Bipyridine-based polybenzimidazole as a nitrogen-rich ionomer and a platinum nanoparticle support for enhanced fuel cell performance

Highlights

• Poly(bipyridine-5,5′-bibenzimidazole) polymer with a Nitrogen-rich structure.

• Polymer electrolyte membrane fuel cells with enhanced proton conductivity.

• Polymer functionalization of pristine-multiwalled carbon nanotube.

• A membrane electrode assembly with enhanced electrochemical performance.

Abstract

High density nitrogen supporters, in particular nitrogen-based polymeric supporters of metal catalysts is a topic of high interest to boost the catalytic activity of oxygen reduction reaction (ORR), and accordingly improve fuel cell (FC) performance. Here, we offer a promising membrane electrode assembly (MEA) of a nitrogen-rich and highly conductive polymer; namely, poly(bipyridine-5,5′-bibenzimidazole) (BipyPBI). The MEA comprised BipyPBI as the membrane and platinum(Pt)/BipyPBI-functionalized multiwalled carbon nanotubes as the electrodes. The fabricated materials characterized by NMR, XPS, TEM, XRD, and TGA, and catalytically evaluated by CV, LSV, current-potential, and power density measurements. The fuel cell performance diagnosed by electrochemical impedance spectroscopy (EIS). The use of BipyPBI provided highly active fuel cell electrodes with a homogenous distribution of the Pt catalyst on the surface of carbon nanotubes. Notably, the onset overpotential of ORR lowered by ∼10 mV, the half-wave potential positively shifted by a 41 mV, and the diffusion-limiting current increased by 36 mA/mgPt, when the BipyPBI-based electrode compared to the conventional PBI-based electrode. Importantly, the power density of BipyPBI-based MEA reached 0.893 W/cm² (1.48 W/mg_Pt) compared to 0.549 W/cm² (0.94 W/mg_Pt) for the PBI-MEA. The EIS results indicated an improvement in the ohmic and charge transfer resistances of BipyPBI-based MEA, thanks to the high density nitrogen structure of BipyPBI which enhanced the proton conduction and the reaction kinetics. These results enrich the fuel cell research, and stimulate researchers engaged in related fields.

Introduction

Polymer electrolyte based-fuel cells (PEFC) are promising candidates for future energy because they provide a zero-emission of carbon dioxide with high energy outputs [1], [2]. PEFC consists of a proton conductor membrane (PEM) and two carbon-based electrodes. Understanding how the membrane structure and carbon electrodes influence the fuel cell performance and efficiency have attracted the academic and the industrial interests, since these two parameters have been considered the main roadblocks for the wide spread applications of fuel cell technology [3], [4], [5].

Pristine-multiwalled carbon nanotube (p-MWCNT) is one of the most promising catalyst supports for fuel cell electrodes, because it possesses a well-developed porosity, a high surface area, a high electrical conductivity [6], [7], and is more stable against corrosion compared to the other carbon supports [8], [9]. However, p-MWCNT has no functional groups or binding sites for metal catalyst loading. Thus, it has to be physically or chemically functionalized [10]. Polymer wrapping is a potential physical activation technique of p-MWCNT [11], because it provides surface functional groups capable of homogenously attaching the metal catalyst in a nanoscale, while retaining the pristine structure of MWCNT which is the key property for high electrical conductivity [12]. In addition, the polymer coating of carbon supports leads to a decrease of direct contact of metal catalyst with the carbon surface, the process that leads to an improved stability against carbon corrosion, and accordingly an improved durability of the catalyst [13]. Some conducting polymers have been introduced to physically functionalize the surface of p-MWCNTs for energy applications. For example, the poly(sodium 4-styrenesulfonate) and polypyrrole polymers have been used to wrap the p-MWCNTs, yielding composites with remarkable stability of the loaded metal catalysts under harsh conditions [14], [15]. Therefore, searching and testing new functional polymers for wrapping of p-MWCNTs is topic of high interest, indeed.

