Planar current collector design and fabrication for proton exchange membrane fuel cell

Highlights

• The uniformity of surface resistance of the developed current collector is good.

• The corrosion behavior of the developed current collector is very slow.

• Larger contact area of the proposed current collector has higher cell performance.

• The stability test of the assembled proton exchange membrane fuel cell is good.

Abstract

This paper proposes a novel planar type lightweight current collector for proton exchange membrane fuel cells (PEMFC) designed for low power portable applications. The proposed lightweight current collector, which is composed of a substrate, electrical conduction layer and corrosion resistance layer, combines the conventional metal sheet/mesh for current collecting and substrate together to reduce the possible distortion during operation caused by the mismatch due to large different mechanical properties between components. The current collector adopts FR-4 as the substrate material. The electrical conduction layer is made via coating a copper thin film using a thermo-evaporation layer. The corrosion resistance layer is made via coating a graphene thin film using spin coating and a vacuum oven process. Fabricated current collector sheet resistance measurements are conducted. The complete current collectors are assembled into a single cell PEMFC with both forced convection air-breathing cathode and self-air-breathing cathode. The related performance and stability experiments were conducted to investigate the feasibility for further applications.

Introduction

In recent years portable electric product development has grown quickly with people relying more on Internet technology applications such as smart phones. The proton exchange membrane fuel cell (PEMFC) is one potential solution as a portable charger source. The bipolar plates or current collectors are the critical fuel cell components which comprise the bulk of the PEMFC unit weight as well as volume [1]. For portable applications in the range of a few watts, a planar type PEMFC module is more suitable than a vertical PEFMC stack because the planar type PEMFC is free air-breathing and does not need extra power for the fan or air pump at the cathode side. In addition, the planar type PEMFC module could be made thinner.

The micro-electro-mechanical system (MEMS) and some related technologies have been intensively applied to construct the micro fuel cell. The MEMS technology applied to construct a planar type micro fuel cell was first proposed by Lee et al. [2]. Cha et al. [3] adopted a metal lift off process to shrink the direct methanol fuel cell (DMFC) size. Lu et al. [4] applied the MEMS technique to deposit metal powder onto the wafer surface for use as current collectors in a micro fuel cell. Yun et al. [5] adopted the electro deposition process to coat the gold-titanium and gold-nickel onto stainless fuel cell current collectors to reduce the electrical resistance as well as increase the corrosion-resistance. Feng et al. [6] developed a passive DMFC stack with ten planer cells connected in series. The current collectors were made of 316 L stainless steel mesh with Au electro-deposited onto the surfaces. The DMFC stack performances were tested at different operating conditions based on the potential portable applications. They also indicated that the DMFC stack would have more stable output voltage when combined with a DC-DC converter and produce a larger current pulse discharge when combined with super capacitors. Chen et al. [7] adopted the MEMS technique to produce micro channels into a silicon substrate to construct a micro fuel cell. Alanis-Navarro et al. [8] used a sputter technique to produce metalized Poly (methyl methacrylate) (PMMA) for constructing a micro-fuel cell. Hsieh and Huang [9] proposed a planar array module type micro fuel cell via adopting the electroforming technique to produce micro channels into a thin cooper substrate. Mani et al. [10] investigated the effect of nitrides on 316 L stainless steel bipolar plate corrosion behavior for PEMFC. The corrosion on the SS bipolar plate surface occurs because the SS bipolar plates are exposed to a highly acidic medium during PEMFC operations. They utilized the physical vapor deposition (PVD) technique to coat TiN, TiAlN mono-layer and TiN/TiAlN bi-layer onto the 316 L stainless steel surface. Their results showed a TiN mono-layer coating that presented excellent corrosion resistance and electrical conductivity and TiN/TiAlN bi-layer coatings that presented good corrosion resistance. Both of these coatings could be potential coating materials on the 316 L SS bipolar plates for PEMFC. Zhao and Li [11] designed, fabricated and tested a type of passive anion membrane ethanol fuel cell (AEM DEFC) stack. This stack consists of two back-to-back independent tank single cells which could avoid the cross reaction between two cells. In their study, two AEM DEFC stacks with four cells connected in series were assembled to a toy car as its power source to show the feasibility of future applications. Baroutaji et al. [12] developed an air-breathing PEMFC using metallic open pore cellular foam (OPCF) as the current collector and flow distributor. The OPCF material is nickel. In order to enhance the corrosion resistance due to PEMFC electrochemical reactions and hydrophobicity, the polytetrafluoroethylene (PTFE) coating was applied to the OPCF surface at both the anode and cathode sides. The results showed that OPCF with the PTFE coating exhibited higher cell performance than OPCF without PTFE. Through the above studies the MEMS related technique was adopted to fabricate the polar plate with flow channel or current collectors. The substrate adopted is silicon wafer or metallic plate. Although electrical conduction is important, the corrosion resistance is also an important issue in the micro fuel cell. A gold or TiN/TiAlN layer is therefore coated onto the surface to reduce the corrosion. Wang et al. [13] adopted stainless steel expanded meshes as flow fields as well as current collectors for the passive DMFC. They structural effects such as the strand widths and assembly patterned were investigated. Their results showed an optimal configuration could help improve the reactant and product management. Renau et al. [14] designed a high temperature PEMFC stack with 40 cells for Unmanned Aerial Vehicle (UAV) in a high altitude mission. In order to reduce the weight, they adopted 5083 aluminum thin sheets as the mono-polar plates, which a 40 mm thick thin layer of NieP (10e12%P) was deposited each surface for corrosion resistance. Chang et al. [15], [16], [17], [18], [19] developed a type of flexible PEMFC using the polydimethylsiloxane (PDMS) with flow channels which coated with Ag nanowires as the current collector. They proposed stiffness controlled endplate to improve the cell performance. The effect of bending, twisting and mixing bending and twisting loads on the bendable PEMFC was also discussed. Yuan et al. [20], [21] adopted a porous metal-fiber sintered plate (PMSFP) as the methanol barrier for the passive DMFC to reduce of the methanol crossover. They further developed a composite anode structure which combined a sintered porous fiber felt and a vertical parallel-fence or an open parallel fence. The CO2 bubble behavior and its effect on the cell performance for different anode designs were also investigated.

