Catalyst loading for Pt-nanowire thin film electrodes in PEFCs

Among electrocatalysts with novel nanostructures in low temperature polymer electrolyte fuel cells (PEFCs), Pt nanowires (Pt-NWs), as one-dimensional (1-D) nanomaterials, are recognized as promising candidates. It has also been reported that the excellent catalytic performance of the nanostructure benefited from their unique 1-D features, but also bring unusual shapes and bulky specific volumes, which make Pt-NWs difficult to fabricate into fuel cell electrodes by any conventional procedures. To understand the effect of catalyst loading on the Pt-NW electrode structure, Pt-NW thin film electrodes of various catalyst loadings were examined towards the oxygen reduction reaction (ORR) ability at the cathode side in low temperature PEFCs. SEM, XRD and electrochemical impedance spectroscopy (EIS) measurements were performed to help understanding and elucidating the electrode role under ‘real’ conditions. The results showed a similar optimal catalyst loading as compared with conventional GDEs with spherical electrocatalysts, but exhibiting a different electrode structure with increasing Pt-NW loading, although a similar larger mass transfer resistance was observed at high Pt-NW loading. The mechanism is further discussed in this paper.     

A novel catalyst layer with carbon matrix for Pt nanowire growth in proton exchange membrane fuel cells (PEMFCs)

Novel catalyst layers for proton exchange membrane fuel cells (PEMFCs) were investigated by in-situ growing of Pt nanowires (Pt-NWs) on carbon matrix. The Pt-NWs grew on the matrix along the thickness direction with a length of 10–20 nm and a diameter of 4 nm. In-situ cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and polarization experiments were employed to characterize the electrochemical performance of the Pt-NWs electrodes. The results showed that the predominantly {111}-oriented facets and oxygen access of the Pt-NWs structure contribute to the higher performance in comparison with that of the conventional catalyst layers. This work is advantageous for fuel cell catalyst layer design by allowing the controlled modification of both Pt distribution and pore size.     

Effect of platinum amount in carbon supported platinum catalyst on performance of polymer electrolyte membrane fuel cell

The performance of polymer electrolyte membrane fuel cells fabricated with different catalyst loadings (20, 40 and 60 wt.% on a carbon support) was examined. The membrane electrode assembly (MEA) of the catalyst coated membrane (CCM) type was fabricated without a hot-pressing process using a spray coating method with a Pt loading of 0.2 mg cm⁻². The surface was examined using scanning electron microscopy. The catalysts with different loadings were characterized by X-ray diffraction and cyclic voltammetry. The single cell performance with the fabricated MEAs was evaluated and electrochemical impedance spectroscopy was used to characterize the fuel cell. The best performance of 742 mA cm⁻² at a cell voltage of 0.6 V was obtained using 40 wt.% Pt/C in both the anode and cathode.     

Evaluation of electrochemical performance for surface-modified carbons as catalyst support in polymer electrolyte membrane (PEM) fuel cells

The electrochemical performance of platinum (Pt) catalyst deposited on various functionalized carbon supports was investigated and compared with that of a commercial catalyst, Pt on Vulcan XC-72 carbon. The supports employed were graphitic or amorphous with a wide range of surface areas. Cyclic voltammetry (CV) and rotating disk electrode (RDE) studies on the supported catalysts indicated equivalent platinum catalyst activities. Fuel cell performance was determined for membrane electrode assemblies (MEA) fabricated from the supported catalysts. The use of high surface area supports did not necessarily translate into a higher electrochemical utilization of platinum. Electrochemical impedance spectroscopy (EIS) measurements indicated lower ohmic losses for low surface area carbon MEAs. This is explained by the supported catalyst electrode microstructures and their intrinsic resistivities. Correlation of all data indicates that for low surface carbons, nature of the support does not significantly affect the Pt catalytic activity. The influence of the support is more critical when high surface area carbons are used because of the vastly different electrode morphology and resistivity.     

