Advanced solutions to improve the water management in electrochemical hydrogen compressors

EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 1/10 A0704 Advanced solutions to improve the water management in electrochemical hydrogen compressors Giuseppe Sdanghi* (1), Ma Anta Diop (1), Jérôme Dillet (1), Moritz Branco (2), Thomas Prouvé (3), Sophie Didierjean (1), Gael Maranzana (1) (1) LEMTA, Université de Lorraine, CNRS, 54500 Vandœuvre-lès-Nancy/France; (2) ESA, ESTEC, 2200 AG, Noordwijk/the Netherlands; (3) CEA-IRIG, DSBT, 38000, Grenoble/France; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract Electrochemical hydrogen compressors (EHCs) have several advantages over mechanical compressors: they are vibration and noise free and can be very cost effective. Although suitable for applications in which low and moderate pressures are required, EHCs have one major drawback: the water management. To achieve good performance, the polymeric membrane must be optimally humidified. Without adequate humidification, the operating conditions can damage the membrane, affecting overall performance and efficiency In this work, we present an EHC system in which the water management is controlled by a passive countercurrent membrane water exchanger. This device allows drying the produced hydrogen flow while simultaneously humidifying the low-pressure hydrogen fed to the compressor. This highly flexible drying system can achieve a dew point temperature of less than −30 °C. A dryer structured in this way avoids conventional drying methods such as TSA or PSA cycles, which are very demanding in terms of power and heat rejection. At the same time, it also avoids the use of conventional hydrogen humidification methods, which require expensive auxiliary equipment. Several membrane electrode assemblies (MEAs) have been used in this study and the effect of their properties on the compression performance has been investigated. Current densities up to 4 A cm-2 were achieved at low voltage and low temperature (up to 0.4 V and 35 °C), with pumped flows greater than 0.8 mg s-1 [1]. The highest efficiencies (> 60) were achieved in a range of low voltages and current densities, but the efficiency of the EHC decreased dramatically as the voltage was increased. However, this condition is essential to achieve high current densities, which are converted into high compressed hydrogen flows. An optimum must therefore be found between performance and cost. Nevertheless, the flexibility of this compressor, with its wide range of flow rates and total absence of vibration, should be highlighted. Indeed, such a system is suitable for aerospace applications (e.g., to produce cold energy in a Joule-Thomson cryocooler), but also for terrestrial hydrogen applications, such as injection into gas pipelines or storage in underground salt caverns. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 2/10 Introduction Both reciprocating and centrifugal hydrogen compressors are very mature technologies, and are widely used worldwide to compress hydrogen. However, they are characterized by very high capital and operating costs, and a number of inconveniences such as noise, vibration and large size [1]. Electrochemical compressors (EHCs) appear to be an attractive alternative. They are nonmechanical compressors, which means no noise or vibration, as well as low operating costs and high efficiency, since compression is isothermal [2]. In addition, they are also very compact, and have proven to be economically advantageous for pressures up to 120 bar [3]. The higher the current applied to an EHC, the more high-pressure hydrogen is produced. However, in order to take advantage of large flows, a significant amount of energy must be consumed, with a consequent drop in efficiency. EHC has one major drawback namely the water management [4]. To achieve good performance, EHC requires a certain amount of external water to ensure adequate hydration of the membrane. Two main mechanisms regulate the water content within the MEA, i.e., the electro-osmotic migration and the back-diffusion [5]. In low pressure applications, water transport from the anode to the cathode predominates, due to the low water content resulting from back-diffusion compared to electro-osmotic migration [6]. Particularly at high current densities, low amounts of liquid water resulting from back-diffusion can be insufficient to humidify the membrane, leading to drying. On the other hand, prolonged operation can result in water condensation in the low-pressure