The printed-circuit board (PCB) technique was also adopted to produce a planar type of fuel cell that is flexible for the serial connection among cells as well as integrated with a control circuit. This was first proposed by O'Hare et al. [22] and Schmitz et al. [23] in the same year. Kim et al. [24] developed an air-breathing planar stack using flexible PCB as a current collector. The current collector is designed and fabricated via an Au-coated layer electro-deposited onto a non-conductive polyimide film. The opening shape and ratio effects at the cathode side are also discussed in this research. However, the cost of coating the Au layer is expensive. Kuan and Chang [25] investigated the degradation performance due to process-induced damage when assembling a DMFC using the PCB technique. In order to improve the surface degradation due to the significant mechanical characteristic differences between the FR4 substrate and thin metal current collectors of the PCB-based fuel cell, Sung et al. [26] and Kuan et al. [27] proposed a lightweight current collector using the thermos evaporation technique to coat a copper electric conduction layer and a nickel corrosion resistance layer onto a FR4 substrate as the current collectors for the DMFC. Yuan el al [28]. adopted the PCB process to make a lightweight current collector and fabricate a disc type DMFC. The current collector is made of copper clad aluminum sheet with the surface coated with gold. Wang et al. [29] designed and fabricated a four cell modular pass DMFC stack. The stack consists of four planar type unit cells connected in series. The 316 L stainless steel plates are adopted as the current collector substrate material. In order to prevent corrosion and reduce electrical resistance, an Au thin layer was coated onto the surface of each current collector via magnetron sputtering ion plating technology. A fuel reservoir is placed at the center of each module with each side consisting of two cells in series. A PCB is placed onto the electrical conductor top with connectors among the cells. The DMFC stack is fabricated via connecting multiple DMFC modules. Through a review of above studies the PCB-based fuel cell is prominent for application in portable devices. The planar type PEMFC/DMFC adopts metal mesh and metal plate as the current collectors. The substrate/flow board of the planar type DMFC is generally made of silicon, PCB, or acrylic. The current collector is clamped between the MEA and substrate/flow board which might lead to a mismatch or warp due to unbalanced clamping force or significant difference in mechanical properties. In addition, metallic current collector corrosion is also an important issue. Coating Au onto the surface is the most popular way to reduce corrosion but the material cost is high. Therefore, preventing current collector surface corrosion during the electrochemical reaction is a very important issue.

