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.     

Development of a catalytic combustor for a stationary fuel cell power generation system

The anode off-gas of high temperature stationary fuel cell stacks still includes fuel components such as hydrogen, carbon monoxide, and hydrocarbon due to the innate characteristics of the fuel cell operation. Even though the anode off-gas has fuel contents, the flammability is very limited due to the vapor concentration of the anode off-gas. A catalytic combustor is applied as an off-gas combustor so as to utilize the waste energy of anode off-gas by stimulating a chemical reaction over selected operating conditions. Temperature and flow uniformity in the radial direction are very significant factors of the durability, because the catalytic combustion is carried out on the surface of the catalyst site. On the other hand, the catalyst selection is also very important due to the composition of the anode off-gas. In this study, the flow uniformity is presented prior to a catalyst screening test. From the results of the screening test, where three commercially available catalysts are tested, KIMM-I and KIMM-II are selected as candidates for a catalytic combustor of anode off-gas.     

Cobalt molybdenum carbides as anode electrocatalyst for proton exchange membrane fuel cell

Cobalt molybdenum (Co-Mo) carbides were prepared by the carburization of Co-Mo oxides at temperatures of 723–973 K in a stream of CH₄/H₂ gas. The carburized catalysts were evaluated using a single-stack fuel cell and three-electrode cell. The results showed high activities for the anodic electrooxidation of hydrogen over the Co-Mo catalysts carburized at 873 and 923 K. The 873 K carburized Co-Mo catalyst had the highest activity and achieved 10.9% of the performance of a commercial Pt/C catalyst in a single-stack fuel cell. The XRD, TPC, TPR and XPS results showed that the Co-Mo oxycarbide in the bulk and on the surface are the active species for the hydrogen oxidation reaction.     

Improved thermal stability,properties, and electrocatalytic activity of sol-gel silica modified carbon supported Pt catalysts

An improvement in the thermal properties of catalysts used in PEM type fuel cells was achieved by siliceous additions. A sol-gel method was developed to deposit controlled amounts of a siliceous composition on Vulcan XC72 carbon (VC). The catalysts designated as Pt/(VC–SiO₂) were obtained by reduction of H₂PtCl₆ in an aqueous solution with NaBH₄. A nanoscale Pt particle formation was observed on the support materials having a range 2.7–5.1 nm. The thermal stability study for Pt/(VC–SiO₂) catalysts demonstrated that the presence of a siliceous phase conferred an increased resistance to Pt nanoparticle agglomeration at 600 °C. In addition, a decrease in low temperature mass loss was observed. Electrochemical properties evaluated by cyclic voltammetry coupled with a rotating disk electrode (RDE) showed improvements with moderate SiO₂ addition. The synthesized catalysts performance was as good as the performance of the control catalyst (46 wt% Pt/VC, Tanaka). Unfortunately, fuel cell performance experiments showed an unwanted hydrophillic behavior of carbon-silica composite aerogel supports at high current density values. The C–SiO₂ aerogel composite catalyst support seems suitable for low current applications.     

Pt nanowire/Ti3C2Tx-CNT hybrids catalysts for the high performance oxygen reduction reaction for high temperature PEMFC

The activity of catalyst could be enhanced by the temperature rising, so it is a suitable way to reduce the noble metal loading for the catalyst. However, the corrosion of carbon supports will be remarkable in the high temperature proton exchange membrane fuel cells (HT-PEMFC, >100 °C). This report demonstrated a novel Ti₃C₂T_x and CNT hybrid material as the catalytic support, and Pt nanowires (Pt NWs) is loaded on the hybrid support to construct the catalyst for HT-PEMFC. The Pt NWs/Ti₃C₂T_x-CNT performs higher electrochemical activity, better stability than that of commercial Pt/C. The mass activity and specific activity of Pt NWs/Ti₃C₂T_x-CNT catalysts are 3.89 and 3.02 times as that of Pt/C, respectively. The power densities of HT-PEMFC showed 155.4 mW cm⁻² and 182 mW cm⁻² at 150 and 180 °C, respectively.     

