# Analysis of local current density, temperature, and mechanical pressure distributions in an operating PEMFC under variable compression

El Mahdi Khetabi, Khadidja Bouziane, Xavier François, Remy Lachat, Yann Meyer, Denis Candusso

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## ► To cite this version:

El Mahdi Khetabi, Khadidja Bouziane, Xavier François, Remy Lachat, Yann Meyer, et al., Analysis of local current density, temperature, and mechanical pressure distributions in an operating PEMFC under variable compression. Applied Energy, 2025, 394, pp.126187. 10.1016/j.apenergy.2025.126187. hal-05086730

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Submitted on 27 May 2025

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# Analysis of local current density, temperature, and mechanical pressure distributions in an operating PEMFC under variable compression

El Mahdi Khetabi<sup>a</sup>, Khadidja Bouziane<sup>a</sup>, Xavier François<sup>b</sup>, Remy Lachat<sup>c</sup>, Yann Meyer<sup>d</sup>, Denis Candusso<sup>a,*</sup>

## HIGHLIGHTS

* The effects of clamping on local phenomena in a 225 cm² PEMFC are studied.

* Maps of load current, temperature, and mechanical stress are analysed.

* The compression from 0.35 to 1.55 MPa homogenises the local currents/temperatures.

* This homogenisation is linked to better stress distribution and ohmic loss decrease.

* The enhancement of local distributions is correlated with higher cell voltages.

## GRAPHICAL ABSTRACT

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■ Measurements of local current density (j), temperature (T), mechanical pressure (p) under compression.

■ Spatial and statistical analysis of maps.

■ Identification of correlations and causal links between:

- Local observations: j, T, and p.

- Global measurements: current - voltage curves, EIS / RHF records,

changes in cell and GDL thicknesses, ex-situ tests of GDL resistances.

## ARTICLE INFO

### Keywords:

PEMFC

Mechanical assembly

Clamping pressure

In-situ characterisation

Local current density

Temperature mapping

Pressure-sensitive film

## ABSTRACT

This article investigates the impact of mechanical compression on local phenomena within an operating PEMFC with a large active area (225 cm²). It explores the distributions of current density, temperature, and mechanical pressure, building on previous global characterisations (cell voltage, polarisation curves, and EIS). The study finds that mechanical compression (0.35–1.55 MPa) enhances the uniformity of current density and temperature distribution, reducing the risk of hotspots that can impair PEMFC performance and durability. The spatial analysis reveals that this homogenisation effect is mainly due to improved pressure distribution with increased compression. The regions with higher mechanical pressure correlate with higher local current density and temperature, which improves performance by reducing ohmic resistance. However, excessive compression at high current density and relative humidity can lead to water management issues. Overall, the results support the positive effect of pressure homogenisation on PEMFC performance, as observed in previous studies.

<footnote><sup>a</sup> Univ. Gustave Eiffel, Univ. Paris Saclay, ENS Paris Saclay, CNRS, SATIE, FCLAB, 90010 Belfort Cedex, France</footnote>

<footnote><sup>b</sup> Univ. Bourgogne Franche-Comté, UTBM, FCLAB, 90010 Belfort Cedex, France</footnote>

<footnote><sup>c</sup> ICB, CNRS Univ. Bourgogne Franche-Comté, UTBM, 90000 Belfort, France</footnote>

<footnote><sup>d</sup> Univ. Savoie Mont Blanc, SYMME, 74000 Annecy, France</footnote>

<footnote>* Corresponding author.
E-mail address: denis.candusso@univ-eiffel.fr (D. Candusso).</footnote>
# 1. Introduction

The influence of mechanical compression on Proton Exchange Membrane Fuel Cell (PEMFC) performance has been fairly widely studied, primarily through ex-situ characterisations of cell components (especially Gas Diffusion Layers - GDLs [1]) and cell numerical modeling, with some research also involving in-situ testing and analyses (mostly based on polarisation curve records) [2]. In any case, further exploration of the impact of clamping pressure and study of mechanical-electrical coupling remain crucial for optimising existing Fuel Cell (FC) designs, investigating new component materials, and refining assembly techniques. These efforts are indeed essential for scaling up production while maintaining PEMFC performance and durability. According to the current research, it is still difficult to judge the best assembly load according to a certain index in practical application. Typically, the average clamping force for PEMFCs can range from around 1 to 5 MPa, but more often around 1–2 MPa. The optimal values depend on the clamping methods, FC components and design (e.g. materials, structures, surface properties, thicknesses), and operating conditions (cell temperature, gas pressures and flows, humidity rates in the FC) [2–5]. The determination of cell performance, especially using polarisation curve and Electro-chemical Impedance Spectroscopy (EIS) measurements, is therefore recommended to evaluate the behaviour of PEMFCs under different assembly loads. Achieving a delicate equilibrium in compression forces, ensuring cell tightness and adequate component contact without inducing adverse effects (e.g. potential loss of porosity in the GDLs and mass transport issues; more broadly, deterioration of cell materials), is crucial for optimising FC performance, efficiency, and durability.

The results of our experimental investigations detailed in two previous articles [3,4] highlighted the impacts of mechanical compression on the global performance of a PEMFC. These studies, conducted using a specialised mechanical compression unit (as shown in the Graphical abstract), were based on analyses of polarisation curves, cell voltage [3], and EIS measurement [4]. They demonstrated that mechanical

compression improves PEMFC performance up to 1.55 MPa. This improvement is primarily due to a significant reduction in ohmic resistance and enhanced charge transfer processes, which result from better contact between components, outweighing any potential increase in mass transport resistance. Mechanical compressions above 1.55 MPa did not yield any further performance improvements and, in some cases, even led to a slight decline in FC efficiency due to increased gas diffusion losses at higher compression levels (from 1.7 to 2 MPa). Our tests and analyses identified 1.55 MPa as the optimal compression level, striking a balance between ensuring effective electrical contact and avoiding damage to the GDL and other components.

Fig. 1 presents examples of polarisation curves from our previous studies, recorded under varying levels of mechanical stress. The U-I plot [3] features two zoomed-in sections at medium and high current densities, highlighting the corresponding changes in cell voltage and Resistance at High Frequency (RHF, measured by EIS between 1 and 10 kHz [4]) as compression increases. A clear trend emerges: with increasing mechanical compression, cell voltage rises and RHF decreases, particularly within the 0.35 to 1.1 MPa range.

Building on this study of the overall performance at the cell scale, this new article aims to thoroughly elucidate the quantitative effects of mechanical compression on the local phenomena within a PEMFC featuring a large active area (i.e. commercial size: 225 cm²), aligning with potential real-world applications. When considering the compression in full-scale FC applications, the distribution factor of gas flow, current, temperature, and mechanical stress in the cell starts playing a significant role.

Several research teams have previously studied the distribution of mechanical pressure in PEMFCs. Beyond assembly pressure, the clamping configuration - such as the number and positioning of clamping bolts relative to the cell geometry - can significantly impact FC performance. Various PEMFC assembly configurations have been documented in the literature, with particular focus on the point-load design, which has received considerable attention [6–11]. In this

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Example of polarisation curves measured for 12 levels of mechanical compression (left); inserts (a) and (b) are zoomed-in regions at medium and high current densities. The related voltage and RHF evolutions as a function of mechanical compression: (a) at 0.6 A cm⁻² and (b) at 0.9 A cm⁻² (right). The tests were conducted at a cell temperature of 60 °C, with air and H₂ flow rates of 19.1 Nl/min and 2.4 Nl/min, respectively, under anode and cathode backpressures of 50 kPa, and gas Relative Humidities (RH) of 50 %.</figcaption></figure>
design, the bolt configuration and clamping torque are critical factors influencing the uniformity of pressure distribution within the FC components. An optimised clamping design minimises interfacial contact resistance, ensures uniform pressure distribution, and maintains gas-tight operation, all of which contribute to enhanced PEMFC performance. For instance, it was reported in [12] that a uniform pressure distribution can boost the maximum power density of a PEMFC by approximately 12% compared to an uneven distribution. Conversely, an uneven pressure distribution can degrade FC performance and may even cause irreversible damage to components, such as the formation of hotspots [13,14], which reduce the FC lifetime.

To date, the literature predominantly reports two techniques for investigating pressure distribution in PEMFCs: pressure-sensitive thin films [8,15–31] and piezoresistive mapping sensors / arrays [32,33]. While pressure-sensitive films have thus far been used mainly in off-line FCs (i.e., non-operating PEMFCs), this technique still offers valuable insights into the pressure distribution across FC components. This technique analyses pressure distribution based on the principle that mechanical compression causes the microcapsules within the pressure-sensitive film to rupture, releasing a colour-forming material that is absorbed by the film. The intensity of the released colour correlates with the degree of compression applied: greater mechanical compression results in a more intense colour. Pressure-sensitive films, with thicknesses typically less than 200 µm across all compression ranges [34], are much thinner than GDLs and gaskets (for instance, a gasket might be 0.5 mm thick compared to 90 µm for the pressure-sensitive film in [8]). As a result, these films are reported to have a negligible impact on the internal compression distribution [8]. Consequently, they remain a suitable choice for investigating pressure distribution across PEMFC components. Gatto et al. [33] examined the impact of assembly pressure and gasket materials on PEMFC performance and mechanical pressure distribution. They utilised a pressure mapping sensor (Tekscan #5076, USA [35]), which had a pressure resolution of approximately 10 kPa. Their study focused on a single-cell PEMFC equipped with a carbon cloth GDL. Mechanical compression was applied by adjusting the clamping torque on the assembly bolts, which varied from 7 to 13 N.m. It was observed that as the clamping torque increased, the clamping plates of the assembly fixture deformed, resulting in uneven pressure distribution. It was reported that the centre of the GDL experienced lower pressure (below 0.7 MPa) compared to its edges. The pressure concentration at the edges increased with higher clamping torque, eventually reaching the saturation limit of the sensor used (over 10 MPa in some regions with a clamping torque starting at 7 N.m). Similar findings were noted in [32]. Additionally, the authors in [33] highlighted that both gasket material and thickness influence pressure distribution for the same assembly pressure, potentially affecting PEMFC performance, which aligns with results from [36,37].

