In-situ experimental characterization of the clamping pressure effects on low temperature polymer electrolyte membrane electrolysis

Highlights

• LT-PEME ohmic and activation resistances decreases at higher clamping pressure.

• H₂ crossover decreases with increasing clamping pressure of LT-PEME.

• H₂O crossover decreases at higher clamping pressure of LT-PEME.

• Ratio of H₂ in O₂ at the anode side decreases with increasing clamping pressure.

• H₂ crossover increases with increasing current density at fixed clamping pressure.

Abstract

The recent acceleration in hydrogen production's R&D will lead the energy transition. Low temperature polymer electrolyte membrane electrolysis (LT-PEME) is one of the most promising candidate technologies to produce hydrogen from renewable energy sources, and for synthetic fuel production. LT-PEME splits water into hydrogen and oxygen when the voltage is applied between anode and cathode. Electrical current forces the positively charged ions to migrate to negatively charged cathode through PEM, where hydrogen is produced. Meanwhile, oxygen is produced at the anode side electrode and escapes as a gas with the circulating water.

The effects of clamping pressure (P_c) on the LT-PEME cell performance, polarization resistances, and hydrogen and water crossover through the membrane, and hydrogen and oxygen production rate are studied. A 50 cm² active area LT-PEME cell designed and manufactured in house is utilized in this work.

Higher P_c has shown higher cell performance this refers to lower ohmic and activation resistances. Water crossover from anode to cathode is slightly decreased at higher P_c resulting in a slight decrease in hydrogen crossover from cathode to anode. Also, the percentage of hydrogen in the produced oxygen at the anode side is significantly reduced at higher P_c and at lower current density.

Introduction

Hydrogen is considered an important element in direct use for synthetic fuel production, a promising energy carrier, and future replacement for fossil fuel energy sources. Integrating LT-PEME with renewable energy sources makes it one of the most suitable candidate technologies to produce hydrogen directly [1], [2], [3]. LT-PEME cells split water into hydrogen and oxygen when an electric potential is applied between anode and cathode. Electrical current forces the hydrogen ions to migrate to negatively charged cathode through the PEM, where hydrogen is produced. Meanwhile, oxygen is produced at the anode side electrode and removed from the cell as a gas with the circulating water. Liquid water and oxygen crossover from anode to cathode and hydrogen crossover in the opposite direction through the membrane takes place at a very small portion compared to the gases production rate on both sides [4].

Most recent research on LT-PEME has focused on operating conditions such as operating temperature, cathode-anode high differential operating pressure, flow field design, stack development, two-phase flow analysis, precious materials electro-catalyst reduction, low cost and durable PEM and current distributors, scale up of LT-PEME, numerical modeling of complex phenomena taking place inside LT-PEME during operation [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Also, a lot of studies have been carried out on the effect of clamping pressure on PEM fuel cell characteristics [18], [19], [20], [21], [22], [23]. However, the asymmetrical catalysts and electrodes materials on both sides of the LT-PEME: platinum based catalyst with carbon paper/cloth porous transport layer (PTL) electrode on the cathode side and iridium based catalyst with titanium felt (TF) electrode on the anode side, liquid water is used as reactant at the anode side, and the high operating differential pressure between cathode and anode of the LT-PEME. Comparing with symmetrical platinum based catalyst with carbon paper/cloth PTL electrode on both sides, humidified/dry hydrogen reactant at the anode side and humidified oxygen reactant at the cathode side, and zero differential pressure between cathode and anode of PEM fuel cell, respectively. All of these differences might cause different effects of clamping pressure on the LT-PEME than PEM fuel cell. Thus, more experimental and analytical studies of the clamping pressure effect on LT-PEME characteristics are necessary.

Conventional LT-PEME usually clamped with bolts around the active area. Thus, P_c can be calculated from clamping force (F_c) applied on the cell divided by the contact area given by:

$$P_c=\frac{F_c}{A_c}$$

Stainless steels bolts are commonly used to compress the cell components against each other as shown in Fig. 1.

Thus F_c can be given by Ref. [24]:

$$F_c=\frac{N\times \tau }{f\times D}$$

where N is the number of bolts distributed symmetrically around the active area of the membrane, τ is the applied torque on each bolt (N·m), f is the friction coefficient (0.2 for steel bolts), and D is the nominal bolt diameter (m). It should be borne in mind that A_c has different dimensions by different researchers in PEM fuel cell which are considered very similar to LT-PEME cell. Hassan et al. [25] defined it as the active area of the membrane, Ahmad et al. [18] defined it as the PTL and the sealing gasket surface area, and Mehboob et al. [24] referred to it as the total contact area between bipolar plate and end plate. These different definitions of A_c might lead to inaccurate calculations of P_c from F_c. Furthermore, the ribbed design of the flow field is another source of error, because the area of the ribs is the only area that compresses against the electrodes, and this makes A_c less than the membrane active area. Thus, to be more accurate, A_c can be defined as the contact area between the flow field plate and the electrode.

Small efforts were made to study the effect of P_c on the LT-PEME characteristics. Siracusano et al. [26] concluded that, a modest enhancement in the 3-cells stack performance was observed at higher compression due to better electrode-electrolyte interface especially at the cathode side. Meanwhile, no significant modifications were noticed on the polarization curves for each cell in the 3-cell stack, and a slight improvement in polarization resistances at higher compression was observed. As shown earlier, the compression force is directly related to the applied torque on each bolt in bolted LT-PEME cell. Selamet et al. [27], [28] studied the effect of bolt torque on LT-PEME cell performance, durability, and contact resistant. They found that, the optimized applied bolt torque is important for homogeneous water distribution and consequently higher durability, performance, and lower contact resistant. They also, concluded that, increasing the bolt torque of a single LT-PEME cell at 0.5 Acm⁻² and ambient temperature results in more contact points between cell layers and consequently the contact resistant was reduced and the cell performance was developed. However, beyond a certain torque the performance starts to decrease due to the mass transport limitation [29]. Awsthi et al. [30] dynamically modeled LT-PEME system under a wide range of operating conditions; they found that, increasing the compression pressure decreases the cell performance. This because, in their model increasing the cell pressure resulted in increasing the partial pressure of the species which in turn increase the open circuit voltage calculated by Nernst equation. This agrees with Marangio et al. [31] who concluded that, the kinetic of the charge transfer could be reduced at higher counter pressure resulting in higher cell polarization. Furthermore, the asymmetrical components on the two sides of the LT-PEME might lead to un-equal compression force on both sides of the membrane due to the difference in the mechanical properties such as elastic and plastic deformation for both electrodes, and the TF pores and fibers diameters are larger and much harder than the PTL pores and fibers diameters. Thus, inhomogeneous compression against the TF might damage the catalyst layer by sinking the TF fibers in the catalyst layer at the anode side [5].

Therefore, the effects of P_c on the LT-PEME on cell performance, ohmic and activation resistances, water, oxygen, and hydrogen crossover rates through the membrane, and hydrogen and oxygen production rates at different current densities are in-situ experimentally demonstrated in this work. A 50 cm² single LT-PEME cell is utilized, and all the measurements are conducted in real time at 70 °C and atmospheric pressure. Furthermore, to avoid inhomogeneous P_c by bolting the cell around the active area, a precise compression set-up uses plate like piston which is in contact with the whole surface of the end plate resulting in uniform compression pressure on the end plate surface and maybe on the active area as well, hence all LT-PEME cell layers are aligned with each other.