Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC: Part II: Degradation mechanism and durability enhancement of carbon supported platinum catalyst

Polymer electrolyte membrane fuel cell (PEMFC) technology has advanced rapidly in recent years, with one of active area focused on improving the long-term performance of carbon supported catalysts, which has been recognized as one of the most important issues to be addressed for the commercialization of the PEMFCs. The cathode catalyst layer in PEMFCs typically contains platinum group metal/alloy nanoparticles supported on a high-surface-area carbon. Carbon support corrosion and Pt dissolution/aggregation are considered as the major contributors to the degradation of the Pt/C catalysts. If the platinum particles cannot maintain their structure over the lifetime of the fuel cell, change in the morphology of the catalyst layer from the initial state will result in a loss of electrochemical activity. This paper reviews the recent advances in the stability improvement of the Pt/C cathodic catalysts in PEMFC, especially focusing on the durability enhancement through the improved Pt–C interaction. Future promising strategies towards the extension of catalysts operation life are also prospected.     

Electrochemical activity and durability of platinum nanoparticles supported on ordered mesoporous carbons for oxygen reduction reaction

A facile procedure for synthesizing platinum nanoparticles (NPs) studded in ordered mesoporous carbons (Pt–OMCs) based on the organic–organic self-assembly (one-pot) approach is reported. These Pt–OMCs, which can be easily fabricated with controllable Pt loading, were found to possess high surface areas, highly accessible and stable active sites and superior electrocatalytic properties pertinent as cathode catalysts for hydrogen–oxygen fuel cells. The enhanced catalytic activity and durability observed for the Pt–OMC electrocatalysts are attributed to the strengthened interactions between the Pt catalyst and the mesoporous carbon that effectively precludes migration and/or agglomeration of Pt NPs on the carbon support.     

Synthesis and characterization of Pt–Se/C electrocatalyst for oxygen reduction and its tolerance to methanol

Pt–Se/C catalyst for oxygen reduction reaction (ORR) was prepared by a modified organic colloid method with sodium citrate and triphenyl phosphine as complexing agents. The active components were highly dispersed on the carbon black support. The addition of Se improved the dispersion of platinum significantly and reduced the particle size to be less than 1.8 nm. The catalyst showed similar activity compared to Pt/C catalyst, and had a higher tolerance to methanol than Pt/C catalyst. The catalyst was characterized with X-ray diffraction (XRD) and transmission electron microscope (TEM). Electrochemical measurements showed that the synthesized Pt–Se/C catalyst had a four-electron transfer mechanism for oxygen reduction.     

Effect of pretreatment on Pt–Co/C cathode catalysts for the oxygen reduction reaction

Carbon supported Pt and Pt–Co electrocatalysts for the oxygen reduction reaction in low temperature fuel cells were prepared by the reduction of the metal salts with sodium borohydride and sodium formate. The effect of surface treatment with nitric acid on the carbon surface and Co on the surface of carbon prior to the deposition of Pt was studied. The catalysts where Pt was deposited on treated carbon the ORR reaction preceded more through the two electron pathway and favored peroxide production, while the fresh carbon catalysts proceeded more through the four electron pathway to complete the oxygen reduction reaction. NaCOOH reduced Pt/C catalysts showed higher activity that NaBH₄ reduced Pt/C catalysts. It was determined that the Co addition has a higher impact on catalyst activity and active surface area when used with NaBH₄ as reducing agent as compared to NaCOOH.     

Selective deposition of gold onto Pt sites in Pt/C catalysts: Electron microscopy and electrochemical characterization

This work aims to study the selective deposition of gold onto the faces of Pt^o in Pt/C catalyst. The preparation of monometallic Pt/C catalyst was carried out by means of the impregnation of platinum over graphite. The bimetallic catalyst Pt–Au/C was prepared by selectively depositing Au on supported monometallic Pt/C catalyst by means of the reduction “in-situ” of AuCl₄⁻. The surface redox method used in this work was the Refilling method (RE), which consisted in adsorbing hydrogen first on the metal (Pt), and subsequently reducing the AuCl₄⁻ species by contact with the Pt–H interface at low temperature. The catalysts were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), whereas for the electrochemical tests the catalysts were supported on Vulcan XC-72 carbon and they were evaluated by linear and cyclic voltammetry as well as rotating disk electrode (RDE) measurements. The composition of the individual metal particles of the solids indicated the presence of both metals, Pt and Au. The results indicated that a partial Au coating of certain Pt facets is possible, which indicates that the coating mechanism is selective and could influence the catalytic properties of the bimetallic Pt–Au/C catalysts. As a result of this blockage we observed a reduction of the catalytic activity of the bimetallic catalyst with respect to the Pt/C catalyst. The electrochemical characterization showed a Tafel slope of −94 mV dec⁻¹ for the Pt–Au/C sample, with a 0.62 transfer coefficient, showing the effect of the blockage of the active platinum sites.     

