Investigation of the performance of a direct borohydride fuel cell with low Pt/C catalyst loading under different operating conditions

Highlights

• The effect of operating parameters on the performance of a DBFC was investigated.

• The highest performance was obtained at 80 °C operating temperature.

• The operating temperature has a more dominant effect on power than fuel flowrate.

• Low fuel flowrate causes high of catalyst.

• Operating parameters should be optimized for low loadings for better performance.

Abstract

Fuel cells are promising alternative energy converters in terms of preventing pollution, efficiency, and noise. Direct borohydride fuel cells (DBFCs) which are defined as a sub-class of polymer electrolyte membrane fuel cells (PEMFCs) and direct liquid fuel cells (DLFC) have increased attention recently since they offer a solution for hydrogen storage problem. However, the commercialization of DBFC is hindered by the need of high platinum loadings. Therefore, reducing the platinum content is crucial to develop cost-effective DBFC without compromising performance. This research focuses on the effects of operational parameters on the DBFC performance with low level Pt/C catalyst loading (anode: 0.32 mg/cm², cathode: 0.36 mg/cm²). The gas diffusion electrode was prepared by spray-coating technique. The peak power density of 19.95 mW/cm² was obtained at 80 ^°C when 1 mL/min was used as a flow rate of fuel.

Introduction

Direct borohydride fuel cells (DBFCs) have increased attention recently as a sub-class of polymer electrolyte fuel cells (PEMFC) because of the following advantages. Compared to direct methanol fuel cells (DMFCs), DBFCs have higher energy density (DBFC: 9.3 kWh/kg – DMFC: 6.08 kWh/kg), higher theoretical cell voltage (DBFC: 1.64 V - DMFC: 1.21 V) and has faster anode reaction kinetics. Theoretically, complete and direct electro-oxidation of borohydride generates a maximum of eight electrons. DBFC can reach 1.64 V theoretically, which is approximately 1.3 times compared with the PEMFC and 1.35 times compared with the DMFC. One of the important advantages of a DBFC compared to PEMFC and DMFC is that anode catalyst CO poisoning is not a problem [[1], [2], [3], [4], [5], [6]]. With all these benefits, DBFCs are promising FC types especially for mobile and portable applications as power suppliers. On the other hand, technologies using the compounds of boron are particularly important for Turkey because a large proportion of global sources of boron are located within the country.

Sodium borohydride can only be used in an aqueous alkali solution as a fuel in DBFC because, BH₄⁻ is not stable in neutral or acidic medium. Oxygen, air, or hydrogen peroxide can be used as oxidants in DBFCs. DBFC anode reaction in alkaline medium of borohydride is as shown in Eq. (I) [7,8].

NaBH₄ + 8OH⁻ = NaBO₂ + 6H₂O + 8e⁻ (E⁰_anode = −1.24 V)

The cathode reaction with oxygen as the oxidant is as shown in Eq. (II)

2O₂ + 4H₂O + 8e⁻ = 8OH⁻ (E⁰_cathode = 0.40 V)

When oxygen is employed as the oxidant at the cathode, the overall cell reaction is shown as Eq. (III).

NaBH₄ + 2O₂ = NaBO₂ + 2H₂O (E⁰_cell = 1.64 V)

The development of an efficient, stable, and economic catalyst for borohydride oxidation is a major challenge in developing cost-efficient DBFCs. There are primarily four types of catalysts for DBFC anodes and they can be listed as precious metals (Pt, Pd, Au), transition metals (Ni, Cu), hydrogen storage alloys, and bimetallic catalysts [[9], [10], [11]]. Among these, Pt, Pd, and Ni show rapid electrode kinetics, high power density, fuel utilization, and, consequently, good power efficiency for fuel cell performance [[12], [13], [14]]. However, in DBFCs, carbon-supported platinum catalyst is still the most commonly used and high-performance electrocatalyst for both BH₄⁻ oxidation and oxygen reduction [11,15,16]. Boyaci et al. investigated the Pt-based PtRu/C catalyst loading (0.7 mg/cm² at anode, 1 mg/cm² at cathode) effects on DBFC and reported 108.5 mW/cm² power density at 80 °C [17]. Olu et al. reported a half cell test results that 158 mW/cm² at ambient conditions with 0.5 mg/cm² Pt loaded anode electrode [11].

