# Dynamic electrical degradation of PEM electrolyzers under renewable energy Intermittency: Mechanisms, diagnostics, and mitigation strategies -A comprehensive review

Fahad Maoulida, Damien Guilbert, Mамadou-Baïlo Camara, Brayima Dakyo

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Fahad Maoulida, Damien Guilbert, Mамadou-Baïlo Camara, Brayima Dakyo. Dynamic electrical degradation of PEM electrolyzers under renewable energy Intermittency: Mechanisms, diagnostics, and mitigation strategies -A comprehensive review. Renewable and Sustainable Energy Reviews, 2025, 225, pp.116170. 10.1016/j.rser.2025.116170. hal-05212130

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# Dynamic electrical degradation of PEM electrolyzers under renewable energy Intermittency: Mechanisms, diagnostics, and mitigation strategies – A comprehensive review

Fahad Maoulida* ID, Damien Guilbert ID, Mамadou-Baïlo Camara ID, Brayima Dakyo

Université Le Havre Normandie, GREAH, 76600, Le Havre, France

## ARTICLE INFO

**Keywords:**

PEM electrolyzer

Electrical degradation

Transient overvoltage

ON/OFF cycling

RES

Performance and durability

Advanced electrical management

## ABSTRACT

Proton Exchange Membrane (PEM) electrolyzers are pivotal for green hydrogen production, yet their widespread deployment is hindered by durability issues under real-world dynamic conditions, particularly when powered by intermittent renewable sources. This review explores the main degradation mechanisms affecting PEM electrolyzers, focusing on electrical, thermal, and cycling-induced aging phenomena. Operating at high current densities (up to 3 A cm⁻²) accelerates internal electrical degradation, with ohmic resistance rising by ~50 µV/h due to titanium oxide formation in porous transport layers. Elevated temperatures (>100 °C) intensify membrane thinning, hotspot development, and hydrogen bubble accumulation reducing device lifetime from 35,000 to 8700 h (e.g., between 333 K and 353 K). Frequent ON/OFF cycling further degrades performance through catalyst dissolution and electrode delamination, causing voltage drifts of 20–50 µV/h. The originality of this work lies in its multiphysics and integrative perspective, systematically linking these degradation pathways with corresponding mitigation strategies, all supported by experimental data. Key solutions include dynamic electrical control (up to 80 % effective), advanced thermal regulation (up to 70 % effective), and real-time diagnostics via Electrochemical Impedance Spectroscopy (EIS). By combining physical modeling with experimental insights, this review offers a comprehensive framework for understanding and reducing degradation in PEM electrolyzers. It provides valuable guidance for improving their durability and reliability under variable operating regimes. Ultimately, this work aims to support the accelerated industrial adoption of PEM technology in renewable energy systems subject to dynamic and fluctuating conditions.

## 1. Introduction

Green hydrogen has rapidly emerged as a cornerstone of the global energy transition, driven by the urgent need to decarbonize industrial processes, transportation, and energy storage. This rising demand is further intensified by increasing fossil fuel prices and the implementation of stringent low-carbon policies aimed at reducing greenhouse gas emissions[1–3].

Hydrogen possesses a notably high gravimetric energy density (approximately 120 MJ/kg); however, its low volumetric energy density (around 8 MJ/L in liquid form, compared to 32 MJ/L for gasoline) poses significant challenges in terms of logistics and storage [4,5]. Among the various hydrogen production pathways, water electrolysis stands out as a strategic solution, enabling the conversion of surplus renewable electricity into storable chemical energy [6]. The hydrogen produced

through this method can be directly used as an energy carrier, injected into natural gas grids, or transformed into valuable chemical derivatives such as methanol and ammonia, thereby enhancing its versatility and compatibility with existing energy infrastructures[6–9]. Fig. 1 presents a schematic overview of the key pathways envisioned for hydrogen production and integration within renewable energy systems.

Moreover, recent sustainability-oriented studies underscore the broader environmental implications of hydrogen integration. For instance, Hung Van Tran [10] demonstrates that the adoption of renewable energy, in conjunction with green growth and globalization, plays an asymmetric yet significant role in enhancing environmental sustainability thereby reinforcing the strategic relevance of green hydrogen in global decarbonization efforts.

Among the various electrolysis technologies, Proton Exchange Membrane (PEM) electrolyzers stand out due to their compact design,

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: fahad.maoulida@univ-lehavre.fr (F. Maoulida).</footnote>
fast dynamic response, and suitability for fluctuating operating conditions [11,12]. These systems can operate at current densities exceeding 3 A cm⁻² and, under optimized conditions, can achieve operational lifetimes of up to 100,000 h [13–16]. As of 2023, the total global installed capacity of electrolyzers reached approximately 1.4 GW, with China accounting for a dominant share of 80 % [17]. While alkaline electrolyzers still represent around 60 % of the market, PEM technology is rapidly gaining traction due to its superior operational flexibility and compatibility with variable renewable energy sources. By 2030, the global installed capacity is projected to increase significantly, with estimates ranging from 230 GW to 520 GW [17].

From an economic standpoint, the global hydrogen roadmap targets production costs of $2/kg by 2026 and $1/kg by 2031 [18]. Achieving these milestones would render green hydrogen more cost-competitive than natural gas-derived hydrogen (grey hydrogen), especially in regions such as China and India where renewable electricity is abundant and natural gas prices remain elevated [9]. This anticipated shift in the global hydrogen economy is expected to favor green hydrogen adoption, particularly in markets where it will outperform blue hydrogen in terms of affordability and sustainability [19–28].

Despite these promising developments, PEM electrolyzers face significant economic challenges primarily due to their reliance on scarce and costly platinum group metals (PGMs), such as iridium [15,16, 29–32]. At projected manufacturing scales of 10 GW per year, global iridium supplies would become critically insufficient, necessitating the advancement of efficient recycling technologies and the exploration of alternative catalyst materials [33–36].

Beyond cost, long-term durability remains a major bottleneck to the widespread deployment of PEM electrolyzers. Achieving the ambitious target of 120,000 h of operational life by 2050 will require overcoming complex and interrelated degradation mechanisms electrical, thermal, and mechanical in nature [32,34,35,37–50]. The intrinsic intermittency of renewable energy sources, particularly solar and wind, exacerbates these issues. Frequent power fluctuations induce transient overvoltages, elevate ohmic resistance, and accelerate the degradation of critical components such as bipolar plates and current collectors [51–54]. In addition, harsh operating conditions namely acidic environments and elevated temperatures further compromise the structural and chemical stability of PEM system components [39].

This review focuses exclusively on Proton Exchange Membrane (PEM) electrolyzers used for water electrolysis in green hydrogen production and does not encompass PEM fuel cells. It aims to provide a comprehensive and systematic analysis of the degradation mechanisms affecting these systems, with a dual focus on electrical and thermal aspects. Special emphasis is placed on internal degradation sources (e.g., catalysts, membranes, and internal resistances), as well as external aggravating factors particularly the impact of renewable energy intermittency, which presents a major operational challenge in current energy integration scenarios.

PGM-based catalysts such as RuO₂ and IrO₂ at the anode for the oxygen evolution reaction (OER), and platinum at the cathode for the hydrogen evolution reaction (HER) are essential for maintaining the electrochemical performance and efficiency of PEM systems [55–58]. However, efforts to reduce catalyst loading in order to lower costs inadvertently intensify degradation mechanisms [59]. These include catalyst dissolution, passivation, and agglomeration, all of which contribute to a progressive loss of electrochemically active surface area and a corresponding increase in cell resistance [60,61].

Durability testing has shown that catalyst loading significantly influences the rate of performance degradation. Cells with reduced catalyst loadings (0.5 mg cm⁻²) exhibit a voltage decay rate of approximately 15 µV h⁻¹, whereas those with higher loadings (1.6 mg cm⁻²) show much lower degradation rates, on the order of 5 µV h⁻¹ [39].

To provide insight into current research directions, Table 1 presents a classification of recent scientific publications based on the specific degradation mechanisms investigated. This synthesis highlights the most frequently studied phenomena (thermal, electrical, mechanical, chemical, etc.) and identifies existing gaps within the current body of literature.

An in-depth analysis of 39 recent publications (summarized in Table 1) reveals notable trends in how degradation mechanisms affecting PEM electrolyzers are reported. To better visualize and compare the relative importance attributed to each degradation category, we employed a normalized scale ranging from 0 % to 100 %. This scale reflects not the absolute frequency of citation, but rather the emphasis, depth, and recurrence with which each mechanism is discussed by the authors. Therefore, the percentages serve as both

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Overview of key scenarios for hydrogen and renewable electricity production and integration.</figcaption></figure>
<table><caption>**Table 1**

Classification of articles according to the types of degradation studied in PEM electrolyzers.</caption><thead><tr><th>Ref.</th><th>Subject studied</th><th>Thermal</th><th>Electric</th><th>Mechanics</th><th>Chemical</th><th>Materials/ Catalysts</th><th>Operational</th><th>Others</th></tr></thead><tbody><tr><td>[62]</td><td>Influence of temperature and current density on membrane degradation</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[63]</td><td>Influence of the mode of operation on degradation</td><td>✓</td><td>✓</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[64]</td><td>Catalytic layer materials for electrode engineering</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[27]</td><td>High-performance PEM electrolysis under high current with Ir/Fe-N-C</td><td>✓</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[65]</td><td>Properties of Nafion membranes under PEM electrolysis</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[66]</td><td>Tightening pressure and relaxation of constraints</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[67]</td><td>Stability under high current density</td><td>✓</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[68]</td><td>Dissolution of platinum in PEM electrolyzers</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[69]</td><td>Impact of intermittent operation on lifespan</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[70]</td><td>Advances in PEM water electrolysis</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[52]</td><td>Cell failure: dissolution and thinning of the membrane</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[71]</td><td>Accelerated degradation protocols for iridium catalysts</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[72]</td><td>Durability in dynamic operation</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[73]</td><td>Complete review of PEM electrolysis</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[74]</td><td>Pure mechanical failures in membranes</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[75]</td><td>Durability of sintered porous transport layers</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[76]</td><td>Effect of cyclic compression on the GDL</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[77]</td><td>Impact of iron and hydrogen peroxide</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[78]</td><td>Radicals in the Membranes of PEMFC</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[28]</td><td>In situ structural and chemical degradation of catalysts during operation</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[79]</td><td>Chemical degradation of membranes</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[80]</td><td>Degradation and mitigation under dynamic ON/OFF operation</td><td>✓</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[81]</td><td>Effects of iron ions on degradation</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td></tr><tr><td>[32]</td><td>Degradation mechanisms and mitigation strategies</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[82]</td><td>Catalyst durability via ultra-thin IrRuOx ALD films</td><td>✓</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[83]</td><td>Iridium electrodes/iridium oxide</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[84]</td><td>Iridium oxide catalyst supported</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[85]</td><td>Degradation mechanisms of electrocatalysts</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td><td>x</td></tr><tr><td>[86]</td><td>Cycling tests at low and high load</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[87]</td><td>Harmonized EU protocols for electrolysis testing</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>✓</td></tr><tr><td>[88-90]</td><td>Harmonized testing methods</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>✓</td></tr><tr><td>[91]</td><td>Review of durability testing protocols</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>x</td><td>✓</td></tr><tr><td>[92]</td><td>High current density effects</td><td>x</td><td>✓</td><td>x</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>[21]</td><td>Robust anode architecture and mitigation strategies</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[93]</td><td>Dynamic degradation and novel CCM benchmarking</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr><tr><td>[22]</td><td>Comprehensive assessment of degradation in electrolysis technologies</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>[94]</td><td>Oxygen evolution catalyst degradation mechanisms</td><td>x</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td><td>x</td></tr></tbody></table>

Legend.

