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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.33 No.4 pp.181-190
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2023.33.4.181

MXene Based Composite Membrane for Water Purification and Power Generation: A Review

Seohyun Kim, Rajkumar Patel†
Energy and Environmental Science and Engineering, Integrated Science and Engineering Division, Underwood International College, Yonsei University, Incheon 21983, South Korea
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr; http://orcid.org/0000-0002-3820-141X)
July 27, 2023 ; August 24, 2023 ; August 25, 2023

Abstract


Wastewater purification is one of the most important techniques for controlling environmental pollution and fulfilling the demand for freshwater supply. Various technologies, such as different types of distillations and reverse osmosis processes, need higher energy input. Capacitive deionization (CDI) is an alternative method in which power consumption is deficient and works on the supercapacitor principle. Research is going on to improve the electrode materials to improve the efficiency of the process. A reverse electrodialysis (RED) is the most commonly used desalination technology and osmotic power generator. Among many studies conducted to enhance the efficiency of RED, MXene, as an ion exchange membrane (IEM) and 2D nanofluidic channels in IEM, is rising as a promising way to improve the physical and electrochemical properties of RED. It is used alone and other polymeric materials are mixed with MXene to enhance the performance of the membrane further. The maximum desalination performances of MXene with preconditioning, Ti3C2Tx, Nafion, and heterostructures were respectively measured, proving the potential of MXene for a promising material in the desalination industry. In terms of osmotic power generating via RED, adopting MXene as asymmetric nanofluidic ion channels in IEM significantly improved the maximum osmotic output power density, most of them surpassing the commercialization benchmark, 5 Wm-2. By connecting the number of unit cells, the output voltage reaches the point where it can directly power the electronic devices without any intermediate aid. The studies around MXene have significantly increased in recent years, yet there is more to be revealed about the application of MXene in the membrane and osmotic power-generating industry. This review discusses the electrodialysis process based on MXene composite membrane.


electrodialysis , RED , Nafion , MXene

정수 및 발전을 위한 맥신(MXene) 복합막에 관한 고찰

김 서 현, 라즈쿠마 파텔†
연세대학교 언더우드학부 융합과학공학부 에너지환경융합전공

초록


폐수 처리는 담수 공급의 수요를 맞추고 동시에 환경 오염을 제어하기 위한 가장 중요한 기술 중 하나이다. 여러 종류의 증류법과 역삼투 공정과 같은 다양한 기술은 더 높은 에너지 투입을 필요로 한다. 축전식 탈염(CDI) 기술은 전력 소 비가 매우 적어 슈퍼커패시터 원리에 기반한 대안으로 떠오르고 있다. 공정의 효율성을 향상시키기 위해 전극 재료를 개선하 기 위한 연구가 계속되고 있다. 역전기투석은 가장 일반적으로 사용되는 담수화 기술 및 삼투압 발전기이다. 역전기투석의 효 율을 향상시키기 위해 수행된 많은 연구 중, 맥신(MXene)은 이온교환막 및 2차원 나노유체 채널로서 역전기투석의 물리적 및 전기화학적 특성을 향상시킬 수 있는 유망한 방법으로 떠오르고 있다. 맥신은 단독 사용뿐만 아니라 다른 물질들이 맥신 과 혼합되어 복합막의 성능을 더욱 향상시킨다. 전처리를 거치거나 Ti3C2Tx, 나피온 등을 포함한 이종구조를 가진 맥신은 각 각 최대 담수화 성능 측정 결과를 통해 담수화 산업에서 유망한 재료로 맥신의 잠재력을 입증했다. 역전기투석을 통한 삼투 압 발전 산업에서 이온교환막에서 비대칭 나노유체 이온 채널에 맥신을 사용함으로써 최대 삼투압 출력 밀도를 크게 향상시 켰으며, 대부분 상용화 기준값인 5 Wm-2를 넘었다. 일정 개수의 단위체를 연결함으로써 매개체의 도움 없이 전자기기에 직접 적으로 전력을 공급할 수 있는 수준의 전압이 출력됐다. 본 리뷰에서는 맥신 복합막을 기반으로 한 전기투석 공정의 최근 연 구들에 대해 설명한다


    1. Introduction

    Due to the increasing demand for freshwater and the side effects of the energy generated by fossil fuels caused by the growing population, the necessity for clean, renewable, and sustainable energy sources and desalination technology is rising as an urgent problem for humanity. Currently, the primary energy sources are mostly fossil fuels such as petroleum, natural gas, and coal, which emit air pollutants and produce greenhouse gas. Thus, many studies of desalination technologies that can acquire freshwater from brackish water and energy harvesting technologies are being performed to improve energy efficiency in an environmentally protective manner[1-4].

