Recycling electronic scrap to make molten carbonate fuel cell cathodes

The paper aims to examine the possibility of improving the manufacturing process for MCFC cathodes. using noble, semi-precious, and rare earth metals sourced from waste electric and electronic equipment (WEEE). As MCFC components are not particularly sensitive to ceramic and metal impurities. The addition of noble metals recovered from WEEE as catalysts are economically justifiable. The reported experimental research revealed the positive impact of MCFC cathode fabricated with 20% recycled electronic scrap. Especially the cell with powder marked as 4/1 enjoyed much better performance operating at 550 °C than the reference cell. During the operation at a temperature of 650 °C, the cell with powder marked as 4/1 has almost the same performance as the reference cell, i.e., 1.01 V OCV and power density of 0.13 A/cm². The cell with cathode with 4/2 powder has the worst performance – current density of 0.09 A/cm² and OCV of 0.97 V.     

On the oxygen vacancies optimization through Pr co-doping of ceria-based electrolytes for electrolyte-supported solid oxide fuel cells

Praseodymium-doped ceria electrolyte powders are synthesized by a co-precipitation method using ammonium carbonate in little excess to fabricate a stable electrolyte-supported solid oxide fuel cell able to operate in hydrogen conditions. Raman and X-ray Photoelectron Spectroscopy are employed for the electrolyte characterization to check the distribution of vacancies and the initial oxidation state of Pr that influences the transport mechanism under the real operation of SOFCs. The optimum Pr concentration in the electrolyte is found to be 6 mol% of Pr and 14 mol% of Gd (sample 6Pr). The electrolyte-supported cells fabricated with this composition are sintered at 1250 °C for 3 h and tested in different gas conditions and operating temperatures, showing a maximum power density of 305.31 mW·cm-2 at 530.36 mA·cm-2 (750 °C) in wet hydrogen conditions. Compared to standard cells fabricated with a gadolinium-doped ceria electrolyte sintered at 1500 °C, the 6Pr has long term stability performances with a power density loss of 17% after 100 h of operation. The results demonstrate the eligible use of this electrolyte under real operating environments.     

Optimization of the protonic ceramic composition in composite electrodes for high-performance protonic ceramic fuel cells

A composite electrode in protonic ceramic fuel cells (PCFCs) is composed of an ionic conductor, an electronic conductor, and catalysts. To determine the contribution of the properties of the ionic conductor to the electrode performance, three different proton conductors: Ba(Ce_0.75Y_0.15)O_3−δ(BCY), Ba(Ce_0.45Zr_0.30Y_0.15)O_3−δ (BCZY), and Ba(Zr_0.75Y_0.15)O_3−δ (BZY), were used to fabricate composite electrodes for PCFCs. Moreover, their effects on the anode and cathode performances were investigated systematically. In the cathodes, the BCZY and BCY scaffolds showed a better performance than the BZY scaffold. However, in the anodes, the BZY scaffold showed a superior performance to the BCZY or BCY scaffold. This exhibited that the requirements of ion-conducting scaffolds in composite electrodes were different in cathodes and anodes and that the fuel cell performance could be optimized by choosing appropriate ionic conductors for each electrode.     

Triple-component composite cathode for performance optimization of protonic ceramic fuel cells

The cathode of a protonic ceramic fuel cell must be able to facilitate ion and electron transfer, while simultaneously possessing a high catalytic activity for steam generation and the dissociation of gas-phase molecules. In this study, the performance of a cathode for protonic ceramic fuel cells is optimized by employing a triple-component composite cathode design, which integrates proton conductors, mixed electronic–ionic conductors, and a catalytic layer. Additionally, two other composite cathodes are fabricated for comparison. Owing to its higher electrical conductivity but lower catalytic activity, the composite cathode with protonic ceramic and (Ba_0.95La_0.05) (Fe_0.8Zn_0.2)O_3-δ (BLFZ) exhibits lower ohmic resistance but poor catalytic activity compared to the composite cathode with protonic ceramic and Ba(Co_0.4Fe_0.4Zr_0.1Y_0.1)O_3-δ (BCFZY). The triple-component cathode is fabricated by infiltrating BCFZY into a composite cathode composed of BLFZ and protonic ceramic, and both the ohmic and non-ohmic resistances of the cathode are optimized in CH₄ and H₂ fuels. In particular, the performance of CH₄ fuel is significantly improved by adopting a triple-component cathode. These results suggest a possible contribution of the oxygen reduction reaction at the cathode to the reformation of CH₄ at the anode.     

