High temperature PEM fuel cells

There are several compelling technological and commercial reasons for operating H₂/air PEM fuel cells at temperatures above 100 °C. Rates of electrochemical kinetics are enhanced, water management and cooling is simplified, useful waste heat can be recovered, and lower quality reformed hydrogen may be used as the fuel. This review paper provides a concise review of high temperature PEM fuel cells (HT-PEMFCs) from the perspective of HT-specific materials, designs, and testing/diagnostics. The review describes the motivation for HT-PEMFC development, the technology gaps, and recent advances.

HT-membrane development accounts for 90% of the published research in the field of HT-PEMFCs. Despite this, the status of membrane development for high temperature/low humidity operation is less than satisfactory. A weakness in the development of HT-PEMFC technology is the deficiency in HT-specific fuel cell architectures, test station designs, and testing protocols, and an understanding of the underlying fundamental principles behind these areas. The development of HT-specific PEMFC designs is of key importance that may help mitigate issues of membrane dehydration and MEA degradation.     

Development of tellurium-modified carbon catalysts for oxygen reduction reaction in PEM fuel cells

Tellurium (Te)-modified carbon catalyst for oxygen reduction reaction was prepared through chemical reduction of telluric acid followed by the pyrolysis process at elevated temperatures. The catalyst was found to be active for oxygen reduction reaction. High-temperature pyrolysis plays a crucial role in the formation of the active sites of the catalysts. When the pyrolysis was conducted at 1000 °C, the catalyst exhibited the onset potential for oxygen reduction as high as 0.78 V vs. NHE and generated less than 1% H₂O₂ during oxygen reduction. The performance of the membrane–electrode assembly prepared with the Te-modified carbon catalyst was also evaluated.     

Stability of platinum based alloy cathode catalysts in PEM fuel cells

Corrosion and surface area changes of platinum (Pt) based catalysts supported on carbon were evaluated using an accelerated durability test (ADT). The results obtained using the ADT were correlated to the performance of the Pt based catalysts in the fuel cell. The catalytic activity and dissolution rate of the alloying metal from the Pt-alloy catalysts were estimated in the same time domain. A strong correlation was observed between the amount of the alloying metal dissolved and the oxygen reduction reaction (ORR) activity of the Pt-alloy catalysts. The Pt catalyst exhibited loss of active surface area, and a resulting decrease in the ORR activity was observed. The Pt/C and Pt–Co/C catalysts showed similar behavior in both ADT and in the fuel cell testing. Cross-sectional studies by electron microprobe analysis of the membrane electrode assembly after fuel cell testing revealed cobalt dissolution followed by diffusion into the membrane.     

Chip-embedded thin film current collector for microfluidic fuel cells

Microfluidic fuel cells are an attractive candidate for low-power applications and provide a unique advantage over traditional fuel cells by elimination of the membrane. More importantly, microfluidic fuel cells enable a simple single-layer structure similar to common lab-on-chip devices, which makes conventional microfabrication or micromachining techniques readily applicable. Microfabrication is a preferable fabrication tool for microscale devices due to the benefits of high precision and repeatability at relatively low cost. However, the performance of most microfluidic fuel cells reported to date was negatively influenced by intrinsic contact resistances arising due to the highly porous nature of the electrodes. In the present work, a chip-embedded thin film current collector for vanadium fueled microfluidic fuel cells is proposed, fabricated, and evaluated as a potential mitigation strategy. The micromachining based thin film process is compatible with the overall cell fabrication, comprising photolithography and soft lithography, and does not require a substantial modification of the original cell design. Cells with and without current collectors are directly compared experimentally: the cell with current collectors demonstrates a 79% increase in peak power density, indicating that the contact resistance is significantly reduced by this approach. A volume specific peak power density of 6.2 W cm⁻³ is achieved, which is significantly higher than for previously reported microfluidic fuel cells. Electrochemical impedance spectroscopy (EIS) analysis is carried out to measure the combined ohmic cell resistance and confirmed a 32% reduction using the current collectors, which shows a good agreement with slope decrements in the polarization curves.     

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.     