Chemically synthesized reduced graphene oxide-carbon black based hybrid catalysts for PEM fuel cells

In this study, hybrid (synthesized rGO and commercial carbon black (CB) in various weight ratios) supported Pt catalysts were synthesized by using the supercritical carbon dioxide deposition (scCO₂) technique for PEM fuel cells. In hybrid materials, rGO to CB weight ratios were changed in between 90:10 to 50:50 which were compared to their plain materials. The physicochemical and the electrochemical characteristics of the materials were examined by using BET, XRD, TGA, TEM, contact angle and roughness measurements, CV and PEM fuel cell performance tests. All these characterizations showed that the hybrid supported Pt catalysts were successfully synthesized. TEM images of the catalysts confirmed the highly dispersed and small nanoparticle formation (1.9–2.9 nm) via scCO₂ deposition technique. Among the hybrid supported catalysts, catalyst having rGO:CB ratio of 70:30 showed the best PEM fuel cell performance. Electrochemical characterization either fuel cell performance test or CV results indicated significantly enhancement in activity with an increase in CB amount in the support.     

Development and performance of a K–Pt/γ-Al2O3 catalyst for the preferential oxidation of CO in hydrogen-rich synthesis gas

Proton exchange membrane fuel cells are widely employed in micro combined heat and power cogeneration (micro-CHP) systems, and the feed to them should be essentially free of CO. CO preferential oxidation is an effective method for the thorough removal of CO from synthesis gas. A series of K–Pt/γ-Al₂O₃ catalysts are prepared and tested for their CO cleaning capabilities. The catalyst is prepared from potassium nitrate acid, chloroplatinic acid and γ-Al₂O₃ powder by normal or ultrasonic impregnation. The catalyst performance is investigated in a micro-reactor system. The effects of K loading, Pt loading, ultrasonic processing, space velocity, O₂-to-CO ratio and operation temperature on catalyst performance are studied. A CO concentration of less than 10 ppm is achieved when the CO concentration in the feed gas is 0.45%. It was found that both ultrasonic processing and the addition of K promote the catalyst performance. The 15K1.0Pt/Al–U catalyst exhibits the best performance.     

Size-controlled synthesis of Pt nanoparticles and their electrochemical activities toward oxygen reduction

Pt/C catalysts with high activity toward oxygen reduction reactions are fabricated in the presence of FeCl₃ as an additive to control the Pt particle size. The polyol method is used for fabrication; 20 wt% Pt/C catalysts are produced with various amounts of FeCl₃ under different fabrication conditions such as pH and temperature. Adding FeCl₃ significantly reduces Pt particle size, which is believed to occur as a result of the interference of Pt reduction rate by Fe³⁺ ions. The average Pt particle size obtained using the fabrication conditions of Pt:Fe = 1:3 (mole ratio), pH = 10, and 130 °C is around 2.0 nm, which is much smaller than that of commercial or homemade Pt/C produced in the absence of FeCl₃. The electrochemically active surface area of this catalyst is 60.6 m²/g. The mass activities at 0.9 and 0.85 V are 15.3 and 42.7 mA/mg_Pt, respectively, which is about 1.8-2.1 times higher than those of commercial Pt/C catalysts. PEMFC test results show that the performance of the Pt/C catalyst fabricated with FeCl₃ additive is better than that of as commercial catalyst.     

Metal foam-supported Pd–Rh catalyst for steam methane reforming and its application to SOFC fuel processing

Pd–Rh/metal foam catalyst was studied for steam methane reforming and application to SOFC fuel processing. Performance of 0.068 wt% Pd–Rh/metal foam catalyst was compared with 13 wt% Ni/Al₂O₃ and 8 wt% Ru/Al₂O₃ catalysts in a tubular reactor. At 1023 K with GHSV 2000 h⁻¹ and S/C ratio 2.5, CH₄ conversion and H₂ yield were 96.7% and 3.16 mol per mole of CH₄ input for Pd–Rh/metal foam, better than the alumina-supported catalysts. In 200 h stability test, Pd–Rh/metal foam catalyst exhibited steady activity. Pd–Rh/metal foam catalyst performed efficiently in a heat exchanger platform reactor to be used as prototype SOFC fuel processor: at 983 K with GHSV 1200 h⁻¹ and S/C ratio 2.5, CH₄ conversion was nearly the same as that in the tubular reactor, except for more H₂ and CO₂ yields. Used Pd–Rh/metal foam catalyst was characterized by SEM, TEM, BET and CO chemisorption measurements, which provided evidence for thermal stability of the catalyst.     

