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Abstract

The issue of water flooding in proton exchange membrane fuel cells (PEMFC) under conditions of humidity levels leads to significant mass transfer losses. To address this challenge, a fiber-structured microporous layer (MPL) with high porosity is developed, and investigates the impact of carbon nanotube (CNT) on its heat treatment process. The inclusion of CNT facilitates the cyclization of polyacrylonitrile in the pre-oxidation process. Thereby reduces the fracture of nanofibers, improved the microstructure and the porosity of the membrane during carbonization. The carbon nanofiber membranes containing CNT shows a higher oriented arrangement of carbon grains after graphitization. The high porosity of the fiber membrane MPL confers exceptional mass transfer capabilities. At 100% RH, the MPL sample CFP-2C improves the maximum power density and a maximum current density of PEMFC by 47.5% and 55.3%, respectively, compares to the conventional carbon black particles MPL.

Keywords

Microporous layer fuel cell carbon nanofiber membrane carbon nanotube electrospinning

Introduction

In the face of escalating environmental challenges and the energy dilemma, the advancement of innovative energy technologies serves as a pivotal strategy for fostering a low-carbon economy. Proton exchange membrane fuel cell (PEMFC) is considered a promising electrochemical power generation device due to its high energy conversion efficiency, high power density, almost zero emissions, and ability to operate continuously at low temperatures.^1–8

The gas diffusion layer (GDL) is a crucial component of PEMFC. It ensures the uniform supply of reaction gas to the catalyst layer (CL) and facilitates the flow of electrons generated in the CL to the bipolar plate (BP).^9–12 Additionally, the water produced by the cathode reaction in PEMFC must also be expelled through the GDL. In practical applications, the performance of PEMFC is heavily reliant on its water management capacity.^13–15 As current density increases, so does the water produced by the cathode reaction, potentially leading to electrode flooding due to the flow field plate’s drainage limitations, especially at high current densities.^16–19 Water saturation in the GDL pores hinders oxygen transport to the catalyst layer and inundates active reaction sites, markedly degrading PEMFC performance.^20,21 Introducing MPL into the GDL has been proven to be an effective strategy for enhancing the water management capability of PEMFC.²²

MPL is primarily composed of conductive carbon black powder and hydrophobic substances, such as polytetrafluoroethylene (PTFE). Compared to the GDL substrate, the smaller pore size of the MPL generates a higher capillary pressure difference, facilitating water removal.²³ Additionally, the MPL can reduce the contact resistance between the GDL and the CL, minimizing ohmic losses.²⁴ The pore structure and component ratio of MPL directly impacts its performance. Research by Nagai et al.²⁵ demonstrated that the cathode MPL (Cell-M2) with cells containing large micron-sized pores enhances water management throughout the GDL by effectively forming water paths, thereby reducing the amount of liquid water in the entire cathode GDL. The pore-forming agent²⁶ can effectively optimize the pore structure of MPL and enhance its mass transfer capacity at high humidity. Zhao et al.²⁷ prepare MPL with a gradient pore structure by regulating the amount of pore-forming agent sodium bicarbonate (NaHCO₃), which increased the capillary pressure difference in GDL and enhanced the stability of water and gas transport in fuel cells. Guo et al.²⁸ developed a directional gradient pore size distribution in MPL using a hard template method, resulting in improved water management and higher output power density in PEMFCs. The content and type of hydrophobic agent also significantly influence MPL performance. Chi²⁹ observed that with an increase in PTFE content, the hydrophobicity of the cathode catalyst layer increased. When the PTFE content reached 50%, the maximum power density of MEA reached 856 mW·cm⁻². Latorrata et al.³⁰ utilized perfluoropolyether (PFPE) instead of PTFE to create MPL with higher hydrophobicity through spray deposition. However, the addition of a hydrophobic agent may result in pore blockage and carbon black aggregation. Coupling agents such as polydimethylsiloxane (PDMS)³¹ and hexadecyltrimethoxysilane (HDTMS)³² have been employed to hydrophobically treat the surface of carbon black particles, ensuring the original pore structure of MPL while enhancing its hydrophobicity, thereby improving PEMFC performance.

