Performance degradation of direct formic acid fuel cell incorporating a Pd anode catalyst

Electrochemical and physical analysis is employed to verify the performance degradation mechanism in direct formic acid fuel cells (DFAFCs). The power density of a single cell measured at 200 mA cm⁻² decreases by 40% after 11 h of operation. The performance of the single cell is partly recovered however, by a reactivation process. Various analytical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) are used to investigate the mechanism of performance degradation. The analytical results show that the electrolyte membranes in the DFAFC are stable for 11 h of operation after the reactivation process. The major factors causing performance degradation in the DFAFC are an increment in the anode charge-transfer resistance and a growth in the particle size of the Pd anode catalyst. The anode charge-transfer resistance, confirmed by EIS, increases with operation time and is due to poisoning of the catalyst surface. Although it is not clear what chemical species poisons the catalyst surface, the catalyst surface is cleaned by the reactivation process. Performance losses caused by surface poisoning are completely recovered by the reactivation process. Increase in catalyst size induces a reduction in active surface area, and the performance loss caused by the growth in catalyst size cannot be recovered by the reactivation process.     

Performance increase of microfluidic formic acid fuel cell using Pd/MWCNTs as catalyst

This paper shows that the combination of an O₂ saturated acidic fluid setup (O₂-setup) and a composite of Pd nanoparticles supported on multiwalled-carbon nanotubes (Pd/MWCNTs) as anode catalyst material, results in the improvement of microfluidic fuel cell performance. Microfluidic fuel cells were constructed and evaluated at low HCOOH concentrations (0.1 and 0.5 M) using Pd/V XC-72 and Pd/MWCNTs as anode and Pt/V XC-72 as cathode electrode materials, respectively. The results show a higher power density (2.9 mW cm⁻²) for this cell when compared to the value reported in the literature that considers a commercial Pd/V XC-72 and 3.3 mW cm⁻² using a Pd/MWCNTs with a 50% less Pd loading than that commercial Pd/V XC-72.     

Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation

Carbon nanofiber (CNF) supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as stabilizing agent and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles are controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.85 to 3.62 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the further increasing of the pH value to 6.5 leads to an increased particle size and the formation of PdO phase in the synthesized Pd/CNF catalyst. The Pd/CNF catalyst synthesized at the pH value of 6.0 exhibits superior catalytic activity and stability for formic acid electrooxidation due to its small particle size and uniform size distribution.     

An ambient aqueous synthesis for highly dispersed and active Pd/C catalyst for formic acid electro-oxidation

An experimentally simple process is reported in aqueous solution and under ambient conditions to prepare highly dispersed and active Pd/C catalyst without the use of a stabilizing agent. The [Pd(NH₃)₄]²⁺ ion is synthesized with gentle heating in aqueous ammonia solution without formation of Pd(OH)_x complex intermediates. The adsorbed [Pd(NH₃)₄]²⁺ on the surface of carbon (Vulcan XC-72) is reduced in situ to Pd nanoparticles by NaBH₄. The Pd/C catalyst obtained is characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that highly dispersed Pd/C catalyst with 20 wt.% Pd content and with an average Pd nanoparticle diameter of 4.3-4.7 nm could be obtained. The electrochemical measurements show that the Pd/C catalyst without stabilizer has a higher electro-oxidation activity for formic acid compared to that of a Pd/C catalyst prepared in a traditional high temperature polyol process in ethylene glycol.     

An effective Pd@Ni-B/C anode catalyst for electro-oxidation of formic acid

The Pd@Ni-B/C catalysts were prepared by using Ni-B/C with different amounts of Ni-B alloys as supports. The structure, morphology and element valence state of these catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The electro-catalytic performance of these catalysts for formic acid oxidation was investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO stripping experiments. It was found that an appropriate amount of Ni-B alloys plays an important role in enhancing catalytic activity and resistance to CO poisoning and improving stability of Pd based catalysts, all of which imply that Pd@Ni_1.61B_0.0199/C is promising for probable applications in direct formic acid fuel cell field.     