Carbon supported Pt–Co (3:1) alloy as improved cathode electrocatalyst for direct ethanol fuel cells

In direct alcohol fuel cells, ethanol crossover causes a less serious effect compared to that of methanol because of both its smaller permeability through the Nafion^® membrane and its slower electrochemical oxidation kinetics on a Pt/C cathode. The main interest in direct ethanol fuel cells (DEFCs) is to find an anode catalyst with high activity for the oxidation of ethanol. However, due to the low activity of pure platinum for the oxygen reduction reaction (ORR), research on cathode electrocatalysts with improved ORR and the same or improved ethanol tolerance than that of Pt are also in progress. In this work, a commercial carbon supported Pt–Co (3:1) electrocatalyst (E-TEK) was investigated as cathode material in DEFCs and the activity compared to that of Pt. In the cathodic potential region (0.7–0.9 V versus RHE) Pt/C and Pt–Co/C showed the same activity for the oxidation of crossover ethanol. But the performance of Pt–Co/C as cathode material in DEFCs in the temperature range 60–100 °C is better than that of Pt/C both in terms of mass activity and specific activity, due to an improved activity of the alloy for oxygen reduction.     

Pt/C/MnO2 hybrid electrocatalysts for degradation mitigation in polymer electrolyte fuel cells

Pt/C/MnO₂ hybrid catalysts were prepared by a wet chemical method. Pt/C electrocatalysts were treated with manganese sulfate monohydrate (MnSO₄·H₂O) and sodium persulfate (Na₂S₂O₈) to produce MnO₂. The presence of MnO₂ was confirmed by FTIR spectroscopy. Rotating ring–disk electrode (RRDE) experiments were performed on electrodes prepared using the hybrid electrocatalysts to estimate the amount of hydrogen peroxide (H₂O₂) formed during the oxygen reduction reaction (ORR) as a function of MnO₂ content. Pt/C/MnO₂ (5% by weight of MnO₂) hybrid electrocatalysts produced 50% less hydrogen peroxide than the baseline Pt/C electrocatalyst. The hybrid electrocatalysts were used to prepare membrane electrode assemblies that were tested at 90 °C and 50% RH at open circuit with pure hydrogen as fuel and air as the oxidant. The fluoride ion concentration was measured using an ion selective electrode. The concentration of F⁻ in the anode condensate over 24 h was found to be reduced by a factor of 3–4 when Pt/C/MnO₂ replaced Pt/C as the catalyst. Through cyclic voltammetry and RRDE kinetic studies, the lower ORR activity of the acid treated hybrid electrocatalysts was attributed to catalyst treatment with acid during MnO₂ introduction. The activity of the hybrid catalyst was improved by switching to a water-based synthesis.     

High catalytic performance and stability of Pt/C using acetic acid functionalized carbon

Carbon (Vulcan XC-72, Cobat Corp.) is pretreated using acetic acid (HAC) before the Pt deposition by microwave assisted glycol method. TEM and XRD results indicate that 3 nm Pt nano-particles are uniformly dispersed on the surface of modified XC-72. In order to examine the interaction between Pt nano-particles and carbon, Pt/C-HAC and commercial Pt/C (Johnson Matthey Corp.) are calcined at 500 °C for 2 h under nitrogen atmosphere. The average Pt particle size of Pt/C-HAC after calcination is only 10–12 nm in diameter while commercial Pt particles grow up to 25–35 nm with a broad size distribution. Meanwhile, electrochemical studies of Pt/C-HAC reveal higher activity and stability for both methanol oxidation and oxygen reduction than that of Pt/C-JM. The pore structure and surface composition are investigated by BET and XPS, which implies that much microporous structure and carbonyl functional groups on carbon surface are obtained after HAC treatment. The high catalytic performance and stability might mainly be due to the strong interaction between Pt nano-particles and carbon by carbonyl functional groups. Therefore, HAC treatment is proved to be a facile and effective method for carbon as the support for Pt as fuel cell catalyst.     

