The electrocatalytic activity and stability of Pt/PLS@CN_x toward oxygen reduction reaction (ORR) are studied by cyclic voltammetry (CV) and steady state polarization measurements. Upon loading Pt (20 wt%), the catalysts exhibit superior catalytic performance during ORR, surpassing the conventional Pt/C (Vulcan XC-72) catalysts. High electrocatalytic activity and good stability can be attributed to the nitrogen atom incorporation and SiO₂ component in PLS.     

Improved ORR activity of non-noble metal electrocatalysts by increasing ligand and metal ratio in synthetic complex precursors

In an effort to improve oxygen reduction reaction (ORR) activity by increasing the catalytic active site density in carbon-supported non-noble metal catalysts, several nitrogen-containing catalysts were synthesized through a heat treatment process at 900 °C using precursor complexes of Fe(II) and tripyridyl triazine (TPTZ). Fe to TPTZ mole ratios of 1:2, 1:3, 1:4, 1:5, 1:6, and 1:7 were used to prepare the precursor complexes. X-ray diffraction and surface electrochemical techniques were used to characterize these catalysts (Fe–N_x/C), and revealed that when the amount of TPTZ in the precursor complex was increased, the decomposition of Fe–N_x sites, which are considered active sites for the ORR, was effectively reduced, resulting in higher Fe–N_x site density and thus improving the catalysts’ ORR activity. This beneficial effect was validated through rotating disk electrode tests and analysis of the ORR kinetics catalyzed by these catalysts. The obtained results showed that as the Fe to TPTZ mole ratio in the precursor complex was decreased, the catalytic ORR activity of Fe–N_x/C increased monotonically in the mole ratio range of 1:2–1:6. Therefore, increasing the amount of ligand in the precursor metal complex was demonstrated to be an effective way to reduce the decomposition of ORR active site density and thereby enhance the ORR activity of non-noble metal catalysts.     

The role of the WO x ad-component to Pt and PtRu catalysts in the electrochemical CH3OH oxidation reaction

High surface area carbon-supported platinum-based catalysts, Pt/C, PtWO _x /C, PtRu/C and PtRuWO _x /C, were prepared via a chemical reduction route using single metal precursor salts. The catalyst particles were found to be in the nanoscale range, and the addition of Ru clearly decreased the particle size. The Ru was found to be partially incorporated into the face centered cubic lattice of Pt and to form a single Ru catalyst component. X-ray diffraction and X-ray photon spectroscopy did not provide evidence for electronic interactions between WO _x and Pt as well as WO _x and Ru. However, the addition of tungsten to the PtRuWO _x /C catalyst resulted in a high degree of catalyst particle agglomeration. Both Ru containing catalysts showed significantly higher activities for the CH₃OH oxidation reaction in terms of Pt + Ru mass as well as electroactive Pt + Ru surface area than the Pt/C and PtWO _x /C catalysts. The addition of tungsten appeared to mainly result in some ‘physical’ modification of the catalytically active Pt and Ru surface components such as differences in electroactive surface area rather than promotion of the CH₃OH oxidation reaction via a true catalytic mechanism.     

Characterization of methanol-tolerant Pd–WO3 and Pd–SnO2 electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

Palladium (Pd) catalysts containing nanosized metal oxides, tungsten oxide (WO₃) and tin oxide (SnO₂), supported on carbon black (Pd–MO_x/C) were synthesized, and the effect of the metal oxide on the oxygen reduction reaction (ORR) in a direct methanol fuel cell (DMFC) was investigated. The SEM images showed that the Pd nanoparticles were highly dispersed on the carbon black, and the metal oxide particles were also distributed well. Pd/C and Pd–WO₃/C catalysts as cathode materials for the ORR in DMFCs showed activity similar to or better than that of Pt/C, whereas Pd–SnO₂/C showed no improvement in catalytic activity.     