Effect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC)

In the present study, degradation and recovery in cell performance of direct formic acid fuel cells (DFAFCs) are investigated. For DFAFC tests, palladium (Pd) and platinum (Pt) are used as anode and cathode catalysts, respectively, and are applied to a Nafion membrane by catalyst-coated membrane (CCM) spraying. As multiple repeated DFAFC operations are performed, the cell performance of DFAFC is steadily degraded. This behavior is ascribed to the electrooxidation of Pd into Pd-OH, which occurs between 0.1 and 0.55 V. To investigate the dependency of the cell performance on the Pd-OH and to evaluate how the cell performance is regenerated, cyclic voltammetry (CV) tests are executed. In CV experiments where the voltages applied to the DFAFC single cell are lower than 0.7 V vs. DHE, the cell performance is further deactivated due to continuous production of Pd-OH. Conversely, in CV experiments where the voltage is higher than 0.9 V vs. DHE, cell performance is reactivated due to redox reactions of Pd-OH into Pd-O and Pd-O into Pd. ATR-FTIR and XPS are used to confirm the transformations of Pd.     

Design of experiment techniques for fuel cell characterisation and development

Designs of Experiments (DoE) can be of immediate relevance for various research works conducted in the Fuel Cell (FC) area. DoE techniques allow efficient test definitions for rapid conceptions and well-organised characterisations of FC materials and components, individual cells, stacks or even complete generators. In the DoE method, some statistic-based models can be proposed in pre-stages of physical models. The statistical/numerical relations are used to predict the behaviour of the investigated systems as a function of various operating parameters. Some control strategies can also be developed to optimise relevant criteria like FC voltage, fuel consumption, and maximal electrical power or stack lifetime.     

Synthesis and optimization of PtRu/TiO2-CNF anodic catalyst for direct methanol fuel cell

Direct methanol fuel cell (DMFC) is a promising power source technology, but it has been unable to be successfully commercialized due to its high cost and low kinetic oxidation. Both problems stem from one of its main components, the catalyst. Therefore, this study is focused on determining and optimizing the electrocatalyst parameters of a high-performance DMFC. The electrocatalyst, PtRu/TiO₂-CNF, is produced by the deposition method and is subjected to electrochemical measurement and cyclic voltammetry (CV) to measure half-cell performance in a DMFC. The optimization process involved two main phases, a screening process followed by response surface methodology (RSM). The resulting optimum parameters were then used for the single cell performance testing. The results show that the mathematical model suggested by RSM is adequate for the optimization of the parameter levels. The optimum parameters suggested by RSM are a PtRu composition of 30.25% and a catalyst loading of 0.59 mg/cm², resulting in almost perfect agreement between the measured current density (603.06 mA/mg_PtRu) and the predicted value (600.63 mA/mg_PtRu). The current density obtained in this study is the highest among other researchers in the same field.     

Methane decomposition with a minimal catalyst: An optimization study with response surface methodology over Ni/SiO2 nanocatalyst

Nowadays, methane cracking in the presence of an efficient catalyst is one of the most investigating areas aiming hydrogen and nanocarbon synthesis. This research contribution systematically investigated the influence of methane partial pressure (P_CH4), decomposition temperature, and weight of Ni/SiO₂ nanocatalyst (n-Ni/SiO₂) on carbon nanotube (CNT) yield. The optimum reaction condition for optimal methane cracking resulted in maximum CNT yield is derived using Design Expert Software. A series of experiments conducted to develop a quadratic polynomial model for CNT yield using response surface methodology. Surprisingly, the optimum catalyst quantity was the lowest (0.30 g) in the experimented parameter range, which exhibited the highest CNT production at 610 °C temperature and 0.8 atm P_CH4. The minimal catalyst quantity for the optimum CNT production, which needs only 0.26% of the total volume of the pilot plant reactor, is a breakthrough finding in methane cracking research. It could help to overcome the reactor blockage limitation issues of the process in large scale applications. Thanks to the uniquely supported n-Ni/SiO₂ catalyst prepared via co-precipitation cum modified Stöber method. The fresh and used catalysts investigated using different types of characterization techniques such as XRD, BET, Raman spectra, HRTEM, and FESEM-EDX. Characterization results evidenced the presence of differently structured CNTs formed at optimum reaction conditions.     

