Periodic Reporting for period 1 - LOWCOST-PBI-HTPEMFC (Novel binder-ionomer-free electrodes enable ultra-low Pt loading electrodes for low cost High Temperature proton exchange membrane fuel cells based in phosphoric acid-doped polybenzimidazole membranes)
Reporting period: 2018-08-15 to 2020-08-14
This project addresses the issue of the precious metal content of the electrodes of polybenzimidazole-based High Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFCs). It´s well known that the high amount of precious metal (Pt and Pt-alloys) incorporated as catalyst in the electrodes of PEMFCs is one of the main factors limiting the worldwide commercialization of this promising and zero emissions technology. This issue is even more aggravated in the case of HT-PEMFCs whose state-of-the-art Pt loading is about 10 times higher than its homologous Low Temperature (LT-) PEMFCs based in perfluorosulphonic acid membrane (Nafion TM). In spite of being the most widespread kind of PEMFC, LT-PEMFCs has not yet reached the expected penetration in the automobile and green-energy sector due to the lack of an infrastructure for fuel (hydrogen) production and distribution. Nowadays, the most common source of H2 are the already existing fossil fuels as hydrocarbons (e.g.: methane or natural gas). By means of steam reforming of hydrocarbons can be obtained an H2-rich fuel that can be used in PEMFCs. But this fuel contains CO (in the volume percent range) that poisons the catalyst and deteriorates the fuel cell performance. At the operating temperature of LT-PEMFCs (60-80ºC) only 10-20 ppm of CO can be tolerated, so, a purification of the gas is needed to lower the CO content down to the tolerable range. In contrast, the operational temperature of HT-PEMFCs (120-200ºC) allow tolerating CO impurities in a concentration up to 1000 times higher than its LT-PEMFCs counterpart. This high CO tolerance makes possible HT-PEMFCs to be fueled directly from the reformer without purification steps what makes a simpler and more efficient fuel cell system.
In spite of the promising prospects of this technology, there is still a major issue to overcome: the high amount of precious metal catalyst (Pt or Pt alloys) incorporated in the electrodes of the HT-PEMFCs. The state-of-the-art Pt loading is currently around 1 mgPtcm-2. In contrast, LT-PEMFCs run at similar efficiency with electrodes loaded to around 0.1 mgPtcm-2. So the goal for the HT-PEMFC technology is to diminish the amount of catalyst in the electrodes without compromising the performance. This ambitious objective has been addressed in this project by a paradigm shift regarding to the traditional composition of the catalytic layer of the electrode.
In spite of the promising prospects of this technology, there is still a major issue to overcome: the high amount of precious metal catalyst (Pt or Pt alloys) incorporated in the electrodes of the HT-PEMFCs. The state-of-the-art Pt loading is currently around 1 mgPtcm-2. In contrast, LT-PEMFCs run at similar efficiency with electrodes loaded to around 0.1 mgPtcm-2. So the goal for the HT-PEMFC technology is to diminish the amount of catalyst in the electrodes without compromising the performance. This ambitious objective has been addressed in this project by a paradigm shift regarding to the traditional composition of the catalytic layer of the electrode.
The work developed in this project can be divided in different actions aimed to lower the Pt loading of the electrodes of HT-PEMFCs:
i) Reconceptualization of the catalytic layer composition of HT-PEMFCs. A deep understanding of the role of the different constituents of the catalyst layer (i.e. ionomer, binder, electrolyte and catalyst) derived in a new concept of catalytic layer that resulted simpler, economically cheaper and more efficient than the conventional catalytic layer. The composition of conventional catalytic layers was based in the catalyst particles ( Pt/C or Pt-alloy/C) and a binder (usually polytetrafluoroethilene, PTFE) homogeneously distributed throughout the catalytic layer. The studies carried out in this project demonstrated that the polymeric binder blocks the catalytic sites hindering the electrochemical reaction in the electrode and also introduce serious mass transport limitations in the catalytic layer that lead to a decreased performance of the HT-PEMFC. A new concept of electrode was created, the binderless electrode, whose catalyst layer composition was just the Pt/C catalyst particles. It was argued that the intrinsic mesoporosity of the catalytic layer was enough to, via capillary forces, suction the excess of PA electrolyte from the PA-doped PBI membrane and redistribute it homogeneously throughout the catalytic layer in a net of open pores (void of acid) that allow for the gas permeation, and mesopores (filled with acid) that allow for the proton conduction. The fact of an augmented kinetic of the electrodes together with a lower mass transport limitation of the reactant gases, favored by this kind of arquitecture of catalytic layer, made possible to lower the Pt loading of HT-PEMFCs by keeping the state-of-the-art performance. Thus, membrane electrode assemblies (MEAs) with cathode electrodes loaded to 0.1 mgPtcm-2, one order of magnitude less than the state of the art (1 mgPtcm-2), rendered as the standards in HT-PEMFCs at 1 mgPtcm-2: 0.6V at 0.2 mgPtcm-2 and peak power density about 420 mW cm-2 (under ambient pressure and fueled with H2/Air).