✓: Topic explicitly addressed in the publication.

x: Topic not addressed in the publication.

qualitative and semi-quantitative indicators of reporting intensity across the reviewed studies.

Using this approach, electrical degradation mechanisms clearly emerge as the most extensively addressed, representing 32 % of the reported content. These mechanisms are primarily associated with

variations in current density, voltage ripple effects, and temperature fluctuations factors that are critical to the electrochemical stability of the cell. Other degradation contributors, such as catalyst deterioration, suboptimal operating conditions, and secondary chemical reactions, are also discussed but receive comparatively less emphasis.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Distribution of degradation mechanisms studied in the literature.</figcaption></figure>
Thermal and mechanical stresses, although less frequently addressed (each accounting for 10.5 % of the content), are highlighted in several studies for their significant role in long-term performance deterioration, especially under dynamic loading conditions. Additional degradation phenomena such as membrane hydration imbalance, metal corrosion, and contamination by ionic species are rarely the main focus of investigations, yet are acknowledged as relevant within the broader degradation landscape, particularly during long-duration operational scenarios (as illustrated in Fig. 2).

Despite the growing number of publications on water electrolyzer degradation, comprehensive reviews focusing specifically on PEM systems remain limited. In particular, there is a lack of integrated analyses covering multiphysics degradation interactions, long-term reliability, and performance under transient regimes (e.g., ON/OFF cycles and intermittency caused by renewable energy sources).

This article seeks to address these gaps by offering a targeted review of dynamic degradation mechanisms specific to PEM electrolyzers. Special attention is given to the integration of advanced diagnostic tools such as Electrochemical Impedance Spectroscopy (EIS) and to the development of mitigation strategies tailored to the variable and demanding conditions characteristic of RES-powered electrolysis systems.

This review aims to provide a comprehensive and original perspective on the degradation of proton exchange membrane (PEM) electrolyzers under dynamic operating conditions, particularly those driven by renewable energy sources. It focuses on identifying and analyzing the key mechanisms of degradation electrical, thermal, and transient and evaluating their interactions in real-world applications. Special attention is given to aging phenomena specific to intermittent power supply, as well as to strategies for diagnosis and mitigation. The objective is to unify diverse findings from recent experimental and modeling studies into a coherent analytical framework, while offering insights for future research and development.

To achieve this, the remainder of the review is structured as follows.

* Section 2 presents the methodological framework used to select and synthesize relevant literature from both experimental and modeling studies.

* Section 3 outlines the fundamental operating principles of PEM electrolyzers, covering electrical behavior, equivalent circuit representations, and the contributions of various overpotential components.

* Section 4 investigates the mechanisms of electrical degradation induced by current, with particular attention to high current densities, transient regimes, and ON/OFF cycling.

* Section 5 focuses on temperature-induced degradation phenomena, including hot spot formation, membrane thinning, and thermal cycling effects.

* Section 6 provides a detailed overview of diagnostic approaches for assessing degradation, encompassing electrochemical techniques (e.g., EIS), advanced spectroscopic tools, and post-mortem analyses.

* Section 7 discusses state-of-the-art mitigation strategies and real-time diagnostic frameworks designed to enhance the durability and operational stability of PEM systems, including dynamic electrical control and thermal management approaches.

* Section 8 concludes the review with forward-looking perspectives, highlighting the importance of integrated multiphysics modeling, standardized diagnostic protocols, and techno-economic assessments tailored to renewable-powered electrolyzer systems.

By bridging various domains of degradation phenomena and assessment strategies, this review contributes to a better understanding of PEM electrolyzer aging under realistic dynamic conditions, paving the way for more durable and efficient hydrogen production technologies.

## 2. Methodology

### 2.1. Selection methodology of the analyzed studies

The comparative analysis of degradation mechanisms presented in this work is based on an expert-driven selection of over 30 experimental publications spanning the period 2008–2024. This selection did not follow a formal systematic protocol such as PRISMA, but was conducted using rigorous qualitative and quantitative criteria based on experimental relevance, clarity of reported results, and their applicability to the study of dynamic aging phenomena.

The included studies exhibit the following characteristics.

* Direct experimental measurements related to both electrical and thermal degradation;

* Temporal profiles of current, voltage, temperature, or internal resistance ($R_{int}$) under dynamic or stationary conditions;

* A variety of operational regimes (continuous, ON/OFF cycling, sinusoidal) and stress types (thermal, electrical, hydration);

* Standardizable degradation indicators, such as voltage drift rate (µV/h), increase in ohmic resistance, efficiency loss, or system lifetime under various mitigation strategies.

Particular attention was paid to the diversity of degradation types covered (catalyst, membrane, interfaces), to ensure a balanced representation in the synthesis. The selected studies are listed in the appendix, along with the associated inclusion criteria.

## 3. Operation of the PEM electrolyzer

### 3.1. Fundamental electrochemical reactions and operating principle

PEM water electrolysis, initially developed by General Electric in the 1960s [73,95,96], is a promising green hydrogen production method. It relies on a solid polymer membrane that conducts protons ($H^+$) while being impermeable to electrons and gases. The core reactions are.

* Anode (Oxidation of water):

$$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^- \quad (\text{Eq.1})$$

* Cathode (Reduction of protons):

$$4H^+(aq) + 4e^- \rightarrow 2H_2(g) \quad (\text{Eq.2})$$

* The overall reaction is written:

$$2H_2O(l) \rightarrow 2H_2(g) + O_2(g) \quad (\text{Eq.3})$$

This process ensures high hydrogen purity [97–100] and is structured around three voltage loss mechanisms: activation, ohmic, and concentration overpotentials (Fig. 3).

### 3.2. Equivalent electrical model and dynamic behavior

As proposed by Guilbert et al. [101], the PEM cell’s dynamic behavior can be modeled via an equivalent circuit (Fig. 4) comprising.

* Resistances:

* $R_a, R_c$: electrode resistances

* $R_{mem}$: membrane resistance

* Capacitances:

* $C_a, C_c$: double-layer capacitances at electrode/electrolyte interfaces

The voltage $v_{ini}$ denotes the initial voltage applied across the cell (e.
<figure><img src="image_5.png" /><figcaption>Fig. 3. General principle of operation of PEM electrolysis.</figcaption></figure>

g., in a step-response experiment), while $v_e(t)$ is the output cell voltage, and $i_e(t)$ is the current flowing through the system.

The total voltage across the electrolyzer can be decomposed into three main contributions.

1. The reversible voltage $E_{rev}$, which corresponds to the theoretical Nernst potential required to split water under ideal conditions and is a function of temperature, pressure, and gas concentration.

2. The activation overpotentials at the anode and cathode ($v_{act,a}(t)$ and $v_{act,c}(t)$), which are due to the kinetics of the electrochemical reactions.

3. The ohmic losses, caused by the resistances in the electrodes and membrane.

Taking these into account, the total cell voltage can be expressed as [102]:

$$v_e(t) = E_{rev} + v_{act,a}(t) + v_{act,c}(t) + R_{mem} \cdot i_{el}(t) \quad (\text{Eq.4})$$

Where.

* $E_{rev}$ is the reversible cell voltage, typically around 1.23 V at standard conditions,

* $v_{act,c}$ and $v_{act,a}$ are the activation overpotentials at the cathode and anode,

* $R_c$, $R_a$, and $R_{mem}$ are the ohmic resistances at the cathode, anode, and membrane,

* $i_{el}(t)$ is the instantaneous cell current.

This equivalent circuit model accurately simulates the cell's electrical behavior under both steady-state and dynamic conditions. As the system ages, key parameters such as the ohmic resistances $R_a$, $R_c$, and especially $R_{mem}$ increase due to factors like membrane dehydration, contamination, or thinning. Concurrently, the double-layer capacitances $C_a$ and $C_c$ decrease, reflecting the loss of active surface area and interface deterioration. These changes, driven by operational stress and thermal cycling, degrade the cell's efficiency and responsiveness. As shown in Fig. 4, this evolving model is critical not only for performance simulation but also for assessing long-term degradation and guiding predictive maintenance strategies.

### 3.3. Electrical and thermal losses and degradation phenomena in PEM electrolyzers

This dynamic electrical model is essential not only for simulating normal operating performance but also for predicting long-term degradation trajectories and optimizing preventive maintenance strategies. The primary contributors to ohmic resistance include.

* The membrane (e.g., Nafion®), which governs proton conductivity while preventing gas crossover [65,103].

* The catalyst-membrane interfaces, where degradation increases charge transfer resistance [104].

* The current collectors and porous transport layers, which ensure current conduction and uniform gas distribution [48,53,105,106].

Fig. 5 schematically represents these key components, illustrating their respective roles within the PEM electrolyzer structure and highlighting the origins of electrical losses across the system [48].

#### 3.3.1. Overpotentials and thermal contributions

The overall cell voltage deviates from its thermodynamic value due to overpotentials.

* Activation overpotential results from slow reaction kinetics at the electrodes. It increases with catalyst aging (e.g., $IrO_2$ and Pt/C dissolution) [39,73,107].

<figure><img src="image_6.png" /><figcaption>Fig. 4. Equivalent electrical schematic of the PEM electrolyzer.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Structural schematic of PEM electrolyzer components and origins of internal electrical losses [48].</figcaption></figure>

* Concentration overpotential arises from gas accumulation or liquid/ gas two-phase transport limitations at high current densities [53, 105,106,108,109].

