Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC: Part II: Degradation mechanism and durability enhancement of carbon supported platinum catalyst

Polymer electrolyte membrane fuel cell (PEMFC) technology has advanced rapidly in recent years, with one of active area focused on improving the long-term performance of carbon supported catalysts, which has been recognized as one of the most important issues to be addressed for the commercialization of the PEMFCs. The cathode catalyst layer in PEMFCs typically contains platinum group metal/alloy nanoparticles supported on a high-surface-area carbon. Carbon support corrosion and Pt dissolution/aggregation are considered as the major contributors to the degradation of the Pt/C catalysts. If the platinum particles cannot maintain their structure over the lifetime of the fuel cell, change in the morphology of the catalyst layer from the initial state will result in a loss of electrochemical activity. This paper reviews the recent advances in the stability improvement of the Pt/C cathodic catalysts in PEMFC, especially focusing on the durability enhancement through the improved Pt–C interaction. Future promising strategies towards the extension of catalysts operation life are also prospected.     

Functionalization of carbons for Pt electrocatalyst in PEMFC

One of the limitations of conventional carbon-supported Pt electrocatalyst in Proton Exchange Membrane Fuel cell (PEMFC) is the carbon corrosion during start-up and shut-down of a fuel cell. The present work investigates the stability of three different carbon blacks and their functionalized forms as supports for Pt electrocatalysts. Acetylene black (AB) as a low surface area carbon shows a higher degree of graphitization and effective functionalization than Vulcan carbon and Ketjan black. Electrochemical tests on corrosion studies show that the AB and its functionalized form (f-AB) as support for Pt electrocatalyst exhibits good electrochemical activity with an ECSA of 52 m²g^-1 and 78 m²g^-1 and excellent corrosion resistance with minimum ECSA loss of 6% and 16% for Pt/AB and Pt/f-AB, respectively, satisfying the DoE target (<40% ECSA loss.) This makes the Pt/f-AB a promising durable electrocatalyst for PEMFC with improved activity and durability.     

Composite‐supported Pt catalyst and electrosprayed cathode catalyst layer for polymer electrolyte membrane fuel cell

A major limitation of the conventional polymer electrolyte membrane fuel cell (PEMFC) catalysts is the fast oxidative degradation of their carbon black supports. Complete replacement of carbon black is difficult because of its low‐cost and high electrical conductivity. Reported here are the development and optimization of composite‐supported Pt catalysts and the electrosprayed cathode catalyst layer with these catalysts for PEMFC. These catalysts are supported by a composite of carbon black (Vulcan XC‐72R) and the electrochemically much more stable carbon‐embedded niobium‐doped titanium dioxide nanofibers (C/Nb_0.1Ti_0.9O₂). Four different catalyst supports with 20 wt.% Pt were prepared by air spraying and electrospraying to compare their activity and stability. Vulcan XC‐72R and C/Nb_0.1Ti_0.9O₂ were tested as pristine support materials for comparison as well as 1:3 and 3:1 mixtures by weight of the two pristine support materials (composite supports). The amount of Nafion in the catalyst ink was optimized for each catalyst layer by a volumetric method. An increase in carbon black content of the support layer from 0% to 100% increases the performance of these catalysts in H₂/air PEMFCs but also increases the loss of oxygen reduction reaction mass activity. The best balance between PEMFC performance and durability was obtained for the Pt catalyst with 25% carbon black in the support layer, while the highest initial oxygen reduction reaction mass activity was obtained for the catalyst with 75% carbon black content. Copyright © 2017 John Wiley & Sons, Ltd.     

Performance and degradation of high temperature polymer electrolyte fuel cell catalysts

An investigation of carbon-supported Pt/C and PtCo/C catalysts was carried out with the aim to evaluate their stability under high temperature polymer electrolyte membrane fuel cell (PEMFC) operation. Carbon-supported nanosized Pt and PtCo particles with a mean particle size between 1.5 nm and 3 nm were prepared by using a colloidal route. A suitable degree of alloying was obtained for the PtCo catalyst by using a carbothermal reduction. The catalyst stability was investigated to understand the influence of carbon black corrosion, platinum dissolution and sintering in gas-fed sulphuric acid electrolyte half-cell at 75 °C and in PEMFC at 130 °C. Electrochemical active surface area and catalyst performance were determined in PEMFC at 80 °C and 130 °C. A maximum power density of about 700 mW cm⁻² at 130 °C and 3 bar abs. O₂ pressure with 0.3 mg Pt cm⁻² loading was achieved. The PtCo alloy showed a better stability than Pt in sulphuric acid after cycling; yet, the PtCo/C catalyst showed a degradation after the carbon corrosion test. The PtCo/C catalyst showed smaller sintering effects than Pt/C after accelerated degradation tests in PEMFC at 130 °C.     

