Composite Catalyst Layers Research Articles

Improved Vapor-Feed Direct Methanol Fuel Cell by Hydrophobic/Hydrophilic Composite Catalyst Layers via Kelvin Equation

Improved Vapor-Feed Direct Methanol Fuel Cell by Hydrophobic/Hydrophilic Composite Catalyst Layers via Kelvin Equation

Direct-methane solid oxide fuel cells with an in situ formed Ni–Fe alloy composite catalyst layer over Ni–YSZ anodes

Coking on Ni surfaces limits the direct application of methane-based fuels in SOFCs with Ni-cermet anodes. Loading an anodic catalytic layer with a high catalytic activity for CH4 conversion can effectively protect the Ni-based anode from coking and increase the cell durability. In this work, a Ni–Fe alloy composite catalyst was prepared by reducing perovskite La0.7Sr0.3Fe0.8Ni0.2O3-δ (LSFN) and then evaluating its catalytic activity in the partial oxidation of CH4. The catalyst was applied on a conventional Ni–8 mol.% Y-stabilized ZrO2 (YSZ) anode for methane SOFCs using two methane-containing fuels (97% CH4–3% H2O and 30% CH4–70% air). The catalyst-modified cells showed much higher performances and durability than the conventional cell using a Ni–YSZ anode, indicating the potential application for direct-methane SOFCs.

Imaging ionomer in fuel cell catalyst layers with synchrotron nano transmission x-ray microscopy

This work presents a novel characterization method for distinguishing ionomer phase within porous composite catalyst layer. Quantifying ionomer distribution in a catalyst layer of polymer electrolyte fuel cells (PEFCs) is critical to understanding the morphological and transport properties of this key component. Current methods to image ionomer rely on staining it with heavy ions, such as cesium, Cs, to enable detection with hard X-rays, however if the catalyst layer contains electrocatalyst, such as platinum, Pt, then the challenge is to distinguish between Cs-stained ionomer and Pt. These two materials have very similar X-ray attenuation coefficients. Using synchrotron transmission x-ray microscopy (TXM) and two energy imaging, above and below Pt electrocatalyst absorption edge, the ionomer distribution is quantified. The method is applied to a PEFC catalyst layer and compared against the method of single energy imaging, which is not able to separate the two phases.

Cooperative activation effect on H2O adsorption in MnO–Co catalyzed steam methane reforming

Steam methane reforming is a very important chemical process in hydrogen production and solid oxide fuel cells (SOFCs). Cobalt (Co) is an important catalyst for dry and steam methane reforming. However, previous studies have confirmed that metal Co surfaces only have weak adsorption activity for H2O, which is evidently unfavorable for steam reforming. In this work we used first-principles simulations to study the activity of MnO–Co catalysts for the adsorption of H2O. Compared with the Co (111) surface and pristine Co clusters, the MnO–Co catalytic layer has a stronger adsorption capability for H2O because of the introduction of the MnO substrate, which is crucial for improving the steam reforming reaction and inhibiting carbon disposition in SOFCs. The cooperation mechanism between MnO and Co is discussed based on the analysis of electronic structures. The conclusions from this work are universal for other metal-oxide composite catalyst layers.

(La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ-Ce0.8Gd0.2O1.9 scaffolded composite cathode for high temperature CO2 electroreduction in solid oxide electrolysis cell