Effect of carbon black additive in Pt black cathode catalyst layer on direct methanol fuel cell performance

Vulcan XC-72R, Ketjen Black EC 300J and Black Pearls 2000 carbon blacks were used as the additive in Pt black cathode catalyst layer to investigate the effect on direct methanol fuel cell (DMFC) performance. The carbon blacks, Pt black catalyst and catalyst inks were characterized by N₂ adsorption and scanning transmission electron microscopy (STEM) with Energy dispersive X-ray (EDX) spectroscopy. The cathode catalyst layers without and with carbon black additive were characterized by scanning electron microscopy, EDX, cyclic voltammetry and current-voltage curve measurements. Compared with Vulcan XC-72R and Black Pearls 2000, Ketjen Black EC 300J was more beneficial to increase the electrochemical surface area and DMFC performance of the cathode catalyst layer. The cathode catalyst layer with Ketjen Black EC 300J additive was kept intimately binding with the Nafion membrane after 360 h stability test of air-breathing DMFC.     

Carbon nanotube supported Pt electrodes for methanol oxidation: A comparison between multi- and single-walled carbon nanotubes

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.     

Pt nanowire growth induced by Pt nanoparticles in application of the cathodes for Polymer Electrolyte Membrane Fuel Cells (PEMFCs)

Improving cathode performance at a lower Pt loading is critical in commercial PEMFC applications. A novel Pt nanowire (Pt-NW) cathode was developed by in-situ growth of Pt nanowires in carbon matrix consisting Pt nanoparticles (Pt-NPs). Characterization of TEM and XRD shows that the pre-existing Pt-NPs from Pt/C affect Pt-NW morphology and crystallinity and Pt profile crossing the matrix thickness. The cathode with Pt-NP loading of 0.005 mg_Pt-NP cm^?2 and total cathode Pt loading of 0.205 mg_Pt cm^?2 has the specific current density of 89.56 A g_Pt^?1 at 0.9 V, which is about 110% higher than that of 42.58 A g_Pt^?1 of the commercial gas diffusion layer (GDE) with Pt loading of 0.40 mg cm^?2. When cell voltage is below 0.48 V, the Pt-NW cathode has better performance than the commercial GDE. It is believed that the excellent performance of the Pt-NW cathode is attributed to Pt-NP induction, therefore producing unique Pt-NW structure and efficient Pt utilization. A Pt-NW growth mechanism was proposed that Pt precursor diffuses into the matrix consisting of pre-existent Pt-NPs by concentration driving, and Pt-NPs provide priority sites for platinum depositing at early stage and facilitate Pt-NW growth.     

Thermal and electrochemical stability of tungsten carbide catalyst supports

The thermal and electrochemical stability of tungsten carbide (WC), with and without a catalyst dispersed on it, have been investigated to evaluate the potential suitability of the material as an oxidation-resistant catalyst support. Standard techniques currently used to disperse Pt on carbon could not be used to disperse Pt on WC, so an alternative method was developed and used to disperse Pt on both commercially available WC and on carbon for comparison of stability. Electrochemical testing was performed by applying oxidation cycles between +0.6 V and +1.8 V to the support-catalyst material combinations and monitoring the activity of the supported catalyst over 100 oxidation cycles. Comparisons of activity change with cumulative oxidation cycles were made between C and WC supports with comparable loadings of catalyst by weight, solid volume, and powder volume. WC was found to be more thermally and electrochemically stable than currently used carbon support material Vulcan XC-72R. However, further optimization of the particle sizes and dispersion of Pt/WC catalyst/support materials and of comparison standards between new candidate materials and existing carbon-based supports are required.     