compartment, leading to flooding [7]. Both situations lead to a dramatic reduction in performance. This demonstrates the importance of water management in EHC, where water balance is necessary and must be carefully achieved. In EHCs, water is generally supplied by an external source, humifying the hydrogen supplied on the low-pressure anode side [8]. Humidification of the membrane is achieved during operation by electro-osmotic migration. Liquid water can also be supplied to the cathode compartment, to promote the humidification of the membrane by diffusion and to compensate for the electro-osmotic drag, thus achieving balanced water transport throughout the EHC. This solution certainly looks promising, but a pressure-resistant pump must be used to recirculate high-pressure liquid water into the cathode compartment, increasing the cost. The aim of this study was to develop an EHC in conjunction with its water management system to feed a Joule-Thomson cryocooler for aerospace applications. The conventional EHC architectures described above are not feasible in this context due to volume and weight constraints. Therefore, we have developed a non-conventional EHC architecture in conjunction with an advanced water management system, with the aim of producing highpressure hydrogen at a very low dew point temperature. 1. Scientific Approach This study focused on testing the feasibility of an EHC with an advanced water management system, and identifying the most suitable assembly materials that would give the best performance through an experimental approach. In addition to the need of minimizing the ohmic losses, a major challenge in EHC is to limit hydrogen permeation through the membrane, which is large when the pressure difference between the cathodic and anodic compartments is increased. The developed system has two important innovative features: (i) the EHC includes an advanced water management system capable of preventing water condensation at the low anodic compartment; (ii) very dry high-pressure hydrogen is produced by using a water EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 3/10 membrane dryer. To avoid the presence of liquid water on the anode side due to the water condensation (which can occur especially during transient operation), a temperature gradient is imposed between the lowand the high-pressure compartment [9]. The use of a water membrane exchanger in conjunction with an EHC is a fairly new concept. Water management using a passive membrane exchanger offers several advantages over alternative methods (cyclonic separation, cold trap and molecular sieve), particularly in terms of weight and energy consumption. The principle consists of adsorbing the overflow of water from the humidified flow at high pressure towards the low pressure one, which has a lower relative humidity. The passive water membrane exchanger, which allows water to be transferred by simple permeation (since there is no power supply) enables the hydrogen flow supplied to the anode compartment of the EHC to be humidified and the high-pressure hydrogen flow produced to be dried. A closed loop can thus be created. 2. Experiments Materials Titanium flow field plate were used on both anode and cathode side. Pin-type channels were machined onto the anode plate and on an active surface of 25 cm2. On the anode side, porous platinum-coated sintered titanium with an averaged pore size of 5µm and thickness of 1mm was used as porous transfer layer (PTL). A Sigracet 28 BA carbon GDL equipped with an MPL was used between the Ti-PTL layer and the MEA. Several MEA were used in the present study: Nafion XL (≈ 27.5 µm thick), Nafion™ HP (≈ 22.5 µm thick) and an MEA supplied by Hyplat with a hydrophobic treatment by the addition of PTFE additives on the anode side (≈ 30 µm thick). The catalyst loading for all the MEAs was 0.3 mgPt cm-2. The active surface for these MEAs was 25 cm2. On the cathode side, a Sigracet 28 AA layer was used as GDL. Two layers-PTLs were used to collect water by capillary action: a first layer consisting of Pt-coated Ti felts (GLD20 with an average pore size of 40 μm), and a second layer made of deployed-Ti with a higher pore size (≈ 0.5 mm). Regarding the water membrane exchanger, an uncoated Ti-PTL was used at the lowpressure compartment, while carbon Sigracet 28 AA layers were used as GDL at the highpressure compartment. As no chemical reaction occurred in the water exchanger, a Nafion™ 212 membrane was used in place of a MEA. Experimental Setup The experimental setup used in the present