    Unlike traditional desalination technologies like reverse osmosis (RO), electrodialysis, and distillation, which demand high cost, excellent energy consumption, and cause secondary pollution, the recently suggested capacitive deionization (CDI) has become a promising desalination technology based on its low energy demand, capital, and far less possibility of secondary pollution. To enhance the low-desalination capacity performed by conventional carbon-based CDI, the introduction of novel electrode materials has been an essential task in recent years[5-7].

    In the energy harvesting industry, salinity gradient power (SGP) has risen as an effective alternative that can generate power regardless of environmental conditions without any pollutant emission. As SGP is generated from the concentration difference between freshwater and seawater instead of brine water, which is less accessible, it is accepted as a promising renewable energy with no emissions of CO2, also known as blue energy. Reverse electrodialysis (RED), one of SGP, is often used to convert chemical energy into electrical energy by alternating stacks of cation and anion exchange membranes (CEMs and AEMs)[8-9]. The classification of the review is presented in Scheme 1.

    2. MXene Based Membrane Fabrication

    The formula of MXene is Mn+1XnTx (n=1-3), where M is an early transition metal (i.e., Ti, Cr, Mo, V, Nb), X is C and/or N and Tx represents surface terminations (i.e., O, OH, and/or F)[10]. The 2D MXene membrane shows excellent mechanical strength, flexibility, and hydrophilic, stable state in the aqueous state due to the abundant O and OH groups on the surface. Das et al. prepared MXene based nanocomposite membrane for wastewater treatment[11]. Ti3AlC2 was etched with HF to remove aluminum and stacked MXene layer generated. It was further agitated by sound wave to exfoliate the stacked structure and mixed with monomer for in situ polymerization and nanocomposite was generated. The schematic of the preparation is presented in Fig. 1.

    SEM image of MXene and preparation of nanocomposite is presented in Fig. 2.

    Huang et al. prepared antifouling membrane made up of TiO2 composite with MXene[12]. Kang et al. prepared MXene decorated membrane for ion sieving process[13].

    3. Capacitive Deionization (CDI)

    In capacitive deionization saline solution was fed through a pair of electrodes which are separated by a spacer which is presented in Fig. 3[14]. There is no redox reaction involved in this process and it is purely based on non-Faradic nature.

    In case of the membrane capacitive deionization anion and cation exchange membranes are placed next to the cathode and anode respectively. It has better charge and ion adsorption efficiency than CDI.

    Mansoor et al. fabricated a flowing electrode capacitive deionization system as shown in Fig. 4[15]. The characterization of Ti3C2Tx by SEM and Raman spectroscopy is presented in Fig. 5. In this CDI system, Ti3C2Tx is used as an electrode of ammonia. It showed a 60% ion removal at a constant voltage of -1.20 to 1.20 V and adsorption of 460 mg/g.

    Liu et al. used NaTi2(PO4)2 (NTP) on carbon as an electrode in CDI system and found out that the salt adsorption capacity is 124.72 mg/g at a constant voltage of 1.8 V[16]. At the same time the electrode possesses excellent stability until 100 cycles.

    MXene, a promising 2D material for ion exchange membranes, was mixed with Ti3 C2Tx to enhance the hydrophilic and conductivity[17]. By blending method, Ti3C2Tx MXene was mixed with polyvinylidene fluoride (PVDF) cation exchange membranes (CEMs) as nanofillers to improve the anti-fouling ability of the membrane and reduce the membrane resistance.

    The results indicated that 3 %wt MXene/PVDF CEM showed the best performance among the other CEMs, all of which successfully achieved less energy consumption and higher mass transfer capacity compared to unmodified CEMs. Specifically, 3 %wt MXene/PVDF CEM had areal resistance of 3.3 Ω⋅ cm2, implying the formation of conductive networks within the membrane matrix attributed to Ti3 C2Tx. Furthermore, strong interaction between PVDF and MXene resulted in the enhancement of thermal and mechanical stability of the membranes.