Different kinds of carbon materials can be used to improve the mass transfer ability of MPL. A large number of studies have investigated the effect of graphene on carbon black MPL.³³ For example, Leeuwne et al.³⁴ prepare a MPL containing graphene. Due to the unique scalar structure and high conductivity of graphene, the ohmic transport losses during electron transport are reduced, thereby obtaining higher power density. Carbon nanotubes (CNT) are also commonly used as modified additives for carbon black MPL.³⁵ And the curly shape of CNT helps to construct a pore structure of appropriate size. For example, XC-72 and CNT³⁶ are used to prepare MPL, the permeability and conductivity of the MPL are improved due to the introduction of CNT. Similar studies have shown that vapor-grown carbon fibers³⁷ and carbon nanorods³⁸ can also improve the performance of PEMFC. It is also a new development trend to prepare MPL by doping other components in carbon materials. Wang et al.³⁹ not only significantly improved the durability of PEMFC by adding CeO₂ to MPL, but also reduced the transmission resistance. Chen et al.⁴⁰ prepared graphene sponge-nano-CeO₂ composite and used it as the MPL of PEMFC. The MPL exhibits excellent performance at different humidity levels. However, the traditional particle MPL composed of carbon black and binder has poor permeability, and the dense stacking structure of carbon black particles is not conducive to the transmission of gas.⁴¹

MPL produced through electrospinning exhibit greater porosity and permeability compared to those prepared using the conventional slurry method. Nevertheless, the utilization of carbonized electrospun nanofibers as MPLs remains inadequate. This study focuses on the fabrication of continuous carbon nanofiber membranes through electrospinning and subsequent heat treatment. The impact of CNT incorporation on the microstructure, pore size distribution, and permeability of carbon nanofiber-based MPL during heat treatment was investigated. Furthermore, the practical efficacy of carbon nanofiber membranes as MPL in PEMFC is assessed. By adding carbon nanotubes to the microporous layer of the fiber structure, the microstructure of the membrane was improved to solve the problem of PEMFC flooding. Compared with the existing research, the carbon nanofiber membrane structure containing CNT has unique advantages in improving the mass transfer ability.

Experimental

Materials

Polyacrylonitrile (Mw ≈ 150,000) was supplied by Sigma Aldrich, n-butanol was supplied by Sinopharm Chemical Reagent Corporation. N, N-dimethylformamide (DMF) was supplied by Sinopharm Chemical Reagent Corporation, and carboxylated carbon nanotubes (CCNT) with a purity greater than 98% were supplied by NANO TIME. Phenolic resin (PF9501B) was provided by Jinan Yuantai Chemical Co., Ltd. The 60% weight Polytetrafluoroethylene (PTFE) emulsion was provided by Daikin. The carbon fiber paper was homemade, and the commercial catalyst-coated film (CCM film) was purchased from SHENGERNUO.

Preparation of fiber MPL

PAN (9 wt%) was added to DMF and stirred at room temperature for 10 h. Subsequently, CNT was added incrementally, and the spinning solution was obtained by stirring at room temperature for 5 h. The CNT content ranged from 0 wt% to 3 wt% of PAN. The spinning reception distance was set at 150 mm, the ambient relative humidity was 20%, the temperature was 25°C, the spinning voltage was 50 kV. Spinning for 20 min, and the nanofiber film was directly formed on the surface of carbon fiber paper (CFP) coated with phenolic resin. The improved bonding of nanofiber film and CFP was achieved by hot pressing process (140°C, 3 MPa, 90 s) to prepare composite carbon fiber paper. Pre-oxidation was carried out by increasing the temperature from 180°C to 280°C at 1.5°C/min under air atmosphere and holding for 30 min. After cooling, the composite carbon fiber paper was carbonized at a rate of 2°C / min in the temperature range of 300°C ∼ 600°C, and applied at a carbonization rate of 10°C/min. Finally, graphitization was performed at 2800°C for 30 min to transform nanofiber film into electrospinning fibrous microporous layers (E-MPLs) (Figure 1).

Figure 1.

Preparation process of carbon nanofiber membrane and E-MPL.

Assembly of single cell

GDL was impregnated in 20 wt% PTFE for 1 min. After drying, calcined at 350°C for 3 mins for hydrophobic treatment. The MPLs with different CNT contents (1 wt%, 2 wt% and 3 wt%) were prepared and named CFP-1C, CFP-2C, and CFP-3C, respectively. Additionally, a conventional particulate MPL, CFP-BC, was established as a comparison group.