study consisted of an EHC and a water membrane exchanger in series. Downstream of the water membrane exchanger, a fastresponse Dew Point Hygrometer (Easidew EA2 Michell Instruments) allowed the dew point temperature of the pumped hydrogen flow to be measured, while a back-pressure regulator (BROOKS SLA5820S) was kept closed to allow the pressure to increase progressively, and opened once the desired pressure was reached. A BROOKS SLA5850S flowmeter was used downstream of the pressure regulator to measure the flow of hydrogen returning to the low-pressure compartment of the water membrane exchanger and deduce the faradic efficiency. To avoid water condensation on the anode compartment, a temperature gradient was established over the cell, with the anode warmer than the cathode compartment. The temperature of each compartment was controlled independently by two thermal baths using Pt100 sensors. In addition, gas lines were overheated to prevent water condensation. Pressure transducers were used in both the anode and cathode compartments (RS 1365045). An ITECH IT-M3903D-10-340 high-power DC power supply was used to power the EHC. Data were automatically acquired using an RS 485 interface and the LabVIEW® software. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 4/10 i-V curves were recorded by gradually increasing the voltage over a limited, optimized voltage range for each membrane (0.15-0.4 V, with a step of 0.05 V). Several average temperatures between 20 and 55 °C were tested. A temperature gradient of 10 °C was also applied between the lowand high-pressure compartments, the former being warmer than the latter to avoid water condensation. All experiments were performed with a hydrogen pressure of 100 bar at the cathode side. Operating protocol Liquid water was firstly introduced into the system at the cathode compartment of the compressor cell, to humidify the Ti felt, and hence the membrane. A hydrogen flow (≈ 6 NL h-1) was supplied to the membrane exchanger (where it was humified), and hence to the compressor cell. A highly humid high-pressure hydrogen flow (saturated at the temperature of the high-pressure plate) was produced while the system was provided with electrical power. The humidified high-pressure hydrogen flow passed through the membrane exchanger. Water diffused from the high-pressure compartment to the low-pressure compartment, humidifying the incoming low-pressure and dry hydrogen flow in countercurrent. As a result, humidified low-pressure hydrogen was progressively fed into the EHC. 3. Results The polarization i-V curves at different operating temperatures obtained when using a MEA with PTFE additives on the anode side are shown in Figure 1a. The range of investigated voltages was between 0.15 and 0.4 V, due to the occurrence of a limiting current density, especially at lower temperatures, for voltages higher than 0.4 V. Figure 1 - a) i-V curves recorded in the range of voltages 0.1-0.4 V using an MEA with a hydrophobic treatment on the anode side, at 100 bar under several temperature conditions; b) Pumped hydrogen flow obtained at 100 MPa in the EHC equipped with the same MEA as a function of the power supplied The behavior of the i-V curves was found to be almost linear in the low voltages range (< 0.25 V), whereas the current density increased sharply, approaching a limiting value, at higher voltages. This behavior has already been observed in our previous work [5], and is related to the water transport across the membrane. Indeed, the total resistance of the EHC increases when increasing the current density, as a consequence of the reduction in the a) b) EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 5/10 membrane proton conductivity resulting from the membrane dehydration. This phenomenon is particularly pronounced at low temperatures. A significant increase in current density was observed when increasing the operating temperature for a given voltage. A current density of approximately 2.4 A cm-2 was obtained at an average temperature of 55 °C and at 0.4 V, while almost half the value, 1.2 A cm-2, was obtained at about 20 °C at the same voltage applied. Figure 1b shows the pumped hydrogen flow rate obtained as a function of the power supplied. The highest hydrogen flow at 100 bar was obtained at an average temperature of 55 °C (23 NL h-1) while supplying about 22 W of electrical power. Approximately 19 NL h-1 of hydrogen at 100 bar was produced when supplying 14 W at 55 °C, whereas a lower value, 11.7 NL h-1, was obtained when supplying the