Research on the impact of clamping pressure on the distribution of local current densities is relatively limited. For example, Peng et al. [38] found that mechanical compression helps to homogenise current density distribution, thereby enhancing PEMFC performance. They observed that regions with higher contact pressure experience greater current flow, while areas with reduced contact pressure exhibit lower current density. Studies on the effects of clamping pressure on the mapping of local current densities have also been conducted through modelling and simulation, typically at the land (or rib) and channel scale of the cell in cross-section, including considerations of possible GDL intrusion into the channel [39,40]. In some instances, these simulations have been complemented by experimental work [41].

In summary, existing literature highlights that the clamping process in PEMFCs using conventional fasteners results in an uneven pressure distribution across the FC components. It is well-documented that this inhomogeneity worsens with increased clamping torque, leading to higher stress concentrations at the edges while the centre experiences lower compressive loads. Consequently, developing new compression methods is crucial to mitigating the adverse effects of uneven pressure

distribution on FC performance.

Several clamping mechanisms designed to optimise pressure distribution in PEMFC stacks have been detailed in the patent literature, including compression retention systems that utilise external springs [42,43], compliant strapping [44], and overlapping sheets [45]. Millichamp et al. reviewed a variety of compression assembly methods described in patents [46]. However, there is a noticeable gap in scientific literature comparing these reported assembly methods [46]. Most existing research has concentrated on pressure distribution in PEMFCs using traditional assembly procedures, such as point-load designs with standard fasteners like bolts and nuts. Future research could advance the field by evaluating and comparing these novel compression retention systems, potentially leading to optimised clamping methods.

In this new article, the investigations focus on studying the distributions of i) current, ii) temperature, and iii) mechanical pressure within the cell. Correlations with the in-situ results previously reported in [3,4] are provided throughout the paper. Building on the work presented in [3,4], this study explores the effects of mechanical compression across 12 levels, ranging from 0.35 to 2 MPa in increments of 0.15 MPa. The lower limit of 0.35 MPa is chosen to ensure cell tightness, while the upper limit of 2 MPa is selected to avoid causing irreversible damage to the cell components. The analyses are conducted for two load current densities (0.6 A and 0.9 A·cm⁻²) and two levels of gas Relative Humidity (50 %RH and 100 %RH). The experimental results aim to offer a comprehensive analysis of PEMFC assembly design improvements by providing guidelines to better understand how mechanical compression impacts cell performance and integrity.

This article is organised as follows: Section 2 describes the experimental setup and the campaigns conducted in this study. Section 3 presents an analysis of the results, including spatial and quantitative assessments of how mechanical compression affects local phenomena within the PEMFC, with a focus on current and temperature distributions. Section 4 provides an analysis of the mechanical pressure distribution within the cell, as observed using sensitive films. The article also includes correlations with ex-situ results. Finally, Section 5 offers conclusions and future outlooks.

## 2. Experimental

This section provides an overview of the test stand, PEMFC assembly, experimental campaigns, and the specialised devices used for local measurements in this study.

### 2.1. Test stand and PEMFC components

A specially designed setup allows for both mechanical and electrical characterisation of a single PEM cell under various operational conditions and compression levels. This equipment includes:

- A PEMFC test bench (G7805 from Greenlight Innovation® [47]),

- A Mechanical Compression Unit (MCU) (baltlicFuelCells [48]): featuring four pneumatic cylinders that apply controlled mechanical constraints (up to 2.15 MPa) on the single cell assembly with an active area of 225 cm². The thickness variation (i.e. displacement) of the single cell under operating conditions is measured using a dedicated sensor with a dynamic resolution of 1 µm [3,5]. A picture of the MCU is included in the Graphical abstract.

- A device for measuring local currents and temperatures within the cell, with further details provided in Sections 2.3 and 2.4.

This equipment is used to investigate a single cell with the following individual components:

- Membrane Electrode Assembly (MEA), including a Nafion™ XL membrane [49,50] and Catalyst Layers (CLs) with platinum loadings of 0.2 and 0.4 mg Pt·cm⁻² at the anode and cathode, respectively.
- A non-woven carbon paper GDL, coated with a MPL and PTFE-treated (Sigracet® 38 BCE from SGL Carbon [51,52]).

- Flow Field Plates (FFPs) in graphite (grade FU 4369 HT from Schunk [53]), with a parallel serpentine design, featuring 12 parallel channels, with a land width and channel width of 0.96 and 1 mm, respectively (Section 2.6).

Additional details about the test apparatus along with the used PEMFC components can be found in [3–5].

In this study, two experimental campaigns were conducted using the same setup and PEMFC components. The first campaign focused on analysing the mapping of current and temperature, along with displacement measurements, while the second investigated the mechanical pressure distribution across the active area of the FC.

## 2.2. Operating conditions

As previously mentioned, this article aims to establish correlations with the results reported in [3,4], where the overall performance of the PEMFC was evaluated as a function of mechanical compression. To ensure comparability, the operating conditions were maintained identical to those in the previous experiments, which were considered

optimal for the electrical performance of the cell (i.e., a cell temperature of 60 °C, air and hydrogen flow rates of 19.1 and 2.4 NL.min⁻¹ respectively, and anode and cathode backpressures of 50 kPa). In this study, we utilised the same mechanical compression profile as shown in Fig. 3(b) of [3] and in the Graphical abstract. Referred to as the “randomised profile”, this approach encompasses the entire mechanical compression range examined here, from 0.35 to 2 MPa in 0.15 MPa increments. The rationale for using this randomised profile is to eliminate the materials' memory effect, thereby mitigating issues such as water accumulation that can occur with a gradual increase in mechanical compression.

A PEMFC conditioning protocol was performed before each experimental campaign reported in this article to ensure that the operating conditions remained sufficiently stable. Detailed information on this conditioning protocol is available in [3,5]. It is important to note that the measurements of local current and temperature distributions, along with displacement measurements, were conducted alongside the experimental investigations in [3]. However, the pressure-sensitive film measurements were carried out off-line (with the FC not in operation), using a custom-developed mechanical conditioning protocol. Further details on this protocol will be provided later in this subsection.

<figure><img src="image_6.png" /><figcaption>Fig. 2. (a) The measurement principle of the local currents and temperatures device; (b) Picture of the anode current collector side of the S++® sensor; (c) Position of the S++® measurement device in the PEMFC assembly.</figcaption></figure>
## 2.3. Mapping of current density

A current and temperature distribution measurement device (S++®) simulation services [54], Germany) was installed in the PEMFC assembly, between the anode FFP and the current collector. This measurement device, i.e. S++® current scan shunt [55], was connected to a control station. This allows collecting current and temperature distributions data conjointly using the CurrentVIEW software provided by S++® [54]. This device was designed and assembled in order to have minimum interference with the phenomena taking place within the PEMFC (Fig. 2). This device has been specifically designed for our 225 cm² FC with 21 × 21 and 7 × 7 points (or segments) for local current densities and temperatures measurements, respectively. The sensor of the segments has a thickness of 700 µm and uses a low ohmic shunt resistor for local current density measurements, and it is directly connected to the anode flow field of the PEMFC assembly. The sensor is made from a special copper layer with gold plated contact segments in order to prevent corrosion and to have a low electrical contact resistance with the FFPs; the operating principle of the local currents and temperatures measurements is described in Fig. 2(a). The left side is connected to the anode FFP while the right side is connected to the current collector side of the MCU (Fig. 2(c)). The current measurement zone is divided into 441 insulated segments, each segment has an area of 1.96 cm². Fig. 2(b) depicts the segments of the measurement device. The local currents collected by the segments flow through the shunt resistors. The current of the segment is obtained by measuring the voltage drop across the shunt resistor with a maximum reading measurement error of 1 %. An automated calibration process is conducted during each experimentation in order to compensate for the variation in the measured resistance due to the temperature coefficient related to the resistance of the copper layer.

The used current mapping device has the advantage of a good spatial resolution, with 21 × 21 measurement segments (i.e. 441 in total) per 225 cm² active area. Therefore, each segment measures the current passing through a surface area of approximately 0.51 cm² (i.e. measurements provided in Ampere per segment). In order to obtain units in A·cm⁻², all the current measurements collected by the S++® device were multiplied by a conversion factor corresponding to 441<sub>segments</sub>/225<sub>cm²</sub> = 1.96.

In this study, ten 21 × 21 current density distribution arrays were collected for each of the 12 randomised levels of mechanical compression. Each array was recorded following a 5-min stabilisation period, at a frequency of 0.5 Hz (one reading every two seconds), which is considered appropriate for our study conducted under steady-state conditions. The current density values from these ten arrays were averaged to produce a single array of mean values for each compression level. To standardise the scale between the results at 0.6 and 0.9 A·cm⁻², the averaged arrays were normalised by dividing by the average current density value. This standardisation process converts the results into a percentage scale relative to the measured average current density value. Thus, a standardised current density value of 1 represents either 0.6 or 0.9 A·cm⁻² (i.e., 100 % of the tested average value), depending on the specific current density level tested. This approach allows for a clear interpretation of current density distributions by using a recognised reference value (the average current density value).