    Hybrid capacitive deionization (HCDI), with a faradic ion storage mechanism, has been rising as a promising desalination technology with high charge efficiency and desalination capacity. To improve its low long-term stability and desalination rate, the study has synthesized MoS2/MXene heterostructure via hydrothermal method, making 3D “mutually supported” structure by combining 2D MXene and 2D MoS2 nanoflakes[18]. MoS2/MXene heterostructure electrode- based HCDI system showed much increased desalination capacity and desalination rate, each reaching 23.98 mg g-1 and 4.6 mg g-1 min-1. Outstanding cycling stability was also recorded with only a 4% decrease in desalination capacity over 100 cycles, showing that this work has suggested a promising way to improve the efficiency of the HCDI system.

    Pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were investigated in terms of discharge potential, half-cycle length (HCL), and flow rate[19]. Salt adsorption capacity (SAC) and rate (SAR) were profoundly increased up to 152% with lower discharge potentials, whereas longer HCL decreased SAR by 54% while increasing SAC by 32%, meaning less impact on the desalination performance. The flow rate was recognized as an important factor in controlling the desalination performance, resulting in a decrease of both SAC and SAR by 20% as flow rates increased. Pre-conditioned MXene electrodes, compared to activated carbon cloth (ACC) electrodes, showed 30% lower gravimetrical performance, but 162% higher volumetric performance, remarkably outperforming ACC. Overall, optimizing the operating conditions of MXene will be the key to maximizing desalination performance.

    Xu et al. prepared TiC3C2Tx electrode by intercalating with bacterial cellulose nanofibers coated with conducting polypyrrole polymer for application in asymmetric capacitive deionization systems[20]. The performance of the system is presented in Fig. 6. This system has excellent desalination properties by removing 17.56 mg/g chloride ion at 1.2 V. The efficiency of the electrode is intact until 30 cycles.

    4. Reverse Electrodialysis

    Reverse electrodialysis (RED) being the most commonly used salinity gradient power (SGP) for a new alternative to the renewable energy system, requires the ion exchange membrane (IEM) with low thickness and high permselectivity. Fig. 7 and 8 demonstrate the schematic of bench scale RED system[21].

    Jang et al. fabricated a concrete-structured Nafion@ MXene/Cellulose acetate (NMC) composite cation exchange membrane (CEM) using a facile dip casting method with electrospun MXene/CA nanofiber as a framework, and Nafion ionomer as a filler[22]. Compared to layered NMC, casted Nafion, and nanofiber, NMC CEM showed improved physical/electrochemical properties such as water uptake (WC) 18.7%, areal resistance (AR) 1.62 Ω cm-1, ion exchange capacity (IEC) 1.33 meq g-1, and permselectivity 99.2%. In addition, high mechanical stability, open-circuit voltage (OCV) of 2.77 V, and power density of 2.30 W m-2 were obtained in the RED system.

    As membrane-based reverse electrodialysis (RED) is regarded as the most efficient osmotic power generator, this study proposed oppositely charged Ti3C2Tx MXene membranes (MXMs) with confined 2D nanofluidic ion channels[23]. In addition to typical surface-chargegoverned ion transport, negatively and positively charged 2D lamellar MXene nanochannels showed superior ion selectivity and electrochemical energy conversion efficiency. The maximum power density, 4.6 Wm-2, was obtained, which value is closest to 5 Wm-2, the commercialization benchmark. This was done by mixing the river water (0.01 M NaCl) and artificial sea water (0.5 M NaCl). Connecting ten tandem MXMRED stacks resulted in an output voltage high up to 1.66 V, which can power the electronic devices directly. The physical confinement and MXene’s excellent chemical stability also enabled the MXM-based RED system to exhibit high long-term stability.