A complementary study on novel PdAuCo catalysts: Synthesis,characterization, direct formic acid fuel cell application,and exergy analysis

At present, Pd containing (10–40 wt%) multiwall carbon nanotube (MWCNT) supported Pd monometallic, Pd:Au bimetallic, and PdAuCo trimetallic catalysts are prepared via NaBH₄ reduction method to examine their formic acid electrooxidation activities and direct formic acid fuel cell performances (DFAFCs) when used as anode catalysts. These catalysts are characterized by advanced analytical techniques as N₂ adsorption and desorption, XRD, SAXS, SEM-EDX, and TEM. Electronic state of Pd changes by the addition of Au and Co. Moreover, formic acid electrooxidation activities of these catalysts measured by CV indicates that particle size changes in wide range play a major role in the formic acid electrochemical oxidation activity, ascribed the strong structure sensitivity of formic acid electrooxidation reaction. PdAuCo (80:10:10)/MWCNT catalyst displays the most significant current density increase. On the other hand, lower CO stripping peak potential obtained for PdAuCo (80:10:10)/MWCNT catalyst, attributed to the awakening of the Pd-adsorbate bond strength down to its optimum value, which favors higher electrochemical activity. DFAFCs performance tests and exergy analysis reveal that fuel cell performances increase with the addition of Au and Co which can be attributed to synergetic effect. Furthermore, temperature strongly influences the performance of formic acid fuel cell.     

Deactivation/reactivation of a Pd/C catalyst in a direct formic acid fuel cell (DFAFC): Use of array membrane electrode assemblies

Palladium-based catalysts exhibit high activity for formic acid oxidation, but their catalytic activity decreases quite rapidly under direct formic acid fuel cell (DFAFC) operating conditions. This paper presents a systematic study of the deactivation and electrochemical reactivation of a carbon supported palladium catalyst (Pd/C) employing anode arrays in a DFAFC. Deactivation of Pd/C is caused by the electro-oxidation of the formic acid, and does not occur significantly at open circuit. Its rate increases sharply with increasing formic acid concentration but is only dependent on potential at high cell voltages. Reactivation can be achieved by driving the cell voltage to a reverse polarity of −0.2 V or higher. The use of array membrane electrode assemblies allows the rapid generation of statistically significant information on differences between catalysts, and the effects of operational parameters on the deactivation and reactivation processes.     

High activity of Pd-WO3/C catalyst as anodic catalyst for direct formic acid fuel cell

Pd nanoparticles supported on the WO₃/C hybrid are prepared by a two-step procedure and the catalysts are studied for the electrooxidation of formic acid. For the purpose of comparison, phosphotungstic acid (PWA) and sodium tungstate are used as the precursor of WO₃. Both the Pd-WO₃/C catalysts have much higher catalytic activity for the electrooxidation of formic acid than the Pd/C catalyst. The Pd-WO₃/C catalyst prepared from PWA shows the best catalytic activity and stability for formic acid oxidation; it also shows the maximum power density of approximately 7.6 mW cm⁻² when tested with a small single passive fuel cell. The increase of electrocatalytic activity and stability is ascribed to the interaction between the Pd and WO₃, which promotes the oxidation of formic acid in the direct pathway. The precursors used for the preparation of the WO₃/C hybrid support have a great effect on the performance of the Pd-WO₃/C catalyst. The WO₃/C hybrid support prepared from PWA is beneficial to the dispersion of Pd nanoparticles, and the catalyst has potential application for direct formic acid fuel cell.     

