质子交换膜技术深度解析：从分子结构到PEM电解制氢应用

Introduction to Proton Exchange Membranes and Proton Conduction Principles

Proton Exchange Membranes (PEMs) are ion-conductive polymer films characterized by high proton conductivity and excellent physical and chemical stability. Based on their fluorine content, PEMs can be categorized into perfluorinated, partially fluorinated, and non-fluorinated types. Currently, PEM electrolyzers predominantly utilize perfluorosulfonic acid (PFSA) membranes, which are generally thicker than those used in fuel cell applications. The manufacturing process for PFSA membranes is complex and has long been dominated by American and Japanese companies, such as Chemours' Nafion® series, Dow's XUS-B204, Asahi Glass's Flemion®, and Asahi Kasei's Aciplex®-S. The base resin of PFSA membranes, perfluorosulfonic acid resin, is a copolymer formed via free-radical copolymerization of polytetrafluoroethylene (PTFE) and perfluorovinyl ether sulfonic acid. Its molecular structure consists primarily of a fluorocarbon backbone with ether side chains bearing sulfonic acid groups. While PFSA ionomers from different manufacturers share similar chemical structures, they differ mainly in side-chain architecture and ion exchange capacity.

质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。（Proton Exchange Membrane, PEM）是一种具有高质子传导率表征质子膜传导质子能力的参数，是电阻率的倒数，单位S/cm。主要受膜的含水量和使用温度影响。和良好物理化学稳定性的离子导电聚合物薄膜。根据含氟量，可分为全氟、部分含氟及非氟质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。。目前PEM电解槽主要使用全氟磺酸质子交换膜以聚四氟乙烯和全氟烯醚磺酸为共聚单体制成的质子交换膜，具有憎水的聚四氟乙烯骨架和亲水的全氟烯醚磺酸基侧链，以Nafion系列为代表，是目前PEM电解槽的主流选择。，其厚度通常比燃料电池用膜更厚。全氟磺酸膜的制备工艺复杂，长期由美国和日本企业垄断，例如美国科慕公司的Nafion®系列、陶氏公司的XUS-B204膜、日本旭硝子公司的Flemion®膜以及日本旭化成公司的Aciplex®-S膜。全氟磺酸树脂以聚四氟乙烯和全氟烯醚磺酸为共聚单体经自由基共聚而成，分子链结构主要由碳氟主链和带磺酸基的醚支链构成。各公司产品的化学结构相似，主要差异在于侧链结构和离子交换容量衡量质子交换膜内磺酸基团浓度的物理量，单位g/mol。通过酸碱滴定法测定，是影响膜质子电导率和微相分离结构的关键变量。。

At the molecular level, PFSA membranes feature a hydrophobic PTFE backbone and hydrophilic perfluorovinyl ether sulfonic acid side chains. This structure imparts excellent mechanical properties, chemical stability, and proton conductivity. Nafion® membranes from Chemours are the de facto standard in PEM water electrolysis (PEMWE) stacks due to their high proton conductivity, robust chemical and mechanical stability, and effective gas barrier properties.

从分子结构看，全氟磺酸膜具有憎水的聚四氟乙烯骨架和亲水的全氟烯醚磺酸侧链，这使其具备良好的力学性能、化学稳定性和质子传导性能。目前PEM电解水制氢电解槽基本选用科慕公司的Nafion®系列膜，主要因其具有高质子传导性、良好的化学与机械稳定性以及优异的气体阻隔性能。

A PEM is a selectively permeable membrane that allows protons to pass through. Within the membrane, microphase separation occurs between the hydrophobic polymer backbone and the hydrophilic sulfonic acid groups, forming interconnected hydrophilic channels that facilitate proton transport. The proton transfer process is complex and involves multiple steps, with several established mechanisms. The classical proton conduction mechanisms are categorized into three types: the Grotthuss (hopping) mechanism, the Vehicle mechanism, and the Surface mechanism.

质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。是一种选择性透过膜，能够传导质子。在膜内，聚合物电解质的疏水骨架与亲水基团发生微相分离指质子交换膜中聚合物电解质的疏水骨架与亲水基团在微观尺度上发生分离，形成亲水区域（离子团簇），这些区域构成膜内质子传递的通道。，形成亲水通道，实现质子的传递。质子传递过程包含多个步骤，机理复杂，已有较多研究。经典的质子传递机理主要分为三类：跳跃机理、运载机理和表面机理。

1. Grotthuss (Hopping) Mechanism: In this mechanism, protons "hop" along a chain of hydrogen-bonded water molecules (often referred to as a "proton wire") from one side of the electrolyte to the other. This occurs through the continuous rearrangement of hydronium ion (H3O+) configurations. Protons exist primarily as hydronium ions, only momentarily dissociating into free H+ to overcome the energy barrier for breaking a hydrogen bond before re-forming a new one. This aligns with the characteristic ease of proton solvation.