* Mass transport limitations are primarily influenced by the properties of the microporous and porous layers (MPLs, PTLs), impacting gas-liquid separation and diffusion paths [110–113].

Thermal losses are intrinsically linked to these overpotentials. Local overheating from activation or resistive losses accelerates membrane thinning, catalyst sintering, and drying initiating a thermo-electrical degradation process.

### 3.3.2. Typology of losses and aging mechanisms

Fig. 7 illustrates the main categories of electrical losses in PEM electrolyzers, which are closely linked to aging mechanisms.

* Ohmic losses arise primarily from membrane resistance, poor interfacial contact, and inadequate electrical pathways in bipolar plates. A key factor is membrane hydration insufficient water content significantly raises internal resistance, especially in the cell core, leading to increased energy losses [114–118][119].

* Activation losses are associated with catalyst limitations. Despite the use of noble metals (e.g., Ir, Pt), prolonged operation at high voltages (>1.5 V) promotes catalyst dissolution, reducing electrochemical activity. Additionally, gas bubble accumulation on active sites further impairs reaction kinetics, as highlighted in Fig. 6 [121–126].

* Mass transport losses result from inefficient two-phase flow management. Poorly designed PTLs and bipolar plates hinder gas evacuation, causing localized flooding or gas blanketing that reduces efficiency. These effects are particularly pronounced in lateral and porous zones, as shown in Fig. 7 [53,105,106,108,109,127,128].

### 3.4. Comparative review of electrolysis technologies and lifetimes

To provide a holistic understanding of the main electrolysis

<figure><img src="image_8.png" /><figcaption>Fig. 6. Cross-sectional view of gas and mass transport limitations in PEM electrolyzer structure.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Typology of electrical losses in a PEM electrolyzer.</figcaption></figure>
technologies, a comparative review is conducted focusing on four major systems: Alkaline, Anion Exchange Membrane (AEM), Proton Exchange Membrane (PEM), and Solid Oxide Electrolysis Cell (SOEC). This comparison encompasses technical, operational, and economic performance metrics based on reference literature [73,129,130].

### 3.4.1. Overview of performance characteristics

A detailed criterion-based comparison is presented in Table 2, highlighting the operating conditions, efficiency, power density, hydrogen production rate, durability, and cost parameters of each technology.

Alkaline electrolysis, the most mature and widely commercialized technology, operates at moderate temperatures (70–90 °C) and pressures (approximately 30 bar), with typical current densities ranging from 0.25 to 0.8 A/cm². It offers relatively low capital expenditures (800–1500 €/kW), extended operational lifetimes (55,000 to 120,000 h), and low degradation rates (0.25–1.5 % per year). However, its dynamic performance is constrained by slow start-up times (1–2 h) and limited flexibility under intermittent operating conditions [73,129,130].

Anion exchange membrane (AEM) electrolysis, still at an early stage of development, provides full load flexibility (0–100 %), rapid cold start capability (5–10 min), and very low capital costs (approximately 200 €/kW). Despite its promising characteristics, the long-term durability of this technology remains uncertain due to limited operational data and field validation [73,129–131].

Proton exchange membrane (PEM) electrolysis combines compact design, high responsiveness, and elevated current densities (1–6 A/cm²), enabling hydrogen production rates of up to 1255 $Nm^3/h$. Its fast start-up time (less than 10 s) makes it particularly well suited for integration with variable renewable energy sources. Nonetheless, PEM systems rely on noble metal catalysts, which significantly increase capital costs (1400–2100 €/kW) and maintenance expenses (3–5 % of capital per year). The associated degradation rates are moderate, typically ranging from 0.5 % to 2.5 % annually [73,129,130].

<caption>**Table 2**</caption>

Comparative performance characteristics of major electrolysis technologies [73, 129–131].

<table><thead><tr><th>Parameters</th><th>Alkaline</th><th>AEM</th><th>PEM</th><th>SOEC</th></tr></thead><tbody><tr><td><strong>Operating conditions</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Temperature [°C]</td><td>70–90</td><td>60–80</td><td>50–80</td><td>700–900</td></tr><tr><td>Pressure [bar]</td><td>30</td><td>35</td><td>50</td><td>15</td></tr><tr><td>Current density [A.cm<sup>-2</sup>]</td><td>0.25–0.8</td><td>0.2–2</td><td>1–6</td><td>0.3–1.0</td></tr><tr><td><strong>Flexibility</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Load flexibility (% of nominal value)</td><td>10–100</td><td>0–100</td><td>0–100</td><td>-100/+100</td></tr><tr><td>Cold start</td><td>1–2 h</td><td>5–10 min</td><td>5–10 min</td><td>heures</td></tr><tr><td>Hot start</td><td>1–5 min</td><td>&lt;10 s</td><td>&lt;10 s</td><td>15 min</td></tr><tr><td><strong>Efficiency</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Stack efficiency (based on LHV)</td><td>63–71 %</td><td>60–83</td><td>60–68 %</td><td>~100 %</td></tr><tr><td>Stack specific energy consumption [kWh.Nm<sup>-3</sup>]</td><td>3.8–5.2</td><td>3.95–5.2</td><td>4.3–5.8</td><td>3</td></tr><tr><td>System efficiency (PSI)</td><td>51–60 %</td><td>&lt;75</td><td>46–60 %</td><td>76–81 %</td></tr><tr><td>System specific energy consumption [kWh.Nm<sup>-3</sup>]</td><td>4.0–5.47</td><td>4.2–5.5</td><td>4.53–6.1</td><td>3.7–3.9</td></tr><tr><td><strong>Power and Production</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Stack power [MW]</td><td>6.5</td><td>2</td><td>5</td><td>&lt;0.01</td></tr><tr><td>Hydrogen production [Nm<sup>3</sup>.h<sup>-1</sup>]</td><td>1400</td><td>462</td><td>1255</td><td>&lt;10</td></tr><tr><td>Cell surface area [m<sup>2</sup>]</td><td>&lt;3.6</td><td>&lt;0.03</td><td>&lt;0.3</td><td>&lt;0.06</td></tr><tr><td><strong>Durability</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Lifetime [kh]</td><td>55–120</td><td>&lt;2</td><td>60–100</td><td>8–20</td></tr><tr><td>Degradation [% per year]</td><td>0.25–1.5</td><td>-</td><td>0.5–2.5</td><td>3–50</td></tr><tr><td><strong>Economic aspects</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Capital cost [€/kW]</td><td>500–800</td><td>200</td><td>1400–2100</td><td>&gt;2000</td></tr><tr><td>Maintenance cost [% of capital cost per year]</td><td>2–3</td><td>-</td><td>3–5</td><td>n.a.</td></tr></tbody></table>

Solid oxide electrolysis cell (SOEC) technology, operating at elevated temperatures (700–900 °C), offers exceptional energy efficiency (approaching 100 %) and low specific energy consumption (~3 kWh/Nm³). Despite its theoretical advantages, SOEC systems are hindered by short operational lifespans (8000 to 20,000 h) and high degradation rates (3–50 % per year), with capital costs frequently exceeding 2000 €/kW [73,129,130,132].

### 3.4.2. Trade-off summary of advantages and limitations

To synthesize the trade-offs between performance, cost, and reliability, a summary matrix of advantages and disadvantages is provided in Table 3.

This table reorganizes the findings from Table 2, offering a quick comparison across dimensions such as flexibility, efficiency, cost, and long-term stability.

### 3.4.3. Visual comparative analysis

To complement the tabulated data, a comparative radar chart was developed based on normalized performance criteria (efficiency, current density, flexibility, cost, durability, and degradation rate). The normalization (0–1 scale) was derived from both quantitative literature values and qualitative insights from 30 peer-reviewed publications.

As seen in Fig. 8: SOEC excels in efficiency and energy consumption, but its high degradation and short lifespan restrict it to niche applications with access to high-temperature waste heat [114]. PEM offers the best dynamic response and high production rates, making it highly suitable for integration with intermittent renewable energy. However, its cost and reliance on rare metals pose economic constraints [62,115].

* AEM appears promising due to its low cost and operational flexibility, but its long-term stability is yet to be fully demonstrated.

* Alkaline systems provide balanced durability and cost-effectiveness but underperform in dynamic contexts due to limited flexibility and slow startup [73,129,130].

Recent studies highlight the critical need to associate degradation rates with clearly defined experimental conditions including temperature, pressure, electrolyte composition, current density, and operational modes in order to ensure meaningful comparisons between technologies. Table 4, below compiles the most representative and recent degradation data from peer-reviewed literature, detailing the associated test conditions for each electrolyzer type.

<caption>**Table 3**</caption>

Advantages and disadvantages of major electrolysis technologies.

<table><thead><tr><th>Technology</th><th>Advantages</th><th>Disadvantages</th></tr></thead><tbody><tr><td>Alkaline</td><td><ul><li>Mature and proven technology</li><li>Low capital cost (800–1500 €/kW)</li><li>Long lifetime (55–120 kh)</li><li>Low degradation rate (0.25–1.5 %/year)</li></ul></td><td><ul><li>Low current density (0.25–0.8 A/cm²)</li><li>Moderate flexibility (10–100 %)</li><li>Slow start-up time (1–2 h)</li></ul></td></tr><tr><td>AEM</td><td><ul><li>Very low capital cost (~200 €/kW)</li><li>Full flexibility (0–100 %)</li><li>Fast cold start (5–10 min)</li></ul></td><td><ul><li>Durability still uncertain</li><li>Moderate performance</li><li>Technology under development</li></ul></td></tr><tr><td>PEM</td><td><ul><li>High current density (1–6 A/cm²)</li><li>High hydrogen production rate (up to 1255 $Nm^3/h$)</li><li>Excellent flexibility (0–100 %)</li><li>Rapid cold and hot start (<10 s)</li></ul></td><td><ul><li>High capital cost (1400–2100 €/kW)</li><li>Moderate degradation rate (0.5–2.5 %/year)</li><li>Dependence on noble metals (Iridium, Platinum)</li></ul></td></tr><tr><td>SOEC</td><td><ul><li>Exceptional efficiency (~100 %)</li><li>Very low specific energy consumption (~3 kWh/Nm³)</li><li>Suitable for integration with high temperature heat sources</li></ul></td><td><ul><li>Limited lifetime (8–20 kh)</li><li>High degradation rate (3–50 %/year)</li><li>Low flexibility</li><li>Very high capital cost (>2000 €/kW)</li></ul></td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Comparative radar chart Illustrating the strengths and weaknesses of Alkaline, AEM, PEM, and SOEC electrolysis technologies across six key performance indicators.</figcaption></figure>

Among all technologies, PEM electrolysis stands out for its unique combination of high responsiveness, compact design, and relatively moderate degradation rates under dynamic operating conditions. Unlike AEM systems, which lack long-term stability, and SOECs, which suffer from severe thermal degradation, PEM electrolyzers offer a balanced trade-off between performance, durability, and adaptability particularly well-suited for integration with intermittent renewable energy sources.