same amount of electrical power but at 20 °C. This means that it was possible to increase by about 62% the amount of hydrogen compressed at 100 bar in an EHC equipped with the PTFE-treated MEA when raising the averaged operating temperature by 35 °C. The improvement in performance obtained when increasing the operating temperature can be explained by the decrease in the ohmic resistance, a well-known characteristic. As evidence, the ohmic resistance calculated at 20 °C (as the slope of the i-V curve) was equal to 0.27 Ω cm-2, and decreased by about 70% and down to 0.158 Ω cm-2 when the temperature was increased to 55 °C. The main effect of the temperature increase is the improvement of the proton conductivity of the membrane. The higher the proton conductivity the lower the ohmic resistance. Figure 1a also shows a slight improvement in the electrochemical performance obtained when applying a thermal gradient of 10°C between the anodic and the cathodic compartments, compared to the results obtained when keeping both compartments at the same temperature. When the anode is warmer than the cathode side, water condensation can be prevented, as previously discussed. This phenomenon is more pronounced in dynamic regimes and at the start of experiments. However, the application of a thermal gradient only gives a small improvement in performance under steady-state conditions. Figure 1b also shows the comparison between the measured hydrogen flows produced at 100 bar and the theoretical ones calculated in accordance with Faraday's Law, as a function of the power applied. We considered that the difference between these two amounts could be used as an empirical basis to evaluate the hydrogen permeation rate across the membrane. Interestingly, it was found that the permeation rate was higher at low temperatures (around 20°C), while it tended to become negligible at higher temperatures. At 60 °C, no difference was observed between the measured and the theoretical hydrogen flows. To explain this trend, we considered the effects of both the assembly components and the membrane's hydration level. First, we placed a Microporous Layer (MPL) between the PTL and the MEA at the low-pressure compartment. The elimination of permeation at high temperatures can be explained by the fact that the membrane is well-supported by small pores when an MPL is used, resulting in a flat, undeformed membrane. Conversely, when the membrane is supported by large pores, as with a PTL alone, it experiences tensile stress due to pressure and undergoes a reduction in thickness. This results in greater hydrogen permeability. Conversely, the increased hydrogen permeation observed at low temperatures was probably due to the higher hydration level of the membrane. Gas permeation of proton exchange membranes is particularly enhanced when they are fully hydrated. In fact, hydrogen permeability is approximately ten times greater than in the solid phase [10]. Furthermore, according to the water sorption measurements found in the literature [11,12], the water content of the membrane is generally higher at low temperatures. The hydrophobic treatment at the anode side has probably contributed to keeping the membrane well hydrated, especially at low temperatures. Indeed, the membrane hydration EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 6/10 occurred by diffusion from the hydrophilic cathode compartment to the hydrophobic anode compartment during the transient regime, which could have facilitated the permeation of hydrogen across the membrane in the same sense. To better evaluate the performance of the MEA with a PTFE-treated anode, we compared the i-V obtained at an average temperature of 35 °C with those of two other MEAs, consisting of Nafion XL (≈ 27.5 µm) and Nafion HP (≈ 22.5 µm) membranes, having the same catalyst loading but without PTFE treatment on the anode side (Figure 2). Figure 2 - Comparison between three different MEAs tested with the anode temperature at 40 °C and the cathode temperature at 30 °C The performances achieved when using the Nafion HP MEA were superior to those obtained using both a Nafion XL and a PTFE-treated MEAs. As evidence, a current density of 2.7 A cm-2 was measured when applying a voltage of 0.4 V, which is 50% higher than that obtained when using a PTFE-treated MEA (1.8 A cm-2) and 26% higher than that measured with a Nafion XL MEA (around 2 A cm-2). This improvement was due to both the lower thickness of the membrane and the higher Ion Exchange Capacity (IEC) of Nafion HP. In fact, the