   跳跃机理：在此机理中，质子通过水合氢离子构型的不断转变，沿着由氢键连接的水分子网络（常被称为“质子导线”）从电解质一侧“跳跃”传递到另一侧。质子绝大部分时间以水合氢离子形式存在，仅在氢键间“跳跃”的瞬间克服能垒解离为独立的H+，这符合氢离子易溶剂化的特点。

2. Vehicle Mechanism: This mechanism posits that protons are not transferred as free H+ but migrate as composite ions like H3O+, with no actual proton (H+) transfer between these ions. Unprotonated species like H2O diffuse in the opposite direction. Here, H2O acts as the "vehicle" for proton migration. It's important to note that while vehicle mechanism involves local kinetics, it differs from simple molecular diffusion, as the latter does not involve the counter-diffusion of neutral carriers.

   运载机理：该机理强调质子并非以H+形式传递，而是以H3O+等复合离子形式进行整体迁移，且复合离子间没有H+的直接传递；未结合质子的H2O等载体则进行反向扩散。H2O等在此过程中充当质子迁移的“运载工具”。需注意，运载机理体现局部动力学，但与简单分子扩散不同，后者不涉及中性载体的逆向扩散。

3. Surface Mechanism: In this model, protons travel along the hydrophilic channels by interacting with fixed anionic sites (e.g., -SO3-). The resistance for proton diffusion via this mechanism is relatively high because the distance between sulfonate groups is greater than the free path length for proton hopping.

   表面机理：该机理是指质子借助亲水通道上的固定阴离子基团（如-SO3-）进行传递。由于-SO3-间距大于质子跳跃的自由程，质子通过表面机理扩散的阻力较大。

The Role of PEM in an Electrolyzer Stack

During PEM electrolyzer operation, the proton exchange membrane provides a transport channel that selectively allows the passage of water molecules and hydronium ions. It transports protons from the anode to the cathode, establishing the essential ionic pathway within the cell. The PEM serves three critical functions:

在PEM电解槽运行过程中，质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。提供了选择性允许水分子和水合氢离子通过的传输通道，将质子从阳极输送到阴极，在电解槽内部形成离子传递通路。PEM在电解槽中主要发挥以下三种作用：

1. Solid Electrolyte: It acts as a solid electrolyte, conducting protons generated at the anode to the cathode to participate in the Hydrogen Evolution Reaction (HER), thereby providing the pathway for proton transport.

   作为固态电解质：将阳极反应产生的质子传导至阴极参与析氢反应，为质子传递提供通道。

2. Gas Separator: It physically separates the reaction products (hydrogen and oxygen) at the cathode and anode, preventing their cross-permeation and ensuring gas purity.

   隔绝反应气体：隔绝阴极侧和阳极侧的反应产物（氢气和氧气），避免氢氧混合。

3. Physical Support: It provides a physical support structure for the catalyst layers on both the cathode and anode sides.

   提供物理支撑：为阴极和阳极的催化剂层提供物理支撑。

Key Performance Parameters of Proton Exchange Membranes

1. Proton Conductivity

Proton conductivity, measured in S/cm, characterizes a membrane's ability to conduct protons and is the reciprocal of resistivity. An empirical formula shows that conductivity is related to the membrane's water content (λ) and operating temperature (T). Higher water content generally leads to higher conductivity. In practical PEMWE applications where water supply is ample and the membrane is fully hydrated, conductivity is primarily temperature-dependent. Within a certain temperature range (at constant relative humidity), higher temperatures result in greater proton conductivity.

质子传导率表征质子膜传导质子能力的参数，是电阻率的倒数，单位S/cm。主要受膜的含水量和使用温度影响。（单位：S/cm）表征膜传导质子的能力，是电阻率的倒数。经验公式表明，电导率与膜的含水量λ和使用温度T相关。含水量越大，电导率越高。在实际PEM电解槽应用中，供水通常充足，膜已充分水合，因此电导率主要与温度相关。在一定温度范围内（相对湿度不变时），温度越高，质子传导率表征质子膜传导质子能力的参数，是电阻率的倒数，单位S/cm。主要受膜的含水量和使用温度影响。越大。

2. Ion Exchange Capacity (IEC)

Ion Exchange Capacity (IEC), with units of mmol/g or meq/g, measures the concentration of sulfonic acid groups within the membrane. The relationship between IEC and proton conductivity is complex. At low IEC, the sulfonic acid sites available for proton transfer are too far apart, lacking spatial continuity, which increases proton transfer resistance and lowers conductivity. As IEC increases, the density of sulfonic acid groups rises, shortening the distance between adjacent groups within the proton transfer channels and making proton "hopping" easier. However, excessively high IEC can cause the membrane to absorb too much water and swell excessively, diluting the concentration of sulfonic acid groups and paradoxically reducing proton conductivity.