## 4. Current-induced electrical degradation

PEM electrolyzers are prone to multiple electrical degradation phenomena that directly affect their long-term durability and efficiency. Among the most impactful are those triggered by high current densities and electrochemical interfacial resistances, which lead to increased

<caption>**Table 4**</caption>

Reported degradation rates in electrolyzer technologies with experimental conditions.

<table><thead><tr><th>Electrolyzer Type/Study</th><th>Degradation Rate</th><th>Experimental Conditions</th><th>Ref.</th></tr></thead><tbody><tr><td>Alkaline</td><td>0.25–1.5 %/year</td><td>70–90 °C; ~30 bar; KOH electrolyte; 0.25–0.8 A/cm²</td><td>[73,129,130, 133]</td></tr><tr><td>AEM</td><td>Not stabilized yet</td><td>Room temperature; deionized water; 0–100 % load; Fumasep membrane</td><td>[73,129–131, 134]</td></tr><tr><td>PEM</td><td>0.5–2.5 %/year<br>22.7 µV/h at 1 A cm⁻²</td><td>1–6 A/cm²; 50–80 °C; Nafion; deionized water<br>Same as above</td><td>[73,129–131, 135]</td></tr><tr><td>Voltage drift: 20–50 µV/h</td><td>Room temperature; 2.5 A/cm²; Nafion 112; 12 h cycling</td><td>[73,92, 129–131]</td></tr><tr><td>TiOx:Ti ratio × 6 after 4000 h</td><td>Nafion 115; 3 A/cm²; 80 °C; 100 % RH</td><td>[47,73, 129–131, 136]</td></tr><tr><td>Nominal voltage rise over 1000 h</td><td>2.5 A/cm²; 70 °C; solar-mode Cycling; Nafion membrane</td><td>[47,73, 129–131]</td></tr><tr><td>Pt dissolution, catalyst delamination</td><td>60 °C; Nafion 212; 30 s ON/OFF cycles; 3 A/cm²</td><td>[47,48,73, 120, 129–131]</td></tr><tr><td>50.0 µV/h at 3 A cm⁻²</td><td>Nafion 117; 80 °C; 100 % RH</td><td>[73,92,95, 129–131]</td></tr><tr><td>3–50 %/year</td><td>700–900 °C; atmospheric pressure; O²⁻ ceramic electrolyte</td><td>[138–140]</td></tr></tbody></table>

ohmic losses and gradual performance decay. These mechanisms are particularly exacerbated under dynamic or harsh operational conditions. For instance, titanium oxide formation within porous transport layers (PTLs) at elevated current densities has been shown to significantly increase the high-frequency resistance (HFR), with Su et al. [92] reporting a drift of 50.0 µV/h at 3 A cm⁻², compared to 22.7 µV/h at 1 A cm⁻². Similarly, Prestat et al. [137] observed a sixfold increase in TiOx: Ti surface ratio after 4000 h of operation, indicating severe electrode oxidation. These findings highlight the intensification of ohmic losses under aggressive current regimes. In parallel, transient operation modes including frequent ON/OFF cycles, voltage fluctuations, and accidental electrical overstress promote degradation mechanisms such as platinum dissolution, interfacial corrosion, thermal hotspots, and loss of catalytic activity, all contributing to premature component failure and reduced efficiency [47,48,120,141–143].

To consolidate these findings, Table 5 summarizes the key degradation sources, their associated experimental evidence, and the quantitative impacts when available. The table includes methodologies such as EIS, thermal modeling, post-mortem analyses, and voltage monitoring used across multiple studies (see Table 6).

This section builds on a targeted review of 25 scientific articles focused on electrical degradation in PEM electrolyzers. A strong consensus highlights current density, ON/OFF cycling, and interfacial oxidation as the key drivers of performance decline trends increasingly emphasized in recent literature due to the growing integration of intermittent renewable energy. Fig. 9 summarizes these findings through a normalized distribution, reflecting the frequency and depth with which each degradation mechanism is discussed across the selected studies. All articles were equally weighted, and percentages represent the relative prominence of each mechanism within the literature.

The figure illustrates that the most frequently emphasized mechanisms are the effects of current density on ohmic resistance and the degradation of electrical interfaces and current collectors, each accounting for 27.3 % of the reported content. These are followed by the impact of dynamic cycling (ON/OFF or power fluctuations) at 18.2 %, which reflects the increasing concern for RES-powered systems. Joule heating, polarization curve drift, and electrical overstress due to accidental overloads each represent 9.1 % of the degradation mechanisms discussed.

Fig. 9 provides a normalized and structured overview of how electrical degradation sources are characterized in the literature, not based on citation count, but on the way, they are discussed and weighted by the authors in their respective studies.

### 4.1. Impact of current density on ohmic resistance

Studies have shown that increasing current density accelerates MEA degradation, particularly by increasing HFR [92,137]. EIS measurements reveal that ohmic losses dominate under high current conditions, with Su et al. [92] reporting up to 50.0 µV/h at 3 A cm⁻². Prestat et al. [137] also confirmed titanium PTL oxidation, with the TiOx:Ti ratio increasing by a factor of 6 after 4000 h. Further research [32,47,48,120, 142,143] indicated that oxide growth, corrosion, and membrane aging amplify these effects.

### 4.2. Joule losses under elevated current

Joule heating results in localized temperature rises that deteriorate membrane and catalyst layer integrity [120,142]. Noor Azam [142] modeled this phenomenon using showing that temperature increases accelerate structural degradation and reduce performance.

$$P = I^2 \cdot R \qquad (\text{Eq.5})$$

Where *P* denotes the dissipated power (W), *I* the electric current (A), and *R* the electrical resistance (Ω).
<table><caption>**Table 5**
Summary of current-induced electrical degradation in PEM electrolyzers.</caption><thead><tr><th>Degradation Source</th><th>Authors (Ref.)</th><th>Observed Effect</th><th>Quantitative Value</th><th>Experimental Conditions</th><th>Methodology</th></tr></thead><tbody><tr><td>High current density and ohmic resistance</td><td>Su et al. [92]</td><td>Increase in HFR due to TiOx formation</td><td>50.0 μV/h at 3 A/cm² 22.7 μV/h at 1 A/cm²</td><td>80 °C, 2 bar, Nafion 117 membrane</td><td>Electrochemical Impedance Spectroscopy (EIS)</td></tr><tr><td>Oxidation of titanium transport layer</td><td>Prestat et al. [137]</td><td>TiOx/Ti ratio increased sixfold after 4000 h</td><td>Ratio increased by a factor of 6</td><td>80 °C, 1.5 A/cm², 2 bar, Nafion membrane</td><td>Post-mortem analysis, EIS</td></tr><tr><td>Interfacial corrosion and membrane aging</td><td>Fouda-Onana et al. [120], Rakousky et al. [143]</td><td>Membrane degradation and titanium corrosion</td><td>Not quantified</td><td>80 °C, 1.5–2.5 A/cm², 1.5 bar</td><td>SEM, EIS</td></tr><tr><td>Joule effect under high current</td><td>Noor Azam et al. [142]</td><td>Local overheating and resistance increase due to TiOx</td><td>P = I²·R</td><td>85 °C, 3 A/cm², 2 bar, Nafion membrane</td><td>Thermal modeling, EIS</td></tr><tr><td>Polarization curve drift</td><td>Fouda-Onana et al. [120]</td><td>Progressive voltage rise, loss of catalytic activity</td><td>Approx. 50 μV/h</td><td>80 °C, 2 A/cm², 2 bar</td><td>Polarization curve analysis, EIS</td></tr><tr><td>Dynamic ON/OFF cycling</td><td>Kuhnert et al. [47,48]</td><td>Pt dissolution, catalyst layer detachment, membrane cracking</td><td>Residual current improves durability</td><td>25–80 °C, 0–3 A/cm², 0–2 bar</td><td>ON/OFF cycling tests, EIS</td></tr><tr><td>Initial voltage drift at startup</td><td>Barbir et al. [136]</td><td>Gradual stabilization after 1000–2000 h of operation</td><td>20–50 μV/h</td><td>80 °C, 2 bar, Nafion membrane, 1.8 A/cm²</td><td>Real-time voltage monitoring</td></tr><tr><td>Current collector degradation</td><td>Noor Azam et al. [142], Kuhnert et al. [47]</td><td>Titanium passivation, increase in contact resistance</td><td>Not quantified</td><td>80 °C, 10 kHz ripple, 3 A/cm²</td><td>EIS, contact resistance measurements, SEM</td></tr><tr><td>Electrical overstress and ripple currents</td><td>Parache et al. [141]</td><td>Increase in overvoltage and HFR under ripple currents; titanium mesh degradation; mass transport losses</td><td>96.6 μV/h (10 kHz triangular), 34.6 μV/h (1 kHz triangular), 12.6 μV/h (300 Hz sinusoidal), 19.7 μV/h (DC ref); HFR slope: 10.8, 4.6, 3.9, 5.0 μΩ/h respectively</td><td>25 °C, 1 bar, 2 A ± 10 %, Nafion membrane, 0.125 A/cm², 3000 h test (E1–E4); ripple from converter or power supply</td><td>Long-term aging test, Polarization curve, EIS, Nyquist analysis</td></tr></tbody></table>

<table><caption>**Table 6**
Key Electrical degradation mechanisms in PEM Electrolyzers: Effects, mitigation strategies, and visualization.</caption><thead><tr><th>Phenomenon</th><th>Effects</th><th>Solutions</th><th>Suggested Placement</th><th>Visual Aid</th></tr></thead><tbody><tr><td>ON/OFF Cycling</td><td>Pt dissolution, catalyst delamination, membrane cracking</td><td>Maintain residual current, use batteries</td><td>Below Polarization Curve</td><td>Circular symbol, delaminated layers</td></tr><tr><td>Current Collector Degradation</td><td>T<sub>i</sub> passivation (T<sub>i</sub>O<sub>x</sub> formation), 10 kHz ripples, high contact resistance</td><td>Protective coatings, optimized interfaces</td><td>Drift</td><td>T<sub>i</sub>O<sub>x</sub> symbol, ripple waveform</td></tr><tr><td>Electrical Surges/Overstress</td><td>Thermal fatigue, increased gas permeability, catalyst degradation</td><td>Fast breakers, dynamic control, early fault detection</td><td>Enhance Electrical Overstress block</td><td>Lightning symbol, damage icon</td></tr><tr><td>Thermal Effects (Joule Heating)</td><td>Local heating, membrane and catalyst aging</td><td>Passive or active cooling if needed</td><td>Near Local Heating block</td><td>Heat symbol, flame, equation: P = I²R</td></tr><tr><td>Performance Loss/ Voltage Drift</td><td>Gradual voltage rise, reduced catalytic activity</td><td>Catalyst regeneration, optional replacement</td><td>Side panel with polarization curve</td><td>Voltage curve graph (U vs I)</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Normalized distribution of electrical degradation sources.</figcaption></figure>

### 4.3. Polarization curve drift

A gradual increase in cell voltage during operation reflects component aging and loss of catalytic activity, with ohmic losses becoming dominant at higher current densities [120].