Effect of carbon black support corrosion on the durability of Pt/C catalyst

The work intends to clarify the effect of carbon black support corrosion on the stability of Pt/C catalyst. The corrosion investigations of carbon blacks with similar structures and characteristics were analyzed by cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS). The results indicate that a higher oxidation degree appears on the Black Pearl 2000 (BP-2000) support, i.e. BP-2000 has a lower corrosion resistance than Vulcan XC-72 (XC-72). The durability investigation of Pt supported on the two carbon blacks was evaluated by a potential cycling test between 0.6 and 1.2 V versus reversible hydrogen electrode (RHE). A higher performance loss was observed on the Pt/BP-2000 gas diffusion electrode (GDE), compared with that of Pt/XC-72. XPS analysis suggests that higher Pt amount loss appeared in the Pt/BP-2000 GDE after durability test. X-ray diffraction (XRD) analysis also shows that Pt/BP-2000 catalyst presents a higher Pt size growth. The higher performance degradation of Pt/BP-2000 is attributed significantly to the less support corrosion resistance of BP-2000.     

Pore-scale study of effects of different Pt loading reduction schemes on reactive transport processes in catalyst layers of proton exchange membrane fuel cells

Reducing the Platinum (Pt) loading while maintaining the performance is highly desired for promoting the commercial use of proton exchange membrane fuel cells (PEMFCs). Different methods have been adopted to fabricate catalyst layers (CLs) with low Pt loading, including utilizing lower Pt/C catalysts (MA), mixing high Pt/C catalysts with bare carbon black particles (MB), and reducing CL thickness while maintaining high Pt/C ratio (MC). In this study, self-developed pore-scale model is adopted to investigate the performance of the three Pt reduction methods. It is found that MA shows the best performance while MB shows the worst. Then, effects of Pt dispersion are further explored. The results show that denser Pt sites will result in higher local oxygen flux and thus higher local transport resistance. Therefore, MA method, which shows the better Pt dispersion, leads to improved performance. Third, CLs with quasi-realistic structures are investigated. Higher tortuosity resulting from the random pores produces higher bulk resistance along the thickness direction, while MA still exhibits the best performance. Finally, improved CL structures are investigated by designing perforated CL structures. It is found that adding perforations can significantly reduce the bulk transport resistance and can improve the CL performance. It is demonstrated that CL structure plays important roles on performance, and there are still huge potentials to further improve CL performance by increasing Pt dispersion and optimizing CL structures.     

High surface area graphite as alternative support for proton exchange membrane fuel cell catalysts

The suitability of a high surface area graphite (HSAG) as proton exchange membrane fuel cell (PEMFC) catalyst support has been evaluated and compared with that of the most popular carbon black: the Vulcan XC72. It has been observed that Pt is arranged on the graphite surface resulting in different structures which depend on the catalysts synthesis conditions. The influence that the metal particle size and the metal-support interaction exert on the catalysts degradation rate is analyzed. Temperature programmed oxidation (TPO) under oxygen containing streams has been shown to be a useful method to assess the resistance of PEMFC catalysts to carbon corrosion. The synthesized Pt/HSAG catalysts have been evaluated in single cell tests in the cathode catalytic layer. The obtained results show that HSAG can be a promising alternative to the traditionally used Vulcan XC72 carbon black when suitable catalysts synthesis conditions are used.     