离子交换容量衡量质子交换膜内磺酸基团浓度的物理量，单位g/mol。通过酸碱滴定法测定，是影响膜质子电导率和微相分离结构的关键变量。（IEC，单位：mmol/g或meq/g）是衡量膜内磺酸基团浓度的物理量。IEC与质子电导率关系复杂。IEC较低时，用于质子传递的磺酸位点相距较远，缺乏空间连续性，质子传递阻力大，电导率低。IEC提高，磺酸基团密度增加，基团间距离缩短，降低了质子“跳跃”难度。但IEC过高时，膜过度吸水溶胀会稀释磺酸基团浓度，反而导致质子电导率下降。

For PEMs, increasing IEC not only reduces the distance between sulfonic acid groups but also influences the microphase-separated structure. Higher IEC leads to a larger hydrophilic phase volume, making it easier for ionic clusters to form continuous channels. From a polymer synthesis perspective, IEC is perhaps the most readily adjustable variable for improving microphase separation and tuning ionic channels. However, higher IEC often adversely affects other membrane properties, such as dimensional stability and mechanical strength.

对质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。而言，提高IEC在减小磺酸基团距离的同时，会影响膜的微相分离指质子交换膜中聚合物电解质的疏水骨架与亲水基团在微观尺度上发生分离，形成亲水区域（离子团簇），这些区域构成膜内质子传递的通道。结构。高IEC下，膜内亲水相体积增大，离子团簇更易形成连续通道。从合成角度看，IEC可能是改善微相分离指质子交换膜中聚合物电解质的疏水骨架与亲水基团在微观尺度上发生分离，形成亲水区域（离子团簇），这些区域构成膜内质子传递的通道。、调控离子通道最易操控的变量。但较高的IEC通常会对膜的其他性能（如尺寸稳定性、机械性能）产生不利影响。

Measurement Method (Acid-Base Titration):

1. Soak a weighed dry membrane sample in a 2 mol/L NaCl solution for 24 hours to fully exchange H+ ions.
2. Remove the membrane, rinse repeatedly with deionized water, and collect the rinse solution with the original NaCl solution.
3. Titrate the collected solution with a standardized 0.05 mol/L NaOH solution, using phenolphthalein as an indicator.
   The IEC is calculated as: IEC = (C_NaOH * V_NaOH) / m_dry, where C_NaOH is the NaOH concentration, V_NaOH is the titrant volume, and m_dry is the dry membrane weight. This method measures the amount of H+ ions displaced from the membrane to determine the sulfonic acid group concentration per unit mass.

测试方法（酸碱滴定法）：

1. 将称重后的干膜样品浸入2 mol/L NaCl溶液中24小时，充分置换膜内的H+。
2. 取出膜样品，用去离子水反复冲洗表面，将洗液收集至原NaCl溶液中。
3. 以酚酞为指示剂，用0.05 mol/L的NaOH标准溶液滴定步骤2中的收集液。
   IEC计算公式为：IEC = (C_NaOH * V_NaOH) / m_dry，其中C_NaOH为NaOH浓度，V_NaOH为滴定消耗体积，m_dry为干膜质量。该方法通过测量从膜中置换出的H+量来衡量单位质量膜内的磺酸基团浓度。

3. Tensile Strength

Tensile strength, measured in MPa, is the maximum tensile stress a membrane specimen can withstand before failure under specified conditions of temperature, humidity, and strain rate. It is calculated as the maximum force divided by the original cross-sectional area (thickness × width). The measurement method is consistent with that for conventional polymer films and characterizes the physical strength of the membrane in a given direction. Literature reports indicate that PFSA membranes can achieve tensile strengths up to 34 MPa at room temperature, which is sufficient for applications in both fuel cells and electrolyzers.

拉伸强度（单位：MPa）是在给定温度、湿度和拉伸速率下，标准膜样断裂前所承受的最大拉伸应力（最大力与原始横截面积的比值）。测试方法与普通膜材料一致，用于表征膜在某一方向上的物理强度。文献报道，全氟磺酸膜在室温下的抗拉强度可达34 MPa，能够满足燃料电池和电解槽的应用要求。

4. Water Uptake

Water is crucial for PEM function, as proton transport in PFSA membranes is heavily dependent on water molecules.