The long-term effects of mechanical compression, such as those influencing liquid water management, were distinguished from transient phenomena occurring within the PEMFC by averaging the ten 21 × 21 current density distribution arrays. This approach excluded short-term changes, such as the movement of small liquid water droplets within the FFPs, that occur on a timescale of 2 s per measurement. By averaging the ten arrays of current density and temperature distributions, as will be described later, the results reflect changes occurring within the FC over a period of 20 s.

## 2.4. Temperature mapping

The S++® temperature distribution measurement device features a 7 × 7 array of segments (a total of 49) across an active area of 225 cm². Thus, each segment measures the local temperature over an area of approximately 4.6 cm², representing about 2 % of the total surface. It can be assumed that the temperature remains relatively uniform within each segment. The measurement principle is based on voltage drop measurement in a series of thermally controlled resistors (i.e. thermistors) as depicted in Fig. 2(a). The system can operate at temperatures up to 100 °C with a resolution of 0.1 °C. Similarly to the current density mapping, ten 7 × 7 temperature distribution arrays were measured for each level of mechanical compression, concurrently with the current density arrays. The local temperature values from these ten arrays were averaged to produce a single array of mean temperature values for each compression level.

## 2.5. Mapping of mechanical pressure

Achieving a uniform mechanical pressure distribution within PEMFC components is crucial for optimising current and temperature distributions. This article investigates the use of Prescale® pressure-sensitive films (Fujifilm [56], Japan) to measure pressure magnitude and distribution within the active area of the FC. The operating principle of Prescale® films and their assembly procedure for our experiments will be described in detail later. We employed Prescale® LLW films, selected for their pressure measurement range of 0.5 to 2.5 MPa [56], which aligns well with the mechanical compression range studied (0.35 to 2 MPa).

## 2.6. Analysis of the local phenomena evolution with mechanical compression

As previously noted, this article aims to provide a comprehensive understanding of how mechanical compression affects local phenomena within an operating PEMFC. This includes analysing the mapping of current, temperature, and mechanical pressure. Since this investigation is conducted locally, we chose to analyse these phenomena following 9 distinct zones, as shown in Fig. 3. These zone numbers will be used for the rest of this article as conventional locations. Fig. 3 also depicts the reactants inlets / outlets configuration. The position of the origin point (0,0) and the direction of the coordinate system are also shown in Fig. 3. The x and y arrows represent the horizontal and vertical direction of the measurement points / segments, respectively. The heating / cooling medium (i.e. deionised water in our study) flows at the cathode from the 3rd zone to the 7th zone, whereas it flows at the anode from 1st zone to the 9th zone. This information will be useful for the spatial analysis of

<figure><img src="image_7.png" /><figcaption>Fig. 3. Presentation of the locations of the 9 regions used in this study. The figure depicts a superimposed image of the anode and cathode FFPs channels.</figcaption></figure>
the current, temperature, and mechanical pressure distributions investigated in this work.

### 3. Analysis of the current and temperature distributions as a function of compression

Analysing distributions of current and temperature is challenging, especially in PEMFCs with a large active area (225 cm² in our study). Using PEMFC with such a large active area can present a number of

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Standardised current density distribution 2-D maps at 0.6 A.cm⁻² and 50 %RH. (a) 0.5 MPa, (b) 0.8 MPa, (c) 1.1 MPa, (d) 1.4 MPa, (e) 1.7 MPa, and (f) 2 MPa. Colour bars show the corresponding colours of each level of the standardised current density. The related histograms (g) of the standardised current density distribution at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa. The blue and red arrows represent the ideal effects of mechanical compression on the current density distribution at very low and very high current density regions, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
issues [57] that can be due for instance to i) the inhomogeneity of electrochemical reaction sites distribution, ii) liquid water management, iii) difference in local electrical and thermal conductivities, iv) manufacturing process issue [58], v) damage occurred during the operational lifetime of the FC components, and vi) design of FFPs. These effects can lead, in the worst case, to the creation of regions with high current densities leading to the creation of high temperature regions that could potentially damage the PEMFC components through the creation of hotspots [13,14,59,60]. These effects could eventually decrease the FC lifetime or even cause premature failure of the PEMFC [61,62]. The objective of our study is not to go through all the plausible effects of the nonhomogeneous distributions of the current and temperature, as it is not the aim of our research work, but rather to give a thorough analysis of these distributions as a function of mechanical compression.

Following the approach in [3,4], the investigations conducted in this work are carried out at both current densities of 0.6 and 0.9 A.cm⁻², related with the ohmic and the mass transport regions, respectively. Each current density is applied at a relative humidity of 50 %RH and 100 %RH. Since the goal of this study is to find correlations between the local phenomena and the global performance of the PEMFC, the procedure followed for the local phenomena analysis is the same as the one formerly used in [3,4]. Results at 0.6 A.cm⁻² will be further detailed in this article. Comparisons between results at 0.6 and 0.9 A.cm⁻² will be addressed as well.

### 3.1. Analysis of the distributions of current and temperature at 0.6 A.cm⁻²

This section provides a comprehensive analysis of the distributions of the current and the temperature at a medium current density of 0.6 A.cm⁻² as a function of mechanical compression. Each distribution of the current and temperature is analysed at both relative humidity levels of 50 %RH and 100 %RH. First, we present the spatial analysis of the heterogeneities in the distributions at 6 levels of mechanical compression (from 0.5 to 2 MPa with 0.3 MPa increments). Then, each spatial distribution analysis will be followed by a statistical analysis covering all the 12 levels of mechanical compression investigated in this study (i.e. from 0.35 to 2 MPa with 0.15 MPa steps).

#### 3.1.1. Current density distribution analysis at 0.6 A.cm⁻²

- **Results at 50 %RH:** The measurement results of the standardised current density distribution at 6 levels of mechanical compression, from 0.5 MPa to 2 MPa with 0.3 MPa steps, are depicted in Fig. 4. These results are represented in 2-dimensional contour maps for a PEMFC operating at an average current density of 0.6 A.cm⁻² and a relative humidity of 50 %RH. The colour bars represent the corresponding colours for each level of the standardised current density. In this study, we have chosen to represent the standardised current density distribution on a scale from 0 to 2. This scale is intended to include the entire standardised current density values that were measured in all the tested operating conditions. For instance, at 0.9 A.cm⁻² and 100 %RH, the maximum standardised current density attained a local value of 2, which corresponds to 200 % of the average current density (i.e. 1.8 A.cm⁻²). This effect is related to assembly pressure issues at low mechanical compression levels. A similar impact was reported in [58] by Baumann et al. This effect will be discussed in greater detail later in the paper. The abscissa and ordinate axes in Fig. 4(a)–(f) (and for all the 2-D contour maps in this article) represent the x-axis and the y-axis shown in Fig. 3, whereas the axes labels represent the numbers of the segments.

Before analysing the effects of mechanical compression on the current density distributions shown in Fig. 4(a)–(f), it is important to first examine the factors contributing to spatial inhomogeneity that are not related to mechanical compression. The inhomogeneity of the current

density can arise from various factors [38], which can sometimes be challenging to identify depending on the experimental means available (e.g. issues related to components manufacturing, damage during FC operation, uneven catalyst loading [63]). The following explanations aim to clarify the potential effects impacting the current density distribution shown in Fig. 4(a)–(f).

Regarding spatial current inhomogeneity, the four corners of the current density maps in Fig. 4(a)–(f) exhibit relatively low or even zero current density across all levels of mechanical compression. This observation is mainly due to the presence of the orifices of the reactant's inlets / outlets (depicted in Fig. 3) where no electrical conducting material is present. A similar effect was emphasised by Baumann et al. [58]. These authors created a defect site with no conducting material and reported the same observation as no current was flowing through this defect site.

During our experimentations, hydrogen and air were used as reactants in co-flow mode, meaning that, as shown in Fig. 3, the air was fed from the bottom left to the top right, whereas the hydrogen was fed from the top left to the bottom right of the FFPs. This configuration is intended to favour a uniform membrane humidification distribution as the anode inlet is located near the cathode outlet (i.e. top zones 7, 8, and 9 in Fig. 3) so that water transport mechanisms, namely back-diffusion, helps to homogenise the membrane humidification. However, it can be observed from Fig. 4(a)–(f) that the current maps present a large spatial inhomogeneity, especially in the horizontal direction. Zones 1 through 6 exhibit relatively higher current densities compared to zones 7 through 9. This relatively low current density at the top region of the active area results from a depletion of the oxygen contained in the air flow near the cathode outlet. The remaining oxygen concentration near the outlet is not sufficient enough to reach similar current density levels as in the first locations (i.e. zones from 1 to 6) of the active area of the PEMFC. In fact, the available oxygen is progressively consumed while approaching the air outlet. A similar current density distribution was reported by Alaefour et al. [64], who noted that local current densities are significantly higher in zones near the reactant inlet compared to those near the reactant outlet in a co-flow configuration. This effect was attributed to the reactant depletion at the outlet regions of the active area. Similar observations were also reported in [65,66].

