Phosphoric Acid Fuel Cell Cathodes Research Articles

Phosphate tolerance of nitrogen-coordinated-iron-carbon (FeNC) catalysts for oxygen reduction reaction: A size-related hindrance effect

The greatest challenges facing the large – scale commercialization of the intermediate temperature phosphoric acid fuel cell (PAFC) technology are: (i) the high cost of Pt catalyst that is required to overcome the slow kinetics of oxygen reduction reaction (ORR) on the PAFC cathode, and (ii) deactivation of the Pt catalyst by exposure to the phosphoric acid electrolyte. Inexpensive carbon-based materials, particularly iron-nitrogen-coordinated carbon supported catalysts (FeNC), are promising cathodes for PAFCs. The higher ORR activation energy and phosphate poisoning resistance of FeNC present a clear advantage over the Pt-based commercial catalysts that are easily susceptible to deactivation after exposure to phosphate anions. While the immunity of FeNC catalysts to phosphate anion poisoning has been reported previously, this work explores the counter-intuitive and conditional nature of this poisoning resistance based on the properties of the carbon support used in the preparation of the FeNC catalyst. A combination of electrochemical measurements with characterization experiments performed using X-ray photoelectron spectroscopy, infrared spectroscopy and in-situ X-ray absorption spectroscopy is used to explore the phenomenon of phosphate poisoning on FeNC catalysts synthesized from two different carbon supports. Results from these studies reveal a “size-related hindrance effect” wherein the ORR active Fe centers in FeNC remain inaccessible to larger anions such as phosphate when the Fe sites are present deeper inside the pores of a carbon support with high porosity. However, FeNC catalyst is poisoned by phosphate anions when synthesized using a carbon support with low porosity. This work helps gain insights into the nature of active sites in FeNC catalysts that will be useful in designing precious-metal free ORR catalysts for intermediate temperature fuel cell technologies.

Characterization and fault diagnosis of PAFC cathode by EIS technique and a novel mathematical model approach

Considerable ongoing research exists in the area of fuel cells for power and distributed power generation. Of the various types of fuel cells, phosphoric acid fuel cell (PAFC) is a mature technology and is under limited production in various parts of the world. Electrochemical impedance spectroscopy (EIS) is a powerful tool which can be used for characterization of PAFC cathodes. In EIS, the electrode response is analyzed against an equivalent electrical circuit for diagnostics. A shortcoming of this approach is that multiple circuits can match the same response and correlating the physical electrode parameters with the circuit components may become quite difficult. To avoid this, a mathematical model of the PAFC cathode that is easy to solve is developed. The relative value and position of the maximum phase angle with respect to frequency is proposed as a diagnostic marker. A diagnostics table based on this marker is developed using the simulation of the mathematical model and the results are experimentally verified.

Step response analysis of phosphoric acid fuel cell (PAFC) cathode through a transient model

Transient state response analysis of phosphoric acid fuel cell (PAFC) cathode is important to understand various competitive processes like diffusion, reaction and product back diffusion occurring at various layers of the composite cathode. A two-dimensional unsteady state model for simulating PAFC cathode is developed as an extension of the previously developed steady-state model [S. Roy Choudhury, M.B. Deshmukh, R. Rengaswamy, A two-dimensional steady-state model for phosphoric acid fuel cells (PAFC), J. Power sources 112 (2002) 137–152]. The transient model is solved to study the impact of various parameters such as Tafel slope, diffusivity etc on the step response of the fuel cell. The effect of partial pressure variation in bulk gas for large sized PAFC cathode is also analysed. Trend analysis based on the model output is also experimentally verified using a small unit cell setup. The effect of various parameters on the settling time of the cathode, as revealed in this study, suggests possible development of a diagnostic tool employing such transient model.

Modelling of phosphoric acid fuel cell cathode behaviour

A mathematical model for phosphoric acid fuel cell (PAFC) cathodes has been developed. The model, based on equations that take activation and diffusional problems into account, provides a description of the electrochemical behaviour of the electrode. These studies allow optimization of the morphological characteristics of this cell component and improvement in the performance of the whole fuel cell. In particular, the sensitivity analysis carried out on the model provides information on the influence of single parameters. Accordingly, the as optimized cathode allowed a performance of 700 mV at 200 mAcm−2 in air; the corresponding cell potential also increased from 500 mV at 150 mAcm−2 to 510mV at 200 mAcm−2.

Agglomerate Model for DC and AC Response of Phosphoric Acid Fuel Cell Cathode

An agglomerate model which simulates both dc and ac responses of phosphoric acid fuel cell cathodes with various oxidant compositions and dc current densities, aiming to reduce ambiguities in the analysis, has been developed and was applied to the in situ analysis of the laboratory cell by curve fitting to the experimental data. It was demonstrated that the model could fairly represent the data with reasonable values of fitting parameters.

Five Percent Platinum‐Tungsten Oxide‐Based Electrocatalysts for Phosphoric Acid Fuel Cell Cathodes

A Pt‐tungsten oxide‐based electrocatalyst has been fabricated by an inexpensive chemical route for use as an oxygen cathode in 99% phosphoric acid at 180°C. The effect of % (wt/wt) on the Pt‐tungsten oxide/C‐based electrode performance was studied. The electrocatalytic properties for the oxygen reduction reaction (ORR), e.g., exchange current density and mass activity of a 5% Pt‐40% ‐based electrode were found to be twice as high as those of 10% Pt, which contains double the amount of platinum. The Tafel slope and specific activity of the two electrodes are similar. It was shown that an increase in its electrochemically active surface area was the only reason for the performance of the 5% Pt‐40% ‐based electrode. The electrocatalytic parameters of the 5% Pt‐40% ‐based electrode for the ORR were compared to those of the 2% Pt‐1% ‐based electrode.