Further performance enhancement of a DME-fueled solid oxide fuel cell by applying anode functional catalyst

In order to increase the coking resistance of SOFCs operating on DME fuel, a Pt/Al₂O₃–Ni/MgO mixture catalyst was investigated for internal partial oxidation of DME. Catalytic test demonstrated the mixture catalyst has higher activity for DME partial oxidation and lower CH₄ selectivity than the individual Pt/Al₂O₃ and Ni/MgO catalysts. O₂-TPO analysis demonstrated that the mixture catalyst also had much higher coke resistance than sintered Ni-YSZ anode, especially at high O₂ to DME ratio. Raman spectroscopy of the carbon-deposited catalysts demonstrates that the graphitization degree of carbon is reduced with introducing O₂ into DME, and the carbon deposited on the mixture catalyst is almost in amorphous structure. Two operation modes of the mixture catalyst for indirect internal partial oxidation of DME, i.e, directly depositing on the anode surface and locating in the anode chamber were tried. The performance of the cells operating on DME fuel through both operation modes was studied by I–V polarization test and EIS characterization. The cells delivered attractive peak power density of around 750 mW cm⁻² by operating on DME-O₂ mixture gas, modestly lower than 1012–1065 mW cm⁻² operating on pure hydrogen fuel at 700 °C. The direct deposition of Pt/Al₂O₃–Ni/MgO onto anode surface to perform as a functional layer and a DME to O₂ ratio of 2:1 in the mixture gas is preferred to minimize coke formation and maximize power output for the cell to operate on DME fuel.     

Tailoring performance of Co–Pt/MgO–Al2O3 bimetallic aerogel catalyst for methane oxidative carbon dioxide reforming: Effect of Pt/Co ratio

Co–Pt/MgO–Al₂O₃ bimetallic aerogel catalysts were synthesized via a sol-gel combined with supercritical drying method. The catalysts were characterized by XRD, BET, HRTEM, STEM-HAADF, XPS, H₂-TPR, H₂-TPD, TG/DSC, FESEM and their catalytic performances in CH₄ oxidative CO₂ reforming were evaluated. The H₂ spillover effect between Pt and Co enhanced the reducibility of the catalyst, while the strong metal-support interaction (SMSI) effect in the bimetallic aerogel catalysts confined the agglomeration of metal particles. Pt/Co ratio played a key role on the existence of surface metal species, leading to different catalytic performances. The optimal Pt/Co ratio was Pt/Co = 0.02 w/w, on which a 50% higher activity in terms of CH₄ conversion than monometallic Co or Pt aerogel catalysts was obtained. Whereas the impregnated catalyst with an identical composition showed a much lower activity. The Co–Pt aerogel catalysts also showed high resistance to inactive carbon formation. The oxidation temperature of the carbon species deposited on the spent Co–Pt aerogel catalyst was only 275 °C and no filamentous or graphitic carbon was identified, disclosing that the formation of inactive carbon was inhibited due to the synergy between Co and Pt and the SMSI effect.     

Effect of Pt–Pd/C coupled catalyst loading and polybenzimidazole ionomer binder on oxygen reduction reaction in high-temperature PEMFC

Polybenzimidazole (PBI) was studied as an ionomer binder at varying ratios (1–7) in a 20–40 wt% Pt–Pd/C cathode-coupled catalyst layer for the oxygen reduction reaction (ORR) in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). Catalytic activity was examined by CV and LSV, while the properties of the catalysts were characterized by FESEM-EDX, N₂ adsorption–desorption, XRD and FTIR. The results showed that the distribution of metals on the carbon surface, carbon wall thickness and the interaction between ionomer and coupled catalysts affected the ORR performance. The fabricated membrane electrode assembly with 5:95 PBI: 30 wt% Pt–Pd/C catalyst ratio exhibited the best performance and highest durability for HT-PEMFC at 170 °C, yielding a power density of 1.30 Wcm⁻² with 0.02 mg_Pt/cm Pt loading. This performance of ultra-low metal loading of coupled Pt–Pd/C electrocatalyst with PBI binder was comparable to those reported by other studies, highlighting a promising catalyst for fuel cell application.     

Part-load performance characteristics of a lean burn catalytic combustion gas turbine system

The purpose of this study is to compare the part-load performance of a lean burn catalytic combustion gas turbine (LBCCGT) system in three different control modes: varying fuel, bleeding off the fuel mixture flow after the compressor and varying rotational speed. The conversions of methane species for chemical process are considered. A 1D heterogeneous plug flow model was utilized to analyze the system performance. The actual turbomachinery components were designed and predicted performance maps were applied to system performance research. The part-load characteristics under three control strategies were numerically investigated. The main results show that: the combustor inlet temperature is a significant factor that can significantly affect the part-load characteristics of the LBCCGT system; the rotational speed control mode can provide the best performance characteristics for part-load operations; the operation range of the bleed off mode is narrower than that of the speed control mode and wider than that of the fuel only mode; with reduced power, methane does not achieve full conversion over the reactor at the fuel only control mode, which will not warrant stable operation of the turbine system; the thermal efficiency of the LBCCGT system at fuel only control strategy is higher than that at bleed off control strategy within the operation range.     