The development of a portable power generation system is also a popular topic via integrating a fuel cell stack and fuel supply mechanism. Barbera et al. [30] developed a planar and monopolar ministack DMFC for portable applications. The ministack plates were designed and fabricated via the modular approach using the PCB technique in which the anode and cathode of each cell are internally connected. The current collector plates are gold-plated. The ministack is composed of two strings with a fuel reservoir between them. Each string contains six cells. Masdar et al. [31] designed and fabricated a 6-cell passive DMFC stack with a hexagonal shape methanol solution reservoir in which each side face is assembled with a planar type DMFC cell. The large methanol reservoir increases the DMFC operation time without refilling the fuel. The related tests were conducted based on single cell DMFC performance. As water management is important for the PEMFC, Amara and Nasrallah [32] investigated the behavior of water droplets in the PEMFC micro channel and they found that a hydrophobic micro channel exhibits better droplet evacuation capability than the hydrophilic micro channel. Wang et al. [33] proposed an approach to feed hydrogen, which is released by a LiBH4 solution, into a planar type MEMS micro silicon fuel cell. Moreover, Lee and Wang [34] integrated a MEMS fuel cell and NaBH4 hydrogen generation mechanism to compose a micro space power system applied for Nano-satellites. Gang and Kwon [35] developed a portable power plant that integrated a commercial PEMFC stack, NaBH4-based hydrogen generator, DC-DC converter, micro-processed controller and data monitoring device. In the proposed system the required hydrogen gas based on the electrical load demand adjusts the hydrogen production rate using a fuel pump. The experimental results show the feasibility of the developed power plant in mobile applications. In order to increase the fuel utilization, Li et al. [36], [37], [38] developed a methanol evaporator such that the methanol vapor was produced through pure methanol vaporization. This vapor could be adopted as the fuel rather than a low concentration methanol solution. They also demonstrated a DMFC stack fed with pure methanol via vaporizing the pure methanol through a vaporizer and then mixing it with the generated carbon dioxide and water vapor through the cathode side in the gas mixing layer. Although the liquid feed passive DMFC is expected to be applied in portable applications for its main advantages of operating near room temperature and simpler structure, there are still some challenges that need to be overcome, such as non-uniform fuel distribution, produced water flooding at the cathode, carbon dioxide bubbles due to the cell orientation at the anode and methanol crossover when the cell temperature is increased [39].

According to the related literature review, developing a lightweight current collector is one of the most popular topics for portable fuel cell applications. In Micro DMFC development the functional components, supplies management and packaging technology are the key issues to be investigated. The current collector is one of the six key functional components of the micro DMFC [40]. However, the current collector is made of metal sheet, which has significantly different properties from the FR-4 substrate. The large difference in thermal expansion and mechanical strength might lead to device distortion because of the mismatch between those two components.

The main objective of this paper is to propose a novel planer type light weight current collector that combines the metal sheet and substrate via electrical conduction and corrosion resistance thin film fabrication onto the FR-4 substrate. The developed current collector improves the possible mismatch between the current collector and substrate in the convectional mini/micro fuel cell due to significantly different mechanical properties. The proposed current collector adopts FR-4 as the substrate material. The electrical conduction layer is made via a copper thin film coating using a thermal evaporation process. The corrosion resistance layer is made via coating a graphene thin film using spin coating and a vacuum oven process. In order to show the feasibility of the proposed current collector in future applications, the completed current collectors are assembled into a single cell PEMFC and the related performance and stability experiments were performed.