Methanol tolerant electrocatalysts for the oxygen reduction reaction

Direct methanol fuel cells (DMFCs) represent an interesting alternative in obtaining electricity in a clean and efficient way. Portable power sources are one of the most promising applications of passive DMFCs. One of the requirements in these devices is to use high alcohol concentration, which due to methanol crossover causes a considerable loss of fuel cell efficiency. In order to develop methanol tolerant cathodes with suitable activity, different supported catalysts namely PtCo/C and PtCoRu/C, were prepared either via ethylene glycol reduction (EG) with or without microwave heating assistance (MW) or via the alloy method, the latter followed by a thermal treatment in a reducing atmosphere (N₂/H₂). All cathode-catalysts were tested to determine the role of the components in simultaneously enhancing the oxygen reduction reaction (ORR) and discouraging the methanol oxidation reaction. According to the synthesis methodology, X-ray photoelectron spectra showed that the amount of metal oxides on the surface varies, being higher on the PtCo/C EG and PtCoRu/C EG catalysts. The electrochemical characterization of the catalysts was accomplished in a three electrodes electrochemical cell with a glassy carbon rotating disk electrode covered with a thin catalytic film as working electrode. To study the ORR and the influence of different methanol concentrations, linear sweep voltammetry and cyclic voltammetry were employed. The PtCo/C EG, with an important metal oxide amount on the surface, and the PtCoRu/C MW and EG electrodes, both with RuO₂ on their surfaces, were the most tolerant to methanol presence.     

Activity,selectivity, and methanol tolerance of novel carbon-supported Pt and Pt<Subscript>3</Subscript>Me (Me = Ni,Co) cathode catalysts

The activity, selectivity, and methanol tolerance of novel, carbon supported high-metal loading (40 wt.%) Pt/C and Pt₃Me/C (Me = Ni, Co) catalysts for the O₂ reduction reaction (ORR) were evaluated in model studies under defined mass transport and diffusion conditions, by rotating (ring) disk and by differential electrochemical mass spectrometry. The catalysts were synthesized by the organometallic route, via deposition of pre-formed Pt and Pt₃Me pre-cursors followed by their decomposition into metal nanoparticles. Characteristic properties such as particle sizes, particle composition and phase formation, and active surface area, were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For comparison, commercial Pt/C catalysts (20 and 40 wt.%, E-Tek, Somerset, NJ, USA) were investigated as well, allowing to evaluate Pt loading effects and, by comparison with the pre-cursor-based catalyst with their much smaller particle sizes (1.7 nm diameter), also particle size effects. Kinetic parameters for the ORR were evaluated; the ORR activities of the bimetallic catalysts and of the synthesized Pt/C catalyst were comparable and similar to that of the high-loading commercial Pt/C catalyst; at typical cathode operation potentials H₂O₂ formation is negligible for the synthesized catalysts. Due to their lower methanol oxidation activity the bimetallic catalysts show an improved methanol tolerance compared to the commercial Pt/C catalysts. The results indicate that the use of very small particle sizes is a possible way to achieve reasonably good ORR activities at an improved methanol tolerance at DMFC cathode relevant conditions.     

Platelet-like catalyst design for high yield production of multi-walled carbon nanotubes by catalytic chemical vapor deposition

We investigated the effect of catalyst design on the synthesis of multi-walled carbon nanotubes (MWCNTs) by chemical vapor deposition (CVD). A set of highly active supported sol–gel Co–Mo/MgO and Ni–Mo/MgO catalysts was prepared systematically modifying the calcination temperature. First, the evolution of catalysts’ crystallographic phases and their morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron (SEM) and transmission electron (TEM) microscopy. Second, the catalysts were used for the CVD growth of MWCNTs. The resulting materials were analysed by SEM and TEM, Raman and XRD to establish a relation between catalyst design and MWCNT yield. We show that our catalyst synthesis route leads to the formation of laminar non-porous catalyst systems, which at a calcination temperature of 800 °C stabilize in a crystallographic phase of Me_xMg_1−xMoO₄ (Me = Co or Ni). We give evidence that increased MWCNT yields of more than 3000 wt.% with respect to the catalysts are directly related to the aforementioned crystallographic phase. Finally, we propose a growth model based on the continuous exfoliation of platelet-like catalyst systems. This consistently explains the high catalytic activity towards MWCNT production using a non-porous catalyst. Our findings provide important insights for catalyst design strategies towards large-scale MWCNT production.     