Regarding the effects of mechanical compression shown in Fig. 4(a)–(f), it can be observed that the spatial homogeneity of the current density improves with increasing the mechanical compression. This effect is mainly due to the enhancement of the mechanical contact within the FC components. Fig. 4(a)–(f) also shows that the spatial homogenisation effect majorly occurs at mechanical compressions up to 1.4 MPa. This homogenisation effect is mainly visible at the top and bottom regions of the maps, and especially in the 3rd zone. The contact pressure is insufficient in this zone, which results in high electronic resistance and, therefore no current in this area. As a result, most of the current flows through the nearby zones with relatively low electronic resistance. This effect also explains the high standardised current density levels located nearby the 3rd zone. These results align well with the observations of Baumann et al. [58]. These authors reported that a nonconducting region of the FFPs led to the increase in the current flowing through their adjacent conducting regions.

Compared to other PEMFC components, GDLs are most affected by structural deformation under mechanical compression due to their soft and brittle nature [67–69]. The electro-physical characteristics of the GDLs are highly influenced by compressive loads and play a major role in the current density maps shown in Fig. 4(a)–(f). In fact, both the bulk and contact resistances of the GDL decrease as mechanical compression increases [3,4,70], as shown in Fig. S1 and Fig. S2 (Supplementary Material file). This effect was highlighted in the work of K. Bouziane [71,72], conducted as part of the same research project as this study (the MIREPOix project). As the GDL is compressed (Fig. S1), its thickness decreases leading to enhanced fibres connections, and therefore to a reduced bulk and contact resistances [3,4,71,72]. In line with these
effects, Fig. 5 depicts in-situ cell displacement measurements (i.e. measured from our assembled PEMFC) and ex-situ tests of GDL thickness variations (i.e. provided by SGL Carbon) using the same GDL employed in this paper (i.e. Sigracet® 38 BCE). The error bars are derived from experimental measurements and represent the deviations observed during the recordings. Fig. 5 shows that the in-situ and ex-situ measurements of the displacement follow a similar trend. The ex-situ results exhibit larger displacements but also greater dispersions so that the in-situ results fall virtually within the error bars of the ex-situ data. This result shows that the major in-situ displacement is due to the GDL compression, leading to a reduced contact and bulk resistances of the GDL as it was emphasised in [3,4,71,72] and shown in Fig. S1 and Fig. S2 (Supplementary Material file).

Thus, based on the results shown in Fig. 4(a)–(f) and Fig. 5, regions with higher mechanical pressure exhibit lower bulk and contact resistances in the GDL, which enhances current flow through these lower resistance regions. These variations in local resistance can explain the homogenisation in the current density distributions as a function of mechanical compression, as shown in Fig. 4(a)–(f). This effect will be further examined later in the article using pressure-sensitive films. To statistically analyse the results shown in Fig. 4(a)–(f), we present the histograms of the standardised current density distributions in Fig. 4(g). To establish correlations with the previously reported results in [3,4], the histograms are presented at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa. These levels are representing as close as possible the first and second compression ranges as defined in [3,4], namely (0.35–1.55 MPa) and (1.55–2 MPa), respectively. These 3 levels of mechanical compression will be used in histogram figures for the rest of this study and will be represented by 3 colours (blue, orange, and yellow). Other colours may be visible due to the superimposition of two of three of these colours. The histograms shown in Fig. 4(g) can be analysed following 3 regions depending on the value of the standardised current density. These regions are coloured in blue, green, and red in Fig. 4(g) and marked by Latin numerals I, II, and III, respectively. The blue region corresponds to current density levels below 40 % of the average current density, whereas the red region corresponds to current density levels higher than 140 % of the average current density. These two regions are considered critical since they contain very low and very high current density levels that lead to an uneven current density distribution. Finally, the green region is defined in-between the blue and red regions, and designated as the medium current density region.

It can be seen from Fig. 4(g) that mechanical compression simultaneously reduces both very low and very high current density regions.

This improvement is mainly due to the more uniform distribution of mechanical compression, which will be explored in more detail later in the paper. These two combined effects increase the medium current density region, which is beneficial for the FC as they homogenise the current density distribution, as shown in Fig. 5. A visual analysis of the histograms in Fig. 4(g) reveals that the homogenisation effect majorly occurs at mechanical compressions up to 1.4 MPa. The mechanical compression reduces the standardised current density of 0.1 (i.e.10 % of the average current density) by 85.4 % at 1.4 MPa over a total reduction of 95.1 % at 2 MPa, whereas the 1.4 values were eliminated at 1.4 MPa. Further statistical analyses will be addressed by comparing results at 0.6 and 0.9 A.cm⁻² at the end of this subsection.

- Results at 100 %RH: Fig. 6(a)–(f) shows the 2-D contour maps of the standardised current density distributions at 6 levels of mechanical compression from 0.5 to 2 MPa. These standardised current density distributions were measured for the PEMFC operating at an average current density of 0.6 A.cm⁻² and a relative humidity of 100 %RH.

Before discussing the effects of the mechanical compression on the standardised current density distributions shown in Fig. 6(a)–(f), it is important to first analyse the causes of current density inhomogeneity that are unrelated to mechanical compression. Similar to the results at 50 % RH, the current density maps here exhibit large inhomogeneity in the horizontal direction. Fig. 6(a)–(f) shows that the high current values flow through regions near the air inlet (zones 1, 2, and 3), whereas the upper zones (7, 8, and 9) show relatively low current. This inhomogeneity is mainly attributed to the depletion of the oxygen concentration [64,65] as explained previously in the section devoted to the 50 %RH results. This observation corresponds with the findings reported by Hakenjos et al. [73].

Comparing the zones 7, 8, and 9 in Fig. 4(a)–(f) with those in Fig. 6 (a)–(f) reveals that the current density is lower in these zones at 100 % RH compared to 50 % RH. This reduction is due to oxygen depletion near the air outlet. However, the impact of oxygen depletion on current density distribution appears to be more critical at 100 %RH compared to 50 %RH. This effect is mainly attributed to the higher presence of water at 100 %RH [74], which leads to diffusion issues. The water accumulation on the cathode side obstructs oxygen transport, such as through the clogging of FFP channels and GDL pores, resulting in oxygen starvation in zones near the air outlet. The same results were observed at a current density of 0.9 A.cm⁻² (figures and analysis available in [5]). This finding aligns with the results of Iranzo et al. [75] in their in-situ study

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Comparison between in-situ measurement using CMD and ex-situ measurement provided by SGL Carbon. Both measurements are conducted on a GDL type Sigracet® 38 BC.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Standardised current density distribution 2-D maps at 0.6 A.cm⁻² and 100 %RH. (a) 0.5 MPa, (b) 0.8 MPa, (c) 1.1 MPa, (d) 1.4 MPa, (e) 1.7 MPa, and (f) 2 MPa. Colour bars show the corresponding colours of each level of the standardised current density. The related histograms (g) of the standardised current density distribution at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa.</figcaption></figure>

on the effects of PEMFC operating conditions on liquid water distribution. These authors reported that when the FC operates at high relative humidity (i.e. 90 %RH), more water accumulates in the FFP channels. More importantly, the in-situ study [75] highlighted that liquid water gradually accumulates towards the cathode outlet. This accumulation decreases the water removal rate of the FC, which in turn reduces the current flowing through these regions.

Regarding the effects of mechanical compression on the current density distribution, and similar to the results at a relative humidity of 50 %RH, the results at 100 %RH shown in Fig. 6(a)–(f) highlight that mechanical compression improves the homogeneity of the current density distribution. This homogenisation was improved by simultaneously reducing the very low and the very high current density regions, corresponding to the regions with standardised current densities lower
than 0.4 and higher than 1.4, respectively. These observations are emphasised in the histograms shown in Fig. 6(g). This effect is due to the enhanced mechanical contact between the PEMFC components, but also to the decrease in the GDLs contact and bulk resistances with increasing mechanical compression, as emphasised by the ex-situ results reported by K. Bouziane [71,72] (Fig. S1 and Fig. S2 in Supplementary Material file). Since the electrical resistance of the GDL is highly impacted by mechanical compression, even small variations in the bulk and contact resistances of the GDL will result in various current density levels flowing through some particular regions of the active area of the FC. This effect is observed at the bottom and top regions of the current density maps shown in Fig. 6(a)–(f). In line with these results, Peng et al. [38] reported an investigation on the effects of the clamping pressure on the distribution of the local current densities using an in-house measurement device. They found that mechanical compression homogenises the current density distribution and therefore improves the FC performance. The study showed that regions with higher mechanical contact pressure experience greater current flow, while areas with lower contact pressure have reduced current density.

In order to statistically visualise the current density 2-D maps at 100 %RH, Fig. 6(g) depicts the histograms of the standardised current density distribution at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa. The histograms shown in Fig. 6(g) confirm that the major current density homogenisation effect occurs for a mechanical compression up to 1.4 MPa. It can be observed that the mechanical compression reduces simultaneously the very high current density region (i.e. higher than 140 % of the average current density) and the very low current density region (i.e. lower than 40 % of the average current density). Similar to the results at 50 %RH, the histograms in Fig. 6(g) show that mechanical compression reduces the standardised current density of 0.1 (i.e. 10 % of the average current density) by 83,8 % at 1.4 MPa and 94,6 % at 2 MPa. It is also noticeable that all standardised current density values above 1.4 (i.e., 150 %, 160 %, and 170 % of the average current density) vanish when the mechanical compression reaches 1.4 MPa. More statistical analyses are given in the following subsections.