As a promising cathode material for CO2 electroreduction in solid oxide electrolysis cell, (La,Sr) (Cr,Mn)O3 perovskite usually suffers from insufficient electrocatalytic activity and poor stability. We report here that a (La,Sr) (Cr,Mn)O3-based scaffolded composite cathode fabricated through co-loading (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ-Ce0.8Gd0.2O1.9 composite within the porous yttria-stabilized zirconia scaffold, enables the electrolyte-supported solid oxide electrolysis cell to exhibit high electrocatalytic activity and stability towards CO2 electroreduction in comparison with the conventional (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ-Ce0.8Gd0.2O1.9 cathode. This scaffolded architecture design provides micro-sized pores for CO2 transportation, well-connected yttria-stabilized zirconia network for oxygen ion conduction, (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ-Ce0.8Gd0.2O1.9 composite catalyst layer for creating highly active (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ-Ce0.8Gd0.2O1.9-gas three-phase boundaries and increasing surface oxygen vacancies concentration. Furthermore, the intimate interaction between (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ and Ce0.8Gd0.2O1.9 nanoparticles in the composites effectively suppresses particles aggregation. The (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3-δ perovskite-Ce0.8Gd0.2O1.9 fluorite scaffolded composite cathode offers a promising approach to prepare highly active and stable (La,Sr) (Cr,Mn)O3 based cathode for CO2 electroreduction in high temperature solid oxide electrolysis cell.

IrOx/Nafion Catalyst for Oxygen Evolution: Effect of Surface Oxide on Activity and Stability

High catalyst loading is a major barrier for large-scale production of proton exchange membrane water electrolysis (PEMWE) despite its advantages over other electrolysis technologies. [1]. [2,3] Herein, a low iridium loading composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT). In this process, the catalyst synthesis and catalyst-coated membrane (CCM) fabrication are combined into one step. The activity and stability for oxygen evolution reaction (OER) are tested through rotating disk electrode (RDE) measurements and membrane electrode assembly (MEA), respectively. The iridium particles has an average size of ~2 nm resulting in very high surface area and superior activity over planar and electrodeposited IrO2 thin films. In this study, two flame conditions (F-1 and F-2) are used for synthesizing the iridium particles and a correlation between the surface oxide layer and the electrochemical performance is observed. Based on preliminary results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), we hypothesize that the iridium particles exhibit core-shell structure with a metallic iridium core and mixed iridium (IV and III) oxides shell. Higher flame temperature and longer residence time in the flame (F-2 condition) increases the amount of surface oxide and forms a thicker oxide layer. The oxide composition of the particle can be determined from XPS results (Figure 1 a and b) employing the fitting model proposed in reference [4]. The oxide composition for sample F-1 and F-2 are 54% and 90%, respectively. On the other hand, electron diffraction pattern from TEM analysis shows a metallic phase, which suggests the oxides may reside only on the surface since XPS is surface sensitive. Electrochemical measurements have shown that the iridium particles with higher oxide content improves the OER mass activity by ~3 fold over particles with lower oxide content. Furthermore, the catalyst stability in electrolysis test is improved from 250 hours to 4500 hours with the higher oxide content. Detailed surface analysis is carried out using XPS and high-resolution TEM to obtain the chemical state or change of iridium in particles synthesized under different flame conditions and to investigate the role of surface oxide in the enhancement of activity and stability. Figure 1. XPS of iridium particles synthesized in F-1 (a) and F-2 condition (b) fitted with asymmetric Doniach-Sunjic line shape.

Iridium Oxide/Nafion Catalyst for Oxygen Evolution Reaction and Proton Exchange Membrane Water Electrolyzer