Catalytic activity of cobalt and iron phthalocyanines or porphyrins supported on different carbon nanotubes towards oxygen reduction reaction

Non-noble metal catalysts for O₂ reduction were prepared by dispersing iron(II) phthalocyanine, cobalt(II) tetra-tert-butylphthalocyanine, cobalt(II) 2,3,7,8,12,13,17,18-octaethyl-porphine, and cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrine on carbon nanotubes (CNTs) used as high surface area support. Different types of CNTs (SWCNTs, DWCNTs and MWCNTs) were investigated as an effective substitute for commonly used carbon black in carbon-supported phthalocyanines and porphyrins. The oxygen reduction reaction (ORR) activity of those CNT-supported catalysts in alkaline and acidic solutions was studied. The results show that: (i) all catalytic systems including MWCNTs are more efficient for O₂ reduction than those with SWCNTs and DWCNTs, (ii) the oxidative chemical treatment of the CNTs increases the electrocatalytic performance of the corresponding CNT-supported catalysts, (iii) similarly to Vulcan-supported catalysts, iron(II) phthalocyanine gives the best electroactivity among the investigated CNT-supported materials and (iv) finally, the MWCNT-supported iron(II) phthalocyanine catalyst chemically treated in oxidative conditions shows an ORR catalytic activity comparable to a commonly used Pt/C catalyst with similar current densities and a very low overpotential (60 mV).     

Importance of catalyst stability vis-à-vis hydrogen peroxide formation rates in PEM fuel cell electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H₂O₂) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts supported on ketjen black (KB) carbon. The selectivity of these catalysts towards H₂O₂ formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H₂O₂ formation rates in a PEMFC anode and cathode. The effect of H₂O₂ formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H₂O₂ formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.     

Study of Methanol‐tolerant Oxygen Reduction Reaction at Pt–Bi/C Bimetallic Nanostructured Catalysts

The nanostructured platinum–bismuth catalysts supported on carbon (Pt₃Bi/C, PtBi/C and PtBi₃/C) were synthesised by reducing the aqueous metal ions using sodium borohydride (NaBH₄) in presence of a microemulsion. The amount of metal loading on carbon support was found to be 10 wt.‐%. The catalyst materials were characterised by X‐ray diffraction (XRD), X‐ray fluorescence (XRF), transmission electron microscope (TEM) and electroanalytical techniques. The Pt₃Bi/C, PtBi/C and PtBi₃/C catalysts showed higher methanol tolerance, catalytic activity for oxygen reduction reaction (ORR) than Pt/C of same metal loading. The electrochemical stability of these nano‐sized catalyst materials for methanol tolerance was investigated by repetitive cycling in the potential range of –250 to 150 mV_MSE. Bi presents an interesting system to have a control over the activity of the surface for MOR and ORR. All Pt–Bi/C catalysts exhibited higher mass activities for oxygen reduction (1–1.5 times) than Pt/C. It was found that PtBi/C catalyst exhibits better methanol‐tolerance than the other catalysts.     

Facile Preparation of an Excellent Pt/RuO2‐MnO2/CNTs Nanocatalyst for Anodes of Direct Methanol Fuel Cells

RuO₂‐MnO₂ complex supported by multi‐wall carbon nanotubes (CNTs) was firstly synthesised by the oxidation–reduction precipitation of RuCl₃ and KMnO₄ in one step. Then Pt was loaded onto the obtained RuO₂‐MnO₂/CNTs to fabricate a novel anodic catalyst Pt/RuO₂‐MnO₂/CNTs for direct methanol fuel cells (DMFCs). The catalyst was characterised by transmission electron microscopy (TEM), X‐ray diffraction (XRD), temperature programmed reduction (TPR), X‐ray photoelectron spectroscopy (XPS) and BET specific surface areas (BET). Pt nanoparticles were found uniformly dispersed on the surface of CNTs, with the average diameter of about 2.0 nm. The activities of methanol and CO electrocatalytic oxidation were analysed, and the reaction mechanism of methanol electro‐oxidation on Pt/RuO₂‐MnO₂/CNTs catalyst was discussed. The MnO₂ in the catalysts improves the proton conductivity and electrochemical active surface area (EAS) for the catalysts. RuO₂ improves the CO oxidation activity and Pt dispersion. CNTs provide effectively electron channels. Thus, the Pt/RuO₂‐MnO₂/CNTs catalyst has high utilisation of the noble metal Pt, high CO oxidation ability and excellent methanol electro‐oxidation activity, being an outstanding anode catalyst for DMFC.     