- **Statistical analysis of the standardised current density distributions at 0.6 A.cm⁻²:** A statistical investigation is conducted in this subsection in order to provide a quantitative analysis of the standardised current density distributions measured at 0.6 A.cm⁻². This investigation aims to provide thorough comparisons between the results at 50 %RH and 100 %RH by studying the evolutions of the Standard Deviation (SD) and the maximum standardised current density values as a function of mechanical compression. The analysis given here includes all the 12 levels of mechanical compression from 0.35 to 2 MPa with 0.15 MPa steps. Similarly to [3,4], the first and second compression ranges are defined as (from 0.35 to 1.55 MPa), and (from 1.55 to 2 MPa), respectively.

- **Evolution of the Standard Deviation (SD) of the current density distribution at 0.6 A.cm⁻²**

The SDs were measured per each of the 12 averaged arrays of the standardised current density distributions at 0.6 A.cm⁻². The evolutions of the SD as a function of mechanical compression at 50 %RH and 100 % RH are depicted in Fig. 7.

Fig. 7(a) shows that mechanical compression reduces the SD of the standardised current density distribution at both relative humidity levels, with higher SD values at 100 %RH compared to 50 %RH. This difference, averaging 13.4 %, is attributed to the increased presence of water at 100 % RH, which blocks reactant pathways [75] and results in an uneven current density distribution, as discussed in the previous subsection. The reduction in the SD of the current density distribution reaches a plateau starting from 1.55 MPa (Fig. 7(a)), with a reduction of 43.8 % in the first compression range and 1.9 % in the second

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** SD (a) and maximum (b) standardised current density distribution as a function of mechanical compression at 0.6 A.cm⁻². Results at 50 %RH and 100 %RH.</figcaption></figure>

compression range at 50 %RH. This reduction of the SD of the current density distribution was slightly less at 100 %RH, with reductions of 35.2 % in the first compression range and 1.1 % in the second compression range. These results align well with the findings reported in [3,4] using polarisation curves and EIS analyses, where the major performance enhancement of the FC was attained in the first compression range.

- **Evolution of the maximum standardised current density of the distribution at 0.6 A.cm⁻²**

The maximum standardised current density was measured for each of the 12 averaged current density arrays at 0.6 A.cm⁻². The results at 50 %RH and 100 %RH are shown in Fig. 7(b). Mechanical compression is observed to reduce the maximum standardised current density. This reduction was measured at 50 %RH to be 12.5 % at 1.1 MPa, 13.8 % at 1.55 MPa over a total reduction of 14.4 % at 2 MPa, whereas the reduction was more important at 100 %RH with 18.8 % at 1.1 MPa, 19.6 % at 1.55 MPa, compared to 20.1 % at 2 MPa. This reduction in the maximum local current density is beneficial for the PEMFC as it reduces the ohmic losses and, therefore, the possible creation of hotspots that may worsen the cell voltage and durability [13,14,59,60]. This effect is further detailed in the following subsection that covers the temperature distribution analysis.

Similar to the trend observed in the reduction of SD with increasing mechanical compression, the results for maximum standardised current density highlight that the most significant reduction in maximum local current density occurs within the first compression range (from 0.35 to 1.55 MPa). These findings are also consistent with those reported in [3,4] and shown in Fig. 1 regarding the effect of mechanical compression on the global FC performance. The results obtained at 0.9 A.cm⁻² (available in [5]) demonstrated a similar trend. At 0.9 A.cm⁻² and 100 %RH, the data revealed negative effects in both SD and maximum current density evolution in the second compression range (Cf. Section 3.2.1.). This effect is attributed to the increased presence of liquid water at 0.9 A.cm⁻² and 100 %RH, which impairs the water management capability of the PEMFC, leading to inhomogeneous current density distribution. A detailed comparison between the results at 0.6 and 0.9 A.
$cm^{-2}$ will be provided in Section 3.2.

### 3.1.2. Temperature distribution analysis at 0.6 A.cm⁻²

The total range of the local temperatures at 0.6 A.cm⁻² was measured between 63.4 °C and 70.4 °C. This range was used as a fixed scale to present all the 2-D maps shown in this subsection. The aim of

using a fixed temperature scale is to compare the temperature distributions at both tested relative humidity levels: 50 %RH and 100 %RH.

- **Results at 50 %RH:** The temperature mapping at 0.6 A.cm⁻² and 50 %RH is shown in Fig. 8(a)–(f). Analogously to the results of the current density maps, Fig. 8(a)–(f) presents the temperature

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Temperature distributions 2-D maps at 0.6 A.cm⁻² and 50 %RH. (a) 0.5 MPa, (b) 0.8 MPa, (c) 1.1 MPa, (d) 1.4 MPa, (e) 1.7 MPa, and (f) 2 MPa. Colour bars show the corresponding colours of each level of the local temperature. The related histograms (g) of the temperature distribution histograms at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa. The red arrow represents the ideal effect of mechanical compression on the temperature distribution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
distribution at 6 levels of mechanical compression from 0.5 to 2 MPa, with steps of 0.3 MPa. It has to be noted that the temperature measurements have 7 × 7 segments, meaning that the S++® temperature measurement device has one temperature measurement point per 4.6 cm² of the PEMFC active area (considering our 225 cm² active area). This resolution is low compared to the current density measurement, which has 21 × 21 measurement points, representing one current measurement point per 0.5 cm² of the FC active area. This resolution difference between the current and temperature mapping will be considered for the analysis reported hereafter.

The first observation that can be deduced from Fig. 8(a)–(f), with regards to the operating conditions of the PEMFC, is that the measured temperature near the MEA (measured by the S++® device) is higher than the coolant setpoint temperature (i.e. 60 °C). This difference is measured to be 5.6 °C on average when the FC was operated at 0.6 A. cm⁻² and 50 %RH. This temperature difference is explained by the fact that the temperature of the coolant (i.e. deionised water) was controlled by the test bench (Greenlight G7805, see [3,5] for more details) that uses temperature sensors located outside the FC (i.e. located at the test bench) and regulated at 60 °C. This temperature setpoint (i.e. 60 °C) was kept constant during all the investigations reported in this article. Therefore, all the changes in the temperature distributions are considered to be mainly due to the local phenomena taking place within the FC.

Even though it was expected that the high current density regions will correspond to the high temperature ones, it seems that the relationship between these two parameters is not so straightforward in our case. The coolant temperature, and especially its pathway inside the FC assembly, play an important role as well. In fact, the bottom zones 1 and 3 of the temperature maps depicted in Fig. 8(a)–(f) show low temperatures compared to the other zones of the FC. This low temperature is mainly attributed to the design of the cooling circuit. In our PEMFC assembly, the cooling medium enters from zones 1 and 3, of the anode and cathode sides, respectively. The cooling medium enters at a relatively low temperature of 60 °C, which reduces the temperature at the bottom of the FC, as shown in Fig. 8(a)–(f). In addition, the top left region in Fig. 8(a)–(f) shows a relatively high temperature. This temperature increase is explained by the fact that this region corresponds to zone 9 (see Fig. 3) from which the cooling medium at the cathode side is drained from the FC. The coolant medium at the cathode side is introduced from the bottom right region of the FC and is gradually heated by the thermal energy generated from electrochemical reactions and Joule heating (also known as resistive or ohmic heating). As a result, the temperature of the cooling medium increases along the direction of the flow, reaching its highest point at the coolant outlet. The temperature difference between the inlet and outlet was measured to be 4.6 °C. Similar observations were also reported by Peng et al. in [38].

In a similar way to the observations from the current density maps, Fig. 8(a)–(f) shows that mechanical compression tends to homogenise the temperature distribution. This effect is primarily due to the reduction in maximum current density, which lowers ohmic losses and consequently reduces temperature by minimising Joule heating. Moreover, the decrease in the temperature shown in Fig. 8(a)–(f) may also be caused by the reduction in the thermal resistance of the GDL with increased mechanical compression. As a matter of fact, one of the primary functions of the GDL is to conduct the heat generated from the MEA to the cooling plates (via the FFPs) that have means for heat removal. Therefore, reducing the thermal resistance of the GDL leads to a better cooling of the MEA and, in turn, to a reduced temperature. This effect was emphasised by Khandelwal and Mench in [76]. These authors provided a correlation between the mechanical compression and the thermal contact resistance of the GDL. It was shown in [76] that the thermal contact resistance between an aluminium bronze material and the GDL was reduced from 6.7 × 10⁻⁴ to 2.0 × 10⁻⁴ m²KW⁻¹ (i.e. 70.1 % reduction) as the mechanical compression was increased from 0.4 to

2.2 MPa. In their study [60], Xiaohui Yan et al. examined the temperature uniformity in a FC stack, revealing that higher clamping pressure reduces the average temperature at the CL of each individual cell. This effect was also attributed to the deformation of the GDLs under high clamping pressure, which enhances their thermal conductivity in both in-plane and through-plane directions.