Experimental verification for simulation study of Pt/CNT nanostructured cathode catalyst layer for PEM fuel cells

In this study, parametric study on the cathode catalyst layer in a Proton Exchange Membrane (PEM) fuel cell was conducted. Steady-state, two dimensional (2D) and nonisothermal conditions were proposed as critical hypotheses of work in essence. Multi-component mass diffusion along with convection mechanism in a single cell, conduction changes of proton and electron with experimental data and Knudsen diffusion which has a crucial impact on the simulation task in nanoscale, were considered in our study. Moreover, carbon nanotube (CNT), platinum (Pt) and Nafion loading effects as well as the porosity characteristics in a single-phase flow at different catalyst layer (CL) thicknesses were thoroughly investigated. The results presented herein, revealed that the amount of Pt and CNT has more profound effect than catalyst porosity. Based on the results derived, the model presented could be a promising mean to develop and construct a nanostructured catalyst layer. Meanwhile, our modified agglomerate model predicts the performance of fuel cell systems in different experimental conditions.     

Output-feedback voltage tracking control for input-constrained PEM fuel cell systems

An output-feedback voltage control system for nonlinear PEM fuel cells is presented. For voltage tracking around equilibrium operating points, the controller design minimizes the energy ratio between tracking error and normalized command while hydrogen and oxygen flowrates satisfy specified magnitude constraints and closed-loop poles meet desired placement constraints. Time response simulations based on Ballard 5 kW PEM fuel cell system parameters verify the design. Simulated controllers constructed numerically via the linear matrix inequality algorithm elaborate relationships between designed input flowrate and voltage tracking error. With controller design based on the same nominal input flowrate constraints, the achieved voltage tracking capability is comparable to our published state-feedback design study. To reduce voltage tracking error under fixed external resistance, gas flowrate magnitude constraints must be relaxed, requiring more fuel energy to manipulate the system variables for operation away from equilibrium conditions. Whereas state-feedback designs depend on internal state variables which are not always measurable, output-feedback control using only voltage tracking error as measurement simplifies practical implementation.     

Non precious metal catalysts for the PEM fuel cell cathode

Low temperature fuel cells, such as the proton exchange membrane (PEM) fuel cell, have required the use of highly active catalysts to promote both the fuel oxidation at the anode and oxygen reduction at the cathode. Attention has been particularly given to the oxygen reduction reaction (ORR) since this appears to be responsible for major voltage losses within the cell. To provide the requisite activity and minimse losses, precious metal catalysts (containing Pt) continue to be used for the cathode catalyst. At the same time, much research is in progress to reduce the costs associated with Pt cathode catalysts, by identifying and developing non-precious metal alternatives. This review outlines classes of non-precious metal systems that have been investigated over the past 10 years. Whilst none of these so far have provided the performance and durability of Pt systems some, such as transition metals supported on porous carbons, have demonstrated reasonable electrocatalytic activity. Of the newer catalysts, iron-based nanostructures on nitrogen-functionalised mesoporous carbons are beginning to emerge as possible contenders for future commercial PEMFC systems.     

Wet-etching of precipitation-based thin film microstructures for micro-solid oxide fuel cells

In micro-solid oxide fuel cells (μ-SOFCs) ceramic thin films are integrated as free-standing membranes on micromachinable substrates such as silicon or Foturan^® glass ceramic wafers. The processing of μ-SOFCs involves unavoidable dry- or wet-chemical etching for opening the substrate below the free-standing fuel cell membranes. In the first part of this paper current dry- and wet-chemical etchants for structuring of ceria-based electrolyte materials are reviewed, and compared to the etch-rates of common μ-SOFCs substrates. Wet-chemical etchants such as hydrofluoric acid are of high interest in μ-SOFC processing since they allow for homogeneous etching of ceria-based electrolyte thin films contrary to common dry-etching methods. In addition, HF acid is the only choice for substrate etching of μ-SOFC based on Foturan^® glass ceramic wafers. Etching of Ce_0.8Gd_0.2O_1.9?x spray pyrolysis electrolyte thin films with 10% HF:H₂O is investigated. The etch-resistance and microstructures of these films show a strong dependency on post deposition annealing, i.e. degree of crystallinity, and damage for low acid exposure times. Their ability to act as a potential etch-resistance for μ-SOFC membranes is broadly discussed. Guidance for thermal annealing and etching of Ce_0.8Gd_0.2O_1.9?x thin films for the fabrication of Foturan^®-based μ-SOFCs is given.     