### 4.4. Dynamic cycling effects (ON/OFF and rapid fluctuations)

Frequent ON/OFF cycling accelerates degradation, causing platinum dissolution, catalyst layer delamination, and membrane damage [47,48, 120]. Strategies such as maintaining a residual current or incorporating batteries have been shown to mitigate these effects. Barbir et al. [136]
observed voltage stabilization after several thousand hours.

## 4.5. Interface and current collector degradation

Titanium current collectors undergo electrochemical passivation, increasing contact resistance and disrupting current distribution [47,48, 92,120,137,141,144] Current ripple (up to 10 kHz) further aggravates degradation. Protective coatings and optimized interfaces are recommended [67,92,137,143–145].

## 4.6. Electrical overstress (accidental surges)

Electrical surges can cause irreversible damage, including catalyst dissolution, thermal fatigue, increased gas permeability, and mechanical degradation of the membrane [47,48,120,141,142,144,145]. Preventative solutions include fast circuit breakers, dynamic control systems, and early fault detection [120,134,136,137,142]. Table 5 below summarizes the main electrical degradation phenomena affecting PEM electrolyzers, detailing their effects, possible mitigation strategies, suggested diagram placement, and associated visual representations.

# 5. Temperature-induced electrical degradation

Thermal degradation in PEM electrolyzers primarily results from elevated operating temperatures and high current densities, which progressively compromise both the electrical performance and mechanical integrity of the cells. Key contributing factors include the formation of localized hot spots, increased interfacial contact resistance, and repetitive thermal cycling. Despite considerable efforts to optimize operating parameters, achieving reliable thermal durability remains a significant challenge for long-term, stable operation [38,117,146–159]. A comprehensive overview of the principal sources and impacts of temperature-induced electrical degradation is presented in Table 7. This table synthesizes major findings from the literature, detailing quantitative effects, applied methodologies, and critical thresholds related to thermal phenomena such as variations in ionic conductivity, shifts in overpotential, reductions in membrane lifespan, and mechanical weakening.

Fig. 10 illustrates a normalized distribution of thermal degradation sources in PEM electrolyzers, derived from a targeted analysis of 24 peer-reviewed studies summarized in Table 7. Rather than relying on

citation count or journal metrics, this distribution is based on a qualitative evaluation of how prominently and frequently each degradation mechanism was discussed within the selected literature. Each study was given equal consideration, and the percentage values represent the relative emphasis placed on each phenomenon, thus offering a balanced overview of the scientific attention devoted to thermal degradation pathways (see Fig. 11).

The analysis identifies four predominant thermal degradation mechanisms in the recent literature, each contributing equally (20 %) to the overall distribution: (i) the effect of temperature on overall ionic conductivity, (ii) localized overheating and hotspot formation, (iii) repeated thermal cycling, and (iv) hydrogen bubble accumulation near the electrodes or within the flow channels. These phenomena are consistently recognized as critical to the long-term performance of PEM electrolyzers due to their adverse impacts on temperature uniformity, local ohmic resistance, and mechanical integrity.

Two additional mechanisms namely, temperature-current interaction and degradation of electrical connections are less frequently emphasized, each representing 10 % of the reported cases. Despite their lower occurrence, these mechanisms remain significant under transient operating conditions, such as frequent ON/OFF cycling or dynamically varying current loads, where coupled thermal and electrical stresses may accelerate system failure.

This distribution, derived from the analysis of 24 peer-reviewed articles summarized in Table 7, reflects the relative level of scientific attention devoted to each degradation pathway. It highlights the need for improved thermal management strategies, including the reduction of thermal gradients, optimization of heat transfer pathways, and the implementation of multiphysics simulations to mitigate degradation under fluctuating conditions. Ultimately, these findings support a more targeted research agenda by distinguishing between the most impactful and the relatively underexplored thermal degradation mechanisms in PEM electrolyzer systems.

## 5.1. Effects of temperature on overall electrical conductivity

An increase in temperature initially enhances ionic conductivity, as demonstrated by Nafion, whose conductivity increases from 0.191 S cm⁻¹ to 0.211 S cm⁻¹ between 75 °C and 85 °C [148,149]. However, at temperatures exceeding 100 °C, electrolyte evaporation and chemical degradation begin to limit both performance and durability. Operating

<table><caption>**Table 7**
Summary of temperature-related electrical degradation in PEM electrolyzers.</caption><thead><tr><th>Source of Thermal Degradation</th><th>Author(s)</th><th>Observations and Results</th><th>Quantitative Impact (Yes/No)</th><th>Key Quantitative Value</th><th>Methodology</th></tr></thead><tbody><tr><td rowspan="2">Effect of temperature on overall electrical conductivity</td><td>Choi et al. [148,149]</td><td>Increase in Nafion ionic conductivity from 0.191 S cm<sup>-1</sup> to 0.211 S cm<sup>-1</sup> between 75 and 85 °C.</td><td>Yes</td><td>Ionic conductivity: 0.191–0.211 S cm<sup>-1</sup></td><td>Conductivity testing, temperature measurements</td></tr><tr><td>Kai et al. [150]</td><td>Reduction in overpotential from 1.57 V to 1.51 V at 120 °C under pressure (0.22 MPa).</td><td>Yes</td><td>Overpotential reduction: 1.57 to 1.51 V</td><td>Pressurized testing, overpotential analysis</td></tr><tr><td rowspan="2">Localized overheating and hot spots</td><td>Chandesris et al. [117]</td><td>Membrane lifetime decreased from 35,000 to 8700 h when temperature increased from 333 K to 353 K.</td><td>Yes</td><td>Lifetime: 35,000 to 8700 h</td><td>Thermal durability analysis</td></tr><tr><td>Albert et al. [160]</td><td>Hydrogen bubble accumulation at high temperature increases local resistance.</td><td>Yes</td><td>Increased resistance due to bubbles</td><td>Thermal stress testing</td></tr><tr><td>Weakening of electrical connections</td><td>Yoshitake et al. [152]</td><td>Reduction in mechanical strength of Nafion at elevated temperatures.</td><td>Yes</td><td>Decreased tensile strength</td><td>Mechanical strength testing, thermal analysis</td></tr><tr><td>Temperature-current synergy</td><td>Inaba et al. [79]</td><td>Increase in hydrogen crossover current between 60 and 80 °C.</td><td>Yes</td><td>Increased crossover current at higher temperatures</td><td>Thermal simulation, high current testing</td></tr><tr><td>Effect of repeated thermal cycling</td><td>Yoshitake et al. [152], Chandesris et al. [117],</td><td>Microcracks and membrane embrittlement due to temperature fluctuations.</td><td>Yes</td><td>Microcracks and mechanical instability</td><td>Thermal fatigue testing</td></tr><tr><td>Impact of hydrogen bubbles</td><td>Albert et al. [160], Chandesris et al. [117],</td><td>Local increase in electrical resistance caused by bubble accumulation in pores.</td><td>Yes</td><td>Increased local resistance</td><td>SEM imaging, pore analysis</td></tr></tbody></table>
<figure><img src="image_12.png" /><figcaption>Fig. 10. Normalized distribution of thermal degradation sources in PEM electrolyzers.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11. Influence of temperature on the performance and degradation of a PEM electrolyzer.</figcaption></figure>

under pressurized conditions helps maintain electrolyte saturation and reduces overpotential, lowering it from 1.57 V to 1.51 V [117,119,150, 160–162].

Nonetheless, thermal degradation of the membrane particularly through thinning significantly shortens its operational lifespan, reducing it from 35,000 h to 8700 h as the temperature increases from 333 K to 353 K [117].

For optimal performance, PEM cells should operate within a temperature window of 60–80 °C. Beyond this range, the formation of free radicals (e.g., $H_2O_2$) and the loss of sulfonate groups lead to degradation of the polymer backbone, ultimately reducing proton conductivity [117, 119,160–162].

## 5.2. Localized overheating: formation of hot spots and thermal gradients

Hot spots induce thermal gradients that cause local thinning of the membrane, which increases the risk of gas crossover ($H_2$ and $O_2$), potentially compromising electrolyzer integrity and accelerating chemical degradation [62,79,117,119,161–163]. In these zones, hydrogen bubbles may also form, disrupting current distribution [79, 117].

It has been observed that membrane lifetime is significantly reduced at elevated temperatures. The formation of localized hot spots within the cell exacerbates temperature gradients, increases mechanical stresses, and promotes membrane degradation, ultimately reducing durability [117].

## 5.3. Weakening of electrical connections: increased contact resistance

At elevated temperatures, Nafion exhibits reduced mechanical strength, which increases contact resistance (see Fig. 12). This rise in

resistance generates additional ohmic losses and accelerates overall degradation [152,155,156]. Furthermore, oxidation of conductive materials (e.g., $IrO_2$ and $Pt$) further increases resistance and diminishes efficiency [117,119,161]. This higher resistance induces greater heat generation, thereby initiating a self-reinforcing degradation cycle [53, 164], as illustrated in Fig. 13, which shows the mechanisms of electrical weakening and increased contact resistance.

## 5.4. Synergy between elevated temperature and current density: accelerated electrical degradation

High temperatures combined with high current densities intensify

<figure><img src="image_14.png" /><figcaption>Fig. 12. Localized overheating in a PEM electrolyzer: hot spot formation and thermal effects.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 13. Figure Electrical weakening and increased contact resistance.</figcaption></figure>

both thermal and chemical degradation. Inaba et al. [79] reported an increase in hydrogen crossover current between 60 and 80 °C [153,154]. Furthermore, the formation rate of free radicals significantly rises between 55 and 150 °C [161], exacerbating ohmic losses and electrode degradation [117,119,161]. In addition, wear of the gas diffusion layers (GDL) impairs H₂ evacuation, creating electrically isolated regions [78, 162]. Finally, corrosion of stainless steel (316 L) at elevated temperatures releases oxides such as Fe₃O₄ and NiO, which migrate to the cathode and impair performance [137].