Fig. 8(a)–(f) shows that the most significant changes in the temperature distribution occur at mechanical compressions up to 1.4 MPa, which is in good agreement with the results of the standardised current density distribution. In order to statistically analyse the temperature maps depicted in Fig. 8(a)–(f), the histograms of the temperature at 3 levels of mechanical compression (namely 0.5 MPa, 1.4 MPa, and 2 MPa) are shown in Fig. 8(g). As mechanical compression increases, the temperature distribution histograms shift to the left, thus to the low temperature range, as shown by the red arrow in Fig. 8(g). This local temperature reduction is most likely caused by the decrease in the maximum current density, and therefore by the reduction in the heat generated by the electrochemical reaction as well as the Joule heating. It is observed that the number of segments at a temperature of 65.5 °C was reduced by 70 % at 1.4 MPa, over a total reduction of 90 % at 2 MPa. This effect is advantageous for the PEMFC, as it reduces the likelihood of hotspot formation, which can negatively impact both performance and durability [13,14,59,60]. This observation is consistent with the study of Hakenjos et al. [73]. The authors noted that regions with higher temperatures correspond to areas with elevated local current densities. This effect was attributed to the increased heat generated by the electrochemical reactions occurring at the CLs [73]. It was observed across all tested operating conditions and will be discussed in detail in the following subsection.

- Results at 100 %RH: Fig. 9(a)–(f) shows the temperature mapping at 0.6 A.cm⁻² and 100 %RH. The 2-D temperature distribution maps use the same temperature scale as that for 50 % RH, ranging from 63.4 °C to 70.4 °C.

Fig. 9(a)–(f) shows that the bottom zones 1 and 3 exhibit relatively lower temperatures compared to the other zones. Similar to the observations at 50 % RH, this is attributed to the fact that the coolant medium enters the FC through zones 1 and 3, located at the anode and cathode sides, respectively. As the coolant medium flows towards the outlet, it is continuously heated by the thermal energy from the electrochemical reactions and Joule heating. Thus, the maximum temperature is measured at the cathode coolant outlet, as shown in the 7th zone of Fig. 9(a)–(f). The temperature difference between the coolant inlet and outlet was measured to be 5.5 °C.

The central region (zones 4, 5, and 6) of Fig. 9(a)–(f) shows relatively high local temperatures. This effect is most likely due to the higher local current densities in this region, which generate increased local temperatures from both the heat produced by electrochemical reactions [73] and Joule heating. Comparing the temperature distributions at 50 %RH and 100 %RH reveals that the FC operates at a higher temperature at 100 % RH, with an average maximum temperature difference of 1.3 °C between the two conditions. This temperature difference is again attributed to the higher local current densities at 100 %RH compared to 50 %RH, as previously indicated by the current density maps.

Regarding the mechanical compression effects, it can be seen from Fig. 9(a)–(f) that the homogenisation of the temperature distribution improves as the mechanical compression increases. Similar to the results at 50 %RH, the high temperature regions, located at the centre of the temperature maps (zones 4, 5, and 6), decrease in size as mechanical compression increases. This reduction in the high temperature regions is attributed to the decrease in the high current density areas with increasing mechanical compression, as previously observed in the current density maps. Fig. 9(g) depicts the histograms of temperature distribution at 3 levels of mechanical compression. Similar to the results at 50 %RH, Fig. 9(g) shows that mechanical compression shifts the
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Temperature distributions 2-D maps at 0.6 A·cm⁻² and 100 %RH. (a) 0.5 MPa, (b) 0.8 MPa, (c) 1.1 MPa, (d) 1.4 MPa, (e) 1.7 MPa, and (f) 2 MPa. Colour bars show the corresponding colours of each level of the local temperature. The related histograms (g) at 3 levels of mechanical compression: 0.5 MPa, 1.4 MPa, and 2 MPa.</figcaption></figure>

temperature histograms to the left (i.e. to the lower temperature range), which is beneficial for the cell as it lowers the occurrence of high-temperature regions, reducing the likelihood of hotspots that could harm the PEMFC performance and durability [13,14,59,60]. It can be seen that the number of segments at a temperature of 68 °C was reduced by 85.7 % starting from a mechanical compression of 1.4 MPa. Comparing the temperature histograms in Fig. 8(g) and Fig. 9(g) reveals that operating the PEMFC at 100 % RH increases the maximum local

temperature by an average of 2.1 °C. This local temperature increase is mainly attributed to the heating effect due to the high local current densities when the FC is operating at 100 %RH. The next subsection will offer a statistical analysis of the temperature distributions at 0.6 A·cm⁻², comparing the results obtained at 50 %RH and 100 %RH.

- Statistical analysis of the temperature distributions at 0.6 A·cm⁻²: This analysis aims to provide a comprehensive comparison
between the results at 50 %RH and 100 %RH by examining the evolution of the Standard Deviation (SD) and maximum values of the temperature maps as a function of mechanical compression. The results presented in this subsection cover the full compression range, from 0.35 to 2 MPa, in 0.15 MPa increments.

– Evolution of the standard deviation of the local temperature distributions at 0.6 A.cm⁻²

The evolution of the SD of the temperature distribution as a function of mechanical compression is shown in Fig. 10(a).

Fig. 10(a) shows that the SD of the temperature distributions is, on average, 12.5 % higher at 100 %RH compared to 50 %RH. This difference in the SD of temperature distributions aligns well with the 13.5 % average difference observed in the SD of the standardised current density maps between 100 %RH and 50 %RH. Thus, the SD difference of the

temperature distributions is attributed to the higher local current densities at 100 %RH compared to 50 %RH, which result from diffusion issues as previously discussed in this article.

Fig. 10(a) illustrates that mechanical compression enhances the uniformity of the temperature distribution at both 50 %RH and 100 % RH. The major temperature homogenisation effects occur in the first compression range (from 0.35 to 1.55 MPa), with relatively higher reduction at 50 %RH (9 %) compared to 100 %RH (4.7 %), which is most likely due to the current density inhomogeneity at 100 %RH compared to 50 %RH. In the second compression range (from 1.55 to 2 MPa), the SD reduction is only equal to 0.9 % at 50 %RH and 1.1 % at 100 %RH. These results are in good agreement with the results of the SD evolution of the standardised current density distribution as a function of compression (see the comments for Fig. 7(a)). They are also consistent with the global mesurments shown in Fig. 1.

<figure><img src="image_14.png" /><figcaption>Fig. 10. SD of the temperature distribution (a) and maximum local temperature (b) as a function of mechanical compression at 0.6 A.cm⁻². Results at 50 %RH and 100 %RH.</figcaption></figure>
– Evolution of the maximum local temperature at 0.6 A.cm⁻²

Fig. 10(b) shows the evolution of the maximum local temperature as a function of mechanical compression at both relative humidity levels of 50 %RH and 100 %RH. The mechanical compression reduces the maximum local temperature at both 50 %RH and 100 %RH. This effect remains low (i.e. 0.7 °C range in Fig. 10(b)) and it is due to the result of two phenomena: the first one is the reduction in the maximal current density and the second one is the enhancement of the thermal conductivity of the GDL, as previously explained in this paper. The maximum temperature at 100 %RH is higher than at 50 %RH. This difference, averaging 2.1 °C, is attributed to the higher local current densities at 100 %RH compared to 50 %RH, as discussed earlier in this section.

In the first compression range (from 0.35 to 1.55 MPa), the maximum temperature reduction at 0.6 A.cm⁻² is 0.24 % at 50 %RH and 0.44 % at 100 %RH. In the second compression range (from 1.55 to 2 MPa), the reductions are nearly 0 %. The effect of mechanical compression on the maximum local temperature is relatively low compared to the other statistical results reported in this section. However, it is still a valuable effect, as it helps reduce the risk of hotspot formation, which can adversely affect both the performance and integrity of the PEMFC [13,14,59,60]. Additionally, it is noteworthy that the major reduction in the maximum local temperature occurs in the first compression range. This aligns with the results regarding the maximum

standardised current density evolution as a function of mechanical compression range (see Table 2). These results underscore the correlations that exist between the current and the temperature distributions.

As noted at the beginning of this section, the effects of mechanical compression on current density and temperature maps were also investigated for the FC operating at 0.9 A.cm⁻². This analysis employed the same spatial and statistical methods; the results and figures obtained at 0.9 A.cm⁻² can be found in [5]. As the focus of this article is to explore the correlations between the current and temperature distributions and the FC performance assessed in-situ [3,4], the comparison between the results at 0.6 and 0.9 A.cm⁻² is discussed in the following section.

## 3.2. *Comparison between current and temperature distributions at 0.6 and 0.9 A.cm⁻²*

This section offers a detailed comparison of current and temperature distributions at 0.6 and 0.9 A.cm⁻². The comparison follows the same methodology outlined in [3,4], focusing on the two mechanical compression ranges. To recap, the first compression range spans from 0.35 to 1.55 MPa, where significant improvements in FC performance were observed through polarisation curves and EIS analysis. In contrast, the second compression range, from 1.55 to 2 MPa, showed only marginal performance enhancements or even performance degradation (Fig. 1).

<figure><img src="image_15.png" /><figcaption>Fig. 11. Percentage of the reduction in the standardised current density distributions (Max. and SD) for the mechanical compression ranges. Results at 0.6 (left) and 0.9 A.cm⁻² (right).</figcaption></figure>
### 3.2.1. Mechanical compression impacts on the current density distributions

Fig. 11 illustrates the percentage reduction in both the maximum local value and the SD of the standardised current density distributions across different mechanical compression ranges.

Fig. 11 reveals that the most significant reductions in both the maximum standardised current density and its SD occur within the first compression range. The changes observed in the second compression range are notably smaller and, at 0.9 A.cm⁻² and 100 %RH, even show a reverse trend compared to those in the first compression range. This observation aligns well with the findings in [3], which highlight significant FC performance improvements in the first compression range (refer to Fig. 10 in [3] and Fig. 1 of the present article).