Towards more active and stable PdAgCr electrocatalysts for formic acid electrooxidation: The role of optimization via response surface methodology

In this study, multiwall carbon nanotube (MCNT)‐supported Pd (Pd/MWCNT) catalysts are prepared by using NaBH₄ reduction method. In order to maximize the oxidation and reduction of H₂SO₄, synthesis conditions (Pd ratio, molar ratio of NaBH₄/K₂PdCl₄, volume of deionized water, and duration of agitation) are optimized by using response surface methodology (RSM). The optimum synthesis conditions are determined as 58.2% of Pd by weight, 154.6 molar ratio of NaBH₄ to K₂PdCl₄, 19.48 mL of deionized water, and 186.16 min of agitation duration. The effect of electrochemical measurement conditions on the oxidation kinetics of Pd/MWCNT is also investigated by RSM. The optimum electrochemical measurement conditions are found as 10 μL of catalyst mixture, 90°C of H₂SO₄ solution, and 5.5 M H₂SO₄. The Pd/MWCNT, Pd₅₀Ag₅₀/MWCNT, and Pd_65.6Ag_33.6Cr_0.80/MWCNT catalysts prepared under optimized conditions are characterized by using X‐ray diffraction, transmission electron microscopy, N₂ adsorption‐desorption, and inductively coupled plasma mass spectrometry. The crystallite sizes of these catalysts are found as 4.85, 5.66, and 5.26 nm for Pd/MWCNT, Pd₅₀Ag₅₀/MWCNT, and Pd_65.6Ag_33.6Cr_0.80/MWCNT catalysts, respectively. Isotherms of all these catalysts are found to be similar to Type V isotherms with H3 hysteresis loop. The average particle size of Pd₅₀Ag₅₀/MWCNT and Pd_65.6Ag_33.6Cr_0.80/MWCNT catalysts are determined as 5.2 and 9.2 nm, respectively. Electrochemical performance of as‐prepared catalysts is evaluated by using cyclic voltammetry and chronoamperometry. The formic acid electrooxidation (FAEO) activities are found as 18.9, 27.8, and 51.6 mA/cm² for Pd/MWCNT, Pd₅₀Ag₅₀/MWCNT, and Pd_65.6Ag_33.6Cr_0.80/MWCNT, respectively. Pd_65.6Ag_33.6Cr_0.80/MWCNT shows the highest activity and stability. Optimization of synthesis conditions and electrochemical measurement parameters allow us to obtain very good electrochemical activity and stability for FAEO reaction compared with anode catalysts in the literature.     

A novel anode catalyst layer with multilayer and pore structure for improving the performance of a direct methanol fuel cell

A novel anode catalyst layer (CL) has been prepared by ultrasonic‐spray process which combines directly spraying method and catalyst‐coated membrane switchover method, and heated‐stereoscopic process has been used to enhance bond force between CLs and proton exchange membrane in this paper. The scanning electron microscopy, electrochemical impedance spectra and polarization curves show that: the anode outer CL with pores and meshwork structure has increased the electrochemical active surface area and retained the transfer of protons and electrons, and the anode inner CL with compact structure has prevented methanol crossover. And the gradient catalysis for methanol electrochemical catalytic oxidation reaction has been achieved. The open circuit voltage has reached 0.697 V, and the performance has increased from 116.8 mW cm^?2 of traditional membrane electrode assembly (MEA) to 202.6 mWcm^?2 of novel MEA at 80°C. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of Pt/C, Pd/C and PdPt/C anode catalysts on the performance and stability of air breathing direct formic acid fuel cells

Pt/C, Pd/C and PdPt/C catalysts are potential anodic candidates for electro-oxidation of formic acid. In this work we designed a miniature air breathing direct formic acid fuel cell, in which gold plated printed circuit boards are used as end plates and current collectors, and evaluated the effects of anode catalysts on open circuit voltage, power density and long-term discharging stability of the cell. It was found that the cell performance was strongly anode catalyst dependent. Pd/C demonstrated good catalytic activity but poor stability. A maximum power density of 25.1 mW cm⁻² was achieved when 5.0 M HCOOH was fed as electrolyte. Pt/C and PdPt/C showed poor activity but good stability, and the cell can discharge for about 10 h at 0.45 V (Pt/C anode) and 15 h at 0.3 V (PdPt/C) at 20 mA.     