• Microstructural Role: Water plays a key role in the evolution of the membrane's microstructure. Ionic clusters, as primary water reservoirs, not only increase in size upon hydration but also undergo microstructural reorganization. Models show that as water content (λ, the number of water molecules per sulfonic acid group) increases, ionic clusters swell and eventually connect to form continuous proton-conducting channels.
• Role in Proton Transport: Water is central to the proton conduction mechanisms described earlier. At low water content, the proton diffusion coefficient (D_H+) is similar to the self-diffusion coefficient of water (D_H), suggesting the Vehicle mechanism dominates. As water content increases, the difference between D_H+ and D_H grows, indicating that the Grotthuss mechanism becomes predominant due to enhanced water mobility and reorientation capability. At very low water levels, the Surface mechanism may dominate. Furthermore, water affects the dissociation of sulfonic acid groups. Dissociation begins around λ=3 and is complete around λ>6. Increased water content enhances dissociation, generating more hydronium ions and thereby increasing the effective proton mobility.

吸水率对质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。至关重要，因为全氟磺酸膜内的质子传递严重依赖于水分子。

• 微观结构作用：水在膜微观结构演变中发挥关键作用。作为主要储水场所的离子团簇，吸水后除尺寸增大外，其微观结构也会重组。模型显示，随着水含量λ（每个磺酸基团结合的水分子数）增加，离子团簇不断溶胀并相互连接，最终形成连续的质子传递通道。
• 质子传递作用：水在质子传递机理中扮演核心角色。低水含量下，质子扩散系数D_H+与水的自扩散系数D_H相近，表明运载机理主导。随着水含量增加，D_H+与D_H差值增大，主要是因为高水含量下水分子的移动和重取向能力提高，跳跃机理取代运载机理成为主导。极低水含量时，表面机理占主导。此外，水影响磺酸基团的解离，λ≈3时开始解离，λ>6时完全解离。水含量增加能增强解离能力，产生更多水合氢离子，从而提高有效质子迁移率。

Measurement Method:
Water uptake is determined from the mass change of a membrane sample before and after immersion in deionized water.

1. Dry the sample in an oven at 80°C for 24 hours, then weigh to obtain the dry mass (W_dry).
2. Condition the sample at specified temperature and humidity for 24 hours.
3. Quickly remove the sample, blot excess surface moisture with filter paper, and immediately weigh to obtain the wet mass (W_wet).
   Water Uptake (%) = [(W_wet - W_dry) / W_dry] * 100%.

测试方法：
通过测量膜样品在去离子水中浸泡前后的质量变化获得。

1. 将样品在80°C烘箱中干燥24小时，称重得干态质量W_dry。
2. 将样品在给定的温湿度条件下放置24小时。
3. 快速取出样品，用滤纸吸干表面多余水分，立即称重得湿态质量W_wet。
   吸水率 (%) = [(W_wet - W_dry) / W_dry] * 100%。

5. Swelling Rate

The water content of a PEM significantly impacts both its microstructure and macroscopic dimensions.

• Microscopically, water uptake induces microphase separation between the hydrophobic PTFE backbone and hydrophilic sulfonic acid side chains, forming the hydrophilic domains that constitute proton transport channels.
• Macroscopically, the membrane undergoes dimensional changes in length, width, and thickness. For example, Nafion® membranes, whose nominal dimensions are specified under dry conditions (e.g., 23°C, 50% RH), exhibit anisotropic swelling. The change from 50% RH to liquid water immersion at 23°C can be around 10% in thickness and up to 15% in length and width. Therefore, temperature and humidity must be carefully controlled during storage and handling, as changes will alter the membrane's water content and consequently its dimensions. Heating a PEM reduces its water content, leading to shrinkage and potential warping or curling.

质子交换膜具有高质子传导率和良好物理化学稳定性的离子导电聚合物薄膜，能够选择性传导质子，是PEM电解槽和燃料电池的核心组件。的含水量对其微观结构和宏观尺寸影响显著。

• 微观上：吸水导致聚四氟乙烯骨架与亲水磺酸基团发生微相分离指质子交换膜中聚合物电解质的疏水骨架与亲水基团在微观尺度上发生分离，形成亲水区域（离子团簇），这些区域构成膜内质子传递的通道。，形成的亲水区域构成了质子传递通道。
• 宏观上：膜在长度、宽度和厚度方向上均会发生尺寸变化。以Nafion®膜为例，其标称尺寸基于干燥条件（如23°C, 50% RH）制定。从50% RH、23°C状态到水浸泡、23°C状态，厚度变化可达10%左右，长度和宽度变化可达15%左右，且可能呈各向异性。因此，在Nafion®膜的保存和使用中需特别关注温湿度，其变化会影响膜含水量进而改变宏观尺寸。对膜加热会降低含水量，导致膜尺寸收缩并可能产生皱缩弯曲。

Measurement Method:
The test procedure is similar to that for water uptake, but dimensional changes are measured instead of mass changes.

1. Measure the area (S_dry) or the length/width dimensions of the dry sample.
2. Condition the sample at specified temperature and humidity for 24 hours.
3. Quickly measure the area (S_wet) or the length/width dimensions of the conditioned sample.
   Swelling Rate in area (%) = `[(S_w