The platinum loading of the catalyst layer on the anode and cathode sides was 0.12 mg/cm² and 0.48 mg/cm², respectively. The cathode GDL and anode GDL were applied to the CCM membrane and sealed with PET gaskets. The membrane electrode was prepared by hot pressing at 130°C for 2 mins at 10 MPa to obtain the membrane electrode, and the effective area of the tested membrane electrode was 5 cm². The membrane electrode was placed on the graphite plate gas flow channel, and the flow field plate, collector plate, spacer, end plate, and insulating frame of the other electrode were aligned and assembled sequentially to form a single cell.

Measurements

Scanning electron microscope (SEM, Phenom ProX and Hitachi SU8230) was used to characterize the nanofiber membranes and the carbon nanofiber membranes.

X-ray diffractometer (XRD, Smartlab 9 kW, Rigaku) was utilized to analyze the microstructural parameters of the nanofibrous membranes. The X-ray source was Cu Kα with a wavelength of λ = 0.1542 nm, operating in a step scanning mode of 1 s/step with a scanning step size of 0.02°, scanning range from 10° to 90°.

Microconfocal Raman spectroscopy (Renishaw, inVia Qontor) was utilized for Raman testing of the carbon material structure.

Transmission electron microscope (TEM, JEOL, JEM-2100Plus) was used to characterize the microscopic morphology, analyze the crystal type, and determine the crystal arrangement direction. The test conditions were an accelerating voltage of 200 kV and an information resolution of 5 nm.

Fourier transform infrared (FTIR, Nicolet iS10) was used to characterize the fiber membranes treated with different pre-oxidation temperatures.

Thermogravimetric analysis (TGA, NETZSCH STA2500) was raised from room temperature to 950°C under a nitrogen atmosphere at a rate of 10°C/min to investigate the impact of various CNT contents on the residual carbon rate of the samples.

Differential scanning calorimeter (DSC, NETZSCH 214 Polymer) was used to test the exothermic curves of the samples. The temperature was ramped up from room temperature to 500°C under a nitrogen atmosphere at a rate of 10°C/min.

The electrochemical impedance spectroscopy (EIS) of PEMFC was measured by electrochemical work-station. (American Gamry Reference 3000).

Result and discussion

Effect of CNT on the pre-oxidation process

As shown in Figure 2(A),the addition of CNT changes the intensity of the absorption peaks of -CN at 2240 cm⁻¹ and the -C = N at 1600 cm⁻¹. Figure 2(B) that the active sites on the surface of CNT promote the cyclization reaction of polymer molecules, promote the cyclization and dehydrogenation of PAN, and the cyclization rate increases with the increase of carbon nanotube content. The internal space and surface of CNT can provide the place required for the cyclization reaction, making the cyclization process more efficient. When the CNT content reaches 3 wt%, the relative cyclization rate of the fibrous membrane increases to 44%. The original PAN molecular chain will be terminated once every 4∼5 cyclization units due to the helical conformation. However, the anchoring effect of CNT will restrict the movement of the PAN molecular chain. Consequently, the PAN molecular chain exhibites a more planar zigzag conformation, leading to an increase in the length of the cyclization sequence, which would enhance the cyclization of PAN. During the heat treatment process, CNT can be used as a physical template to guide the arrangement of polymer molecules around it, thereby promoting the directional arrangement of fibers. The van der Waals force and π-π interaction between CNT and polymer molecules can enhance the binding force between molecules and contribute to the stability and arrangement of fibers.

Figure 2.

(A) FTIR spectra of fibrous membranes with different CNT contents; (B) relative cyclization rate and degree of cyclization; (C) DSC curves; (D) TG curves.

