Effects of tungsten oxide addition on the electrochemical performance of nanoscale tantalum oxide-based electrocatalysts for proton exchange membrane (PEM) fuel cells

In the present study, the properties of non-platinum based nanoscale tantalum oxide/tungsten oxide-carbon composite catalysts were investigated for potential use in catalyzing the oxygen reduction reaction on the cathode side of a PEM fuel cell. All of the tantalum oxide-based catalysts exhibit high ORR on-set potentials, comparable with the commercial Pt/C catalyst even though oxygen reduction current was limited. The tungsten oxide doping to tantalum oxide improved catalytic performance. The performance enhancement was due to a decrease in resistance polarization with increasing tungsten content mainly due to the decrease in resistance polarization. XPS results indicate that the oxidation state of tungsten is +6 and that of the tantalum is +5, suggesting that excess oxygen is generated in the resulting oxide structure. This compositional effect seems to reduce resistance polarization by altering the surface chemistry of the tantalum oxide and enhancing the reaction steps such as surface diffusion. Maximum performance was achieved with a catalyst containing 32 mol% of tungsten oxide, reaching a mass specific current density of ∼7% that of the commercial Pt/C catalyst at 0.6 V vs. NHE and ∼35% at 0.2 V vs. NHE. In term of area-specific current density, five-fold increase in loading of the doped catalyst leads to a 4-4.5 fold increase in area specific current density at 0.6 V vs. NHE, reaching 66% that of the Pt/C catalyst at 100 rpm and 35% at 2400 rpm.     

Microwave assisted,facile synthesis of Pt/CNT catalyst for proton exchange membrane fuel cell application

The present study aims at developing a high performing Pt/CNT catalyst for ORR in PEM fuel cell adopting modified chemical reduction route using a mixture of NaBH₄ and ethylene glycol (EG) as reducing agent. In order to select the most suitable reduction conditions to realize high performing catalyst, heating of the reaction mixture is done following two methods, conventional heating (CH) or microwave (MW) irradiation. The synthesized Pt/CNT catalysts were extensively characterized and evaluated in-situ as ORR catalyst in PEM fuel cell. A comparison of their performance with the standard, commercial Pt/C catalyst was also made. The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher SSA for Pt/CNT-MWH compared to Pt/CNT-CH. In-situ electrochemical characterization studies revealed higher ESA, lower charge transfer resistance, lower activation over-potential loss and higher peak power density compared to the cathode with Pt/CNT-CH and Pt/C. This study suggests the viability of MW assisted, metal particle deposition as a simple, yet effective method to prepare high performing Pt/CNT catalyst for ORR in PEM fuel cell.     

Power source research at USC: Development of advanced electrocatalysts for polymer electrolyte membrane fuel cells

This paper provides an overview on the development of advanced fuel cell cathode catalysts at University of South Carolina (USC) with the emphasis on the stability of non-precious metal and Pt alloy catalysts. Nitrogen-modified carbon composite (NMCC) catalysts were developed for the oxygen reduction reaction (ORR) through the pyrolysis of cobalt (iron)-nitrogen chelate followed by the treatment combination of pyrolysis, acid leaching, and re-pyrolysis. A promising stability was observed for 1050 h fuel cell operation under current density of 200 mA cm⁻² as evidenced by a potential decay rate as low as 40 μV h⁻¹. The performance degradation mechanism of the NMCC-based fuel cell is discussed. Pt and PtPd hybrid catalysts are developed that use a NMCC, which is itself active for the ORR, instead of a conventional carbon black support. The stability test at 1 A cm⁻² indicated that the Pt/NMCC hybrid catalyst (new Pt-Co/C) is more stable than the conventional Pt-Co/C without the Co leaching out. The PEM fuel cell accelerated stress test (AST) for supports and catalysts demonstrated that their stability changes in the order: Pt₃Pd₁/NMCC hybrid catalyst > Pt/NMCC hybrid catalyst > conventional Pt/C catalyst. Moreover, the hybrid catalysts exhibit higher mass activity than the Pt/C catalysts.     