A synthesis of CO-tolerant Nb2O5-promoted Pt/C catalyst for direct methanol fuel cell; its physical and electrochemical characterization

A Pt-Nb₂O₅/C electrocatalyst was synthesized by a two-step process as an anode material in direct methanol fuel cell (DMFC). The Pt-Nb₂O₅/C catalysts heat-treated at different temperatures (400 and 500 °C) in flowing N₂ were characterized by various methods such as inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoemission spectroscopy (XPS). The heat-treated Pt-Nb₂O₅/C catalyst at 400 °C showed the best electrochemical activity for CO and methanol oxidations among the prepared catalysts. The XPS results showed the electronic structure change of Pt, indicating a formation of interaction between Pt and Nb₂O₅. It is suggested that a synergistic effect between Pt and Nb₂O₅ enhances the electrocatalytic activity for CO and methanol oxidations. We believe that Nb₂O₅-promoted Pt/C catalyst may be regarded as one of the attractive candidates as an anode material in DMFC.     

Performance optimization of microbial electrolysis cell (MEC) for palm oil mill effluent (POME) wastewater treatment and sustainable Bio-H2 production using response surface methodology (RSM)

Microbial electrolysis cells (MECs) are a new bio-electrochemical method for converting organic matter to hydrogen gas (H₂). Palm oil mill effluent (POME) is hazardous wastewater that is mostly formed during the crude oil extraction process in the palm oil industry. In the present study, POME was used in the MEC system for hydrogen generation as a feasible treatment technology. To enhance biohydrogen generation from POME in the MEC, an empirical model was generated using response surface methodology (RSM). A central composite design (CCD) was utilized to perform twenty experimental runs of MEC given three important variables, namely incubation temperature, initial pH, and influent dilution rate. Experimental results from CCD showed that an average value of 1.16 m³ H₂/m³ d for maximum hydrogen production rate (HPR) was produced. A second-order polynomial model was adjusted to the experimental results from CCD. The regression model showed that the quadratic term of all variables tested had a highly significant effect (P < 0.01) on maximum HPR as a defined response. The analysis of the empirical model revealed that the optimal conditions for maximum HPR were incubation temperature, initial pH, and influent dilution rate of 30.23 ^°C, 6.63, and 50.71%, respectively. Generated regression model predicted a maximum HPR of 1.1659 m³ H₂/m³ d could be generated under optimum conditions. Confirmation experimentation was conducted in the optimal conditions determined. Experimental results of the validation test showed that a maximum HPR of 1.1747 m³ H₂/m³ d was produced.     

A hybrid multi-level optimization approach for the dynamic synthesis/design and operation/control under uncertainty of a fuel cell system

During system development, large-scale, complex energy systems require multi-disciplinary efforts to achieve system quality, cost, and performance goals. As systems become larger and more complex, the number of possible system configurations and technologies, which meet the designer’s objectives optimally, increases greatly. In addition, both transient and environmental effects may need to be taken into account. Thus, the difficulty of developing the system via the formulation of a single optimization problem in which the optimal synthesis/design and operation/control of the system are achieved simultaneously is great and rather problematic. This difficulty is further heightened with the introduction of uncertainty analysis, which transforms the problem from a purely deterministic one into a probabilistic one. Uncertainties, system complexity and nonlinearity, and large numbers of decision variables quickly render the single optimization problem unsolvable by conventional, single-level, optimization strategies.To address these difficulties, the strategy adopted here combines a dynamic physical decomposition technique for large-scale optimization with a response sensitivity analysis method for quantifying system response uncertainties to given uncertainty sources. The feasibility of such a hybrid approach is established by applying it to the synthesis/design and operation/control of a 5 kW proton exchange membrane (PEM) fuel cell system.