From Table 1, with the increase of CNT content, the exothermic enthalpy decreases and the exothermic rate slows down, suggesting that CNT can reduce the possibility of heat release concentration in the pre-oxidation process. The trend of the curve in Figure 2(C) indicates that CNT has no significant effect on the peak temperature of the pre-oxidation exotherm. The mass loss rate of the samples is tested by TG. The experimental results show that the mass loss rate of nanofiber membranes at high temperatures decreases significantly with the increase of CNT content. In particular, at 950°C, the residual mass of the sample containing CNT is about 10% higher than that of the sample without CNT. In the temperature range of 600°C–950°C, the addition of CNT can effectively reduce the mass loss rate of nanofiber membranes while increasing the residual mass of the membranes. This is due to CNT’s ability to improve the thermal stability of the fibrous membranes. At high temperature, the molecular structure in the nanofiber membrane will change, such as the fracture of chemical bonds and the volatilization of small molecular substances, which will lead to mass loss. CNT has good thermal properties such as high thermal conductivity, which can make the heat transfer quickly and evenly in the fiber membrane and avoid the occurrence of local overheating. Local overheating can easily lead to excessive breakage of chemical bonds in the local area and rapid volatilization of the material, while CNT makes the heat evenly distributed, thereby reducing the mass loss and improving the thermal stability of the fiber membrane. And CNT enhances the structural integrity of the fibrous membranes through its excellent mechanical properties.

Table 1.

DSC data with different PAN/CNT.

PAN-CNTs	T_i (°C)	T_c (°C)	T_f (°C)	$Δ H$ (J/g)	$\frac{Δ H}{Δ T}$ (J/g/°C)
PAN-0C	282	310	321	567	14.7
PAN-1C	291	310	318	558	20.7
PAN-2C	286	309	318	484	15.1
PAN-3C	288	309	320	461	14.2

T_i: Initial exothermic temperature.

T_f: Termination exothermic temperature.

$\frac{Δ H}{Δ T}$ : exothermic rate.

T_c: Maximum cyclization exothermic temperature.

$Δ H$ : exothermic enthalpy.

Effect of CNT on the carbonization and graphitization process

XRD and Raman tests are performed to evaluate the microcrystalline structure of nanofiber membrane after carbonization. In XRD patterns, the degree of graphitization of a carbon material can be assessed by analyzing the position and intensity of the diffraction peaks at the (002) crystal plane. As depicted in Figure 3(A) and Table 2, the carbonized nanofiber membranes exhibit characteristic broad peaks associated with the (002) crystal plane, ranging from 22° to 25°. The incorporation of CNT does not markedly alter the crystallographic structure of the carbon fibers. In order to further analyze the alterations in the crystal structure, the microcrystalline parameters, including grain size (Lc) and crystal plane spacing (d₀₀₂), were quantified. During carbonization, the d₀₀₂ decreases with the increase of CNT content, while the L_c gradually increased. This indicates that the graphite microcrystals gradually transitioned from a chaotic layer structure to ordered layer structure, and the grains gradually approaches completion and the microcrystalline defects decreased. In Raman spectroscopy, the D peak is associated with defects and disordered structures in carbon materials, while the G peak corresponds to the in-plane vibrations of hybridized carbon atoms. The degree of graphitization of the carbon material can be assessed by calculating the intensity ratio (I_D/I_G) of the D-peak to the G-peak. Figure 3(B) reveals that the addition of CNTs to the carbon nanofiber membranes enhances their structural integrity, diminishes defects, and reduces amorphous regions. Consequently, the intensity ratio of the D-peak to the G-peak (I_D/I_G) significantly decreases, indicating a reduction in disordered and amorphous carbon and an increase in surface graphitization. The addition of CNT during carbonization promotes a more orderly arrangement of carbon atoms.

Figure 3.

Nanofiber membrane with different CNT content: (A) XRD and (B) Raman spectra.

Table 2.

Crystal structure parameters of carbon nanofiber membranes.

CNT concentration (wt%)	2θ (°)	d₀₀₂ (nm)	L_C (nm)	Carbon yield (%)
0	23.574	0.376	3.32	0.49
1	23.747	0.374	3.35	0.50
2	23.863	0.372	3.63	0.57
3	24.736	0.359	3.56	0.61