### 5.5. Impact of hydrogen bubbles on electrical degradation

Hydrogen bubbles increase local electrical resistance by interrupting ionic conduction pathways. At elevated temperatures, bubble accumulation causes contact disruptions, decreases efficiency, and may lead to catalyst layer delamination [79,151]. The membrane's increased flexibility at higher temperatures exacerbates this effect, raising the risk of efficiency losses and electrochemical failure [152,155].

Albert et al. [160] tested membranes immersed at 90 °C for five days,

observing minimal changes in surface resistance and hydrogen permeability. However, under real operating conditions, trapped hydrogen bubbles increase local resistance and promote degradation. Their accumulation within GDL pores and bipolar plates induces micro-contact losses, reducing active contact area and disrupting current distribution [53,78,79,160]. Over time, this can lead to catalyst layer delamination and membrane erosion [117].

### 5.6. Repeated thermal cycling: thermal fatigue and its impact on electrical properties

Thermal cycles (shutdowns, startups, and load variations) induce fatigue that weakens the membrane and generates microcracks. These cracks reduce electrical conductivity and increase ohmic losses, ultimately shortening the system's service life, as illustrated in Fig. 14 [79, 117,152–155,160,162,164,165] (see Fig. 15).

## 6. Degradation analysis

### 6.1. Observable phenomena and affected components

Degradation mechanisms in PEM electrolyzers can be identified through observable phenomena and the release of specific substances, as illustrated in Fig. 17 [166]. An increase in water drag typically indicates membrane degradation, such as cracking or perforation. Similarly, the detection of fluoride or sulfate anions suggests ionomer breakdown. Metallic ions are generally associated with the corrosion of catalysts, PTLs, or bipolar plates (BPPs), although their accumulation within the membrane complicates quantitative evaluation [166].

Carbon degradation, particularly in the cathode and PTL, is revealed by CO₂ emissions, while increased gas permeability indicates chemical or mechanical deterioration. Rising overpotential or ohmic resistance is less specific and may result from the degradation of multiple components [166]. Ex situ analyses or single-cell studies enable precise localization of damage, including surface changes, porosity variations, deposits, and membrane thinning.

### 6.2. Degradation analysis methods

Fig. 16 provides an overview of analytical techniques employed at various scales, including stack-level, single-cell, and ex situ assessments [166]. During operation, water and gas analysis can detect degradation products such as polymer fragments, CO₂, and metallic ions. Short

<figure><img src="image_16.png" /><figcaption>Fig. 14. Flow diagram of electrically driven degradation phenomena in PEM electrolyzers.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 15. Observable degradation phenomena in PEM electrolyzers: CO₂ emissions, metal ions, gas permeability, fluoride/sulfate anions, increased water drag, and elevated ohmic resistance.</figcaption></figure>

operational interruptions allow for electrochemical measurements, including voltage and impedance. Post-mortem analyses (e.g., imaging, cross-sectioning, and EDS) offer in-depth characterization of internal structures.

Fig. 16 also categorizes these methods based on detection timing (in operation, short interruption, ex situ), invasiveness, and applicability (operation, development, research). Monitoring of voltage, EIS, and segmented measurements are particularly valuable for detecting emerging degradation processes and enhancing preventive maintenance strategies [166].

## 6.3. Electrochemical methods

### 6.3.1. I-V curves and voltage monitoring

Polarization curves (I-V) provide insights into performance across different operating regimes. Typically, three distinct regions are observed: activation (low current), ohmic losses (linear region), and mass transport limitations (high current) [90,167]. Deviations or shifts in the polarization curve signal losses in catalytic surface area, increased resistances, or mass transport issues. Complementary analysis using EIS is essential for a detailed interpretation of these changes.

### 6.3.2. Reference electrodes and segmented cells

Incorporating a reference electrode allows measurement of individual half-cell potentials, which is crucial for accurately locating degradation phenomena [168,169]. According to Hinds and Brightman [170], reference electrode configurations generally fall into three categories: edge-mounted (salt bridge), sandwich (dual membranes), and centralized NPL designs [170,171]. Segmented cells are especially useful for mapping local current density and temperature distributions [172–177].

### 6.3.3. Electrochemical impedance spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) plays a pivotal role in separating ohmic losses, associated with HFR, from polarization losses, which dominate at lower frequencies (LFR), as typically visualized in Nyquist plots [178]. To obtain a comprehensive characterization of the electrochemical behavior, a broad frequency spectrum ranging from 0.01 Hz to 1 MHz is generally recommended [88]. Analytical interpretation of EIS data can be conducted using equivalent R-C circuit models, reference electrodes, or more advanced methods such as the Distribution of Relaxation Times (DRT) technique [179,180]. Moreover, the Kraemers-Kronig relations are commonly employed to ensure the internal consistency and physical validity of the impedance spectra [181].

<figure><img src="image_18.png" /><figcaption>Fig. 16. Schematic representation of degradation analysis methods in PEM electrolyzers, categorized by scale (in operation, brief interruption, ex situ) and selection criteria.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 17. Schematic of real-time monitoring of a PEM electrolyzer using EIS.</figcaption></figure>

Fig. 17 illustrates a schematic setup for real-time monitoring of a PEM electrolyzer using EIS, highlighting the integration of impedance measurement tools within the system and the typical acquisition workflow used to track degradation phenomena in operando.

### 6.3.4. Voltammetry

Cyclic and linear voltammetry techniques are widely used to assess catalytic activity and the electrochemically active surface area (ECSA) [88,167,182]. For platinum, the hydrogen adsorption charge is typically about 210 µC cm⁻² [167,183,184], while for iridium, it is approximately 596 µC cm⁻² [185]. Depending on crystallographic orientation, other studies have reported ECSA values ranging from 90 to 130 µC cm⁻² [186]. Additionally, EIS can be employed to evaluate double-layer capacitance in the absence of faradaic processes [187].

### 6.4. Mathematical modeling and analysis

In order to ensure scientific rigor, reproducibility, and traceability, all mathematical formulations incorporated into the dynamic multi-physical model of the PEM electrolyzer were meticulously selected from validated models in the scientific literature. These equations are designed not only to represent the electrochemical and thermal processes, but also to capture the various degradation mechanisms observed in real operating conditions. The following key phenomena are considered in the model.

* Electrochemical losses (activation, ohmic, and diffusion overpotentials),

* Degradation mechanisms (loss of electrochemical surface area, catalyst dissolution, membrane thinning, titanium oxide formation),

* Thermal effects (localized heating, thermal cycling, and heat dissipation),

* Gas dynamics (hydrogen and oxygen production and transport, bubble formation and blockage),

* Dynamic cycling (ON/OFF) is representative of coupling with intermittent RES,

* Hydrogen storage and pressure variation are linked to operational and degradation constraints.

Each mathematical expression is introduced and justified based on its ability to accurately reproduce the physical phenomena and capture the degradation trends reported in experimental studies [188]. Table 7 below presents a synthesis of the main references from the literature that support the mathematical equations used in this dynamic model.

## 7. Strategies for mitigating electrical degradation and diagnostic tools

### 7.1. Summary of strategies

This section provides a comprehensive synthesis of the main strategies reported in the literature for mitigating electrical degradation in PEM electrolyzers. The analysis is based on 24 scientific studies (as listed in Table 8), with a focus on how each strategy is addressed whether through detailed discussion, experimental validation, or advanced modeling rather than merely on citation frequency. The percentages shown in Fig. 16 reflect the depth, rigor, and level of implementation with which these strategies are presented in the literature, offering a qualitative and quantitative representation of their effectiveness and applicability.

Dynamic electrical control and advanced thermal management emerge as the most promising approaches, as they are extensively developed and supported by experimental results, with demonstrated benefits in terms of system stability and durability. In contrast, strategies such as multi-physics modeling and anti-corrosion coatings, while offering significant theoretical potential, are often reported in a more conceptual or exploratory manner, with practical limitations such as high complexity, cost, or computational requirements. Table 9 summarizes the comparative evaluation of these approaches, while Fig. 18 illustrates their relative effectiveness and associated constraints as reported in the scientific literature (see Fig. 19).

Fig. 18 provides a comparative visual assessment of the main strategies reported in the literature for mitigating electrical degradation in PEM electrolyzers. Each strategy is evaluated based on the relative proportion of reported advantages and disadvantages, as identified through a detailed review of 24 scientific studies. This qualitative-quantitative assessment reflects how these strategies are addressed in the literature not only in terms of their theoretical principles but also
<table><caption>**Table 8**
References associated with the mathematical models and equations used.</caption><thead><tr><th>Domain/Block</th><th>Equation(s)</th><th>Physical Phenomenon modeled</th><th>Scientific Ref.</th></tr></thead><tbody><tr><td>Anode/Cathode - Activation overpotential (RC Equivalent Circuit)</td><td>V<sub>act</sub>(t) = R(t)I(t) + 1/C(t) ∫ I(t)dt</td><td>Electrochemical kinetics with double-layer capacitance representation</td><td>[189–193]</td></tr><tr><td>Membrane - Ionic resistance and conductivity</td><td>V<sub>ohm</sub>(t) = R<sub>ion</sub>(t)I(t); R<sub>ion</sub>(t) = φ/Aσ(t)</td><td>Ohm's law + ionic conductivity dependency on temperature and humidity</td><td>[108,161,189,194, 195]</td></tr><tr><td>Thermal block - Energy balance model</td><td>C<sub>th</sub> dT(t)/dt = Q<sub>gen</sub>(t) - Q<sub>loss</sub>(t)</td><td>Conservation of thermal energy</td><td>[160,163] [128, 194,196,197]</td></tr><tr><td>Gas bubbles - Blocked surface area and efficiency loss</td><td>α<sub>bubble</sub>(t) = α<sub>0</sub> + k<sub>bubble</sub>(1 - e<sup>-rt</sup>); I<sub>eff</sub>(t) = I(t)(1 - α<sub>bubble</sub>(t))</td><td>Modeling gas bubbles formation and impact on active area</td><td>[48,145,198]</td></tr><tr><td>Hydrogen Storage - Accumulation and losses</td><td>dn<sub>H₂</sub>(t)/dt = ṁ<sub>H₂,gen</sub>(t) - ṁ<sub>H₂,one</sub>(t) - k<sub>leak</sub>(t)n<sub>H₂</sub></td><td>Ideal gas model and leakage losses</td><td>[199–202]</td></tr><tr><td>ON/OFF Dynamic cycles - Cyclic degradation</td><td>k<sub>eff</sub>(t) = k<sub>normal</sub> + k<sub>stress</sub> Cycle(t)</td><td>Empirical model for dynamic and intermittent operations</td><td>[48,136,203]</td></tr><tr><td>EIS - Complex impedance for diagnostics</td><td>Z(f) = R<sub>s</sub> + 1/(j2πfC<sub>dl</sub>) + R<sub>ct</sub></td><td>Randles circuit model for electrochemical impedance spectroscopy (EIS)</td><td>[49,163,204]</td></tr><tr><td>H₂ Production - Faraday law + faradaic efficiency</td><td>ṁ<sub>H₂</sub> = ε(t)I(t)/2F</td><td>Faraday's law combined with efficiency drop due to degradation</td><td>[103,163]</td></tr></tbody></table>