Sublayers for Pt catalyst electrodeposition electrodes in PEMFC

The effect of sublayers on the deposited platinum (Pt) catalyst layer fabricated by electrodeposition, and on the resulting fuel cell performance, was investigated. The substrate was prepared by applying a hydrophobic sublayer, composed of polytetrafluoroethylene (PTFE) and carbon black, and a hydrophilic sublayer, composed of Nafion and glycerol, onto an uncatalyzed gas diffusion layer prior to the electrodeposition of the Pt catalyst. The hydrophilic sublayer was found to play a substantial role in the Pt electrodeposition, since the structure of the resultant Pt catalyst significantly depended on the presence of the hydrophilic sublayer, the total loading amount and the Nafion to glycerol weight ratio, which in turn affected the fuel cell performance. The hydrophobic sublayer, which did not directly contact with the plating solution, did not show as marked an effect on the Pt deposit structure compared to that of the hydrophilic sublayer, except at high PTFE to carbon black weight ratios (≥70:30). However, the suitable PTFE to carbon black weight ratio in the hydrophobic sublayer was still important for the water management, mass transport of reactant gases and ohmic resistance of the membrane electrode assembly (MEA) during fuel cell operation. In this study, a total hydrophilic loading of 0.8 mg cm⁻² with a Nafion to glycerol weight ratio of 50:50, and a PTFE to carbon black weight ratio in the hydrophobic sublayer of 30:70 was found to yield the best Pt catalyst layer for PEMFC.     

Graphitic mesoporous carbon as a durable fuel cell catalyst support

Highly stable graphitic mesoporous carbons (GMPCs) are synthesized by heat-treating polymer-templated mesoporous carbon (MPC) at 2600 °C. The electrochemical durability of GMPC as Pt catalyst support (Pt/GMPC) is compared with that of carbon black (Pt/XC-72). Comparisons are made using potentiostatic and cyclic voltammetric techniques on the respective specimens under conditions simulating the cathode environment of PEMFC (proton exchange membrane fuel cell). The results indicate that the Pt/GMPC is much more stable than Pt/XC-72, with 96% lower corrosion current. The Pt/GMPC also exhibits a greatly reduced loss of catalytic surface area: 14% for Pt/GMPC vs. 39% for Pt/XC-72.     

Carbon corrosion behaviors and the mechanical properties of proton exchange membrane fuel cell cathode catalyst layer

Carbon corrosion-induced catalyst layer destruction is the primary reason to the performance decay of proton exchange membrane fuel cells (PEMFCs). In this study, the accelerated stress test (AST) on carbon corrosion was conducted, and real-time CO₂ evolution was measured in-situ by non-dispersive infrared (NDIR) analysis. The performance degradation was investigated by the reduction of the current density and the loss of electrochemical active surface area (ECSA) of Pt. The loss of catalyst layer porosity and increase of mass transport resistance were investigated by the visible reduction of porosity and thickness in the cathode catalyst layer (CCL). Further mechanical tensile tests showed that the elastic modulus of CCL remained unchanged initially, and then increased probably due to the compaction of CCL. In the final step, it decreased due to the complete failure of the material. Thus, carbon corrosion was proved to alter the mechanical strength of CCL.     

Tungsten carbide modified high surface area carbon as fuel cell catalyst support

Phase pure WC nanoparticles were synthesized on high surface area carbon black (800 m² g⁻¹) by a temperature programmed reaction (TPR) method. The particle size of WC can be controlled under 30 nm with a relatively high coverage on the carbon surface. The electrochemical testing results demonstrated that the corrosion resistance of carbon black was improved by 2-fold with a surface modification by phase pure WC particles. However, the WC itself showed some dissolution under potential cycling. Based on the X-ray diffraction (XRD) and inductively coupled plasma (ICP) analysis, most of the WC on the surface was lost or transformed to oxides after 5000 potential cycles in the potential range of 0.65-1.2 V. The Pt catalyst supported on WC/C showed a slightly better ORR activity than that of Pt/C, with the Pt activity loss rate for Pt/WC/C being slightly slower compared to that of Pt/C. The performance and decay rate of Pt/WC/C were also evaluated in a fuel cell.     