Nanostructured polypyrrole/carbon composite as Pt catalyst support for fuel cell applications

A novel catalyst support was synthesized by in situ chemical oxidative polymerization of pyrrole on Vulcan XC-72 carbon in naphthalene sulfonic acid (NSA) solution containing ammonium persulfate as oxidant at room temperature. Pt nanoparticles with 3–4 nm size were deposited on the prepared polypyrrole–carbon composites by chemical reduction method. Scanning electron microscopy and transmission electron microscopy measurements showed that Pt particles were homogeneously dispersed in polypyrrole–carbon composites. The Pt nanoparticles-dispersed catalyst composites were used as anodes of fuel cells for hydrogen and methanol oxidation. Cyclic voltammetry measurements of hydrogen and methanol oxidation showed that Pt nanoparticles deposited on polypyrrole–carbon with NSA as dopant exhibit better catalytic activity than those on plain carbon. This result might be due to the higher electrochemically available surface areas, electronic conductivity and easier charge-transfer at polymer/carbon particle interfaces allowing a high dispersion and utilization of deposited Pt nanoparticles.     

Electro-catalytic activity of enhanced CO tolerant cerium-promoted Pt/C catalyst for PEM fuel cell anode

Cerium-promoted Pt/C catalysts were prepared by one-pot synthesis process and applied as an anode material for CO tolerance in PEM fuel cell. Its physical properties were characterized by XRD and TEM techniques, which indicated that Pt nano-particles are highly dispersed on the carbon supports. The investigation focused on examining the CO tolerance in sulfur acid solution of Pt–CeO₂/C compared to Pt/C (JM). The hydrogen oxidation activity was strongly depended on the content of the cerium in the Pt catalyst which was detected by CV, LSV, CO-stripping and EIS techniques. Effect of the anode catalyst poisoning on hydrogen oxidation in the presence of CO was studied in single cells. Pt–CeO₂/C catalyst at the appropriate content of 20% Ce presented a very higher CO tolerant activity. A tentative mechanism is proposed for a possible role of a bi-functional synergistic effect between Pt and CeO₂ for the enhanced electro-oxidation of CO. CeO₂-promoted Pt/C catalyst may be one of the attractive candidates as CO tolerance anode material in PEMFC.     

Enhancement of anodic oxidation of formic acid on palladium decorated Pt/C catalyst

A palladium decorated Pt/C catalyst, Pt@Pd/C, is prepared by a colloidal approach with a small amount of platinum as core. It is found that the catalyst shows excellent activity towards anodic oxidation of formic acid at room temperature and its activity is 60% higher than that of Pd/C. Decoration of palladium shell on the platinum core is supported by XPS results. Due to the use of platinum as core, active components are dispersed very well and the particle sizes are smaller than those of Pd/C. The cyclic voltammetry measurement clearly shows synthetic electro-oxidation effects of formic acid on Pt@Pd/C. It is speculated that the high performance of Pt@Pd/C may result from the unique core-shell structure and synergistic effect of Pt and Pd at the interface. The preparation method for Pt@Pd/C reported in this work will provide additional options for the design of catalysts for direct formic acid fuel cell (DFAFC).     

Electrocatalytic activity of Pt–Pd electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells: Effect of supports

Pt–Pd electrocatalysts supported on different types of support including domestic Hicon Black (HB), multi-walled carbon nanotubes (MWCNT) and titania (TiO₂) were prepared by a combined approach of impregnation and seeding, and compared to that prepared using the commercial Vulcan XC-72 (C). Their oxygen reduction reaction (ORR) activities in an acid electrolyte (0.5 M H₂SO₄) and in a single proton exchange membrane (PEM) fuel cell were evaluated. The type of support was found to affect the Pt–Pd electrocatalyst morphology and ORR activity. The Pt–Pd/C electrocatalyst had the smallest Pt particle size, better catalyst dispersion and a higher Pt:Pd M ratio compared to that of other types of supported Pt–Pd electrocatalysts. However, both in the acid solution and in a single PEM fuel cell, the ORR activities of the Pt–Pd/HB and Pt–Pd/CNT electrocatalysts were comparable to that of the Pt–Pd/C one. The ORR pathway of all supported Pt–Pd electrocatalysts were close to the four-electron pathway.     