Regarding the evolution of SD with mechanical compression, Fig. 11 shows that the reduction in SD during the first compression range diminishes as both current density and relative humidity increase. This effect is mainly attributed to the increased presence of water, which leads to the blockage of GDL pores and FFPs channels and, in turn, tends to cause reactants starvation. This results in uneven current generation, thereby raising the SD of the current density distributions. Moreover, at the highest water content in the FC (i.e. at 0.9 A.cm⁻² and 100 %RH), mechanical compression in the second range actually increases the SD, indicating that compression begins to cause water management issues, resulting in a less uniform current density distribution. Thus, the mechanical compression optimally homogenises the current density when the PEMFC operates with a relatively low water content. However,

excessive compression may induce water management problems.

Fig. 11 also indicates that the reduction in maximum local current density increases with higher relative humidity and current density levels. As these factors rise, the water content within the PEMFC becomes significant, leading to inhomogeneous current density distribution and elevated current densities in specific regions of the GDL. When mechanical compression is applied within the first range, it lowers the contact and bulk resistance of the GDL (Fig. S1. and Fig. S2. in Supplementary Material file), promoting a more even distribution of current and resulting in a reduction in maximum local current density. We can assume that the reduction in maximum local current density positively impacts the decrease in ohmic losses [77]. These findings align well with the results presented in [3] (refer to Fig. 1 in this article and Fig. 10 in [3]), which show a greater increase in cell voltage with mechanical compression in the first compression range at 0.9 A.cm⁻² compared to 0.6 A.cm⁻². Additionally, this decrease in maximum current density also contributes to a reduction in maximum local temperature, a topic that will be explored further in the following subsection.

### 3.2.2. Mechanical compression impacts on the temperature distributions

The reduction (in percentage) of the maximum local value and the SD of the temperature distributions, as a function of the mechanical compression range, is depicted in Fig. 12.

Fig. 12 shows that the SD of the temperature distribution decreased in the first compression range at 0.6 A.cm⁻², aligning with the reduction

<figure><img src="image_16.png" /><figcaption>Fig. 12. Percentage of reduction in the temperature distributions across various mechanical compression ranges. Results at 0.6 A.cm⁻² (left) and 0.9 A.cm⁻² (right).</figcaption></figure>
observed in the SD of the current density map at the same current density. In contrast, at 0.9 A.cm⁻², the SD evolution of the temperature distribution does not exhibit a clear trend with respect to mechanical compression, in particular at 100 %RH. This lack of a clear trend is mainly due to several competing factors that become significant at high current densities, such as the heat generated by electrochemical reactions, Joule heating [78], and diffusion issues. These factors contribute to uneven temperature distribution at 0.9 A.cm⁻², where mechanical compression has only an effect on the reduction in the maximum local temperature. Although the reduction in temperature distribution values shown in Fig. 12 is less pronounced compared to the changes observed in current density distributions (Fig. 11), there is a notable correlation, particularly regarding the maximum temperature reduction in the first mechanical compression range. Specifically, Fig. 12 demonstrates that the decrease in maximum temperature during this range increases with higher relative humidity and current density levels. These observations align with the results on current density distributions reported in Fig. 11. It can be concluded that reductions in high current density regions correspond to decreases in high temperature regions across all tested operating conditions. This finding underscores the significant impact of mechanical compression in mitigating hotspots that can negatively affect FC performance and integrity [13,14,59]. The results also highlight that achieving uniform current and temperature distributions is crucial; otherwise, potential improvements in performance and durability could be compromised. Therefore, optimising mechanical compression is essential for manufacturers aiming to enhance PEMFC performance and lifespan.

## 4. Analysis of the pressure distribution

Previous findings from this study highlight that mechanical compression ranging from 0.35 to 1.55 MPa significantly improves the homogeneity of both current density and temperature distributions. This enhancement positively affects PEMFC performance by reducing ohmic losses (Fig. 1) and minimising the risk of hotspot formation. These benefits are attributed to the improved pressure distribution achieved through increased mechanical compression. This section aims, therefore, to provide a comprehensive analysis of mechanical pressure distribution as it varies with mechanical compression. Pressure-sensitive films are used for this analysis. To maintain clarity throughout the remainder of this article, the term “pressure” will refer to the measurements obtained from these films, while “mechanical compression” will denote the assembly pressure.

### 4.1. Overview of the pressure-sensitive films used

Although pressure distribution and magnitude studies using pressure-sensitive films (also known as pressure indicating films) are typically conducted on off-line PEMFCs [8, 15–22,79,80], this technique remains valuable for analysing internal pressure distribution across PEMFC components.

The pressure-sensitive films used in this study were supplied by Fujifilm (Prescale® [56]). They have thicknesses less than 200 µm for all compression ranges [56]. Importantly, these films have been reported to have a negligible effect on the internal pressure maps of the PEMFC [8]. Therefore, their application for investigating pressure distribution across PEMFC components is appropriate and will be utilised in this study. In this section, super low pressure Prescale® films (LLW), which are optimal for measurements ranging from 0.5 to 2.5 MPa [56], are utilised. These films come in two types: films A and C, as illustrated in Fig. 13.

The pressure distribution and magnitude measurements rely on the principle that mechanical compression causes microcapsules in the film A to rupture, releasing a colour-forming material. This material is then absorbed by the film C. The intensity of the resulting colour corresponds to the applied compression level: greater pressure results in more intense

<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Presentation of the Prescale® LLW placement and measurement principle. The blue arrows indicate the direction of applied mechanical compression. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

colour. By analysing this colour intensity, the magnitude and distribution of the pressure can be accurately assessed.

The pressure distribution investigations were carried out using the same PEMFC components as in the previous studies. The Prescale® film was positioned in place of the MEA, as illustrated in Fig. 13. Following the methodology used for current density and temperature distribution analyses, 6 levels of mechanical compression were investigated, spanning from 0.5 to 2 MPa in 0.3 MPa steps. All measurements were conducted under the conditions recommended by Fujifilm [56], specifically at an ambient temperature of 21 °C and a relative humidity of 54 %RH.

We followed Fujifilm's standardised procedure for applying mechanical compression in our study. After assembling the qCf with the Prescale® LLW films, the mechanical compression was gradually increased over two minutes until the target levels were achieved, which were then held for an additional two minutes. The pressure-sensitive film (Film A) was then digitised and analysed. This section presents a spatial analysis of the pressure maps using the Prescale® Fujifilm LLW, followed by a quantitative assessment via an image processing protocol. Additionally, this section explores the correlations between the pressure distribution results and the current / temperature distributions.

### 4.2. Spatial analysis of the pressure distributions

This subsection provides a spatial analysis of the pressure maps within the FC components and examines the correlations between pressure, current, and temperature distributions. The pressure-sensitive films will be analysed in the same zones as depicted in Fig. 3. The digitalised images of the pressure-sensitive films are presented in Fig. 14.

The pressure maps in Fig. 14 confirm that mechanical compression homogenises the pressure distribution within the FC components. At lower mechanical compression (up to 1.1 MPa), the pressure distribution is visibly uneven, with significant gradients observed between the central, top, and bottom regions. The disparities between contact zones are caused by slight geometric imperfections on the surfaces, such as
<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Pressure distribution images using Prescale® LLW. (a) 0.5 MPa, (b) 0.8 MPa, (c) 1.1 MPa, (d) 1.4 MPa, (e) 1.7 MPa, and (f) 2 MPa. Colour references shown in (g) are provided by Fujifilm and represent the corresponding colours for each pressure magnitude level. (h) shows the coordinate system us, as depicted in Fig. 3.</figcaption></figure>

non-ideal flatness of the various cell components (including the flow field plates) and residual mechanical play in the cell assembly. For example, at 0.5 MPa, the imprint marks of the FFPs ribs are sparse and faint, and they are mostly concentrated in the 5th and 6th zones, indicating a generally low and uneven pressure distribution. This pressure inhomogeneity raises the electronic resistance, providing fewer preferential pathways for electron flow, and consequently increases the local current density in the adjacent zones (5th and 6th zones). As the mechanical compression gradually increases, the top and bottom regions experience higher pressure, leading to a more uniform pressure distribution, particularly from 1.4 MPa onwards. The results in Fig. 14 demonstrate a clear correlation between the pressure distribution and the current / temperature distributions. For example, the four corners clearly display the imprint of the reactants' inlet and outlet orifices, indicating a lack of pressure in these areas. This low pressure magnitude accounts for the minimal or non-existent current flow through these corners. Additionally, it is observed that at low mechanical compression levels, particularly at 0.5 and 0.9 MPa, the top and bottom regions exhibit lower pressure magnitudes compared to other areas. This observation aligns well with the low current densities observed in these regions, especially in the 3rd zone, which consistently shows the lowest pressure magnitude even at relatively higher compression levels (up to 1.4 MPa). This effect accounts for the reduced current density in the 3rd zone, as illustrated in Fig. 4(a)–(f) and Fig. 6(a)–(f). As mechanical compression increases, the colour intensity of the Prescale® films intensifies, indicating an improvement in both pressure magnitude and distribution. Given that the FC was operated in galvanostatic mode, the total current should ideally flow uniformly across the entire active area of the cell. However, the uneven pressure observed at mechanical compressions up to 1.4 MPa results in higher current flow through

regions with lower electronic resistance and higher pressure magnitudes, while regions with low pressure experience very low current flow. This pressure distribution inhomogeneity has been reported to increase the ohmic losses in the PEMFC [77], consequently reducing overall performance. These findings align with the results reported in [3,4] and shown in (Fig. 1), where the most significant performance enhancement with mechanical compression was observed within the first compression range, up to 1.55 MPa, primarily due to the reduction in ohmic resistance. To further validate these observations, the next subsection will quantify the pressure mapping presented in Fig. 14.