Effective factors improving catalyst layers of PEM fuel cell

Cathode catalyst layer has an important role on water management across the membrane electrode assembly (MEA). Effect of Pt percentage in commercial catalyst and Pt loading from the viewpoint of activity and water management on performance was investigated. Physical and electrochemical characteristics of conventional and hydrophobic catalyst layers were compared. Performance results revealed that power density of conventional catalyst layers (CLs) increased from 0.28 to 0.64 W/cm² at 0.45 V with the increase in Pt amount in commercial catalyst from 20% to 70% Pt/C for H₂/Air feed. In the case of H₂/O₂ feed, power density of CLs increased from 0.64 to 1.29 W/cm² at 0.45 V for conventional catalyst layers prepared with Tanaka. Increasing Pt load from 0.4 to 1.2 mg/cm², improved kinetic activity at low current density region in both feeding conditions. Scattering electron microscopy (SEM) images revealed that thickness of the catalyst layers (CLs) increases by increasing Pt load. Electrochemical impedance spectroscopy (EIS) results revealed that thinner CLs have lower charge transfer resistance than thicker CLs. Inclusion of 30 wt % Polytetrafluoroethylene (PTFE) nanoparticles in catalyst ink enhanced cell performance for the electrodes manufactured with 20% Pt/C at higher current densities. However, in the case of 70% Pt/C, performance enhancement was not observed. Cyclic voltammetry (CV) results revealed that 20% Pt/C had higher (77 m²/g) electrochemical surface area (ESA) than 70% Pt/C (65 m²/g). In terms of hydrophobic powders, ESA of 30PTFE prepared with 70% Pt/C was higher than 30PTFE prepared with 20 %Pt/C. X-Ray Diffractometer (XRD) results showed that diameter of Pt particles of 20% Pt/C was 2.5 nm, whereas, it was 3.5 nm for 70% Pt/C, which confirms CV results. Nitrogen physisorption results revealed that primary pores of hydrophobic catalyst powder prepared with 70% Pt/C was almost filled (99%) with Nafion and PTFE.     

To improve the high temperature polymer electrolyte membrane fuel cells performance by altering the properties of catalyst layer

The structure of electrochemical reaction zone and the catalyst layer (CL) thickness affect the performance of high temperature polymer electrolyte membrane fuel cells. In this study, the physical structures and compositions of CL are investigated by electron microscopy and polarization curve techniques. The Pt concentration of Pt/C decides the thickness of CL when the Pt loading is fixed. A higher weight percentage Pt/C contains a lower amount of carbon powder results in a thin CL and limited space for electrochemical reaction. On the other hand, the lower weight percent Pt/C provides larger space and smaller size of platinum catalyst which engenders the electrochemical reaction in CL more easily. The ionomer binds electrocatalysts Pt/C particles together and offers the ion conducting phase. Two different ionomers, Polytetrafluoroethylene (PTFE) and Polyvinylidene difluoride (PVDF), were tested. SEM results showed that PTFE forms a better uniform CL structure than PVDF. With 10 wt% Pt/C, PTFE ionomer possesses a higher gas permeability property which induces a higher reactant flow rate in CL, and consequently results in a 42.9% higher cell potential than the PVDF at 0.4 A/cm² current density output. A proper combination of 10 wt% Pt/C with PTFE ionomer is able to gain 0.62 A/cm² output at 0.3067 V for the HT-PEMFC.     

Ionomer content effects on the electrocatalyst layer with in-situ grown Pt nanowires in PEMFCs

The effects of ionomer contents were investigated in composite electrodes with in-situ grown single crystal Pt nanowires (Pt-NWs) for PEMFCs, including the amount in the carbon matrix and impregnated on the surface of the electrocatalyst layer. The electrocatalyst layer was prepared by growing Pt-NWs directly on the carbon matrix with a simple one-step wet chemical approach at room temperature. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), polarization curve tests, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were employed to evaluate the ionomer effects. The experimental results showed that the ionomer in the carbon matrix had an influence on the ionic conductivity and aggregation and distribution of the Pt-NWs, and the ionomer impregnated on the surface of the electrocatalyst layer affected the mass transport and ionic conductivity. The performance of the MEA was improved by optimizing the ionomer contents.     

Facile synthesis and characterization of highly dispersed platinum nanoparticles for fuel cells

We report a facile synthesis and characterization of highly-dispersed platinum nanoparticles supported on Ketjen carbon black (Pt/C) as electrocatalysts for polymer electrolyte membrane fuel fells (PEMFCs). Pt particles with size of ∼ 2.6 nm were synthesized through adsorption of Pt acetylacetonate on carbon supports and subsequently thermal decomposition. A comparative characterization analysis, including X-ray diffraction (XRD), high resolution transmission electron microscope (HR-TEM), cyclic voltammetry (CV), and hydrodynamic voltammetry measurements, was performed on the synthesized and commercial TKK catalysts. It revealed the details of Pt dispersion on the carbon support, particle size and distribution, electrochemical surface area (ECSA), and oxygen reduction reaction (ORR) activity of the catalysts. It was found that the synthesized Pt/C has similar particle size to that of the TKK catalyst (2.6 nm and 2.7 nm, respectively), but narrower particle size distribution. Accelerated durability tests under potential cycles were performed to study electrochemical degradation of the catalysts in corrosive environments. The synthesized Pt/C displayed significant losses in ECSA and activities after 20 k potential cycles, especially from 5 k to 20 k cycles, though with higher initial values (43% and 79% higher in ECSA and mass activity, respectively).     