CO tolerant Pt electrocatalysts for PEM fuel cells with enhanced stability against electrocorrosion

An optimized route for preparation of Ti_0.8Mo_0.2O₂–C composite supports for Pt electrocatalysts with 75/25 and 25/75 oxide/carbon mass ratio was elaborated using commercial (BP: Black Pearls 2000) and functionalized (FC) carbon materials. The sol-gel-based synthesis resulted in complete Mo incorporation into the rutile-TiO₂ lattice which is a prerequisite for good CO tolerance and high stability. According to the TG and XPS measurements the highest amount of oxygen-containing functional groups was obtained on the BP carbon annealed in nitrogen at 1000 °C and functionalized with HNO₃ and glucose.The electrochemical stability tests for 500 polarization cycles performed on the 20 wt% Pt/Ti_0.8Mo_0.2O₂–C catalysts revealed similarly small performance loss (8.5–11.8%) in case of all support materials. Considering the negative effect of the oxide content of the catalyst layer on the cell resistance, the catalyst with Ti_0.8Mo_0.2O₂/C = 25/75 ratio was chosen as the most promising.     

Investigation of tungsten carbide supported Pd or Pt as anode catalysts for PEM fuel cells

In this contribution, we present results of electrochemical characterization of prepared tungsten carbide supported palladium and platinum and Vulcan XC-72 supported palladium. These catalysts were employed as anode catalysts in PEMFC and results are compared to commercial platinum catalyst. Platinum seems to be irreplaceable as a proton exchange membrane fuel cell (PEMFC) catalyst for both the anode and the cathode, yet the high price and limited natural resources are holding back the commercialization of the PEMFCs. Tungsten carbide is recognized as promising catalyst support having the best conductivity among interstitial carbides. Higher natural resources and significantly lower price make palladium good candidate for replacement of the platinum catalyst. The presented results show that all prepared catalysts are very active for the hydrogen oxidation reaction. Linear sweep voltammetry curves of Pd/C and Pd/WC show existence of peaks at 0.07 V vs. RHE, which is assigned to absorbed hydrogen. H₂|Pd/WC|Nafion117|Pt/C|O₂ fuel cell has almost the same efficiency and similar power output as commercial platinum catalyst.     

Study of different nanostructured carbon supports for fuel cell catalysts

Pt clusters were deposited by an impregnation process on three carbon supports: multi-wall carbon nanotubes (MWNT), single-wall carbon nanohorns (SWNH), and Vulcan XC-72 carbon black to investigate the effect of the carbon support structure on the possibility of reducing Pt loading on electrodes for direct methanol (DMFC) fuel cells without impairing performance. MWNT and SWNH were in-house synthesised by a DC and an AC arc discharge process between pure graphite electrodes, respectively. UV-vis spectrophotometry, scanning and transmission electron microscopy, X-ray diffraction, and cyclic voltammetry measurements were used to characterize the Pt particles deposited on the three carbon supports. A differential yield for Pt deposition, not strictly related to the surface area of the carbon support, was observed. SWNH showed the highest surface chemical activity toward Pt deposition. Pt deposited in different forms depending on the carbon support. Electrochemical characterizations showed that the Pt nanostructures deposited on MWNT are particularly efficient in the methanol oxidation reaction.     