Polybenzimidazole (PBI)-based polymers are receiving considerable attentions as potential materials for designating proton conducting membranes of high temperature-PEFCs, because they provide thermally and mechanically-stable membranes with remarkable proton conduction at both high operating temperatures and non-humidifying conditions [16]. The number of N atoms present in PBI structure has played a significant role in the proton conductivity of PBI-based membranes [17]. The higher the number of N atoms in the repeating unit of PBI, the increase in the number of bonded dopant, and accordingly the improvement of hydrogen bonding network and proton conduction [18]. Therefore, a lot of research efforts have been directed to the synthesis and modification of various PBI-based polymers, aiming to boost the fuel cell performance [19], [20]. The phenyl-PBI polymer has first used to improve the dispersibility of MWCNTs [21]. The obtained PBI-MWCNT composite has displayed remarkable dispersion properties in many solvents, thanks to the hydrophilic surface functional groups of the PBI polymer. The nitrogen atoms of PBI structure have also worked as nucleation sites for a homogenous deposition of FC metal catalysts [22]. A PBI polymer with a high density of N atoms was used to modify the nucleation rate and crystal growth of the Pt metal catalyst, resulting in an enhancement of the Pt catalytic activity [23]. The nitrogen atoms of PBI polymer have also used to increase the stability of the catalyst on the support through a chemical bonding interaction [24]. Accordingly, PBI polymers with higher density of N atoms is a topic of high interest, especially for homogenous and stable loading of metal catalysts of PEFC electrodes [25].

Nitrogen-rich PBI polymers are promising candidates for the PEFCs electrodes, because they provide additional basic sites (Nitrogen atoms) for enhancing both the capability of metal catalyst loading, and the acid-doping level of membrane electrode assembly (MEA) [26]. Very recently, we have described a highly conductive bipyridine-based polybenzimidazole (BipyPBI) membrane with remarkable proton conduction properties at high operating fuel cell temperatures, thanks to the nitrogen-rich structure of BipyPBI polymer which offered high doping levels of the phosphoric acid dopant into the cast membranes [27]. This membrane has shown a 36% improvement in proton conductivity compared to the phenyl-PBI membrane of analogous molecular weight. This study has clearly shown the impact of monitoring the structure and the molecular weight of the BipyPBI polymer to provide promising membranes for high temperature PEFC. Accordingly, the extend of application of BipyPBI in the electrodes of PEFC is envisioned to provide a MEA with improved proton conducting properties and metal catalyst stability.

In the current study, we explore the use of BipyPBI both as a highly conductive ionomer to physically active the surface of p-MWCNT through a polymer wrapping process, and as a functional polymer with considerable number of nucleation sites (nitrogen atoms) available for loading homogenous and highly-active nanoparticles of platinum metal catalyst (PtNP). We perform a characterization process of the fabricated PtNP/BipyPBI/p-MWCNT catalyst by using different spectroscopic tools, including transmittance electron microscope (TEM), thermal gravimetric analysis (TGA), and X-ray powder diffraction (XRD). Also, we evaluate the synergetic effect of BipyPBI in the electrochemical surface area (ECSA) of PtNP/BipyPBI/p-MWCNT and the catalytic activity of ORR in comparison to the conventional PBI-based p-MWCNT catalyst (Benzene-based polybenzimidazole; PBI has been used instead of BipyPBI as an ionomer in the catalyst layer). Next, we assemble MEAs of: 1- BipyPBI film as a conductive membrane and PtNP/BipyPBI/p-MWCNT electrodes (coded; BipyPBI-MEA), and 2- PBI film as a conductive membrane and PtNP/PBI/p-MWCNT electrodes (coded; PBI-MEA) to assess the PEFC performance at 120 °C under non-humidifying condition. Subsequently, we use the electrochemical impedance spectroscopy (EIS) as a non-destructive technique to diagnose the fuel cell performance of the assembled MEAs.