Effect of rare earth oxides for improvement of MCFC

The solubility of rare earth metal oxides and their effect on the NiO solubility have been discussed to stabilize the cathode of molten carbonate fuel cells. The solubility of Ho, Yb, and Nd oxides were 4.4 × 10⁻⁴, 3.4 × 10⁻⁴, and 1.3 × 10⁻³ (mole fraction) at 923 K, respectively. The solubilities of NiO in (Li_0.52/Na_0.48)₂CO₃ with the saturated Ho, Yb, and Nd were 1.57 × 10⁻⁵, 1.41 × 10⁻⁵, and 9.5 × 10⁻⁶, respectively. Among these three, Nd, which has the highest solubility in the carbonates, reduced the NiO solubility most; although, the La reduced the NiO solubility more than Nd.

The logarithm of the solubility of the rare earth metal oxides has a linear relation to the Coulomb force ratio between the rare earth metal and the alkaline metal. Following this relation, the La should have the highest solubility among all the lanthanides. The basicity which NiO solubility closely relates has a linear relationship to the Coulomb force parameter of the melts. Based on these two models, the La would be the best additive to reduce the NiO solubility in Li/Na eutectic carbonate melt, among all the lanthanides.     

Carbon and non-carbon support materials for platinum-based catalysts in fuel cells

Carbon and other platinum-supporting materials have been studied as electrode catalyst component of low-temperature fuel cells. Platinum (Pt) is commonly used as the catalyst due to its high electro-catalytic activity. Current research is now focusing on using either modified carbon-based or non-carbon-based materials as catalyst supports to enhance the catalytic performance of Pt. In recent years, Pt and Pt-alloy catalysts supported on modified carbon-based and non-carbon-based materials have received remarkable interests due to their significant properties that can contribute to the excellent fuel cell performance. Thus, it is timely to review this topic, focusing on various modified carbon-based supports and their advantages, limitations and future prospects. Non-carbon-based support for Pt and Pt-alloy catalysts will also be discussed. Firstly, this review summarises the progress to date in the development of these catalyst support materials; from carbon black to the widely explored catalyst support, graphene. Secondly, a comparison and discussion of each catalyst support in terms of morphology, electro-catalytic activity, structural characteristics, and its fuel cell performance are emphasized. All the catalyst support materials reviewed are considered to be promising, high-potential candidates that may find commercial value as catalyst support materials for fuel cells. Finally, a brief discussion on cost relating Pt based catalyst for mass production is included.     

CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity

Ni based catalysts are usually used for catalyzing the CO₂ methanation to produce synthetic natural gas due to their low cost, though their catalytic activities cannot be comparable with the noble metal counterparts. In order to address this challenge, a series of rare earth (La, Ce, Sm, and Pr) doped Ni based mesoporous materials had been facilely fabricated by the one-pot evaporation induced self-assembly (EISA) strategy and directly employed as the catalysts for CO₂ methanation. These mesoporous catalysts had been systematically characterized by means of X-ray diffraction, N₂ physisorption, transmission electron microscope, X-ray photoelectron spectroscopy, H₂ temperature programmed reduction, CO₂ temperature programmed desorption, and so on. It was found that the Ni species were highly dispersed among the mesoporous framework and the strong metal-framework interaction had been formed. Thus, the thermal sintering of the metallic Ni nanoparticles could be effectively suppressed under CO₂ methanation conditions, promising these mesoporous catalysts with 50 h excellent catalytic stabilities without evident deactivation. Besides, the rare earth dopants could greatly increase the surface basicity of the catalysts and intensify the chemisorption the CO₂. Further, the rare earth elements were also functioned as the electron modifiers, which was also helpful in activating the CO₂ molecule. The apparent activation energies of CO₂ could be obviously decreased by rare earth dopants. As a result, their low-temperature catalytic activity had been greatly intensified over these rare earth elements promoted catalysts.     

Investigation of degradation mechanisms in PEM fuel cells caused by low-temperature cycles

Environmental influences, especially temperatures below the freezing point, can affect the performance and long-term stability of PEMFCs. Within the scope of this research, a completely new test procedure was developed to characterize PEMFC single cells with respect to their long-term stability at temperature cycles between 80 °C and ?10 °C. Using this procedure, the behavior of PEMFC single cells (active surface area of 43.6 cm²) with different cathode-ionomer-to-carbon (I/C) weight ratios (0.5/1.0/1.5) was evaluated. The generated in-situ measurement data clearly demonstrate that the performance of each PEMFC single cell changes individually as a function of the cathode I/C-ratio during the 120 stress cycles. While the MEA with an I/C ratio of 0.5 showed a power loss of ~1.49%, the MEAs with an I/C ratio of 1.0 and 1.5 showed a power loss of about ~7.75% and ~24.7%, respectively. The subsequent post-mortem ex-situ analyses clearly showed how the test procedure and the different I/C-ratios affected the changes in the catalyst layers (CL). The destructive mechanisms responsible for the changes can be divided into two categories: One part was driven by rapid enthalpy change leading to mechanical failure, and the other part, which led to the reduction of cathode CL thickness, was driven by rapid potential changes and potential shifts (overpotentials). This reduction in cathode CL thickness ultimately leads to an accumulation and excessive load of ionomer in the direction of GDL, resulting in a reduction in pore size, a shift in the core reaction area, and high O₂ transport resistance.     