Synthesis and characterization of Pt on novel catalyst supports for the H2 production in the Westinghouse cycle

This work shows the preparation and physicochemical and electrochemical characterization of Pt based catalysts supported on two different catalyst supports (Vulcan carbon and SiC/TiC) with a 40 wt% Pt content for the depolarized SO₂ electro-oxidation in the hybrid sulfur process, which is a promising approach for the hydrogen production. The Pt based catalysts supported on carbon showed the lowest Pt crystallite size, but the electrochemical surface area of the Pt deposited on the Si_0.9CTi_0.1C was the highest of the three catalysts prepared in the lab under the same operation conditions. The Pt catalysts supported on the novel SiC/TiC based material are promising catalysts for this technology as they showed high catalyst activity and durability in sulfuric acid conditions.     

Carbon nano-fiber interlayer that provides high catalyst utilization in direct methanol fuel cell

The introduction of a carbon nano-fiber (CNF) interlayer to the interface between the carbon paper and the catalyst layer was investigated for providing a highly active catalyst layer with PtRu nano-particles on it for the direct methanol fuel cell (DMFC) anode. A precipitation method was used for applying the CNF layer and the catalyst layer. The effects of the loadings of the CNF and the catalyst on the DMFC power generation were evaluated. The CNF interlayer covered the large openings of the carbon paper resulting in a dense and smooth surface. The PtRu black catalyst prepared on the surface of the CNF layer provided a higher power density of DMFC than that obtained by using carbon black, suggesting that the dense and crackless surface of the CNF layer reduced the catalyst loss that leaks into the crack and increases the active reaction sites on the anode surface.     

Influence of ink preparation with the untreated and the burned Pt/C catalysts for proton exchange membrane fuel cells

This research discovers the burning reaction of the Pt/C catalyst on ink preparation and the effect of the untreated and burned Pt/C catalysts for the proton exchange membrane fuel cells (PEMFCs). The platinum nanoparticles on the carbon support aggregate to form bigger cluster sizes due to the burning reaction of the untreated Pt/C catalyst reacting with the Nafion solution or isopropyl alcohol. After several times of “purposely” burning reaction, the specific surface area of the fully burned Pt/C reduces from 150.9 to 46.6 m² g⁻¹ which is 3 times smaller than the untreated Pt/C catalyst. The crystallite size of platinum catalyst changes from 8.4 to 46.2 nm via the calculation of Debye–Scherrer equation from X-ray diffraction (XRD) and the electrochemical surface area (ECSA) obviously decreases from 85.6 to 14.8 m² g⁻¹. The variation of the ratio of Pt/C to Nafion influences the consequent electrochemical performances. Three catalyst coated membranes (CCMs) coated with untreated, fully burned, and partial burned Pt/C catalysts are analyzed and compared in this study. The CCM coated with the untreated Pt/C catalyst shows the best polarization curve which presents the peak power density, 897 mW cm⁻². Moreover, it presents the slowest degradation rate (0.1 mA min⁻¹) at a constant voltage of 0.4 V for 4000 min, even though the result of Nyquist plots is slightly worse than others The work confirms that the burning reaction of Pt/C catalyst influences the electrochemical performance and structural balance of the catalyst layer.     