Proton exchange membrane water electrolysis (PEMWE) offers several advantages over other electrolysis technologies including greater energy efficiency, higher production rates, dynamic responses and more compact design. [1] High loading is a major barrier to the path of large-scale production of PEMWE. [2] The state-of-the-art anode catalysts in conventional PEMWEs are mixed iridium and iridium oxides (IrOx) with typical iridium loading from 1 to 3 mg cm-2, which is too high to meet the long term cost target for energy markets [2]. Meanwhile, translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of electrode stability. [3] Herein, a composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT) which combines the catalyst synthesis and catalyst-coated membrane (CCM) fabrication into one step. The RSDT-derived IrOx/Nafion showed distinctive features that are beneficial to the oxygen evolution reaction (OER) activity and durability. The iridium particles were about 1-2 nm with very high surface area due to the intrinsic property of resistance to sintering [4]. This resulted in high anodic charge proportional to the oxide surface area and superior activity over planar and electrodeposited IrO2thin films. The OER activity can be further enhanced by increasing the amount of surface oxide of iridium which was achieved by increasing the flame length, flame temperature and oxygen flow rate. In Figure 1, F-1 and F-2 corresponds two different flame conditions with iridium surface oxide percentage of 28% and 100%, respectively. Higher surface oxide percentage (F-2) resulted in superior anodic charge (Figure 1a) and OER activity (Figure 1 b,c). The electrolyzer durability of the RSDT-derived IrOx/Nafion catalyst layer was optimized by increasing the amount of iridium oxide on the iridium particle surface and by decreasing the number of IrOx/Nafion agglomerates on the catalyst layer surface. Ongoing studies of CCMs fully fabricated by the RSDT process showed promising electrolyzer durability of 3300 hours (Figure 1d). Analysis of post-durability test samples will be conducted to investigate the effect of long-term operation on catalyst layer integrity and iridium dissolution and migration in the electrolyte membrane. Figure 1. Cyclic voltammetry (CV) of RSDT-derived IrOx/Nafion catalyst with two flame conditions (a). CV recorded in nitrogen-purged 0.1M HClO4 at 25 oC with 20 mV/s scan rate from 0.0 V to 1.5V (vs. RHE). Quasi-stationary polarization curves for OER recorded from 1.45-1.69 V vs. RHE with 20 mV step and 5 min dwell time (b). (c) Compares the OER mass activity and specific activity of RSDT-derived IrOx/Nafion catalysts and pure IrO2 (adapted from [5]) tested with the same procedure. (d) Shows the electrolyzer MEA durability test of RSDT-derived cathode and anode. Test performed using a circular 86 cm2 cell at 80 oC with 1.8 A cm-2current load and 400 psi hydrogen pressure.

Ni‐Al2O3/Ni‐foam catalyst with enhanced heat transfer for hydrogenation of CO2 to methane

Monolithic Ni‐Al2O3/Ni‐foam catalyst is developed by modified wet chemical etching of Ni‐foam, being highly active/selective and stable in strongly exothermic CO2 methanation process. The as‐prepared catalysts are characterized by x‐ray diffraction scanning electron microscopy, inductively coupled plasma atomic emission spectrometry, and H2‐temperature programmed reduction‐mass spectrometry. The results indicate that modified wet chemical etching method is working efficiently for one‐step creating and firmly embedding NiO‐Al2O3 composite catalyst layer (∼2 μm) into the Ni‐foam struts. High CO2 conversion of 90% and high CH4 selectivity of >99.9% can be obtained and maintained for a feed of H2/CO2 (molar ratio of 4/1) at 320°C and 0.1 MPa with a gas hourly space velocity of 5000 h−1, throughout entire 1200 h test over 10.2 mL such monolithic catalysts. Computational fluid dynamics calculation and experimental measurement consistently confirm a dramatic reduction of “hotspot” temperature due to enhanced heat transfer. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4323–4331, 2015

Composition Dependence of the Pore Structure and Water Transport of Composite Catalyst Layers for Polymer Electrolyte Fuel Cells

The material distribution, porous features, and ionomer behavior in the catalyst layer (20wt% platinum on Vulcan carbon impregnated with Nafion) of polymer electrolyte fuel cells were investigated by microscopy, nitrogen adsorption, and water uptake as a function of ionomer loading or relative humidity. As revealed by microscopy, thin ionomer coverage along the carbon surface is observed when ionomer loading is low, while for higher loading, the pore spaces among the carbon agglomerates were mostly flooded. Even at a low ionomer to carbon ratio, the micro-porosity is closed off and the BET surface area decreases by more than 50%. At 100% relative humidity, the water content in the ionomer within the I/C = 0.5 catalyst layer reaches only 4 water molecules per sulfonate, presumably due to the inhibition of swelling by the carbon filler. The resulting structures and properties in the catalyst layer induce a tortuous pathway for water or proton transport in the ionomer phase, particularly at low water content. A distinct feature is observed in the catalyst layers using differential scanning calorimetry (DSC). DSC measurements on the sub-components lead us to conclude that the feature reflects interactions between the ionomer and the carbon or the Pt particles.