A novel Pt/Cr/Ru/C cathode catalyst for direct methanol fuel cells (DMFC) with simultaneous methanol tolerance and oxygen promotion

New carbon supported electrocatalysts Pt/Cr/Ru with distinct compositions and preparation methods were studied with the help of different electrochemical and spectroscopic techniques. The purposes of obtaining these catalysts lie on their possibilities towards methanol/oxygen fuel cells. In this sense, the oxygen reduction reaction and methanol oxidation reaction were analyzed using stationary and fluid dynamic methodologies. Pt_7.8/Ru_1.3/Cr_0.5 and Pt_8.0/Ru_2.0/Cr_0.1 were the most interesting prepared substrates, on which the first one shows the best catalytic properties towards methanol oxidation and the second the finest performance towards oxygen reduction reaction. Reaction orders with respect to oxygen for the oxygen reduction reaction were obtained being equal to ½ at potentials lower than 0.80 V for both catalysts. Polarization curves run for this reaction depicted two Tafel slopes, i.e. 0.09 V dec⁻¹ above 0.8 V and 0.20 V dec⁻¹ below 0.8 V for both catalysts. An analysis of the most likely mechanism for the oxygen reduction was proposed on the base of those reaction orders and Tafel slopes.     

New materials for polymer electrolyte membrane fuel cell current collectors

Polymer Electrolyte Membrane Fuel cells for automotive applications need to have high power density, and be inexpensive and robust to compete effectively with the internal combustion engine. Development of membranes and new electrodes and catalysts have increased power significantly, but further improvements may be achieved by the use of new materials and construction techniques in the manufacture of the bipolar plates. To show this, a variety of materials have been fabricated into flow field plates, both metallic and graphitic, and single fuel cell tests were conducted to determine the performance of each material. Maximum power was obtained with materials which had lowest contact resistance and good electrical conductivity. The performance of the best material was characterised as a function of cell compression and flow field geometry.     

Biogas use in high temperature fuel cells: Enhancement of KOH-KI activated carbon performance toward H2S removal

Activated carbons (ACs) treated with KOH-KI are very effective sorbents for deep H₂S removal, as required by biogas use in high temperature fuel cell systems. For this application, the performance of a commercial KOH-KI treated AC was investigated through a systematic study based on dynamic adsorption tests. With reference to the composition of a real biogas produced in a wastewater treatment plant located in Barcelona, the present work presents a sensitivity performance analysis on singular and synergetic effects of gas matrix, humidity and oxygen on AC KOH-KI performance.The results revealed a positive role of water (up to 90% of relative humidity (R.H.)) for different gas matrices, enhanced by the simultaneous presence of small percentages of oxygen (2%_v). A relevant influence of gas matrix composition was found (except for the case of oxygen addition to dry inlet streams), specifically in terms of a marked negative effect of CO₂ and a significant sorption capacity increase for high percentage of methane. Sulfur dioxide was not detected in the outlet gas-phase for the investigated operating parameters (O₂ 2%_v, R.H. 0–90%, H₂S 100 ppm_v, temperature 45 °C). Therefore, even in the case of further oxidation of adsorbed elemental sulfur to SO₂, this product could be completely removed by AC KOH-KI.     

Stability of lanthanum oxide-based H2S sorbents in realistic fuel processor/fuel cell operation

We report that lanthana-based sulfur sorbents are an excellent choice as once-through chemical filters for the removal of trace amounts of H₂S and COS from any fuel gas at temperatures matching those of solid oxide fuel cells. We have examined sorbents based on lanthana and Pr-doped lanthana with up to 30 at.% praseodymium, having high desulfurization efficiency, as measured by their ability to remove H₂S from simulated reformate gas streams to below 50 ppbv with corresponding sulfur capacity exceeding 50 mg S g_sorbent⁻¹ at 800 °C. Intermittent sorbent operation with air-rich boiler exhaust-type gas mixtures and with frequent shutdowns and restarts is possible without formation of lanthanide oxycarbonate phases. Upon restart, desulfurization continues from where it left at the end of the previous cycle. These findings are important for practical applications of these sorbents as sulfur polishing units of fuel gases in the presence of small or large amounts of water vapor, and with the regular shutdown/start-up operation practiced in fuel processors/fuel cell systems, both stationary and mobile, and of any size/scale.     