Enhanced performance of nanostructured thin film anode through Pt plasma enhanced atomic layer deposition for low temperature solid oxide fuel cells

Thin-film electrode deposited by sputtering has drawn attention due to high surface area and density of reaction sites for low-temperature solid oxide fuel cells. However, the nano-column structure of the sputtered film on the nanoporous anodic aluminum oxide (AAO) substrate has been showing low performances, possibly originated from low in-plane electrical connectivity and limited reaction area at electrolyte/electrode interface. We report here that application of 10 nm thickness of Pt plasma-enhanced atomic layer deposition (PEALD) on the nanoporous Ni-based anode and Gd doped ceria (GDC) deposited by sputtering dramatically enhances anodic reactions, significantly reduces ohmic and polarization resistances (25% reduction in ohmic, 50% reduction in polarization resistances), and improves the power density over 60% compared to the bare cells. It is noteworthy that Pt PEALD deposited on the nanoporous GDC layer shows much-improved performance compared to that deposited on the nanoporous anode structure. This is attributed to the enhanced contact area at Pt/GDC interface by exceptional conformal deposition of Pt PEALD and improved reaction sites from surface of GDC anode interlayer.     

Recent progress in carbon nanotubes support materials for Pt-based cathode catalysts in PEM fuel cells

Support materials have a significant impact on catalytic activity, stability, and performance of catalysts toward the oxygen reduction reaction (ORR). The properties of carbon-based materials have made them an excellent alternative for use as support for nanosized catalysts. Recently, carbon nanotubes (CNTs) have been explored as catalyst support materials, and their properties make them a promissory alternative. Furthermore, catalysts supported on CNTs exhibit higher resistance to electrochemical oxidation, better catalytic performance, and higher durability than catalysts supported on carbon black. In recent years, CNTs have acquired great relevance as catalysts support materials for ORR in acid media. This review addresses the most relevant studies on CNTs modification using methods such as functionalization, doping, and hybrid supports (CNTs-metal oxide) used as supports for Pt-based cathode catalysts in proton exchange membrane fuel cells.     

Design considerations for miniaturized PEM fuel cells

In this paper, we consider the design of a miniaturized proton-exchange membrane (PEM) fuel cell for powering 0.5–20 W portable telecommunication and computing devices. Our design is implemented on a silicon substrate to take advantage of advanced silicon processing technology in order to minimize production costs. The reduced length scales afforded by silicon processing allow us to consider designs that would be prohibited by excessive Ohmic losses in larger systems. We employ a mathematical model to quantify the effects of the secondary current distribution on two competing cell designs. In addition to the design of the cell itself, we discuss key integration issues and engineering trade-offs relevant to all miniaturized fuel cell systems: air movement, fuel delivery and water balance, thermal management and load handling.     

Technical cost analysis for PEM fuel cells

The present cost of fuel cells estimated at about $200 kW⁻¹ is a major barrier for commercialization and use in automotive applications. In the United States the target costs for fuel cell systems for the year 2004 as formulated by PNGV are $50 kW⁻¹. Lomax et al. have estimated the costs of polymer electrolyte membrane (PEM) fuel cells to be as low as $20 kW⁻¹. These estimates are based on careful consideration of high volume manufacturing processes. Recently, Arthur D. Little (ADL) has estimated the cost of a fuel cell system for transportation at $294 kW⁻¹. This estimate considers a fuel processor and directly related balance of plant components. The difference of the cost estimates results from the vastly different design assumptions. Both of these estimates are based on considering a single high volume of production, 500,000 fuel cells per year. This work builds on these earlier estimates by employing the methods of technical cost modeling and thereby including explicit consideration of design specifications, exogenous factor cost and processing and operational details. The bipolar plate is analyzed as a case study. The sensitivity of the costs to uncertainty in process conditions are explored following the ADL design. It is shown that the PNGV targets can only be achieved with design changes that reduce the quantity of material used. This might necessitate a reduction in efficiency from the assumed 80 mpg.     

Metallic bipolar plates for PEM fuel cells

Metallic bipolar plates for Polymer electrolyte membrane (PEM) fuel cells with and without coatings were tested in single cell tests. Current–voltage curves, lifetime curves and the contamination with metal ions were measured. Additionally the surface of the plates was analyzed by several methods. So far the investigations revealed that principally stainless steel covered with a thin coating is suitable as material for bipolar plates in PEM fuel cells. Cell performance is the same as in PEM fuel cells with graphite bipolar plates. Concerning the cost it has to be considered that not only the material itself but also the coating process has to be evaluated.