Electrocatalysis of oxygen reduction on carbon-supported PtCo catalysts prepared by water-in-oil micro-emulsion

Synthesis of carbon-supported PtCo/C using micro-emulsion method including simultaneous procedure and sequential procedures in both acid and alkaline media was reported. UV-vis and electron microscopy were used to characterize the formation, surface morphology and distribution of PtCo nanoparticles. Crystallite structure of catalysts was analyzed from XRD patterns. Catalytic properties of PtCo/C catalysts synthesized were compared with commercial Pt/C using RDE based on both mass activity (MA) and specific activity (SA). PtCo/C catalysts prepared in both acidic and basic conditions showed better performance than commercial Pt/C catalyst. High-temperature heat treatment was found useful only to PtCo/C by sequential procedure. The peroxide yield was also explored using RRDE technique. The H₂O₂ yield results were correlated with SA and R values (ratio of charge transferred about Co and Pt on the surface of catalyst) obtained from CVs in 1 M KOH solution. A sacrificial Co oxidized effect on impediment of adsorption of OH may cause higher catalytic properties and higher H₂O₂ yield to Pt base alloy catalysts.     

Electroreduction of dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalysts—effect of the presence of methanol

Ag/C catalysts with different loading were prepared using a colloidal route to obtain well dispersed catalysts on carbon, with a particle size close to 15 nm. An amount of 20 wt.% Ag on carbon was found to be the best loading in terms of current density and mass activity. The 20 wt.% Ag/C catalyst was then studied and the kinetics towards ORR was determined and compared with that of a 20 wt.% Pt/C catalyst. The number of exchanged electrons for the ORR was found to be close to four with the rotating disk electrode (RDE) as well as with the rotating ring disc electrode (RRDE) techniques. From the RDE results, the Tafel slopes b, the diffusion limiting current density inside the catalytic film (j_l^film) and the exchange current density (j₀) were evaluated. The Tafel slopes b and diffusion limiting current densities inside the catalytic film (j_l^film) were found to be in the same order for both catalysts, whereas the exchange current density (j₀), which is a suitable estimation of the activity of the catalyst, was at least 10 times higher at the Pt/C catalyst than at the Ag/C catalyst. The behavior of both catalysts in methanol containing electrolyte was investigated and it was found that at a low methanol concentration, the Pt/C catalyst was quasi-tolerant to methanol. But, at a high methanol concentration, the ORR at a Pt/C was affected. However, the Pt/C catalyst showed in each case better activity towards ORR than the Ag/C catalyst, even if the latter one was less affected by the presence of methanol than the former one.     

Phosphate adsorption and its effect on oxygen reduction reaction for PtxCoy alloy and Aucore–Ptshell electrocatalysts

The phosphate adsorption characteristics and its effect on oxygen reduction reaction (ORR) were examined for various carbon-supported catalysts (Pt/C, Pt₃Co/C, PtCo/C, and Au_core–Pt_shell/C). Using cyclic voltammetry (CV) and the addition of phosphoric acid, the degree of phosphate adsorption for each catalyst was evaluated based on the intensity of the phosphate adsorption peaks (0.25–0.3 V and 0.5–0.65 V) and on the decrease in the platinum oxidation current (0.9 V). In the N₂O reduction technique, the surface structures were analyzed using N₂O as an electrochemical probe, which showed that as the Co content increased, (i) steps or defects were introduced by surface reconstruction, (ii) the phosphate adsorbed more strongly compared to Pt/C with a preference for the terrace sites, and (iii) the potential of zero total charge (PZTC) shifted to negative potentials. In the case of the Au_core–Pt_shell/C, the phosphate adsorption was found to be weaker than other catalysts, including Pt/C catalyst. The relative ORR activity with PA addition, normalized by that with no phosphate adsorption, was significantly smaller for Co containing alloy catalysts (PtCo/C: 18.2%) and larger for Au_core–Pt_shell (30.2%) compared with the Pt/C catalyst (27.8%), confirming the phosphate adsorption characteristics of each catalyst, as measured by CV and N₂O reduction analysis.     