Highly stable Pt and PtPd hybrid catalysts supported on a nitrogen-modified carbon composite for fuel cell application

The durability and cost of fuel cell cathode catalysts are major technical barriers to the commercialization of fuel cells for vehicle applications. In this work, novel Pt and PtPd hybrid catalysts are developed that use a nitrogen-modified carbon composite (NMCC), which is itself active for the oxygen reduction reaction (ORR), instead of a conventional carbon black support. The fuel cell accelerated stress test (AST) for supports and catalysts demonstrated that the Pt₃Pd₁/NMCC and Pt/NMCC hybrid catalysts possess much higher stability than Pt/C catalysts in polymer electrolyte membrane (PEM) fuel cells. Moreover, the hybrid catalysts exhibit higher mass activity than the Pt/C catalysts. The origin of the hybrid catalysts’ improved performance relative to Pt/C is discussed in light of pore size distribution and surface area analysis, XRD, XPS, and TEM analyses and electrochemical measurements.     

Pt-Pd nanodendrites as oxygen reduction catalyst in polymer-electrolyte-membrane fuel cell

Platinum-palladium (Pt-Pd) bimetallic alloys have shown prospect as electrocatalyst for the oxygen reduction reaction (ORR) in the cathode of polymer-electrolyte-membrane (PEM) fuel cells. This article reports a facile solvothermal synthesis of Pt-Pd bimetallic nanodendrites (Pt-Pd NDs). The characterization with a variety of spectroscopic techniques indicates that the Pt-Pd NDs possess a three-dimensional (3-D) porous structure consisting of interconnected branches of highly alloyed Pt-Pd nanorods (NR). The measurements using rotating disk electrode in electrolyte solution show that the catalyst of Pt-Pd NDs supported on carbon (Pt-Pd NDs/C) possesses a Pt mass activity for ORR that is more than 3 times higher than that of the state-of-the-art Pt/C catalyst, as well as the significantly improved stability due to the branched porous structure. The measurements using membrane-electrode-assembly (MEA) in a single PEM fuel cell indicate the 3-D interconnected dendrite structures make the Pt-Pd NDs/C catalyst significantly advantageous over the nanoparticle Pt/C catalyst in reducing the mass transport and ohmic polarization which would become significant at high current density in MEA.     

Platinum/tin oxide/carbon cathode catalyst for high temperature PEM fuel cell

The performance of high temperature polymer electrolyte fuel cell (HT-PEMFC) using platinum supported over tin oxide and Vulcan carbon (Pt/SnOx/C) as cathode catalyst was evaluated at 160-200 °C and compared with Pt/C. This paper reports first time the Pt/SnOx/C preparation, fuel cell performance, and durability test up to 200 h. Pt/SnOx/C of varying SnO compositions were characterized using XRD, SEM, TEM, EDX and EIS. The face-centered cubic structure of nanosized Pt becomes evident from XRD data. TEM and EDX measurements established that the average size of the Pt nanoparticles were ∼6 nm. Low ionic resistances were derived from EIS, which ranged from 0.5 to 5 Ω-cm² for cathode and 0.05 to 0.1 Ω-cm² for phosphoric acid, doped PBI membrane. The addition of the SnOx to Pt/C significantly promoted the catalytic activity for the oxygen reduction reaction (ORR). The 7 wt.% SnO in Pt/SnO₂/C catalyst showed the highest electro-oxidation activity for ORR. High temperature PEMFC measurements performed at 180 °C under dry gases (H₂ and O₂) showed 0.58 V at a current density of 200 mA cm⁻², while only 0.40 V was obtained in the case of Pt/C catalyst. When the catalyst contained higher concentrations of tin oxide, the performance decreased as a result of mass transport limitations within the electrode. Durability tests showed that Pt/SnOx/C catalysts prepared in this work were stable under fuel cell working conditions, during 200 h at 180 °C demonstrate as potential cathode catalyst for HT-PEMFCs.     

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.     

Oxygen reduction reaction increased tolerance and fuel cell performance of Pt and RuxSey onto oxide-carbon composites

Pt and Ru_xSe_y nanoparticles were selectively deposited onto oxide sites of oxide-carbon composite substrates using the photo-deposition process and compared to conventional carbon support materials. The oxide was essentially anatase phase. Cyclic voltammetry and rotating disk electrode measurements for the oxygen reduction reaction (ORR) in formic acid containing-electrolyte showed a tolerance improvement for ORR of Pt supported on composite substrates. This positive substrate effect on platinum, turned out not to be favorable for Ru_xSe_y catalyst centers. On the other hand, the methanol tolerance for ORR was increased for both nanostructured materials supported on the oxide-carbon composite. Single H₂/O₂ fuel cell results were in agreement with half-cell electrochemical measurements on Pt, showing an improvement of the power density when using the oxide-carbon as substrate for the cathode. The composites were evaluated as cathode catalysts of an HCOOH laminar-flow fuel cell (LFFC) in which commercial Pd/C was used as an anode catalyst. The cathodes with Ru_xSe_y and Pt supported on TiO₂/C improved the specific power density by 15% and 24%, respectively, with respect to carbon as support.     