ii) Impact of the catalyst deposition method on the performance of the fuel cell in the scope of ultra-low Pt loadings. The novel concept of binderless electrode relies in the porous structure of the catalytic layer, which is influenced by the catalyst deposition method. In this project, ultra-low Pt loaded binderless electrodes were built by conventional catalyst deposition methods (ultrasonic-spraying and airbrushing) and by using an emerging coating technique: Electrospraying. The conventional methods gave rise to compact catalytic layers with mean porosity of about 50%, a value increased up to 70% by the electrospraying technique. But the most differentiate feature between the different coating methods was the pore size distribution. Thus, in the regular methods, the pore size distribution was constrained to the micro- and meso-porous range. However, the catalyst layer built from electrospraying exhibited a pore size distribution at a larger scale, from macro-pores (that facilitate the gas permeation of the reactant gas) to micro-pores (that suck the PA from the PA-doped membrane via capillary forces) favoring the creation of triple phase boundaries (TPBs). This arrangement of the catalytic layer lead to a substantially improved performance of the ultra-low Pt loading electrodes prepared by the electrospraying method compared to the regular methods (peak power density of 420 mW cm-2 vs. 350 mW cm-2, respectively).
The results of the project are expected to be disseminated via the conventional platforms in science: publication of papers in open access journals within the field of HT-PEMFCs and participation in international meetings, congresses or seminars in order to provide the highest visibility of the work and to reach the maximum audience.
i) Reconceptualization of the catalytic layer composition of HT-PEMFCs. A deep understanding of the role of the different constituents of the catalyst layer (i.e. ionomer, binder, electrolyte and catalyst) derived in a new concept of catalytic layer that resulted simpler, economically cheaper and more efficient than the conventional catalytic layer. The composition of conventional catalytic layers was based in the catalyst particles ( Pt/C or Pt-alloy/C) and a binder (usually polytetrafluoroethilene, PTFE) homogeneously distributed throughout the catalytic layer. The studies carried out in this project demonstrated that the polymeric binder blocks the catalytic sites hindering the electrochemical reaction in the electrode and also introduce serious mass transport limitations in the catalytic layer that lead to a decreased performance of the HT-PEMFC. A new concept of electrode was created, the binderless electrode, whose catalyst layer composition was just the Pt/C catalyst particles. It was argued that the intrinsic mesoporosity of the catalytic layer was enough to, via capillary forces, suction the excess of PA electrolyte from the PA-doped PBI membrane and redistribute it homogeneously throughout the catalytic layer in a net of open pores (void of acid) that allow for the gas permeation, and mesopores (filled with acid) that allow for the proton conduction. The fact of an augmented kinetic of the electrodes together with a lower mass transport limitation of the reactant gases, favored by this kind of arquitecture of catalytic layer, made possible to lower the Pt loading of HT-PEMFCs by keeping the state-of-the-art performance. Thus, membrane electrode assemblies (MEAs) with cathode electrodes loaded to 0.1 mgPtcm-2, one order of magnitude less than the state of the art (1 mgPtcm-2), rendered as the standards in HT-PEMFCs at 1 mgPtcm-2: 0.6V at 0.2 mgPtcm-2 and peak power density about 420 mW cm-2 (under ambient pressure and fueled with H2/Air).
ii) Impact of the catalyst deposition method on the performance of the fuel cell in the scope of ultra-low Pt loadings. The novel concept of binderless electrode relies in the porous structure of the catalytic layer, which is influenced by the catalyst deposition method. In this project, ultra-low Pt loaded binderless electrodes were built by conventional catalyst deposition methods (ultrasonic-spraying and airbrushing) and by using an emerging coating technique: Electrospraying. The conventional methods gave rise to compact catalytic layers with mean porosity of about 50%, a value increased up to 70% by the electrospraying technique. But the most differentiate feature between the different coating methods was the pore size distribution. Thus, in the regular methods, the pore size distribution was constrained to the micro- and meso-porous range. However, the catalyst layer built from electrospraying exhibited a pore size distribution at a larger scale, from macro-pores (that facilitate the gas permeation of the reactant gas) to micro-pores (that suck the PA from the PA-doped membrane via capillary forces) favoring the creation of triple phase boundaries (TPBs). This arrangement of the catalytic layer lead to a substantially improved performance of the ultra-low Pt loading electrodes prepared by the electrospraying method compared to the regular methods (peak power density of 420 mW cm-2 vs. 350 mW cm-2, respectively).
The results of the project are expected to be disseminated via the conventional platforms in science: publication of papers in open access journals within the field of HT-PEMFCs and participation in international meetings, congresses or seminars in order to provide the highest visibility of the work and to reach the maximum audience.
The main achievement within this project was to lower the Pt loading of the electrodes for PBI-based HT-PEMFCs without compromising the performance output. Durable (thousands of hours) and high performance (420 mW cm-2 at ambient pressure and H2/Air) electrodes with a cathode Pt loading of only 0.1 mgPtcm-2 represent a substantial progress respect to the state-of-the-art electrodes with a Pt loading 1 order of magnitud higher (1mgPtcm-2). This achievement is expected to be extended to other researchers in the field of HT-PEMFCs to further impulse this emerging technology. A decrease in 1 order of magnitud of the catalyst is expected to be attractive to the industry sector of HT-PEMFCs since so far, the high Pt loading of the electrodes was the main concern and an impassable barrier for the commercialization of this kind of PEMFCs.