CNT-based catalysts for H2 production by ethanol reforming

Hydrogen production by steam reforming of ethanol (SRE) was studied using steam-to-ethanol ratio of 3:1, between the temperature range of 150–450 °C over metal and metal oxide nanoparticle catalysts (Ni, Co, Pt and Rh) supported on carbon nanotubes (CNTs) and compared to a commercial catalyst (Ni/Al₂O₃). The aim was to find out the suitability of CNTs supports with metal nanoparticles for the SRE reactions at low temperatures. The idea to develop CNT-based catalysts that have high selectivity for H₂ is one of the driving forces for this study. The catalytic performance was evaluated in terms of ethanol conversion, product gas composition, hydrogen yield and selectivity to hydrogen. The Co/CNT and Ni/CNT catalysts were found to have the highest activity and selectivity towards hydrogen formation among the catalysts studied. Almost complete ethanol conversion is achieved over the Ni/CNT catalyst at 400 °C. The highest hydrogen yield of 2.5 is, however, obtained over the Co/CNT catalyst at 450 °C. The formation of CO and CH₄ was very low over the Co/CNT catalyst compared to all the other tested catalysts. The Pt and Rh CNT-based catalysts were found to have low activity and selectivity in the SRE reaction. Hydrogen production via steam reforming of ethanol at low temperatures using especially Co/CNT catalyst has thus potential in the future in e.g. the fuel cell applications.     

Investigation of the effect of graphitized carbon nanotube catalyst support for high temperature PEM fuel cells

In this study, it is aimed to investigate the graphitization effect on the performance of the multi walled carbon nanotube catalyst support for high temperature proton exchange membrane fuel cell (HT-PEMFC) application. Microwave synthesis method was selected to load Pt nanoparticles on both CNT materials. Prepared catalyst was analyzed thermal analysis (TGA), Transmission Electron Microscopy (TEM) and corrosion tests. TEM analysis proved that a distribution of Pt nanoparticles with a size range of 2.8–3.1 nm was loaded on the Pt/CNT and Pt/GCNT catalysts. Gas diffusion electrodes (GDE) were manufactured by an ultrasonic spray method with synthesized catalyst. Polybenzimidazole (PBI) membrane based Membrane Electrode Assembly (MEA) was prepared for observe the performance of the prepared catalysts. The synthesized catalysts were also tested in a HT-PEMFC environment with a 5 cm² active area at 160 °C without humidification. This study demonstrates the feasibility of using the microwave synthesis method as a fast and effective method for preparing high performance Pt/CNT and Pt/GCNT catalyst for HT-PEMFC. The HT-PEMFC performance evaluation shows current densities of 0.36 A/cm²0.30 A/cm² and 0.20 A/cm² for the MEAs prepared with Pt/GCNT, Pt/CNT and Pt/C catalysts @ 0.6 V operating voltage, respectively. AST (Accelerated Stress Test) analyzes of MEAs prepared with Pt/GCNT and Pt/CNT catalysts were also performed and compared with Pt/C catalyst. According to current density @ 0.6 V after 10,000 potential cycles, Pt/GCNT, Pt/CNT and Pt/C catalysts can retain 61%, 67% and 60% of their performance, respectively.     

Low temperature hydrogen production by catalytic steam reforming of methane in an electric field

Catalytic steam reforming of methane in an electric field (electroreforming) at low temperatures such as 423 K was investigated. Pt catalysts supported on CeO₂, Ce_xZr_1−xO₂ solid solution and a physical mixture of CeO₂ and other insulators (ZrO₂, Al₂O₃ or SiO₂) were used for electroreforming. Among these catalysts, Pt catalyst supported on Ce_xZr_1−xO₂ solid solution showed the highest activity for electroreforming (CH₄ conv. = 40.6% at 535.1 K). Results show that the interaction among the electrons, metal loading, and catalyst support was important for high catalytic activity on the electroreforming. Catalytic activity of the electroreforming increased in direct relation to the input current. Characterizations using X-ray diffraction (XRD), temperature programmed reduction with H₂ (H₂-TPR), and alternate current (AC) impedance measurement show that the catalyst structure is an important factor for activity of electroreforming.     