High performance low temperature carbon composite catalysts for flexible dye sensitized solar cells

Roll-to-roll manufacturing of dye sensitized solar cells (DSSCs) requires efficient and low cost materials that adhere well on the flexible substrates used. In this regard, different low temperature carbon composite counter electrode (CE) catalyst ink formulations for flexible DSSCs were developed that can be simply and quickly coated on plastic substrates and dried below 150 °C. The CEs were investigated in terms of photovoltaic performance in DSSCs by current-voltage measurements, mechanical adhesion properties by bending and tape tests, electro-catalytic performance by electrochemical impedance spectroscopy and microstructure by electron microscopy. In the bending and tape tests, PEDOT-carbon composite catalyst layers exhibited higher elasticity and better adhesion on all the studied substrates (ITO-PET and ITO-PEN plastic, and FTO-glass), compared to a binder free carbon composite and a TiO2 binder enriched carbon composite, and showed lower charge transfer resistance (1.5-3 Ω cm(2)) than the traditional thermally platinized CE (5 Ω cm(2)), demonstrating better catalytic performance for the tri-iodide reduction reaction. Also the TiO2 binder enriched carbon composite showed good catalytic characteristics and relatively good adhesion on ITO-PET, but on ITO-PEN its adhesion was poor. A DSSC with the TiO2 binder enriched catalyst layer reached 85% of the solar energy conversion efficiency of the reference DSSC based on the traditional thermally platinized CE. Based on the aforementioned characteristics, these carbon composites are promising candidates for replacing the platinum catalyst in a high volume roll-to-roll manufacturing process of DSSCs.

Application of low-voltage electrophoretic deposition to fabrication of direct methanol fuel cell electrode composite catalyst layer

In this study, the application of a low-voltage electrophoretic deposition (EPD) approach to the fabrication of DMFC electrode composite (i.e., catalyst/ionomer) catalyst layers using a CNT-supported PtRu (PtRu/CNT) anode nanocatalyst was investigated. In the operation of EPD, the PtRu/CNT electrocatalyst was first well mixed with a suitable amount of Nafion ® solution (ionomer dispersion) with or without the addition of HClO 4 as a supporting electrolyte and then electrophoretically deposited onto a non-catalyzed electrode base at a low applied DC voltage range of 0–5 V for 0–60 min. The resultant composite catalyst layer appeared to be thin and quite smooth exhibiting a lustrous texture particularly when the supporting electrolyte was employed in the suspension. Electrochemical impedance spectroscopy (EIS), however, showed that the coated composite catalyst layer exhibited a fairly high resistance indicating an excessive amount of ionomer was preferably deposited. Application of the fabricated electrode to a DMFC resulted in a cell performance with low but reasonable power density. These test results suggested that the low-voltage EPD could be a feasible approach to effective fabrication of DMFC electrode composite catalyst layers incorporated with CNT-supported electrocatalysts, although significant improvements are deemed to be necessary.

Influence of Catalyst Layer Structure on the Current Distribution of PEMFCs

a modified subcell approach was adopted to evaluate the current density distributions of proton exchange membrane fuel cells (pemfcs) with different electrodes. conventional hydrophobic electrodes showed better performance under flooding conditions compared to hydrophilic electrodes. the thin-film hydrophilic electrode performed better in the absence of liquid water, but it was more readily flooded. a composite catalyst layer was designed with 2/3 of the area from the inlet prepared hydrophilic and the remaining 1/3 area hydrophobic. the composite catalyst layer with commercial scale dimension showed notable enhanced performance in the concentration polarization region. (c) 2004 the electrochemical society.

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm −2 were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm −2 Pt loading.