リン酸形燃料電池空気極の初期性能不安定現象

リン酸形燃料電池空気極の初期性能不安定現象

Influence of the temperature of heat treatment on phosphoric acid fuel cell cathodes

The thermal behaviour of electrode materials during heat treatment from 250–400 °C was studied. The cathode performance changed by two steps depending on temperature of the heat treatment. At the first step (300–320 °C), polytetrafluoroethylene (PTFE) melted unexpectedly, giving hydrophobic character to the catalyst layer. This change drastically improved the cathode performance. At the second step (340–400 °C), support carbon was oxidized by catalytic action of platinum, increasing the PTFE content from 45 to 83 wt %. This caused a gradual decline in the cathode performance. The poor cell performance of the electrode treated below 300 °C is due to the fact that PTFE is not melted at that temperature. Triton X-100 used as a surfactant in the PTFE dispersion disappeared completely from the electrode at these temperatures by oxidation, catalysed by platinum.

Simulation and optimization of porous gas-diffusion electrodes used in hydrogen oxygen phosphoric acid fuel cells—II development of a detailed anode model

A detailed model has been formulated for porous gas-diffusion electrodes which are used as anodes in the phosphoric acid fuel cell (PAFC). This model development parallels previous efforts in the fundamental modelling of the PAFC cathode. The resulting anode model incorporates the elementary steps and fundamental electrochemical rate processes which occur during the platinum catalyzed hydrogen oxidation in the presence of carbon monoxide and includes the various transport processes which arise in porous gas diffusion cathodes. The formulated anode model is presented as a system of equations which can be solved by numerical methods to allow simulation and correlation of fuel cell anode performance.

ChemInform Abstract: Carbons as Supports for Precious Metal Catalysts.

Abstract Activated carbons are attractive as support materials for precious metal catalysts in certain applications. However, the preparation of industrial catalysts can be difficult and unpredictable because of wide batch to batch variations in commercial carbons. This does not mean that carbon manufacturers produce poor quality materials; they are meeting the specifications laid down by those non-catalytic industries which they primarily serve. This paper describes the important features required for chemical process catalysts in terms of active metal location and metal dispersion in relation to activity and selectivity. Redox and acid/base titrations are identified as useful tools for characterising carbons prior to catalyst preparation and can be used as part of the control process to ensure consistent products. The additional properties required for carbon supports for fuel cell applications are corrosion resistance and good electrical conductivity. These are discussed in relation to phosphoric acid fuel cell cathode catalysts and results on platinum/base metal alloy systems on a graphitic support are reported.

Carbons as supports for precious metal catalysts

Activated carbons are attractive as support materials for precious metal catalysts in certain applications. However, the preparation of industrial catalysts can be difficult and unpredictable because of wide batch to batch variations in commercial carbons. This does not mean that carbon manufacturers produce poor quality materials; they are meeting the specifications laid down by those non-catalytic industries which they primarily serve. This paper describes the important features required for chemical process catalysts in terms of active metal location and metal dispersion in relation to activity and selectivity. Redox and acid/base titrations are identified as useful tools for characterising carbons prior to catalyst preparation and can be used as part of the control process to ensure consistent products. The additional properties required for carbon supports for fuel cell applications are corrosion resistance and good electrical conductivity. These are discussed in relation to phosphoric acid fuel cell cathode catalysts and results on platinum/base metal alloy systems on a graphitic support are reported.

An investigation of the effects of electrode preparation parameters on the performance of phosphoric acid fuel cell cathodes

We have investigated the effect of electrode fabrication parameters ( ie Teflon content and sintering temperature) on cathodes for phosphoric acid fuel cells. We evaluated these cathodes in 98% phosphoric acid at 170°C and correlated their performance with their morphological characteristics. Additionally, we derived expressions and values for the oxygen gain under conditions of activation polarization and joint activation and diffusion polarization. We observed that with optimum electrode fabrication parameters, electrode performance was dominated by activation polarization up to current densities of about 0.6 A cm −2. Finally, we observed that the underlying factor affecting electrode performance is the percent acid occupation of the electrocatalyst layer. The optimum value of this parameter is independent of the specific electrode fabrications used in our study.

Carbon substrates for phosphoric acid fuel cell cathodes

High surface area carbons are used as electrocatalyst supports for noble metal particles in phosphoric acid fuel cells. Under operating conditions at high temperature (in excess of 200°C), the carbon support corrodes. An examination of the corrosion characteristics of various carbons as a function of electrode potential, acid temperature and carbon treatments is given. Since efficient catalyzation of the carbon requires small noble metal electrocatalyst particles, a high surface area carbon is required. The influence of heat treatment on commercial carbons shows a decrease in dispersion for the noble metals, although the heat treatment stabilizes the carbons against corrosive degradation. Correlations between the electrochemical properties of the carbons and the X-ray diffraction ( c 2 ) lattice parameters were observed.