<table><caption>**Table 9**
Overview and evaluation of the main strategies for mitigating electrical degradation in PEM electrolyzers.</caption><thead><tr><th>Strategy</th><th>Principle</th><th>Advantages</th><th>Drawbacks</th><th>Ref.</th></tr></thead><tbody><tr><td>Dynamic Electrical Control</td><td>Regulation of current density to minimize overpotentials</td><td>Improves electrical stability</td><td>Complex control required under dynamic operation</td><td>[52,120,136, 137]</td></tr><tr><td>Advanced Thermal Management</td><td>Optimized cooling to reduce thermal gradients</td><td>Mitigates overheating and thermal failures</td><td>Requires advanced cooling systems</td><td>[48,50,51,112, 205]</td></tr><tr><td>Optimization of Interfaces and Connections</td><td>Use of corrosion-resistant materials and anti-oxidation coatings</td><td>Reduces contact resistance</td><td>High material costs</td><td>[48,120,127, 137]</td></tr><tr><td>Real-Time Monitoring and Diagnostics</td><td>Continuous monitoring of internal resistance combined with EIS analysis</td><td>Early detection of degradation phenomena</td><td>High cost of monitoring equipment</td><td>[32,51,92,136, 137,141]</td></tr><tr><td>EIS-Based Predictive Algorithms</td><td>Lifetime modeling of components based on EIS data</td><td>Enables proactive maintenance and increased reliability</td><td>Relies heavily on data quality</td><td>[48,136]</td></tr><tr><td>Ripple Current Control</td><td>Frequency adjustment to limit ohmic losses</td><td>Reduces electrode degradation</td><td>Challenging to control variable frequencies</td><td>[141,206,207]</td></tr><tr><td>Anti-Corrosion Coatings</td><td>Protect current collectors from oxidation</td><td>Enhances electrical durability</td><td>Complex application process</td><td>[204,207–209]</td></tr><tr><td>Integrated Multi-Physics Modeling</td><td>Coupled thermal-electrical simulation</td><td>Improves understanding of complex interactions</td><td>Requires high computational power</td><td>[204,206,208, 210]</td></tr></tbody></table>

<figure><img src="image_20.png" /><figcaption>**Fig. 18.** Comparative analysis of degradation mitigation strategies for PEM electrolyzers.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Fig. 19. Heatmap of degradation mitigation strategies in PEM electrolyzers: Summary of comparative scores.</figcaption></figure>

regarding their reported performance, feasibility, and implementation challenges.

Dynamic electrical control (80 % advantage) and advanced thermal management (70 % advantage) appear as the most effective and well-supported strategies, with high practical relevance and experimental validation. In contrast, integrated multi-physics models and anti-corrosion coatings, although promising, are associated with a higher proportion of disadvantages (60 % and 55 %, respectively), primarily due to complexity, cost, and technical limitations. Intermediate strategies, such as ripple current control and real-time diagnostics, offer a more balanced profile, highlighting their complementary role in predictive maintenance and system optimization.

## 7.2. Optimization of dynamic electrical control

Dynamic control of current density plays a critical role in stabilizing overvoltages and enhancing overall system durability [52,120,136, 137]. High-frequency ripple currents (10 kHz) have been shown to degrade titanium mesh structures [141,206,211], whereas low-frequency sinusoidal ripple (300 Hz) has minimal impact [52,63, 206,212–215]. Active converter control enables frequency adjustments to minimize degradation and extend component lifespan.

## 7.3. Advanced thermal management

Optimized cooling systems are essential to prevent localized overheating and ensure long-term stability of system components [27,47,50, 143,207,209]. Elevated operating temperatures are known to accelerate passivation processes on titanium meshes [204,208,210]. Therefore, active thermal management is indispensable, especially under intensive operating conditions.

## 7.4. Optimization of interfaces and electrical connections

Low-resistance bipolar plates effectively reduce ohmic losses [52, 137]. Although corrosion-resistant materials are more costly, they are strongly recommended to maintain interface integrity [48,120,127, 137]. Furthermore, anti-oxidation coatings, as suggested by Parache [141], are critical to preserve electrical conductivity despite thermal cycling stress [207,209,216–219].

## 7.5. Real-time monitoring and diagnostics

Real-time monitoring, based on continuous measurements such as internal resistance tracking and electrochemical impedance

spectroscopy (EIS), is widely recognized as a key strategy for the early detection of degradation phenomena in PEM electrolyzers [31,50,92, 136,137,141,211,220]. EIS is particularly valuable for distinguishing ohmic losses from mass transport and charge transfer limitations [141], and has been employed as a basis for predictive lifetime modeling by authors such as Barbir [136] and Kuhnert [47].

Based on a detailed review of 8 scientific publications focused on diagnostic and monitoring strategies, as well as a broader analysis of 24 peer-reviewed studies covering the full spectrum of degradation mitigation approaches, we developed a comparative framework structured around four key criteria: technical effectiveness, implementation feasibility, relative cost (inverse), and projected impact on system lifetime. Each strategy was normalized to ensure a 100 % total per row, allowing semi-quantitative, balanced comparisons, as visualized in the heatmap presented in Fig. 17, [37,118].

The results highlight Real-Time Monitoring & Diagnostics, Anti-Corrosion Coatings, and Integrated Multi-Physics Models as the strategies offering the highest impact on system lifetime (over 33 %). Dynamic Electrical Control and Advanced Thermal Management show the most balanced performance, combining high effectiveness (around 26–27 %) and strong lifetime impact (over 28 %). Conversely, Interface and Connection Optimization demonstrates strong effectiveness but suffers from higher associated costs, limiting its feasibility. Ripple Current Control, while simple to implement, ranks lower in terms of effectiveness and lifetime contribution [18,27,47,48,50,120,127,137,143,207, 209].

Though slightly lower in direct mitigation impact, EIS-based predictive algorithms are considered essential tools for intelligent system control and preventive maintenance, especially in systems exposed to dynamic loads. Supporting strategies such as Integrated Multi-Physics Models contribute to a deeper understanding of failure mechanisms but can face implementation complexity. Finally, Anti-Corrosion Coatings are effective for protecting critical interfaces and are relatively simple to integrate [37,104].

Overall, the heatmap confirms that no single strategy is sufficient on its own. A comprehensive approach, combining dynamic current regulation, thermal management, interface engineering, and real-time diagnostics is indispensable to ensure the long-term performance, resilience, and stability of PEM electrolyzers, especially under intermittent renewable energy operation.

In parallel, novel catalyst-support combinations and nanostructured materials have been explored in adjacent electrochemical fields. For instance, Adnan et al. [221] recently demonstrated the effectiveness of Ni/ZnO nanocomposites for photocatalytic degradation, highlighting the potential of such engineered interfaces to enhance charge transport
and stability under harsh conditions. While these findings focus on dye treatment, they may inspire similar approaches in PEM electrolyzer catalyst layer design, particularly regarding long-term resistance to degradation under dynamic operation.

## 8. Conclusion

Proton Exchange Membrane (PEM) electrolyzers are increasingly recognized as cornerstone technologies for green hydrogen production, particularly in energy systems powered by intermittent renewable energy sources (RES). Their compact design, rapid dynamic responsiveness, and ability to operate at high current densities make them highly suitable for decentralized and flexible hydrogen generation. However, ensuring long-term durability under fluctuating real-world operating conditions remains a significant technical challenge.

This review has identified three major degradation pathways affecting PEM electrolyzers.

* Electrical degradation, especially under high current densities, leads to a progressive increase in internal ohmic resistance, particularly the High-Frequency Resistance (HFR). For example, under conditions of 3 A cm⁻² at 60 °C and ambient pressure, voltage drift may reach up to 50.0 µV/h more than twice the rate observed at 1 A cm⁻². This phenomenon is primarily attributed to the gradual formation of titanium oxides (TiOx) within the porous transport layers, as evidenced by a sixfold increase in the TiOx/Ti surface ratio after 4000 h of operation.

* Thermal degradation, while temporarily enhancing the conductivity of Nafion® membranes within the 75–85 °C range, becomes detrimental above 100 °C. At elevated temperatures, membrane thinning, gas crossover, and hydrogen bubble accumulation degrade local conductivity and can trigger mechanical delamination at the membrane-electrode interface.

* Transient degradation, induced by dynamic ON/OFF cycling, causes platinum dissolution and catalyst layer detachment, particularly during cold start-ups and low-pressure operations. These processes contribute to voltage drifts of up to 50 µV/h, highlighting the vulnerability of PEM systems to intermittent load profiles typical of RES-driven applications.

In response, several mitigation strategies have been reported. Among the most promising are dynamic electrical control and advanced thermal management, offering relative effectiveness levels of approximately 80 % and 70 %, respectively, under controlled test conditions (typically 60–80 °C, 1.5–3 A cm⁻², and atmospheric or slightly pressurized environments). These approaches are attractive due to their real-time adaptability, relative ease of implementation, and low additional cost.

That said, alternative mitigation pathways are also gaining attention in the literature. For example, the use of novel catalyst supports, protective coatings (e.g., TiN, ceramic-based layers), and reinforced membranes can help increase intrinsic durability against dynamic degradation. Moreover, system-level solutions such as hybrid architectures with buffer storage (e.g., batteries or supercapacitors) may smooth out current fluctuations, thereby reducing cycling stress on the electrolyzer.

Compared to these material or system-based solutions, dynamic electrical control offers the advantage of operational simplicity, fast response time, and direct integration with existing power electronics, especially in renewable-powered configurations. However, it may require careful calibration to avoid unwanted oscillations or efficiency losses.