On the stability of TiN-based electrocatalysts for fuel cell applications

The transition metal compound - titanium nitride (TiN), with its high electrical conductivity and corrosion resistance, can be a potential fuel cell material, particularly in the development of durable electrocatalysts replacing the state-of-the-art Pt/C. Compared to conventional carbon black, TiN nanoparticle (TiN NP) catalyst supports have a lower rate of corrosion under fuel cell conditions. The current research is a follow up study on the stability of TiN based fuel cell electrodes and its impact on the electrochemical activity of the Pt based electrocatalysts when used as an alternative to the state-of-the-art carbon black support in commercial fuel cell catalyst systems under fuel cell operating conditions. Through this paper, we report that an active behavior of TiN NP can be observed at the optimal conditions of 0.5 M H₂SO₄ and 60 °C. But under increased temperature or acidic conditions, the native layer on TiN tends to dissolve in the electrolyte exposing the underlying nitride surface which then gets passivated with hydroxide groups. Electrochemical and XPS characterization is used to validate our hypothesis of active or passive nature of TiN NP due to presence or absence of surface passivating -OH groups, respectively. The synthesized Pt/TiN electrocatalyst, upon subjecting to accelerated durability test, showed the performance trends which agree well with the active and passive nature of the TiN NP supports.     

Electrochemical impedance analysis with transmission line model for accelerated carbon corrosion in polymer electrolyte membrane fuel cells

The effects of varying the applied voltage and relative humidity of feed gases in degradation tests of polymer electrolyte membrane fuel cells (PEMFCs) were analyzed using electrochemical impedance spectroscopy (EIS). A transmission line model that considers the proton-transport resistance in the cathode catalyst layer was used to analyze impedance spectra obtained from degraded PEMFCs. As the applied cell voltage was increased from 1.3 to 1.5 V to induce accelerated degradation, the cell performance decayed significantly due to increased charge- and proton-transfer resistance. The PEMFC degradation was more pronounce at higher relative humidity (RH), i.e. 100% RH, as compared with that observed under 50% RH. Furthermore, changes in the charge transfer resistance of the electrode accompanied changes in the ionic conductivity in the PEMFC catalyst layer. Although the initial ionic and charge-transfer resistances in the catalyst layer were lower under higher RH conditions, the impedance results indicated that the performance degradation was more significant at higher water contents in the electrode due to the consequential carbon corrosion, especially when higher voltages, i.e. 1.5 V, were applied to the PEMFC single cell.     

Controlling Pt loading and carbon matrix thickness for a high performance Pt-nanowire catalyst layer in PEMFCs

Pt-nanowire (Pt-NW) catalyst layers were prepared by in-situ growing Pt nanowires onto carbon matrix coated on electrolyte membrane surface and used as PEMFC cathodes. The performances of the catalyst layer with various catalyst loadings and carbon matrix thicknesses were evaluated. Scanning electron microscopy (SEM) was employed to observe the morphology and thickness of the Pt-NW catalyst layers, energy-dispersive X-ray spectroscopy (EDS) was used to investigate the Pt distribution along the layers. The polarization curve, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were performed in fuel cells to check the practical electrochemical activity of the Pt-NW catalyst layers. The results showed that the electrochemical surface area (ECSA) and the cell performance exhibited a volcano-type curve with different Pt loadings, while the carbon matrix thickness had an influence on the Pt-NW gradient distribution, the mass diffusion, and the charge transfer in the electrodes.     

The effect of Pt/C agglomerates in electrode on PEMFC performance using 3D micro-structure lattice models

To simulate the Pt/C agglomerate phenomenon in proton exchange membrane fuel cell (PEMFC) cathode, we established 3D two-phase micro-structure lattice models, including C phase (Pt/C particles) and IP phase (mixed ionomer and pore), respectively. Based on these models, we studied the effect of Pt/C agglomerates on theoretical catalyst utilization, IP phase tortuosity and cell performance in PEMFC, and compared the electrode with linear agglomerates to the electrode with spherical agglomerates. Unexpectedly, contrary to the simulation results from macro-models, the electrode with Pt/C linear agglomerates performed better than that with Pt/C spherical agglomerates. In addition, it was found that the electrode with Pt/C agglomerates performed better than that without Pt/C agglomerates, which was different to what we believed before. By analysing the reasons of cell performance change, it was found that mass transport had a more important effect on cell performance than electrochemical reaction.     

Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC: Part I. Physico-chemical and electronic interaction between Pt and carbon support,and activity enhancement of Pt/C catalyst

Carbon-supported platinum or platinum alloys are generally used as the cathodic electrocatalysts in polymer electrolyte membrane fuel cells (PEMFC) to enhance the oxygen reduction reaction (ORR). Main challenges to be addressed in this area are the high electrochemical activity and high stability maintenance for low-Pt-loading catalysts toward the feasibility for fuel cell operation and the reduction of the system cost. The interaction of Pt with carbon support, as well as the interaction of Pt precursor with carbon during the supported catalyst formation, are considered to be beneficial to the improvement of catalytic activity and durability of the electrocatalysts. This paper provides a review of recent advances related to the physico-chemical and electronic interactions at the catalyst–support interface and the catalyst activity enhancement through improved Pt–C interaction, especially focusing on the surface modification of the carbon support to form proper functional groups and chemical links at the Pt/C interface.     