SbOx‐promoted pt nanoparticles supported on CNTs as catalysts for base‐free oxidation of glycerol to dihydroxyacetone

Understanding of selective base‐free oxidation of glycerol to dihydroxyacetone (DHA) over Pt‐based catalysts is of paramount scientific and industrial importance. In this work, a comparative study between differently sized SbO_x‐promoted and unpromoted Pt/CNTs catalysts is carried out to decouple the promoter effects from the metal size effects. The introduction of SbO_x appears to enhance both the glycerol oxidation activity and the DHA selectivity, and the largely sized promoted Pt/CNTs catalysts afford a relatively high DHA yield and less C–C bond cleavage. X‐ray photoelectron spectroscopy measurements reveal that the Sb species are mainly in the form of SbO_x, and the differently sized promoted catalysts show similar metal binding energies. Furthermore, theoretical studies on the promotional effects of SbO_x are carried out by DFT calculations. It is found that the presence of the promoter on the catalyst surface favors the preferential activation of the secondary hydroxyl group. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3979–3987, 2018     

Preparation of Pt-Co catalysts on mesoporous carbon and effect of alloying on catalytic activity in oxygen electro-reduction

Mesoporous carbon (MC)-supported PtCo catalysts were prepared by a sodium borohydride (NaBH₄) reduction method. To increase the alloy degree of PtCo catalyst, the heat treatment was carried out at various temperatures (300–700°C). The heat-treated PtCo catalysts (PtCo/MC-x) had the higher degrees of Pt-Co alloy than that of as-synthesized PtCo catalyst (PtCo/MC). MC supported-PtCo catalyst (PtCo/MC-500) that was treated at 500°C, had the highest activity and lowest overpotential in oxygen electro-reduction (ORR) among the prepared PtCo catalysts. The high alloy degree and favorable chemical states of PtCo/MC-500 are believed to be responsible for the superior activity in oxygen electroreduction compared to the other PtCo catalysts. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

Synthesis of well-dispersed PtRuSnOx by ultrasonic-assisted chemical reduction and its property for methanol electrooxidation

PtRuSnO_x supported on multi-wall carbon nanotubes (MWCNTs) was prepared by ultrasonic-assisted chemical reduction method. The as-prepared catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that Pt exists as the face-centered cubic structure, Ru is alloyed with platinum, while non-noble metal oxide SnO_x exists as an amorphous state. From TEM observation, PtRuSnO_x is well dispersed on the surface of MWCNTs with the particle size of several nanometers. The electrochemical properties of the as-prepared catalyst for methanol electrooxidation were studied by cyclic voltammetry (CV) and chronoamperometry (CA). The onset potential of methanol oxidation on PtRuSnO_x and PtRu catalysts is much more negative than that on Pt catalyst, shifting negatively by about 0.20 V, while the peak current density of methanol oxidation on PtRuSnO_x is higher than that on PtRu. Electrochemical impedance spectroscopy (EIS) studies also show that the reaction kinetics of methanol oxidation is improved with the presence of SnO_x. The addition of non-noble metal oxide SnO_x to PtRu promotes the catalytic activity for methanol electrooxidation and the possible reaction mechanism is proposed.     

Methanol-tolerant PdPt/C alloy catalyst for oxygen electro-reduction reaction

A carbon-supported Pd-based PdPt catalyst (PdPt/C) with a small amount of Pt was prepared by borohydride reduction method and its activity in the oxygen electro-reduction reaction (ORR) was investigated in acidic conditions both with and without methanol. For comparison, carbon-supported Pt (Pt/C) and Pd (Pd/C) catalysts were prepared and the ORR activities were compared. Results revealed that the PdPt/C catalyst showed slightly lower ORR activity in terms of onset potential of oxygen reduction than Pt/C catalyst in 0.1M HClO₄. However, PdPt/C catalyst exhibited enhanced activity toward selective ORR with methanol-tolerant characteristics in 0.1M HClO₄ in the presence of methanol. The PdPt/C catalyst prepared here is suitable for use as a cathodic electrocatalyst in direct alcohol fuel cells after addition of small amount of expensive Pt metal.