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.     

Direct ethanol fuel cells based on PtSn anodes: the effect of Sn content on the fuel cell performance

In the present work, several carbon supported PtSn catalysts with different Pt/Sn atomic ratios were synthesized and characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Both the results of TEM and XRD showed that all in-house prepared carbon supported Pt and PtSn catalysts had nanosized particles with narrow size distribution. According to the primary analysis of XPS results, it was confirmed that the main part of Pt of the as-prepared catalysts is in metallic state while the main part of Sn is in oxidized state. The performances of single direct ethanol fuel cells were different from each other with different anode catalysts and at different temperatures. It was found that, the single DEFC employing Pt₃Sn₂/C showed better performance at 60 °C while the direct ethanol fuel cells with Pt₂Sn₁/C and Pt₃Sn₂/C exhibited similar performances at 75 °C. Furthermore, at 90 °C, Pt₂Sn₁/C was identified as a more suitable anode catalyst for direct ethanol fuel cells in terms of the fuel cell maximum power density. Surface oxygen-containing species, lattice parameters and ohmic effects, which are related to the Sn content, are thought as the main factors influencing the catalyst activity and consequently the performance of single direct ethanol fuel cells.     

A high-performance PdZn alloy catalyst obtained from metal-organic framework for methanol steam reforming hydrogen production

Methanol steam reforming (MSR) is deemed to be an effective way for hydrogen production and Pd/ZnO catalyst were found to exhibit high activity in this reaction. However, their activities are strongly related to the preparations methods. In most cases, these catalysts are synthesized by impregnation or co-precipitation methods, aiming to change the dispersion and stability of Pd nanoparticle to get better performance. Here we report an efficient Pd/ZnO catalyst that was synthesized with zeolitic imidazolate framework-8 (ZIF-8) as the precursor, on which Pd ions was supported after NaHB₄ reduction. Typically, when the catalyst reduced at 300 °C for 2 h, the methanol conversion could reach 97–98% and the CO₂ selectivity is around 86.3% under the reaction condition of 0.1 MPa, water/CH₃OH = 1.2:1 (mol ratio), WHSV_methanol = 43152ml/g_cat*h, catalyst = 0.1 g, our catalyst was found to show much better performance than other Pd@ZnO catalysts prepared by other methods, especially in terms of selectivity which is particularly important for hydrogen fuel cell application considering that Pt electrode could be poisoned by even trace amount of CO. It turned out that the large surface area, enough holes, evenly distributed PdZn alloy activity sites and abundant oxygen vacancies lead to the overall excellent performance of our catalyst.     

Hydrogen production through autothermal reforming of CH4: Efficiency and action mode of noble (M = Pt,Pd) and non-noble (M = Re,Mo, Sn) metal additives in the composition of Ni-M/Ce0.5Zr0.5O2/Al2O3 catalysts

Hydrogen production through autothermal reforming of methane (ATR of CH₄) over promoted Ni catalysts was studied. The control of the ability to self-activation and activity of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was achieved by tuning their reducibility through the application of different types (M = Pt, Pd, Re, Mo or Sn) and content (molar ratio M/Ni = 0.003, 0.01 or 0.03) of additive. The comparison of the efficiency and action mode of noble (M = Pt, Pd) and non-noble (M = Re, Mo, Sn) metal additives in the composition of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was performed using X-ray fluorescence analysis, N₂ adsorption, X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and thermal analysis. The composition-characteristics-activity correlations were determined. It was shown that the introduction of a promoter does not affect the textural and structural properties of catalysts but influences their reducibility and performance in ATR of CH₄. At the similar dispersion of NiO active component (11 ± 2 nm), the Ni²⁺ reduction is intensified in the following order of additives: Mo < Sn < Re ≤ Pd < Pt. It was found that for the activation of Ni and Ni–Sn catalysts before ATR of CH₄ tests, the pre-reduction is required. On the contrary, the introduction of Pt, Pd and Re additives leads to the self-activation of catalysts under the reaction conditions and an increase of the H₂ yield due to the enhanced reducibility of Ni²⁺. The efficient and stable catalyst for hydrogen production has been developed: in ATR of CH₄ at 850 °C over an optimum 10Ni-0.9Re/Ce_0.5Zr_0.5O₂/Al₂O₃ catalyst the H₂ yield of 70% is attained. The designed catalyst has enhanced stability against oxidation and sintering of Ni active component as well as high resistance to coking.