thinner thickness of Nafion HP resulted in a lower ohmic resistance. Moreover, the IEC of Nafion HP (≈ 1.08 meq g-1) is higher compared to that of Nafion XL (≈ 0.9 meq g-1), resulting in an enhanced ion exchange in the membrane, and therefore a higher protonic conductivity at the same temperature. A further consideration to take into account is that the water management method used in this study is better suited to the use of thin membranes. Using thin membranes facilitates the transfer of water from the high-pressure to the lowpressure compartment, resulting in the more efficient humidification of the membrane. Due to the improved performance achieved with a Nafion HP MEA, a long-term experimental test was conducted to evaluate compression performance and identify operating issues. Figure 3 shows the behavior of the cathode pressure, current density, and dew point temperature over a period of approximately 270 operating hours (about 11 days) when applying a voltage of 0.2 V. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 7/10 Figure 3 – Current density (blue), cathode pressure (red) and dew point temperature (black) behaviour in a 270 hours experiment (anode temperature = 40 °C, cathode temperature = 30 °C), when using a Nafion HP MEA and when applying 0.2 V Throughout the entire experiment, the cathode pressure remained stable at 100 bar. The experiment was conducted by applying a temperature gradient between the 40°C anode compartment and the 30°C cathode compartment. Experiments performed at higher temperatures resulted in a collapse of performance after a few hours of operation. This dramatic loss of performance was attributed to the mechanical properties of the membranes, which were affected by the combined effect of the relatively high temperature and the highpressure gradient (100 bar) across the cell. By keeping an average temperature of 35°C over the cell, no membrane failure was observed during 10 days. The current density also remained almost stable at an average value of about 1.5 A cm-2 when applying 0.2 V. After 10 days of operation, a slight decrease of the current density was observed, due to a water management issue, as evidenced by the behavior of the dew point temperature. Figure 3 also shows the performance of the water membrane exchanger used in the present study, whose role was both to humidify the low-pressure hydrogen flow supplied to the EHC and to dry out the produced high-pressure hydrogen flow. Using a passive water exchanger as described in the Experimental Section, a dew point temperature as low as -29 °C was achieved, corresponding to a relative humidity of about 1.5% at 35 °C. Nevertheless, the dew point temperature increased dramatically to 60°C after 11 hours of operation, revealing issues in the water management and the presence of liquid water droplets inside the hygrometer, thus affecting its proper functioning. Since it was no longer possible to record data, the experiment was stopped. In this study, only the feasibility of the water membrane exchanger was tested. The dew point temperatures achieved confirmed that such a device can be a valuable tool to dry highly humid flows of hydrogen. Even lower dew points could be achieved and the durability of the device improved, by increasing the porosity of the diffusion layers, optimizing the exchange surface area or by using different channel configurations to improve the water transfer, e.g., using interdigitated channels. In view of these encouraging perspectives, we sized a stack of EHC to be used in aerospace applications together with its water management system consisting of a water membrane dryer. Figure 4a shows the results of the sizing for a total hydrogen flow of around 400 NL h-1, which is considered ideal for use in a Joule-Thomson cryocooler. EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 8/10 Figure 4 – a) Sizing of an EHC stack equipped with a Nafion HP MEA for different applied voltages (T anode = 40 °C, T cathode = 30 °C) to produce 400 NL h-1 of hydrogen at 100 bar; b) Efficiency of an EHC equipped with a Nafion HP MEA at different operating conditions According to Faraday’s law, the higher the current applied, the higher the hydrogen flow produced by a single EHC cell. This means that in order to produce a given total flow of high-pressure hydrogen, the number of cells required in the stack is low when high power is applied, while a higher number of cells must be used when the applied power is relatively low. As shown in Figure 4a, around 12 cells have to be used