Durability studies on performance degradation of Pt/C catalysts of proton exchange membrane fuel cell

In the present paper, a proton exchange membrane fuel cell (PEMFC) using 20 wt.% Pt/C as anode and cathode catalysts, and ambient air at cathode was operated at a current density of 160 mA cm⁻² for 2250 h. The measurement results showed that electrochemically active specific areas (S_EAS) of both electrode catalysts calculated from CV curves after test evidently decreased. The decay rate of S_EAS of anode catalyst was much lower than that of cathode one. X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), and X-ray photoelectron spectrometry (XPS) were employed to characterize the anode and cathode catalysts before and after the life test. The XRD results showed that their crystal structures were perfect, the particle size of new Pt/C catalyst was about 2.5 nm, however, the particle sizes of anode and cathode ones markedly increased, and were about 4.9 nm and 6.8 nm, respectively, after the life test. Furthermore, the size of cathode catalyst was much bigger than that of anode one after test. The Pt element was also found in Nafion^® film as shown in EDAX result. The XPS results presented that the content of Pt oxidation states in cathode was much more than that in anode, and the corrosion of carbon support in cathode was also more severe than that in anode after the life test. The experimental results indicated that the increase of particle size of Pt/C catalyst was illustrated with the dissolution/redeposition mechanism. The degradation of cathode catalyst for oxygen electroreduction was one of the main factors affecting on the performance decay of PEMFC.     

Improved durability of Pt/CNT catalysts by the low temperature self-catalyzed reduction for the PEM fuel cells

Long-term durability of catalysts is considered as the most critical issue to commercialize the fuel cell vehicles. In this study, we showed highly durable CNT-supported Pt catalysts treated by simple low temperature hydrogen bubbling (LTHB). Even at low temperature, the loss of the oxygen related functional groups which can accelerate the carbon corrosion and Pt agglomeration was observed by the effect of the catalytic cleavage of hydrogen molecules followed by the spillover and dehydration reaction. The 62.5% activity loss was observed in the commercial Pt/C catalyst but only 6.2% for Pt/CNT-R catalyst after high voltage acceleration test. Less agglomeration of Pt particles and loss of active surface area were observed than those of commercial Pt/C catalysts, which also contributed to the less increase in the electrochemical resistance of PT/CNT-R. The polarization resistance increased only 3% for Pt/CNT-R but as high as 700% for commercial Pt/C.     

Morphological influence of graphitic carbon nanofibers by N–F dual-doping on Pt electrocatalytic activity and stability for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Morphology of carbon nanofibers significantly effects Pt nanoparticles dispersion and specific interaction with the support, which is an important aspect in the fuel cell performance of the electrocatalysts. This study emphasizes, the defects creation and structural evolution comprised due to N–F co-doping on graphitic carbon nanofibers (GNFs) of different morphologies, viz. GNF-linearly aligned platelets (L), antlers (A), herringbone (H), and their specific interaction with Pt nanoparticle in enhancing the oxygen reduction reaction (ORR). GNFs–NF–Pt catalysts exhibit better ORR electrocatalytic activity, superior durability that is solely ascribed to the morphological evolution and the doped N–F heteroatoms, prompting the charge density variations in the resultant carbon fiber matrices. Amongst, H–NF–Pt catalyst performed outstanding ORR activity with exceptional electrochemical stability, which shows only 20 mV loss in the half-wave potential whilst 100 mV loss for Pt/C catalyst on 20,000 potential cycling. The PEMFC comprising H–NF–Pt as cathode catalyst with minimum loading of 0.10 mg cm^?2, delivers power density of 0.942 W cm^?2 at current density of 2.50 A cm^?2 without backpressures in H₂–O₂ feeds. The H–NF–Pt catalyst owing to its hierarchical architectures, performs well in PEMFC at the minimized catalyst loading with outstanding stability that can significantly decrease total price for the fuel cell.