The Electrochemical Hydrogen Compression (EHC), also known as "hydrogen pump", is a process which uses electric power to "transport" hydrogen across a PEM membrane, from anode to cathode. This very easy principle has many applications in electrochemistry: from cross-over measurement to hydrogen purification [1] and hydrogen compression for hydrogen storage at high pressure [2]. The process of electrochemical hydrogen compression features several advantages over the equivalent mechanical process: it is potentially more efficient, being the ionic resistance of the membrane the major cause of inefficiency; it is modular, meaning that it can be designed for a wide range of flow performance by simply replicating the single cell design; the device is much simpler to design and to operate, being the electrical current the only parameter to control, which also gives a simple measurement and control of the actual flow during the operation. Furthermore, the efficiency increases at partial flow, while mechanical compressors operate in a narrow range if efficiency must be optimized. Also, there are no moving parts, hence no vibrations and noise, while the reliability of the EHC relies mostly on the mechanical performance of the membrane. What makes this technology even more appealing is its simplicity, which makes it ideal for small size PEFC stacks, especially those intended for portable applications, where cost, efficiency, simplicity and reliability are key issues. While mechanical compressors are not suitable to operate under the typical PEFC conditions these conditions are ideal for the EHC. Hence it is interesting to investigate which are the key parameters to operate these devices and which are the boundary conditions which makes them suitable to replace the mechanical compressors. The typical operating conditions for portable or small size PEFC are a low differential pressure between inlet and outlet pressure along the anode loop (0,5 - 1 bar, depending on cell design and actual flow), a gas flow of 1,2 - 1,5 times the stoichiometric flow and, depending on the stack rated power and application, an open cathode design operating under natural convection or using an axial fan as air blower. The hydrogen line pressure in the range from ambient to 3- 5 bar(abs), depending on design and application, gas temperature typically in the range ambient to 80°C, fully humidified gas with condensed phase of distilled water and a wide variation of flow. Being simplicity a key issue to decrease the full system cost, many auxiliaries are not included, such as the cooling pump and loop, being natural or forced convection through the cathode the typical cooling technique. At the anode, the typical solution to avoid the gas compressor is a timed purge into the ambient. Unfortunately this solution does not only waste part of the fuel but it can not always be applied, especially in closed or confined environments or where strict safety restrictions are considered. In this work, the performance of a composite Ni-TiOx membrane, already tested for fuel cell applications [3, 4], has been evaluated and compared to a reference Nafion NR212 membrane in order to optimize the operation at low pressure for the integration of a EHC in a PEFC stack for portable applications. Temperature and relative humidity effects have been also investigated, resulting in additional application indications. The experimental results have been then evaluated to understand the limitations of EHC technology as replacement technology for small size hydrogen compressors. The experimental results show that the composite membrane gives better performance than the thinner Nafion NR212, thanks to its water retention properties and also giving evidence that the main limitation is the membrane resistance. Hence the best operative conditions have been identified: low current density, to decrease the Ohmic power loss, but also increasing the active area of the MEA (membrane and electrode assembly) and the overall EHC size. The performance have been then compared to a theoretical model of a mechanical compressor with an average efficiency of 60%, suggesting that the EHC can conveniently replace the mechanical compressor under specific assumptions on current density and cell design. Figure 1. Actual performance comparison of a EHC with a TiOx membrane and a mechanical compressor with 60% efficiency at 0,1-0,5-1 A/cm2 .
Performance assessment of alternative membranes for Electrochemical Hydrogen Compressor (EHC) in portable PEM fuel cell applications
F Matera;A Carbone;R Pedicini;
2018
Abstract
The Electrochemical Hydrogen Compression (EHC), also known as "hydrogen pump", is a process which uses electric power to "transport" hydrogen across a PEM membrane, from anode to cathode. This very easy principle has many applications in electrochemistry: from cross-over measurement to hydrogen purification [1] and hydrogen compression for hydrogen storage at high pressure [2]. The process of electrochemical hydrogen compression features several advantages over the equivalent mechanical process: it is potentially more efficient, being the ionic resistance of the membrane the major cause of inefficiency; it is modular, meaning that it can be designed for a wide range of flow performance by simply replicating the single cell design; the device is much simpler to design and to operate, being the electrical current the only parameter to control, which also gives a simple measurement and control of the actual flow during the operation. Furthermore, the efficiency increases at partial flow, while mechanical compressors operate in a narrow range if efficiency must be optimized. Also, there are no moving parts, hence no vibrations and noise, while the reliability of the EHC relies mostly on the mechanical performance of the membrane. What makes this technology even more appealing is its simplicity, which makes it ideal for small size PEFC stacks, especially those intended for portable applications, where cost, efficiency, simplicity and reliability are key issues. While mechanical compressors are not suitable to operate under the typical PEFC conditions these conditions are ideal for the EHC. Hence it is interesting to investigate which are the key parameters to operate these devices and which are the boundary conditions which makes them suitable to replace the mechanical compressors. The typical operating conditions for portable or small size PEFC are a low differential pressure between inlet and outlet pressure along the anode loop (0,5 - 1 bar, depending on cell design and actual flow), a gas flow of 1,2 - 1,5 times the stoichiometric flow and, depending on the stack rated power and application, an open cathode design operating under natural convection or using an axial fan as air blower. The hydrogen line pressure in the range from ambient to 3- 5 bar(abs), depending on design and application, gas temperature typically in the range ambient to 80°C, fully humidified gas with condensed phase of distilled water and a wide variation of flow. Being simplicity a key issue to decrease the full system cost, many auxiliaries are not included, such as the cooling pump and loop, being natural or forced convection through the cathode the typical cooling technique. At the anode, the typical solution to avoid the gas compressor is a timed purge into the ambient. Unfortunately this solution does not only waste part of the fuel but it can not always be applied, especially in closed or confined environments or where strict safety restrictions are considered. In this work, the performance of a composite Ni-TiOx membrane, already tested for fuel cell applications [3, 4], has been evaluated and compared to a reference Nafion NR212 membrane in order to optimize the operation at low pressure for the integration of a EHC in a PEFC stack for portable applications. Temperature and relative humidity effects have been also investigated, resulting in additional application indications. The experimental results have been then evaluated to understand the limitations of EHC technology as replacement technology for small size hydrogen compressors. The experimental results show that the composite membrane gives better performance than the thinner Nafion NR212, thanks to its water retention properties and also giving evidence that the main limitation is the membrane resistance. Hence the best operative conditions have been identified: low current density, to decrease the Ohmic power loss, but also increasing the active area of the MEA (membrane and electrode assembly) and the overall EHC size. The performance have been then compared to a theoretical model of a mechanical compressor with an average efficiency of 60%, suggesting that the EHC can conveniently replace the mechanical compressor under specific assumptions on current density and cell design. Figure 1. Actual performance comparison of a EHC with a TiOx membrane and a mechanical compressor with 60% efficiency at 0,1-0,5-1 A/cm2 .I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