    MXene can be used to synthesize asymmetric nanofluidic ion channels, which can enhance the osmotic energy conversion, by combining tunable surface charge properties and hydrophilic surfaces. Thus, this study suggested flexible, robust, and easy-to-scale-up bioinspired heterogeneous MXene membrane (BHMXM) with an ionic diode-type current[24]. Asymmetric surface charge polarity and high surface charge density of the heterogeneous membrane contributed to highly rectified current, thus enabling the effective avoidance of ion polarization in the osmotic energy conversion system. BHMXM showed power densities of 8.6 Wm-2, and 17.8 Wm-2 at a 500-fold salinity gradient of river water and synthetic seawater, which highly exceeds 5 Wm-2, the commercialization benchmark. The new design of nanofluidic channels based on MXene stimulated the development of large-scale 2D nanofluidic channels with selective ion transport.

    Adopting nanofluidic channels in the ion-exchange membrane-based reverse electrodialysis (RED) devices is a promising way to enhance the performance of osmotic power generators. This study suggested positively and negatively charged MXene fibers (P-MF and N-MF) as channels to produce osmotic energy that depends on the gradient of seawater and artificial river water[25]. The oppositely charged fiber membranes exhibit excellent ion selectivity and transmissibility due to the high surface charge densities (~3.8 mCm-2), and narrowness of the nanochannels (< 2 nm). An output power density of 12.3 Wm-2, higher than the commercialization benchmark (5 Wm-2), was observed consistently for three months. Connecting 10 unit cells provided the output voltage of 1.7 V, implying the oppositely charged fiber membrane can directly power the electronic devices.

    In the trend of using light as a tool to enhance the directional ion transport for osmotic energy conversion, light-induced heat-driven ion transport performance within the MXene membrane was conducted in nanofluidic systems[26]. Temperature difference reached about 19.91°C with the ionic current range from 0 to -37.5 nA as soon as illumination was applied asymmetrically due to the light-induced heat effect. Under the salinity gradient, lamellar MXene membrane with excellent photothermal effects exhibited significantly better osmotic energy conversion output power density up to 1.68 mW·m–2, and more than twice the value under circumstances without illumination. The study has paved the way to control ion transport under the external stimuli in nanofluidic systems for ion-related mass transport, desalination, and energy conversion.

    To overcome the low energetic efficiency of membrane- based reverse electrodialysis (RED), a hetero genous two-dimensional lamellar Ti3C2Tx membrane with surface-charge channels and asymmetric structure was designed[27]. The surface-charge channels exhibited unidirectional ion transport behavior, meaning that a specific transport direction exists within the nanochannels. The performance of the heterogenous Ti3C2Tx membrane as an osmotic energy generator was higher than the commercialization benchmark (5 Wm-2), reaching 16 Wm-2 of an output power density on mixing the river water and natural brine. Theoretical calculations done with Poisson-Nernst-Planck equations confirmed that the asymmetric structure of the membrane successfully weakened the concentration polarization and at the same time maintained the high ion selectivity. A Newly suggested heterogenous Ti3C2Tx membrane has paved the way for artificial nanofluidic membranes and their use for efficient osmotic energy harvesting.

    Due to the high diffusion rate, the anion-selective nanofluidic channel is uncommon but needed for the application of reverse electrodialysis (RED). Here, a two-dimensional layered MXene membrane with positive charge was suggested using the animation of 3-aminopropyltriethoxysilane (APTES) as an efficient osmotic power harvesting tool in terms of anion selectivity[ 28]. The resultant membrane reached the maximum power density of 10.98 Wm-2, higher than the commercial benchmark of 5 Wm-2, at the 50-fold salinity gradient of river water and artificial seawater. Under the regulated temperature, the power density went up to 20.66 Wm-2. 10-h testing resulted in only a 3% loss of output power density, and the applicable anti-pollution ability of the membrane was maintained under the polluted environment of organic pollution or heavy metal ions.

    5. Conclusions and Future Prospects

    In conclusion, the utilization of MXene, both as an ion exchange membrane (IEM) and in combination with other materials, shows great promise in improving the efficiency and performance of reverse electrodialysis, which is the most commonly used desalination technology and osmotic power generator. Numerous studies have been conducted to enhance the physical and electrochemical properties of RED, and MXene has emerged as a leading candidate. The maximum desalination performances of MXene, including MXene with preconditioning, Ti3C2Tx, Nafion, and heterostructures, have been measured, demonstrating the potential of MXene as a promising material in the desalination industry. Moreover, MXene used as asymmetric nanofluidic ion channels in IEM has significantly increased the maximum osmotic output power density, surpassing the commercialization benchmark of 5 Wm-2. Furthermore, by connecting multiple unit cells, the output voltage has reached a level where it can directly power electronic devices without the need for any intermediate aid. While the studies on MXene have evidently increased in recent years, there is still much more to be discovered about its applications in the membrane and osmotic power-generating industry.

