Sodium borohydride hydrogen generator using Co–P/Ni foam catalysts for 200 W proton exchange membrane fuel cell system

Highlights

• Response characteristics of Co–P/Ni foam catalysts are investigated.

• Catalytic activity is improved with increase in PPI (pores per inch) of Ni foam.

• Co–P/Ni foam (110 PPI) catalyst has improved response characteristics.

• The energy density of a 200 W PEMFC system for producing 600 Wh is 252.1 Wh/kg.

• Co–P/Ni foam (110 PPI) catalyst is suitable for fuel cell system.

Abstract

The response characteristics of electroless-deposited Co–P/Ni foam catalysts for sodium borohydride hydrolysis were investigated. The effect of nickel foam geometry on the properties of the catalysts was evaluated. As the PPI (pores per inch) of the nickel foam increased, the hydrogen generation rate per gram of the deposited catalyst increased due to an increase in surface area. The response characteristics of various catalysts were compared under real operating conditions. When a thin nickel foam with high PPI was used, the response characteristics of the catalyst improved due to an increase in the amount of the deposited catalyst and surface area. Finally, a 200 W PEMFC (proton exchange membrane fuel cell) system using electroless-deposited Co–P/Ni foam (110 PPI) catalyst was investigated. The response time to reach a hydrogen generation rate sufficient for a 200 W PEMFC was 71 s, and the energy density of a 200 W fuel cell system for producing 600 Wh was 252.1 Wh/kg. A fuel cell system using Co–P/Ni foam catalysts can be widely used as a power source for mobile applications due to fast response characteristics and high energy density.

Introduction

Small UAVs (unmanned aerial vehicles) are widely used for reconnaissance missions without the risk of loss of life on the battlefield. Fuel cells have attracted attention in the field of aerospace engineering because the batteries that are currently used as power sources for small UAVs limit the duration of these missions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Fuel cells are suitable for military applications owing to their high efficiency, high energy density, low noise, and low vibration.

The combination of PEMFCs (proton exchange membrane fuel cells) and NaBH₄ (sodium borohydride) hydrolysis (Eq. (1)) is a suitable power source for small UAVs [3], [5], [8]. PEMFCs have many advantages, including high power density (300–1000 mW/cm²), high energy density (100–600 Wh/kg), a wide power range (0–100 kW), rapid response characteristics, and simplicity [12], [13], [14], [15], [16], [17]. NaBH₄ hydrolysis is a suitable method to supply hydrogen to PEMFCs for the following reasons: hydrogen purity, simple hydrogen generation system, safe fuel storage, and easy refueling.

$${\text{NaBH}}_4+2{\text{H}}_2\text{O}\to {\text{NaBO}}_2+4{\text{H}}_2$$

NaBH₄ is stable when stored in alkaline solution, with hydrolysis activated by various catalysts including platinum, rhodium, ruthenium, cobalt, and nickel [18]. Platinum [19], [20], [21], rhodium, and ruthenium [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] are very expensive; therefore, cobalt [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52] and nickel [42], [43], [44], [53], [54], [55] are being widely investigated.

A Co/γ–Al₂O₃ catalyst was used for fuel cell powered UAVs in our previous study, but it was not stable for long periods because of poor durability and adhesion of the catalyst [3], [5]. Consequently, Co–P/Ni foam catalysts were investigated based on studies of Co–P/Cu sheet catalysts [41], [47], [48]. Ni foam was used to improve the durability of the support, and electroless plating was used to improve adhesion of the catalyst. We previously determined appropriate catalyst manufacturing conditions such as the coating conditions [50] and bath composition [51]. The performance of the Co–P/Ni foam catalyst was previously evaluated [52]. The catalyst durability was good, but response characteristics were poor because of low surface area [52]. The response characteristics and catalyst durability are both important for mobile applications, such as in small UAVs. Although a number of catalysts for NaBH₄ hydrolysis have been developed by many research groups, to the best of our knowledge, the response characteristics of those catalysts were rarely studied.

Sodium borohydride hydrogen generator using Co–P/Ni foam catalysts with fast response characteristics was investigated for a 200 W PEMFC system in this study. The effect of pore density (pores per inch; PPI) of Ni foam on the properties of the catalyst and catalyst response characteristics was determined. Batch-type hydrogen generators were used in other studies [41], [47], [48], [50], [51], but to evaluate the response characteristics of Co–P/Ni foam catalysts under real operating conditions, a flow-type hydrogen generator was used in this study as well. Moreover, a hydrogen generation test and fuel cell tests for a 200 W PEMFC system were conducted, and the energy density of a 200 W fuel cell system was estimated.