As evident from Figures 4 and 5, alterations in the molecular structure of the fibers during carbonization affect the internal molecular arrangement, thereby impacting the local stress within the fibers and leading to multiple fracture points. However, the reinforcement of the CNT mitigates the fiber fracture during the carbonization process. Because CNT has a certain charge, it can change the electric field distribution around it, so that the electric field force on the droplet is enhanced, which leads to the enhancement of the stretching effect of the droplet in the electrostatic field. The droplets are stretched longer and thinner, so that the diameter of the fiber becomes smaller during the formation of the fiber. The addition of CNT enhances the stretching effect of droplets in the electrostatic field, making the fiber finer. The carbonization treatment ensures that the impurities in the nanofibers are fully volatilized, which makes the internal structure of the fibers more dense. At the same time, due to the release of the space occupied by the impurities, the fiber will shrink as a whole, resulting in a smaller fiber diameter. The porosity of the carbon nanofiber membrane is further improved. Combined with the high magnification SEM image in Figure 5, it can be observed that as the CNT content increases, the number of nanoscale pores on the fiber surface decreases, and the frequency of fiber breaks also decreases. This demonstrates that the inclusion of CNT helps protect the surface of the nanofibers during the carbonization process. As shown in Figure 5(D), When the CNT content is 3 wt%, there are some bumps and agglomerations on the surface of the nanofibers, indicating a high CNT content and relatively poor dispersion. During the preparation process, the high content of CNT tends to agglomerate together due to mutual van der Waals forces and other effects. This agglomeration phenomenon not only affects the uniform distribution of CNT in the fiber, but also negatively affects the fiber properties

Figure 4.

(A) Fiber diameter before and after carbonization; (B) porosity change after carbonization with different CNT contents.

Figure 5.

Carbon nanofiber surface micromorphology under different CNT concentrations: (A) 0 wt% CNT; (B) 1 wt% CNT; (C) 2 wt% CNT; (D) 3 wt% CNT.

From the pore size distribution in Figure 6, the addition of CNT will make the pore size distribution of carbon nanofibrous membranes more uniform. The range of the pore size distribution changes from the initial 36 μm∼54 μm to 29 μm∼44 μm, which is conducive to obtaining fibrous MPLs with larger pore sizes after subsequent graphitization.

Figure 6.

Pore size distribution of fiber membranes under different CNT concentrations: (A) 0 wt% CNT; (B) 1 wt% CNT; (C) 2 wt% CNT; (D) 3 wt% CNT.

The Raman spectra of carbon nanofiber membranes after graphitization treatment are exhibited in Figure 7. With the increase in the content of carbon nanotubes, the ID/IG of carbon nanofiber membranes decreases. Due to the CNT play the role of induced alignment during the graphitization process, which guides the carbon atoms around them to be arranged according to a specific orientation, thus promoting the formation of a more ordered graphitization structure formation and enhance the graphitization degree of carbon nanofiber membranes. The increase in graphitization improves the electrical conductivity of the carbon nanofiber membrane, which helps to reduce the ohmic loss in its mass transfer process.

Figure 7.

(A) Raman spectra of samples containing different CNT contents; TEM image of nanofiber membrane after graphitization at 2800°C: (B) containing CNT, and (C) no CNT.

Figure 7(B) is the TEM image of nanofiber membrane with CNT added after graphitization treatment at 2800°C, and (C) is the TEM image of carbon grains without CNT added after graphitization treatment at 2800°C, and it can be seen that the order of the carbon grains is improved after the addition of CNT, and the CNT induces the arrangement of the carbon grains in the graphitization process.

Electrochemical characterization of single cells with MPL

Figure 8(A) and (B) shows the polarization curves of each group of GDL under different cathode humidification conditions. At low current densities (<0.2 A/cm²), the performance of the cells at 60% relative humidity (RH) and 100% RH is closely comparable. But at medium to high current densities (>0.2 A/cm²), a significant discrepancy in cell performance is observed. CFP-2 C exhibits superior performance over CFP-BC (prepared with the conventional particle MPL) at both 60% and 100% RH, indicating that the self-wetting capability of the fibrous MPL is greater than that of the granular MPL at 60 % RH, and the carbon black particle accumulation structure in the traditional particle MPL is not conducive to the discharge of liquid water at 100% RH. The fiber structure of the fiber MPL is easy to flow gas, which improves the water management function of the fuel cell under high relative humidity.

Figure 8.

Polarization curves for different gas diffusion layers (A) 60% RH; (B) 100% RH; Power density curves for different gas diffusion layers (C) 60% RH; (D) 100% RH.