Nanostructured platinum catalyst layer prepared by pulsed electrodeposition for use in PEM fuel cells

Nanostructured thin catalyst layer with uniform distribution of platinum particles on a GDL useful for PEM fuel cell was obtained by preferential pulsed electrodeposition (PED) from a dilute solution of chloroplatinic acid. A low platinum loading on the electrode was obtained by PED method, without any loss in fuel cell performance compared with electrodes prepared by conventional brush coating method. The electrodeposition was optimized by varying the duty cycle and current density. The fuel cell performance was found to be 350 mA/cm² at an operating voltage of 0.6 V at 60 °C with hydrogen and air as reactants at ambient pressure. The nanostructured thin catalyst layer showed a very less ohmic resistance of 0.00076 mΩ/cm².     

Analytical fuel cell modeling; non-isothermal fuel cells

The isothermal fuel cell model, given in an earlier publication, will be generalized to describe the behaviour of non-isothermal fuel cells of co-flow type. To this end the temperalure distribution inside a fuel cell in steady state is investigated analytically. A simplified relation between the local temperature and the fuel utilization is derived and its practical significance elucidated. Furthermore, it is shown that the solution of the non-isothermal model is accurately approximated by analytical expressions obtained from a so-called quasi-isothermal approach. This new approach yields a similar expression for the cell voltage as derived from the isothermal model. The quasi-isothermal approach is also used to make a clear comparison between the isothermal and the non-isothermal fuel cell model.     

Model-based control of fuel cells (2): Optimal efficiency

A dynamic PEM fuel cell model has been developed, taking into account spatial dependencies of voltage, current, material flows, and temperatures. The voltage, current, and therefore, the efficiency are dependent on the temperature and other variables, which can be optimized on the fly to achieve optimal efficiency. In this paper, we demonstrate that a model predictive controller, relying on a reduced-order approximation of the dynamic PEM fuel cell model can satisfy setpoint changes in the power demand, while at the same time, minimize fuel consumption to maximize the efficiency. The main conclusion of the paper is that by appropriate formulation of the objective function, reliable optimization of the performance of a PEM fuel cell can be performed in which the main tunable parameter is the prediction and control horizons, V and U, respectively. We have demonstrated that increased fuel efficiency can be obtained at the expense of slower responses, by increasing the values of these parameters.     

Inkjet printing of carbon supported platinum 3-D catalyst layers for use in fuel cells

We present a method of using inkjet printing (IJP) to deposit catalyst materials onto gas diffusion layers (GDLs) that are made into membrane electrode assemblies (MEAs) for polymer electrolyte fuel cell (PEMFC). Existing ink deposition methods such as spray painting or screen printing are not well suited for ultra low (<0.5 mg Pt cm⁻²) loadings. The IJP method can be used to deposit smaller volumes of water based catalyst ink solutions with picoliter precision provided the solution properties are compatible with the cartridge design. By optimizing the dispersion of the ink solution we have shown that this technique can be successfully used with catalysts supported on different carbon black (i.e. XC-72R, Monarch 700, Black Pearls 2000, etc.). Our ink jet printed MEAs with catalyst loadings of 0.020 mg Pt cm⁻² have shown Pt utilizations in excess of 16,000 mW mg⁻¹ Pt which is higher than our traditional screen printed MEAs (800 mW mg⁻¹ Pt). As a further demonstration of IJP versatility, we present results of a graded distribution of Pt/C catalyst structure using standard Johnson Matthey (JM) catalyst. Compared to a continuous catalyst layer of JM Pt/C (20% Pt), the graded catalyst structure showed enhanced performance.     

A review of numerical modeling of solid oxide fuel cells

The solid oxide fuel cell (SOFC) is one of the most promising fuel cells for direct conversion of chemical energy to electrical energy with the possibility of its use in co-generation systems because of the high temperature waste heat. Various mathematical models have been developed for three geometric configurations (tubular, planar, and monolithic) to solve transport equations coupled with electrochemical processes to describe the reaction kinetics including internal reforming chemistry in SOFCs. In recent years, considerable progress has been made in modeling to improve the design and performance of this type of fuel cells. The numbers of the contributions on this important type of fuels have been increasing rapidly. The objective of this paper is to summarize the present status of the SOFC modeling efforts so that unresolved problems can be identified by the researchers.