### 4.3. *Quantitative analysis of pressure distribution images*

To perform a quantitative analysis of the Prescale® images depicted in Fig. 14, an image processing protocol was developed and implemented. This protocol is designed to extract the contribution of each pressure magnitude, as indicated in Fig. 14(g), from the Prescale® images presented in Fig. 14(a)–(f). A brief overview of this protocol is provided in Fig. 15, demonstrating the extraction of the 0.9 MPa pressure contribution from the Prescale® image at a mechanical compression of 1.1 MPa. As a reminder, the term “pressure” refers to the mechanical pressure measured by the pressure-sensitive films, while “mechanical compression” refers to the assembly pressure.

The image processing protocol illustrated in Fig. 15 was applied to each level of mechanical compression shown in Fig. 14. The analysis was performed using MATLAB® with a custom-developed code and an image processing toolbox [81]. The protocol was executed as follows:
<figure><img src="image_19.png" /><figcaption>**Fig. 15.** Image processing protocol for analysing Prescale® LLW images: Example of extracting the 0.9 MPa pressure contribution from the 1.1 MPa mechanical compression image.</figcaption></figure>

- **Calculation of average RGB triplets for pressure magnitudes:** the average RGB triplets (red, green, blue) were first calculated from the colour samples provided by Fujifilm, as shown in Fig. 14(g).

- **Extraction of pixels corresponding to each pressure magnitude:** all the pixels matching the average RGB triplets for the investigated pressure magnitude were extracted from the Prescale® images, creating a new image for each pressure level. This process was repeated for each pressure level depicted in Fig. 14(g) and for each mechanical compression level shown in Fig. 14(a)–(f). The resolution of the images remained consistent across all compression levels (560 × 560 pixels).

- **Grayscaling and thresholding:** each extracted image is converted to grayscale and binarised. The total number of black pixels corresponding to the investigated pressure magnitude was then calculated. The percentage of these pixels relative to the total number of pixels in the image (including white pixels) was determined. It is important to note that pressure magnitudes below 0.3 MPa and above 2.6 MPa, which are not represented in Fig. 14(g), were excluded from the analysis as they fall outside the measurable range of the Prescale® LLW film. The percentages of the combined contributions are calculated for each level of mechanical compression (by summing the percentages of contributions corresponding to each pressure magnitude).

The results of this image processing are presented in Fig. 16.

From Fig. 16(a), it is observed that regions of both excessive and insufficient compression are present at all compression levels, though their contributions vary. It is also evident that the results at 1.7 and 2 MPa are nearly identical, indicating that the pressure distribution has achieved optimal homogenisation within this compression range. These results are consistent with the previously reported investigations into the current and temperature distributions, reinforcing that significant homogenisation effects primarily occur in the first compression range up to 1.55 MPa. Fig. 16(a)) also reveals that mechanical compression levels below 1.1 MPa exhibit significant discrepancies in the measured pressure magnitudes. To further analyse this pressure discrepancy, Fig. 16(b)) illustrates the percentages of the combined contributions of all pressure magnitudes, as shown in Fig. 14(g), relative to the mechanical compression levels. By comparing Fig. 16(a) and Fig. 16(b), it is observed that at low mechanical compression levels, up to 1.1 MPa, the pressure magnitude profile (Fig. 16(a)) exhibits substantial inhomogeneity. For instance, at 0.5 MPa, the combined pressure magnitudes account for 25.5 % of the total image pixels (Fig. 16(b)). Furthermore, the 0.3 MPa pressure magnitude accounts for 70.5 % of the total extracted pixels, while the combined contributions from 1.75 MPa, 2.15 MPa, and 2.6 MPa only total 7.2 %. These results indicate that at 0.5 MPa, the pressure distribution within the FC is generally uneven.

In terms of correlations with the previously reported studies, this

pressure inhomogeneity indicates that the current tends to flow preferentially through regions with higher pressure magnitudes (i.e. lower electronic resistance), while regions with lower pressure magnitudes (i.e. higher electronic resistance) see minimal or no current flow. This uneven pressure distribution increases the ohmic resistance of the GDL [82] due to the significant pressure discrepancies, thereby negatively impacting the cell performance. Similar pressure discrepancies can be observed at mechanical compression levels below 1.1 MPa. However, starting from 1.4 MPa, Fig. 15 and Fig. 16 demonstrate that pressure magnitudes achieve their maximum homogeneity. This indicates that the effects of mechanical compression on reducing ohmic resistance have reached a plateau. This finding aligns with the results reported in [3,4] and Fig. 1 and it is consistent with other studies using pressure-sensitive films [21,31]. This quantitative analysis confirms the correlation between mechanical compression, overall cell performance, and the local phenomena occurring within the operating FC.

## 5. Conclusion

Determining the optimal compression force for a PEMFC assembly helps to strike a balance between ensuring optimal electrical and thermal contacts while safeguarding the integrity of the GDLs and other components. This article offers a comprehensive analysis of how mechanical compression quantitatively influences the local phenomena within a large-area FC (225 cm²), providing insights relevant to real-world applications. The investigations are conducted at two different current densities (0.6 and 0.9 A·cm⁻² corresponding to the ohmic and mass transport regions, respectively) and relative humidity levels in the cell (50 %RH and 100 %RH). The mechanical load is varied between 0.35 and 2 MPa in precise increments of 0.15 MPa, ensuring a high degree of precision in the study. The spatial analysis of heterogeneities in the 2D-contour maps of current density and temperature distribution is performed using histograms and statistical metrics: maximum and Standard Deviation (SD) values. The study includes additional measurements and images of pressure distributions obtained using pressure-sensitive films in the non-operating cell. The work also highlights the crucial role of GDLs and their specific mechanical behaviour in influencing current, temperature, and pressure distribution, through analyses of cell displacements and GDL thickness variation, as well as measurements of GDL bulk and contact resistances (Fig. S1 and Fig. S2 in Supplementary Material file), as a function of compression. One of the challenges in the upcoming analyses and explanations is distinguishing between the factors that contribute to spatial inhomogeneity of current and temperature. This requires isolating the effects of mechanical compression from other influences, such as cell design and the positioning of reactant and cooling medium inlets and outlets.

The work confirms the correlations that exist between the current, temperature, and mechanical pressure distributions and the
<figure><img src="image_20.png" /><figcaption>Fig. 16. (a) Percentage of contributions in pressure distributions for each level of mechanical compression. (b) Percentage of combined contributions of all pressure magnitudes, as shown in Fig. 14(g), plotted against the mechanical compression levels.</figcaption></figure>
electrochemical performance of the cell. The results underscore that mechanical compression helps to even out the current density distribution, which in turn homogenises the temperature distribution. This latter effect is beneficial as it reduces the formation of hotspots that have detrimental impacts on the performance and lifespan of the FC. This homogenising effect on current and temperature (evidenced by the reductions in maximum and standard deviation values) is primarily attributed to the improved pressure distribution achieved within the initial compression range (0.35 to 1.55 MPa), as demonstrated by experiments using pressure-sensitive films. Furthermore, this homogenisation was also proven to have positive impacts on the cell performance due to the reduction of the ohmic resistance (lower contact and bulk GDL resistances). The study also reveals that potential water management issues can be caused by excessive compression, at higher current density (0.9 A cm⁻²) and relative humidity (100 %RH). Consequently, mechanical compression is most effective at homogenising the current density distribution when the FC operates with a relatively low water content (50 %RH).

The observations reported in this study align well with previous findings from steady-state characterisations and EIS regarding the overall performance enhancement of the cell, as reported in [3,4] and illustrated in Fig. 1. This improvement was primarily attributed to the significant reduction in ohmic resistance outweighing the increase in mass transfer losses. As a result, the data indicate that compressing the FC beyond a certain threshold (1.55 MPa) does not yield any additional performance benefits and may actually introduce water management issues. This optimal level of compression is also consistent with experimental and simulation results from other research teams [2,10,11,16,24,83,84].

Investigating the effects of mechanical compression on local phenomena within a PEMFC is challenging, as it involves multiple interacting processes. The findings from this study contribute to a deeper understanding of these phenomena under mechanical stress in an operating FC. This study showed that some hard-won performance enhancement, achieved for instance by onerous materials development, can be lost when applying an inadequate mechanical assembly pressure. By integrating both in-situ and ex-situ characterisation techniques and provide guidance on achieving optimal assembly pressure, this comprehensive analysis offers valuable insights for manufacturers aiming to enhance FC performance and durability.

### CRediT authorship contribution statement

**El Mahdi Khetabi:** Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Khadidja Bouziane:** Writing – review & editing, Visualization, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Xavier François:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Remy Lachat:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Yann Meyer:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Denis Candusso:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The "Région Bourgogne-Franche-Comté" is gratefully acknowledged for its support through the ELICOP Project (Ref. 2015C-4944 and 2015-4948) and the co-funding of KHETABI El Mahdi's PhD thesis (Convention N° 2017 Y_07529). The authors would also like to gratefully acknowledge Plastic Omnium and 3 M for the support given to this research project.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apenergy.2025.126187.

### Data availability

Data will be made available on request.

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