The power density (power output per unit area) directly reflects the power output capability of PEMFCs at different scales. It is closely related to the energy conversion efficiency of PEMFCs.Higher power density is usually accompanied by a more efficient energy conversion process. From the power density curves (Figure 8(C) and (D)), it can be found that sample MPL has a power density of 0.5 W/cm² and a maximum current density of 1.2 A/cm² at 100% RH, indicating that the proton exchange membrane can be fully wetted at 100% RH. The mobility of protons from the anode catalyst layer to the cathode catalyst layer is increased, and the reaction gas is more fully reacted in the catalyst layer under high current density. The carbon black in the traditional particle MPL is tightly packed, and small particles are easy to agglomerate, which is not conducive to the outflow of liquid water and the transport of reaction gas to the catalyst layer, increasing the mass transfer impedance, and premature concentration polarization of the sample. Compared with CFP-2C group, the maximum current density and maximum power density of CFP-BC decreased by 40.8% and 33.3% when operating at 60% RH. The pore distribution of carbon nanofibers is uniform, which guarantees gas transmission and drainage ability, making the cell less prone to ‘flooding’ during operation. At 100% RH, the power density of CFP-2C group is about 1.5 times that of the traditional particle MPL, and the current density is still up to 1.2 A/cm², while the current density of the traditional particle MPL is down to 0.7 A/cm². The current density and power density of CFP-1C group also showed an increasing trend at 100% RH.

Setting up electrochemical impedance tests at 100% RH and 60% RH to evaluate the effect of different types of MPL on cell performance. It can be seen from Figure 9 that the two intersection points between the curve and the real axis are respectively ohmic impedance and mass transport impedance. The mass transport capacity in the GDL can be judged by comparing the electrochemical impedance of different cathode diffusion layers under the same test conditions. The loss forms of the cell are different under different current densities. The ohmic loss occurs mainly under low current densities, which is mainly caused by the assembly of each component and the impedance of the proton transmitted in the proton exchange membrane. Mass transfer impedance is mainly generated at high current density. The impedance test is performed under the condition that the current density I = 2.5 A/cm². It can be seen from Figure 9 that the charge transfer impedance of each layer is the same at both high and low relative humidity, while the mass transport impedance of the MPL containing traditional particles is larger, which will cause poor liquid drainage. When the CNT content in the fiber MPL is 2 wt%, the prepared MPL has the best water management function when the cell runs at 100% RH, and the mass transmission impedance is the least 0.4 mΩ.

Figure 9.

Impedance test curves of different gas diffusion layers: (A) 60% RH; (B) 100% RH.

Conclusion

In this work, a carbon nanofiber membrane MPL is prepared by electrospinning technology. The effect of carbon nanotube on the heat treatment process of carbon nanofiber membranes was investigated. With the introduction of CNT, the cyclization of polyacrylonitrile in the pre-oxidation stage increased, and the microstructure and porosity of the fibrous membranes were improved. In addition, CNT has the effect of inducing the arrangement of carbon atoms, and with the increase of CNT content in the fiber membrane, the graphitization of the fiber membrane after graphitization increases. Furthermore, the fiber MPL is applied to a PEMFC in order to evaluate the electrochemical performance of the cell, and compared with conventional carbon black MPL to illustrate the effect of the pore structure of the fiber MPL on the water management effect. The results show that compare with the particulate MPL, the carbon nanofiber membrane MPL has the advantages of high porosity and uniform distribution of macropores, which enables the smooth flow of gas and liquid water, resulting in better electrochemical performance of the PEMFC. Compared with traditional particle MPL, the carbon nanofiber membrane MPL with CNT (CFP-2C), when the relative humidity is 100%, increases the power density and current density of PEMFC by 47.5% and 55.3%, respectively, and decreases the ohmic impedance by 37.5%.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Mingming Yu

Lin Fang

Pibo Ma

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Fabrication of novel carbon nanofiber membrane microporous layer to improve water management ability of PEMFC

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of fiber MPL

Assembly of single cell

Measurements

Result and discussion

Effect of CNT on the pre-oxidation process

Effect of CNT on the carbonization and graphitization process

Electrochemical characterization of single cells with MPL

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References