Polyol synthesis of nanosized Pt/C electrocatalysts assisted by pulse microwave activation

A polyol process assisted by pulse microwave activation was used to prepare efficient Pt/C electrocatalysts for PEMFC applications with reducing cost. Catalysts from pulsed microwave method were compared with a catalyst issued from a classical method, in terms of active surface area, platinum loading and activity towards the oxygen reduction reaction. A design of experiments (DOE derived from the Taguchi method) has been implemented to optimize experimental parameters only related to pulse microwave activation, the intrinsic synthesis parameters (concentration of platinum salt, platinum/carbon weight ratio and pH) being kept constant. Controlled parameters were duration of microwave pulse, maximum temperature and total duration of the synthesis. Considered responses were catalyst active surface area and the Pt/C loading. An optimized configuration of synthesis parameter was proposed. The confirmation experiment revealed a trend in agreement with that expected. Three catalysts (two from pulsed microwave synthesis method and one prepared by the classical method) were characterized by transmission electron microscopy, cyclic voltammetry and CO stripping. Catalysts from pulsed microwave method display higher characteristics than the one prepared by the classical method. The Pt/C catalyst from the confirmation experiment displays the highest catalytic activity toward oxygen reduction reaction.     

Enhanced durability of Pt/C catalyst by coating carbon black with silica for oxygen reduction reaction

A platinum electrocatalyst was presented for oxygen reduction reaction that the durability to potential cycling was enhanced. It was synthesized by coating carbon black with a silica layer, followed by Pt deposition on it. To investigate the durability of the electrocatalyst, two accelerated degradation testing protocols were carried out. Carbon corrosion and platinum metal degradation properties were evaluated under the potential cycling between 1.0 and 1.5 V and between 0.6 and 0.95 V, respectively. Silica-coated catalysts (Pt/CB-SiO₂) showed better stabilities compared to the commercial Pt/C catalyst under both of the two protocols. Commercial Pt/C catalyst initially had better mass activity than silica-coated catalysts but it became similar after the potential cycling of carbon corrosion. TEM showed the platinum particle aggregation and particle density decrease especially for the commercial catalyst by the potential cycling. The silica coating prevents carbon corrosion by blocking the carbon support from direct contact with the oxygen source and preventing the effect of oxygen spillover from the platinum to carbon during the potential cycling between 1.0 and 1.5 V. It also alleviates platinum dissolution in reverse scans by reducing the formation of Pt oxide during potential cycling in between 0.6 and 0.95 V. The results suggest the coating of carbon support can enhance the durability of Pt/C catalyst to the potential cycling.     

Bucky diamond produced by annealing nanodiamond as a support of Pt electrocatalyst for methanol electrooxidation

Bucky diamond (BD) with a nanoscale diamond core surrounded by a fullerene shell was used as a support of Pt electrocatalyst for methanol electrooxidation. BDs were prepared by annealing detonation-synthesized nanodiamond (ND) powders in 10⁻³ Pa vacuum at 900–1100 °C. The electrochemical properties of the BD powders in aqueous solution were investigated. The BDs and NDs supported platinum (Pt) electrocatalysts were prepared using a microwave-assisted reduction method. Higher dispersion of Pt nanoparticles was observed on the BDs than the pristine NDs, indicating a high affinity between BDs and Pt metal. The Pt/BD catalyst had better catalytic activity and higher stability for the methanol electrooxidation in comparison to the Pt/ND prepared at the same conditions. This provides a novel nanoparticle with a high conductivity and a high stability for electrochemical applications.     

Morphological influence of graphitic carbon nanofibers by N–F dual-doping on Pt electrocatalytic activity and stability for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Morphology of carbon nanofibers significantly effects Pt nanoparticles dispersion and specific interaction with the support, which is an important aspect in the fuel cell performance of the electrocatalysts. This study emphasizes, the defects creation and structural evolution comprised due to N–F co-doping on graphitic carbon nanofibers (GNFs) of different morphologies, viz. GNF-linearly aligned platelets (L), antlers (A), herringbone (H), and their specific interaction with Pt nanoparticle in enhancing the oxygen reduction reaction (ORR). GNFs–NF–Pt catalysts exhibit better ORR electrocatalytic activity, superior durability that is solely ascribed to the morphological evolution and the doped N–F heteroatoms, prompting the charge density variations in the resultant carbon fiber matrices. Amongst, H–NF–Pt catalyst performed outstanding ORR activity with exceptional electrochemical stability, which shows only 20 mV loss in the half-wave potential whilst 100 mV loss for Pt/C catalyst on 20,000 potential cycling. The PEMFC comprising H–NF–Pt as cathode catalyst with minimum loading of 0.10 mg cm^?2, delivers power density of 0.942 W cm^?2 at current density of 2.50 A cm^?2 without backpressures in H₂–O₂ feeds. The H–NF–Pt catalyst owing to its hierarchical architectures, performs well in PEMFC at the minimized catalyst loading with outstanding stability that can significantly decrease total price for the fuel cell.     