Sulfated zirconium oxide as electrode and electrolyte additive for direct methanol fuel cell applications

Sulfated zirconium oxide (S-ZrO₂) was used as electrode and electrolyte additive for direct methanol fuel cells (DMFCs). Composite Nafion electrolyte membranes and Pt electrocatalysts, both containing S-ZrO₂ at different content, were prepared. The morphology and catalytic activity of prepared catalysts were investigated by scanning electron microscopy, and voltammetric technique. Results indicated that Pt/S-ZrO₂ catalysts showed enhanced efficiency towards oxygen reduction reaction and increased methanol tolerance as compared to bare platinum. Pt/S-ZrO₂-based carbon cloth electrodes were prepared and assembled as cathode in a DMFC, with Nafion/S-ZrO₂ as composite electrolyte membrane. With respect to bare platinum and Nafion, higher values of current and power density were recorded at 110 °C. The use of S-ZrO₂ both as catalyst and electrolyte additive provided enhanced membrane/electrode interface stability, as revealed by EIS spectra recorded during cell operation.     

Natural aloe vera derived Pt supported N-doped porous carbon: A highly durable cathode catalyst of PEM fuel cell

Evolution of highly durable electrocatalyst for oxygen reduction reaction (ORR) is the most critical barrier in commercializing polymer electrolyte membrane fuel cell (PEMFC). In this work, Pt deposited N-doped mesoporous carbon derived from Aloe Vera is developed as an efficient and robust electro catalyst for ORR. Due to its high mesoporous nature, the aloe vera derived carbon (AVC) play a very vital role in supporting Pt nanoparticles (NPs) with N-doping. After doping N into AVC, more defects are created which facilitates uniform distribution of Pt NPs leading to more active sites towards ORR. Pt/N-AVC shows excellent ORR activity when compared with commercial Pt/C and showing a half wave potential (E_1/2–0.87 V Vs. RHE) and reduction potential (E_red ~ 0.72 V Vs. RHE) towards ORR. Even after 30,000 potential cycles, Pt/N-AVC shows in its E_1/2 only ~5 mV negative shift and lesser agglomeration of Pt NPs is seen in the catalyst. In membrane electrode assembly (MEA) fabrication, Pt/N-AVC as a cathode catalyst in a PEMFC fixture and performance were studied. The Pt/N-AVC shows good performance, which proves the potential application of this naturally available bio derived carbon, which serves as an excellent high durable support material in PEMFC. All these features show that the Pt/N-AVC is the most stable, efficient and suitable candidate for ORR catalyst.     

Control of self-organization of drop-casted Nafion film for improving proton conduction in a polymer-electrolyte-membrane fuel cell to raise its output power density

A simple drop-cast method to directly deposit Nafion polymer electrolyte membrane (PEM) on nanostructured thin-film catalyst layer composed of stacked Pt nanoparticles prepared by pulsed laser deposition (PLD) was demonstrated. Through optimization of solvent composition and drying temperature of Nafion solution to control self-organization of Nafion, a uniform PEM with better bulk and interface microstructures could be produced, leading to a significant improvement in the output current density of a PEM fuel cell over that using reference commercial PEMs. The formation of facile proton conduction pathways in the bulk Nafion membrane resulted in a 35% reduction in ohmic resistance compared to that with the commercial membrane. Moreover, the infiltration of Nafion in the catalyst layer formed suitable proton transport network to render more catalyst nanoparticles effective and thus lower charge-transfer resistance. With the optimized PLD, drop-cast, and hot-pressing conditions, the current density of PEMFCs using drop-casted PEM reached 1902 mA cm⁻² at 0.6 V at 2 atm H₂ and O₂ pressures with a cathode Pt loading of 100 μg cm⁻², corresponding to a power density of 1.14 W cm⁻² and a cathode mass-specific power density of 11.4 kW g⁻¹.     