    The presence of abundant functional groups enhances the properties of the composite membrane and its properties can be easily tuned by the functionalization of various types of group. The long-term stability of MXene is a key issue that need a solution. One of the solutions is to exfoliate and functionalize the 2D structure before the fabrication of the composite polymer membrane.

    Figures

    MEMBRANE_JOURNAL-33-4-181_S1.gif

    Schematic presentation of the topic.

    MEMBRANE_JOURNAL-33-4-181_F1.gif

    Schematic representation of the fabrication method of the PMC nanocomposite (Reproduced with permission from Das et al.[13], Copyright 2021, American Chemical Society).

    MEMBRANE_JOURNAL-33-4-181_F2.gif

    (a) XRD pattern of pure MXene and PMC nanocomposites. (b) XPS survey scan. SEM images of (c) pure MXene and (d, e) PMC nanocomposites with different magnifications. (f) TEM image of PMC nanocomposites. (g) Fabrication approach of the free-standing composite membrane by the vacuum filtration technique (Reproduced with permission from Das et al.[13], Copyright 2021, American Chemical Society).

    MEMBRANE_JOURNAL-33-4-181_F3.gif

    Schematic representation of membrane capacitive deionization (MCDI) system (Reproduced with permission from Gberno et al.[14], Copyright 2020, MDPI).

    MEMBRANE_JOURNAL-33-4-181_F4.gif

    Schematic illustration for a Flowing electrode capacitive deionization (FE-CDI) module for deionization testing, and b FE-CDI unit cell assembled with: (i) titanium current collectors; (ii) vitreous carbon; (iii) carbon cloth; (iv) rubber gaskets; (v) anion and cation exchange membranes; (vi) spacer; (vii) polyester filter felt (Reproduced with permission from Mansoor et al.[15], Copyright 2022, Springer Nature).

    MEMBRANE_JOURNAL-33-4-181_F5.gif

    a XRD patterns of Ti3AlC2 (bottom) and Ti3C2Tx (top) before and after etching, respectively; b Raman spectra of Ti3C2Tx flow electrode; c TEM image of as-etched Ti3C2Tx with arrows highlighting separated layers; Scale bar = 0.2 μm. d SEM image of as-etched Ti3C2Tx with SEM image of Ti3AlC2 (inset); Scale bar (s) = 5 μm. e SEM image of activated carbon (AC) powder particle. Scale bar = 5 μm (Reproduced with permission from Mansoor et al.[15], Copyright 2022, Springer Nature).

    MEMBRANE_JOURNAL-33-4-181_F6.gif

    (a) Concentration variation of the outlet water from a hybrid capacitive deionization cell using different MXene-based electrodes; comparison of the (b) salt adsorption capacity (SAC), Cl– removal capacity, and (c) charge efficiency between MXene-based electrodes at charging voltages of 0.8, 1.0, and 1.2 V; (d) energy consumption (Ev) as a function of the average concentration reduction (Δc) of MXene-based electrodes at 1.0 V (bottom left) and 1.2 V (top right); (e) salt adsorption removal rate (SAR) vs time and (f) SAR vs SAC of MXene-based electrodes at 1.2 V; (g) stability performance of the MBP-72.7% electrode over 30 cycles at 1.0 V (Reproduced with permission from Xu et al.[20], Copyright 2022, American Chemical Society).

    MEMBRANE_JOURNAL-33-4-181_F7.gif

    Schematic diagram of a bench-scale RED system (Reproduced with permission from Oh et al.[21], Copyright 2018, American Chemical Society).

    MEMBRANE_JOURNAL-33-4-181_F8.gif

    Schematic diagram of a bench-scale RED system (Reproduced with permission from Oh et al.[21], Copyright 2018, American Chemical Society).

    Tables

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