Combined method to prepare core–shell structured catalyst for proton exchange membrane fuel cells

This paper reports a modified core–shell structured CuPd@Pt/C catalyst, which was synthesized by combining a two-step reduction method and chemical dealloying step using carbon black Vulcan XC-72R as the supporting material. The physical measurements confirmed that the final catalyst has a complete core–shell structure. The interaction between the core and the shell as well as the particle size, particle size distribution, and morphology of the catalyst particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) indicated that the Pt and Pd on the surface of the catalyst nanoparticles are mainly in the zero valence oxidation state. Cyclic voltammetry (CV) and rotating disk electrode (RDE) tests were used to measure the electrochemical performance, and the results showed that the modified CuPd@Pt/C catalyst has higher electrochemical catalytic activity in catalyzing the oxygen reduction reaction (ORR) than Pt/C, making it possible to reduce the usage of platinum and leading to a promising low-Pt catalyst for proton exchange membrane fuel cells (PEMFCs).     

Graphene based catalyst supports for high temperature PEM fuel cell application

In this study, the effect of graphene nanoplatelet (GNP) and graphene oxide (GO) based carbon supports on polybenzimidazole (PBI) based high temperature proton exchange membrane fuel cells (HT-PEMFCs) performances were investigated. Pt/GNP and Pt/GO catalysts were synthesized by microwave assisted chemical reduction support. X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Brauner, Emmet and Teller (BET) analysis and high resolution transmission electron microscopy (HRTEM) were used to investigate the microstructure and morphology of the as-prepared catalysts. The electrochemical surface area (ESA) was studied by cyclic voltammetry (CV). The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher ECSA for Pt/GNP compared to Pt/GO. The Pt/GNP and Pt/GO catalysts were tested in 25 cm² active area single HT-PEMFC with H₂/air at 160 °C without humidification. Performance evaluation in HT-PEMFC shows current densities of 0.28, 0.17 and 0.22 A/cm² for the Pt/GNP, Pt/C and Pt/GO catalysts based MEAs at 160 °C, respectively. The maximum power density was obtained for MEA prepared by Pt/GNP catalyst with H₂/Air dry reactant gases as 0.34, 0.40 and 0.46 W/cm² at 160 °C, 175 °C and 190 °C, respectively. Graphene based catalyst supports exhibits an enhanced HT-PEMFC performance in both low and high current density regions. The results indicate the graphene catalyst support could be utilized as the catalyst support for HT-PEMFC application.     

Electrochemical study of platinum deposited by electron beam evaporation for application as fuel cell electrodes

Platinum is the most used catalyst in electrodes for fuel cells due to its high catalytic activity. Polymer electrolyte and direct methanol fuel cells usually include Pt as catalyst in their electrodes. In order to diminish the cost of such electrodes, different Pt deposition methods that permit lowering the metal load whilst maintaining their electroactivity, are being investigated. In this work, the behaviour of electron beam Pt (e-beam Pt) deposited electrodes for fuel cells is studied. Three different Pt loadings have been investigated. The electrochemical behaviour by cyclic voltammetry in H₂SO₄, HClO₄ and in HClO₄ + MeOH before and after the Pt deposition on carbon cloth has been analysed. The Pt improves the electrochemical properties of the carbon support used. The electrochemical performance of e-beam Pt deposited electrodes was finally studied in a single direct methanol fuel cell (DMFC) and the obtained results indicate that this is a promising and adequate method to prepare fuel cell electrodes.     

Oxygen reduction stability of graphene-supported electrocatalyst: Electrochemical and morphological evidences

Graphene has been proposed as support or as co-support for Proton Exchange Membrane Fuel cells (PEMFCs) electrocatalysts due to its high electronic conductivity, corrosion stability and high specific surface area compared to common carbon blacks, such as Vulcan. Despite such outstanding properties, comprehensive studies on long-term stability of graphene-supported Oxygen Reduction Reaction (ORR) catalysts are still unavailable or limited.The stability of graphene as support is now studied and compared with carbon black in terms of electrochemical performance and degradation mechanisms. Catalytic layers with Pt/G were prepared and subjected to a set of three different Accelerated Stress Tests (ASTs): cycling at high potentials, Open Circuit Potential (OCP) holding and cycling at low potentials. This study aims to assess if and why graphene is a better candidate as support.The Pt/G catalyst outperformed the reference catalyst in all ASTs, delivering both higher electrochemical active surface area (ECSA) and ORR mass-specific activity. Such electrochemical performance was correlated with morphological evidences associated to the support itself and with Pt nanoparticles.