Electrocatalytic activity of iridium oxide nanoparticles coated on carbon for oxygen reduction as cathode catalyst in polymer electrolyte fuel cell

The iridium oxide nanoparticles supported on Vulcan XC-72 porous carbon were prepared for cathode catalyst in polymer electrolyte fuel cell (PEFC). The catalyst has been characterized by transmission electron microscopy (TEM) and in PEFC tests. The iridium oxide nanoparticles, which were uniformly dispersed on carbon surface, were 2-3 nm in diameter. With respect to the oxygen reduction reaction (ORR) activity was also studied by cyclic voltammetry (CV), revealing an onset potential of about 0.6 V vs. an Ag/AgCl electrode. The ORR catalytic activity of this catalyst was also tested in a hydrogen-oxygen single PEFC and a power density of 20 mW cm⁻² has been achieved at the current density of 68.5 mA cm⁻². This study concludes that carbon-supported iridium oxide nanoparticles have potential to be used as cathode catalyst in PEFC.     

Non-precious metal catalysts synthesized from precursors of carbon, nitrogen, and transition metal for oxygen reduction in alkaline fuel cells

Non-precious metal catalysts (NPMCs) synthesized from the precursors of carbon, nitrogen, and transition metals were investigated as an alternate cathode catalyst for alkaline fuel cells (AFCs). The procedures to synthesize the catalyst and the post-treatment were tailored to refine its electrocatalytic properties for oxygen reduction reaction (ORR) in alkaline electrolyte. The results indicated that the performance of NPMCs prepared with carbon-supported ethylenediamine-transition metal composite precursor and subjected to heat-treatment shows comparable activity for oxygen reduction with Pt/C catalyst. The NPMC exhibits an open circuit potential of 0.97 V and a maximum power density of 177 mW cm⁻² at 50 °C when tested in anion exchange membrane (AEM) fuel cells.     

A noble silver nanoflower on nitrogen doped carbon nanotube for enhanced oxygen reduction reaction

An electrodeposition-approach for the synthesis of silver nanoflowers (AgNFs) on nitrogen doped carbon nanotubes (NCNTs) for the oxygen reduction reaction (ORR) in alkaline media has been developed. The as prepared material (NCNTs-AgNFs) has been characterized by various instrumental methods. The morphological analysis shows the unique rose-like AgNFs are placed onto the NCNTs with better dispersion. The higher population of AgNFs has also been observed onto NCNTs coated glassy carbon (GC) rather than bare GC plate. The X-ray photoelectron spectroscopy shows chemical reduction and N-doping has done successfully with the restoring sp² domain in carbon network. The electrocatalytic activities have been verified using cyclic voltammetry (CV) and hydrodynamic voltammetry techniques in 0.1 M KOH electrolyte. The resulting catalyst system, NCNT-AgNFs, surpasses the performance of Pt/C, in terms of a kinetic current density, better fuel selectivity and durability. It is also noteworthy that the NCNT-AgNFs exhibits a four-electron reduction pathway for ORR with lowering H₂O₂ yield. The admirable performance of NCNT-AgNFs catalyst along with higher durability holds great potential for application in various fuel cells as cathode catalyst.     

Electrophoretically fabricated nickel/nickel oxides as cost effective nanocatalysts for the oxygen reduction reaction in air-cathode microbial fuel cell

The high cost and limited availability of cathode catalyst materials (most commonly Pt) prevent the large-scale practical application of microbial fuel cells (MFCs). In this study, unique Pt group metal-free (PGM-free) nanocatalysts were fabricated using a simple and cost-effective technique called electrophoretic deposition (EPD) to create a high catalytic oxygen reduction reaction rate (ORR) on the cathode surface of MFCs. Among the tested PGM-free catalysts (Ni, Co, and Cd-based), a maximum power density of 1630.7 mW m⁻² was achieved based on nickel nanoparticles. This value was 400% greater than that obtained using a commercial Pt catalyst under the same conditions. This result was due to the uniform deposition of a thin layer of Ni/NiO_x nanoparticles on the cathode, which improved electrical conductivity, catalytic activity, and long-term stability while reducing electron transfer resistance. The fabricated PGM-free catalysts significantly improved MFC performance and accelerated ORR induced by the novel layered morphology of metal/metal oxide nanoparticles.     