to produce 400 NL h-1 of hydrogen at 100 bar at an average temperature of 35 °C and at 400 W, while to produce the same amount of hydrogen but applying 150 W, a 3 times larger EHC with 38 cells needs to be used. A trade-off between energy consumption and size is therefore necessary to find an optimum, which is to apply a relatively low power to an EHC of average size. Figure 4b also confirms the need for a trade-off, by showing the behavior of the total efficiency of an EHC as a function of the pumped hydrogen flow. The efficiency of an EHC can be expressed as follows: 𝜂 = 𝑊𝑡ℎ 𝑊𝑟𝑒𝑎𝑙 =𝑉𝑁𝑒𝑟𝑛𝑠𝑡 𝑉⋅𝐼 − 𝐼𝑙𝑜𝑠𝑠 𝐼 (1) where VNernst is the Nernst voltage, V is the real voltage applied to the EHC to compress hydrogen, I is the current density and Iloss is the current density deriving from the hydrogen permeation across the membrane. The maximum efficiency of around 62% was achieved when low hydrogen flows were produced, i.e., at low current densities (hence, low applied power). It is worth noting that the efficiency of an EHC can be even higher at very low current densities, i.e., when a voltage close to the Nernst potential is applied. However, these operating conditions are not of practical interest as only a small amount of hydrogen is produced at low current densities. The highest efficiencies were achieved when a pumped hydrogen flow of around 5 NL h-1 was produced. Producing more hydrogen at high pressure means that more power has to be supplied to the EHC, which contributes to the increase in overvoltages and hence the decrease in efficiency. As shown in Figure 4b, a significant loss of efficiency was obtained when 5-times higher hydrogen flows were produced. Indeed, the EHC had an efficiency of 15% when producing around 25 NL h-1 of hydrogen at 100 bar. Conclusions An innovative water management system for electrochemical hydrogen compressors (EHC) has been developed in the present study. It consists of producing dry hydrogen at a) b) EFCF 2025: Low-Temp. Fuel Cells, Electrolysers & H2 Processing 1 – 4 July 2025, Lucerne Switzerland https://doi.org/10.5281/zenodo.17244111 A0704 Page 9/10 100 bar by installing a passive water membrane exchanger downstream of an EHC. This water management system allowed achieving dew points temperatures of around -29 °C, corresponding to a relative humidity of about 1.5% at 35°C. The developed EHC had an unconventional architecture. A temperature difference was applied between the anode and the cathode compartments to prevent water condensation at the anode compartment and thus the flooding of the catalytic sites. Under the operating conditions investigated in this study, applying a thermal gradient of 10 °C across the EHC allowed slightly better performances to be obtained than in the case of a uniform temperature throughout the cell. Several MEAs were tested: Nafion XL (≈ 27.5 µm thick), Nafion™ HP (≈ 22.5 µm thick) and an MEA supplied by Hyplat with a hydrophobic treatment by the addition of PTFE additives on the anode side (≈ 30 µm thick). The best performances were obtained when using a Nafion HP MEA, which has both a lower thickness and a higher Ion Exchange Capacity, allowing higher protonic conductivities, higher water transfer rates and thus a lower resistance to be achieved. As evidence, a current density of 2.7 A cm-2 was measured when applying a voltage of 0.4 V, which is 50% higher than that obtained when using a PTFE-treated MEA (1.8 A cm-2) and 26% higher than that measured with a Nafion XL MEA (around 2 A cm-2). The highest efficiencies (> 60%) were achieved in a range of low current densities and voltages close to the Nernst potential, but the efficiency of the EHC decreased dramatically as the voltage was increased. However, this condition is essential to achieve high current densities, which are converted into high compressed hydrogen flows per single cell, thus into a more compact and flexible system. An optimum must therefore be found between energy consumption and capital expenditure. Nevertheless, the flexibility of this compressor, with its wide range of flow rates and total absence of vibration, should be highlighted, being suitable for both terrestrial and aerospace applications. References [1] Sdanghi G., Maranzana G., Celzard A., Fierro V. Review of the current technologies and performances of hydrogen compression for stationary and automotive applications. International Journal of Hydrogen Energy, 102, 150-170 (2019) [2] Sdanghi G., Maranzana G., Celzard A., Fierro V. 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