While this review does not delve into quantitative techno-economic comparisons, we fully acknowledge their importance. Such analyses represent a promising direction for future research, especially to guide the optimal selection of mitigation strategies based on long-term performance, system complexity, and cost trade-offs.

Simultaneously, the implementation of real-time diagnostic techniques, particularly Electrochemical Impedance Spectroscopy (EIS), has proven critical. EIS enables detailed analysis of ohmic, charge-transfer, and mass-transport losses and supports predictive lifetime assessment under varying operational conditions, including temperature fluctuations and dynamic power inputs.

Although simulation tools were not the primary focus of this review, the relevance of emerging approaches such as integrated multiphysics frameworks and hybrid digital twins is strongly emphasized. These methods are increasingly viewed as promising tools for linking physical degradation mechanisms with real-time sensor feedback, enabling adaptive control and health-aware operation.

On the diagnostic front, embedded EIS systems and AI-enhanced fault detection algorithms are gaining momentum, offering new opportunities for continuous monitoring and proactive maintenance, especially in harsh or highly dynamic environments.

Beyond their technical potential, PEM electrolyzers also offer transformative value for broader societal applications. For example, Koestoer et al. [222] demonstrated the use of green ammonia, derived from hydrogen, for the production of sustainable fertilizer tablets in rural agriculture highlighting the relevance of robust, decentralized PEM systems in advancing environmental and socio-economic development.

Moreover, decentralized renewable energy strategies remain particularly relevant in insular and remote regions. A recent SWOT analysis by Cruz-Pérez et al. [223] emphasized the multiple benefits and constraints of deploying hydropower in four archipelagos, pointing to the critical role of local geography and energy governance in ensuring sustainability. These findings reinforce the importance of robust and adaptable technologies such as PEM electrolyzers, especially when integrated into hybrid systems for local energy autonomy.

In conclusion, the long-term sustainability of PEM electrolyzers will depend on the careful integration of durable materials, intelligent control systems, and standardized diagnostic protocols. Future efforts should aim to establish holistic frameworks that bridge degradation physics with system performance and economic indicators, thereby ensuring the deployment of efficient, reliable, and cost-effective green hydrogen solutions aligned with the global energy transition.

<table><caption>List of abbreviations, symbols, and units</caption><thead><tr><th>Acronym/Symbol</th><th>Definition</th><th>Unit</th></tr></thead><tbody><tr><td colspan="3"><strong>Electrical/Electrochemical</strong></td></tr><tr><td>E<sub>rev</sub></td><td>Reversible cell voltage</td><td>V</td></tr><tr><td>I(t)</td><td>Electric current</td><td>A</td></tr><tr><td>I<sub>eff</sub>(t)</td><td>Effective current density</td><td>A/m<sup>2</sup></td></tr><tr><td>j(t)</td><td>Current density</td><td>A/cm<sup>2</sup></td></tr><tr><td>j<sub>0</sub>(t)</td><td>Degraded exchange current density</td><td>A/m<sup>2</sup></td></tr><tr><td>P(t)</td><td>Instantaneous power</td><td>W</td></tr><tr><td>P<sub>Joule</sub></td><td>Joule heating losses</td><td>W</td></tr><tr><td>R<sub>ct</sub></td><td>Charge transfer resistance (EIS)</td><td>Ω</td></tr><tr><td>R<sub>int</sub></td><td>Internal resistance</td><td>Ω</td></tr><tr><td>R<sub>ohm</sub></td><td>Ohmic resistance</td><td>Ω</td></tr><tr><td>R<sub>s</sub></td><td>Series resistance (EIS)</td><td>Ω</td></tr><tr><td>V(t)</td><td>Cell voltage</td><td>V</td></tr><tr><td>V<sub>act</sub></td><td>Activation overpotential</td><td>V</td></tr><tr><td>V<sub>diff</sub></td><td>Diffusion overpotential</td><td>V</td></tr><tr><td>Z(f)</td><td>Complex impedance</td><td>Ω</td></tr><tr><td>Z<sub>re(f)</sub></td><td>Real part of impedance</td><td>Ω</td></tr><tr><td>Z<sub>im(f)</sub></td><td>Imaginary part of impedance</td><td>Ω</td></tr><tr><td colspan="3"><strong>Thermal/Heat Transfer</strong></td></tr><tr><td>C<sub>th</sub></td><td>Heat capacity</td><td>J/K</td></tr><tr><td>h</td><td>Heat transfer coefficient</td><td>W/m<sup>2</sup>-K</td></tr><tr><td>Q<sub>gen</sub>(t)</td><td>Generated heat</td><td>W</td></tr><tr><td>Q<sub>loss</sub>(t)</td><td>Heat losses</td><td>W</td></tr><tr><td>S</td><td>Heat exchange surface area</td><td>m<sup>2</sup></td></tr><tr><td>T(t)</td><td>Cell temperature</td><td>K</td></tr><tr><td>Tamb</td><td>Ambient temperature</td><td>K</td></tr><tr><td>ΔH</td><td>Reaction enthalpy</td><td>J/mol</td></tr></tbody></table>

(continued on next page)
(continued)

<table><thead><tr><th>Acronym/Symbol</th><th>Definition</th><th>Unit</th></tr></thead><tbody><tr><td colspan="3"><strong>Electrochemical/Physicochemical Properties</strong></td></tr><tr><td>α</td><td>Charge transfer coefficient</td><td>–</td></tr><tr><td>β</td><td>Bubble formation constant</td><td>s<sup>-1</sup></td></tr><tr><td>ε(t)</td><td>Degraded Faradaic efficiency</td><td>–</td></tr><tr><td>F</td><td>Faraday constant</td><td>C/mol</td></tr><tr><td>σ(t)</td><td>Ionic conductivity</td><td>S/m</td></tr><tr><td>φ</td><td>Membrane thickness</td><td>m</td></tr><tr><td>λ</td><td>Membrane water content</td><td>–</td></tr><tr><td>D<sub>w</sub></td><td>Water diffusion coefficient</td><td>m<sup>2</sup>/s</td></tr><tr><td>E<sub>a</sub></td><td>Activation energy</td><td>J/mol</td></tr><tr><td colspan="3"><strong>Degradation/Dynamic Operation</strong></td></tr><tr><td>k<sub>deg</sub>(t)</td><td>Dynamic degradation rate</td><td>s<sup>-1</sup></td></tr><tr><td>k<sub>eff</sub>(t)</td><td>Effective degradation rate (cycling)</td><td>s<sup>-1</sup></td></tr><tr><td>k<sub>normal</sub></td><td>Normal degradation rate</td><td>s<sup>-1</sup></td></tr><tr><td>k<sub>stress</sub></td><td>Stress-induced degradation rate</td><td>s<sup>-1</sup></td></tr><tr><td>ω</td><td>ON/OFF cycle frequency</td><td>rad/s</td></tr><tr><td>Cycle(t)</td><td>ON/OFF switching state</td><td>–</td></tr><tr><td>α<sub>bubble</sub>(t)</td><td>Surface blockage by gas bubbles</td><td>–</td></tr><tr><td colspan="3"><strong>Hydrogen/Gas Production</strong></td></tr><tr><td>nH<sub>2</sub></td><td>Molar flow rate of hydrogen</td><td>mol/s</td></tr><tr><td>nH<sub>2</sub></td><td>Stored amount of hydrogen</td><td>mol</td></tr><tr><td>H<sub>2</sub></td><td>Hydrogen</td><td>mol or Nm<sup>3</sup></td></tr><tr><td>O<sub>2</sub></td><td>Oxygen</td><td>mol or Nm<sup>3</sup></td></tr><tr><td colspan="3"><strong>Storage</strong></td></tr><tr><td>P<sub>res</sub></td><td>Reservoir pressure</td><td>Pa</td></tr><tr><td>V<sub>res</sub></td><td>Reservoir volume</td><td>m<sup>3</sup></td></tr><tr><td>k<sub>leak</sub></td><td>Leakage constant</td><td>s<sup>-1</sup></td></tr><tr><td colspan="3"><strong>Materials and Components</strong></td></tr><tr><td>GDL</td><td>Gas Diffusion Layer</td><td>–</td></tr><tr><td>MEA</td><td>Membrane Electrode Assembly</td><td>–</td></tr><tr><td>MPL</td><td>Microporous Layer</td><td>–</td></tr><tr><td>PTL</td><td>Porous Transport Layer</td><td>–</td></tr><tr><td colspan="3"><strong>Diagnostics and Monitoring</strong></td></tr><tr><td>EIS</td><td>Electrochemical Impedance Spectroscopy</td><td>–</td></tr><tr><td>EDS</td><td>Energy Dispersive X-ray Spectroscopy</td><td>–</td></tr><tr><td>SOH</td><td>State of Health</td><td>–</td></tr><tr><td colspan="3"><strong>Economic and Performance</strong></td></tr><tr><td>ROI</td><td>Return on Investment</td><td>%</td></tr><tr><td>CAPEX</td><td>Capital Expenditure</td><td>USD ou %</td></tr><tr><td>OPEX</td><td>Operational Expenditure</td><td>USD/an</td></tr><tr><td>LCOH</td><td>Levelized Cost of Hydrogen</td><td>USD/kg H<sub>2</sub></td></tr><tr><td colspan="3"><strong>General Technologies</strong></td></tr><tr><td>AEM</td><td>Anion Exchange Membrane</td><td>–</td></tr><tr><td>CNG</td><td>Compressed Natural Gas</td><td>–</td></tr><tr><td>PEM</td><td>Proton Exchange Membrane</td><td>–</td></tr></tbody></table>

## Authorship contribution statement

Fahad Maoulida, PhD: Conceptualization, methodology, multi-physics modeling, data analysis and visualization, writing original draft, review and editing. Prof. Damien Guilbert: Supervision, critical review of the manuscript, validation, provision of resources, final approval of the version to be published. Prof. Mamadou Baïlo Camara: Investigation, data validation, critical review, final approval of the version to be published.

## Conflict of interest statement

The authors affirm that there are no financial or personal conflicts of interest that could have influenced the content or outcomes of this study.

## Data availability statement

The data supporting the findings of this study are not publicly available due to confidentiality agreements and restrictions related to the research project.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Fahad MAOULIDA reports financial support was provided by ULHN University. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work was conducted at the GREAH-Laboratory, University of Le Havre Normandie, as part of the research activities of the “Renewable Energy Management and Storage Systems (MERS)” team. The authors gratefully acknowledge the financial support provided by the Norman-die Region and the University of Le Havre Normandie for this research project.

## Data availability

Data will be made available on request.

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