Pd5Cu4Pt oxygen reduction nanocatalyst for PEM fuel cells

Carbon dispersed Pd₅Cu₄Pt nanocatalyst synthesized by chemical reduction with NaBH₄ for the oxygen reduction reaction (ORR) in acid media is investigated. Nanocatalyst is physically characterized by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (DRX). Results demonstrate the formation of conglomerate nanometric particles ranging from 2 to 10 nm in size. Electrochemical activity is demonstrated by cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques. The results show that the onset potential for the ORR on Pd₅Cu₄Pt is shifted by ca. 50 mV to more positive values and enhanced catalytic current densities are observed in comparison to carbon dispersed PdCu and Pd catalysts. The Pd₅Cu₄Pt tested as cathode electrode in a membrane-electrode assembly (MEA) shows a power density of 330 mW cm⁻² at 0.5 V and 80 °C, resulting an attractive low Pt content cathode nanocatalyst for PEM fuel cells.     

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.     

Chemically and thermally reduced graphene oxide supported Pt catalysts prepared by supercritical deposition

Reduced graphene oxide (RGO) is used in many energy applications, especially in Polymer Electrolyte Membrane (PEM) fuel cells, as carbon sourced catalyst support materials. In this study, thermally (T-RGO) and chemically (C-RGO) reduced GO support materials were synthesized for utilization in PEM fuel cells. Pt catalysts were synthesized using supercritical carbon dioxide (SCCO₂) deposition technique over synthesized support materials. Physical (BET, SEM-EDX, FTIR, RAMAN, XRD, TEM, ICP-MS and optical tensiometer) and electrochemical (CV, PEM fuel cell test) characterizations of synthesized support materials and corresponding Pt catalysts were performed. The differences between the structures of thermally and chemically reduced graphene oxide supports and their Pt catalysts were investigated. The ECSA values of the Pt/T-RGO and Pt/C-RGO catalysts are 19.86 m² g^?1 and 6.31 m² g^?1, respectively. The current and power density values of the Pt/T-RGO and Pt/C-RGO catalysts at 0.6 V are 84 mA cm^?2, 80 mA cm^?2 and 50 mW cm^?2, 45 mW cm^?2, respectively. Pt/T-RGO and Pt/C-RGO catalysts showed similar trend in PEMFC environment.     

Electrocatalysis of oxygen reduction when varying the mass ratio of metal nanoparticles to carbon support for catalysts with a 10 to 10 to 80 mol% of Pt and Pd on Ag

The reduction of total Pt-loading in a cathode catalyst without sacrificing performance is one of the key objectives for the large-scale commercialization of proton exchange membrane fuel cell (PEMFC) technology. A core-shell type nanostructured catalyst with a Pt-loading 20 times lower than a commercial catalyst is demonstrated herein to be more active for the electrocatalysis of the oxygen reduction reaction (ORR) in acid electrolyte. The weight ratio of metal nanoparticles on carbon support is the key to achieving the highest ORR activity in a series of silver-based catalysts, all with 10 mol percent of Pt and 10 mol percent of Pd over 80 mol percent of silver (Ag) and supported on untreated Vulcan carbon to form an electrocatalyst (Ag@Pt₁₀Pd₁₀/C) with either 5, 10, 20 or 30 wt% of total metals on carbon; which correspond to a Pt concentration around 1, 2, 3 and 5 wt%, respectively. All metal nanostructures on carbon show a similar morphology, size and structure. Thin films of these four Ag@Pt₁₀Pd₁₀/C catalysts on rotating disk electrodes (TF-RDEs) all shown a 4-electrons pathway for the ORR and give higher exchange current densities (j_o > 3.8 mA/cm²) than a commercial Etek Pt₂₀/C catalyst (j_o = 2.4 mA/cm²). The Ag@Pt₁₀Pd₁₀/C catalyst with 5 wt% of total metals (1 wt% of Pt) on carbon gives the best electrocatalysis; reducing molecular oxygen to water two times faster and generating 25% higher current per milligram of platinum (mass activity) than the commercial catalyst (Pt₂₀/C). Therefore, the Ag@Pt₁₀Pd₁₀/C catalyst with 5 wt% of total metals is a new catalyst for ORR for a PEMFC with a lower Pt loading and cost.