The commercialization of DBFC mainly depends on two important parameters: Cost and power performance. The membrane electrode assembly (MEA) is a key component comprising more than 50% of the cost of a fuel cell. It consists of the polymer electrolyte membrane, gas diffusion layers (GDL), and catalyst layers (CL). Also, the electrochemical reaction takes place in the CL in an MEA [[18], [19], [20]].

Because of the high cost of Pt, lowering the catalyst loading of GDL is another way to reduce the cost of DBFC [21,22]. The increase in Pt loading on the electrode generally leads to improved performance. On the contrary, increasing the total load of catalysts causes an increase in thickness. It affects particle agglomeration and mass transfer resistance of reactants and products. Consequently, catalyst loading is needed to be optimized for DBFC [22,23]. According to the literature review, there are few studies about low Pt loading (<0.5 mg/cm²) DBFC compared to the other types of fuel cells especially PEMFC [[24], [25], [26], [27], [28], [29]].

Both the borohydride oxidation and hydrolysis reaction activities of platinum, palladium, and nickel catalysts are high. The power density of the DBFC in which these catalysts are used is high, but their coulombic efficiency is low due to hydrogen formation (For example, the ratio is 50% for nickel catalyst) [30]. It has been reported that high fuel efficiencies for platinum and palladium electrodes can be achieved at low BH₄⁻concentrations with high anode current [18]. Platinum and its alloys are one of the most widely studied metals as an anode catalyst in DBFC systems. Studies have reported that it gives 2 to 4 electrons in the oxidation reaction depending on the concentration of borohydride [31]. In another study, researchers investigated the borohydride oxidation activity of carbon-supported Pt–Au alloy, which theoretically combines the good reaction kinetics of platinum and the high coulombic yield of gold. When Pt–Au catalyst was compared with pure Pt catalyst, it was observed that higher current density was obtained and the number of electrons transferred in oxidation was close to 8. In the same study, when the results tested in carbon-supported colloidal Pt–Ir and Pt–Ni alloy catalysts were examined, it was seen that both catalysts were the most active catalysts with a power density of 53 mW/cm² at 60 °C [32]. The researchers examined the effect of different Pt loading amounts of PtCoB/Cu catalyst prepared by metal loading and galvanic displacement on BOR in an alkaline environment. Pt particles between 10 and 45 nm were loaded on the CoB/Cu electrode and it was observed that the prepared catalyst was more active than Pt/C and CoB/Cu. It was stated that the catalyst was also active in the borohydride hydrolysis reaction and the highest activity was observed at 70 °C with 14.4 μg/cm² Pt loading [33]. In another study conducted on anode catalysts, Pd-doped Ni–Co/C was used as an anode catalyst in an active and passive state, and it was reported that this electrocatalyst was more active for BOR compared to 10% Pt/C catalyst. However, it was also noted that it was poisoned more easily in the 200-cycle performance test performed. When hydrogen peroxide was used at the cathode as the oxidant, a power density of 76 mW/cm² was obtained with 10% Pt/C, while when a Ni–Co/C catalyst with Pd is used, the fuel cell reached 126 mW/cm² [34].

Research in direct sodium borohydride fuel cells has generally focused on the development of anode materials. Studies on cathode catalysts have remained relatively limited [31]. In addition to the borohydride hydrolysis problem at the anode, another important problem that prevents the development of DBFC fuel cells is the crossover of BH₄⁻ ion through the membrane from the anode to the cathode. This causes efficiency loss due to the a mixed potential and low fuel utilization. In studies to solve this problem, priority has been given to the membrane and the resistance of the electrocatalyst to borohydride oxidation at the cathode [35]. Although Pt catalyst has been widely used due to its high oxygen reduction activity and chemical stability, it is necessary to develop an inexpensive, corrosion-resistant, selective alternative catalyst or to reduce the catalyst loading amount.

In this study, MEA was prepared for DBFC which consist of low catalyst loaded anode (0.32 mg/cm², 0.4 mg/cm² Pt) and cathode (0.36 mg/cm², 0.4 mg/cm² Pt) GDE and Nafion® 115 membrane as polymer electrolyte. The spray-coating method was used to fabricate GDEs. DBFC operation parameters were investigated in terms of temperature and fuel flow rate.