The effect of carbon support treatment on the stability of Pt/C electrocatalysts

A potential cycling test was conducted to evaluate the effect of carbon support treatment on the stability of Pt/C electrocatalysts. The FTIR spectra show that after oxidative treatments, the carbon support became rich in oxygen-containing functional groups. Oxidative treatments of the carbon support increase the interaction between the metal particle and the support, resulting in an improved electrochemical stability of Pt/C catalysts. The Pt/C catalyst prepared from the H₂O₂-treated carbon support exhibits a higher stability than that prepared from the HNO₃-treated carbon support.     

Enhanced Pt utilization in electrocatalysts by covering of colloidal silica nanoparticles

This work aims at enhancing Pt utilization in electrocatalysts by covering of preformed silica nanoparticles. Pt/C electrocatalysts were prepared by reductive deposition of Pt by citrate at moderate temperatures on silica nanoparticles with varying atomic silica to Pt ratios (1.7:1 and 3.3:1) to study the effects of silica to Pt ratio. Considerable voidages were created by inter-situated 10–20 nm silica nanoparticles between support carbon particulates to facilitate mass transfer of reactants and products. This particular method of catalyst preparation increases the Pt metal utilization, and generates a large amount of accessible voidage in the interpenetrating particle network of carbon and silica to support the facile transport of reactants and products. Electrochemical hydrogen adsorption/desorption has shown an increase in electrochemically active surface area by this approach. Methanol electro-oxidation was used as a test reaction to evaluate the catalytic activity. It was found that the Pt catalyst modified with silica at silica:Pt = 1.7:1 atomic ratio was more active than a catalyst prepared when silica to Pt ratio increased to 3.3:1.     

Electrocatalytic activity and durability study of carbon supported Pt nanodendrites in polymer electrolyte membrane fuel cells

In this paper, Pt nanodendrites are synthesized, and their use as an oxygen reduction catalyst in polymer electrolyte membrane fuel cells is examined. When the Pt nanoparticles are shape-controlled in a dendritic form, the Pt nanoparticles exhibit a high mass activity that is nearly twice as high as the commercial Pt/C catalyst for the oxygen reduction reaction. This high activity is only achieved when the Pt nanodendrites are supported on carbon. The unsupported Pt nanodendrites exhibit very poor catalytic activity due to the limited accessibility of the active sites in the catalyst layer of the fuel cells. Based on the durability study of Pt nanodendrites, however, the dendritic structure is not stable during repeated potential cycling test and its structure collapse is the primary reason for the performance loss in the fuel cells.     

Special emphasis towards decorating platinum nanoparticles on carbon to boost cell performance and durability for portable hydrogen-powered fuel cell stack

In recent years, ultrathin layer Pt has intrigued the fuel cell community to a greater extent, particularly for low-temperature hydrogen-powered fuel cell applications. Special emphasis is given in this study to decorate platinum catalyst layers with optimal size on carbon for hydrogen-powered fuel cells. In this work, we report a unique long-time-reduction technique using K₂PtCl₄ as a precursor to selectively deposit Pt nanoparticles that can be dispersed on the support. These electrocatalysts boost oxygen reduction activity with an ultrathin metal loading of 80 μg cm^?2. A long-time-reduction procedure permits selective Pt adsorption on carbon, enabling their strong-affinity/interaction and thereby preserves more than 65% of activity even after 30,000 cycles at 60 °C. Cell performance and stability are enhanced with an ultrathin layer and optimum size formed by closely spaced carbon support and the electrode layer. We also design compact and lightweight indigenous cell components for hydrogen-powered fuel-cell studies. The indigenously designed 8-cell portable stack confirms excellent power output and stability during start-up and shutdown conditions. An indigenous hydrogen-powered portable stack is also subjected to a varsity of multi-segment applications like mobile charging, micro-fan, and LED lights.