WO2020067999A1

WO2020067999A1 - A membrane, related methods and system

Info

Publication number: WO2020067999A1
Application number: PCT/SG2019/050489
Authority: WO
Inventors: Xinwei Chen; Shu Mei Man
Original assignee: Agency For Science, Technology And Research
Priority date: 2018-09-25
Filing date: 2019-09-25
Publication date: 2020-04-02

Abstract

There is provided a membrane comprising a plurality of pore channels coated with an oil layer for selectively allowing movement of gaseous water vapor across the membrane but substantially preventing movement of liquid water across the membrane, wherein the membrane is composed of a composite impregnated with oil adsorbing particles. Also provided is a method of making said membrane, a thermo-osmotic energy conversion system and a thermo-osmotic energy conversion method.

Description

A MEMBRANE, RELATED METHODS AND SYSTEM

TECHNICAL FIELD

The present disclosure relates broadly to a membrane and a method of making a membrane. The present disclosure also relates to a system and a method of using the membrane. BACKGROUND

An estimated 34 billion gigajoules of industrial heat (equivalent to 5.8 billion barrel of oil) is lost yearly due to lack of promising technologies to economically harvest this. Currently, traditional steam techniques and binary cycle can efficiently recover medium (232°C < T < 646°C) and high-grade waste heat (T > 646°C). The main bulk of the industrial waste heat (-60%), however, exists as low-grade heat (T < 232°C) and recovering energy from these sources remains challenging. Indeed, extracting energy from low-grade heat sources such as below

150°C is limited with existing technologies due to the inefficiency and high cost of current technologies, and this is further elaborated as below.

Known technologies that convert low-grade heat to useful energy include: (1) Organic Rankine binary cycle system (ORC), (2) solid-state thermos-electric devices (TE), and (3) thermally regenerative electrochemical cycle (TREC). ORC uses an organic working fluid and operates similarly to Rankine cycle. ORC, however, is unable to tolerate fluctuations in heat source. TE remains costly and inefficient due to the type of materials required. TREC uses metal complexation reactions or temperature dependence of electrochemical redox potentials. TREC, however, is limited by the heat capacity of materials and effectiveness of heat exchangers. In all, these systems achieved limited success in harnessing low-grade heat and in practice these systems only attain <2% efficiencies of the Carnot cycle most of the time.

Table 1 summaries the limitations of each system for easy comparison.

Table 1. A summary of the limitations of existing systems

An alternative more efficient low-grade heat harnessing method is the thermo-osmotic energy conversion (TOEC) process, which in contrast to the other processes described above, can potentially achieve efficiency of -58% of the Carnot cycle, even with fluctuating heat sources.

In TOEC, a hydrophobic porous membrane is placed between a hot sink (TH) and a cold sink (Tc), with a thin gas barrier trapped due to the membrane. The partial vapor pressure difference due to the temperature difference (DT) drives the fluid (in the form of water vapour) across the porous membrane (which occurs when water evaporates from one meniscus) and transports the water vapour across the membrane which then condenses on the opposite meniscus. As mass transfer continues, the cold sink becomes increasingly pressurized. This resultant increase in pressure can then be directed through a turbine to generate electrical energy.

While the emergence of thermo-osmotic systems has been promising, there are several limitations that restrict their usage. Currently, the TOEC process only works efficiently for temperatures below 60°C and is unable to work well at elevated temperatures due to the limitations of the polytetrafluoroethylene (PTFE) membrane it typically uses. For instance, the power significantly drops when the operating temperature increases from 60 to 80°C. This can be attributed to the following main reasons:

(i) As temperature increases, there is a simultaneous loss of hydrophobicity of PTFE (due to the inverse relationship between temperature and hydrophobic effect since the hydrophobic effect is entropy driven and AS > 0) and increase in wetting ability of water (decreased surface tension and increase in kinetic energy as temperature increases help the water molecule overcomes the intermolecular hydrogen bonding). This may result in pore wetting, where the membrane can no longer maintain the thin barrier and mass transfer of liquid water (as opposed to vapor) occurs in the opposite direction.

(ii) Pore distortion of the PTFE membrane; PTFE’s high thermal expansion coefficient of -112 x 10^-6 m/K means that significant distortion of the pores in the PTFE membrane exists at high temperatures, compromising membrane integrity.

In view of the above, there is a need to address or at least ameliorate the above-mentioned problems. There is also a need to provide a membrane that is suitable for TOEC processes and/or for applications that require the use of membranes (e.g. hydrophobic porous membranes) at high temperatures.

SUMMARY

In one aspect, there is provided a membrane comprising a plurality of pore channels coated with an oil layer for selectively allowing movement of gaseous water vapor across the membrane but substantially preventing movement of liquid water across the membrane, wherein the membrane is composed of a composite impregnated with oil adsorbing particles.

In one embodiment, the oil adsorbing particles comprise carbon nanoparticles. In one embodiment, the walls of the pore channels are at least partially lined with carbon.

In one embodiment, the oil has a viscosity in the range of 5 to 15,000 cSt.

In one embodiment, the oil is selected from the group consisting of silicone oil, Krytox oil, halogenated oil, fluoroether based oil, perfluoropolyether (PFPE) oil, perfluoroalkylether (PFAE), perfluoropolyalkylether (PFPAE), fluorosilicone oil, polytrifluorochloroethylene oil and mixtures thereof.

In one embodiment, the composite comprises a curable mixture with oil adsorbing particles dispersed therein coated onto a porous support and cured.

In one embodiment, the oil adsorbing particles dispersed in the curable mixture are in an amount from 10 wt% to 40 wt% of the curable mixture.

In one embodiment, the porous support comprises pores having pore sizes of no more than about 10 microns.

In one embodiment, the membrane is capable of achieving a power density of at least about 0.20 W/m² at 95°C in a thermo-osmotic energy conversion process.

In one aspect, there is provided a method of making a membrane, the method comprising providing a curable mixture having oil adsorbing particles dispersed therein; coating a porous support with said mixture; curing the mixture coated on the porous support to form a composite membrane having a plurality of pore channels thereon, the membrane being impregnated with the oil adsorbing particles; and coating the pore channels with an oil layer.

In one embodiment, the step of providing a curable mixture having oil adsorbing particles dispersed therein comprises mixing a thermosetting resin, a hardener and oil adsorbing particles together.

In one embodiment, the thermosetting resin comprises an epoxy resin.

In one embodiment, the hardener comprises an amine group containing hardener. In one embodiment, the step of coating the pore channels with an oil layer comprises infusing the composite membrane in oil at a temperature above about 100°C for more than about 24 hours. In one aspect, there is provided a thermo-osmotic energy conversion system comprising a hot sink for containing water at a temperature TH that is higher than room temperature; a cold sink for containing water at a temperature Tc that is lower than TH; a membrane as disclosed herein, the membrane disposed between and separating the hot sink and cold sink.

In one embodiment, the thermo-osmotic energy conversion system further comprises an energy conversion unit for converting energy from water vapor flowing from the hot sink to the cold sink, to electrical energy. In one aspect, there is provided a thermo-osmotic energy conversion method comprising creating a pressurized water vapor flow across a membrane as disclosed herein from a hot sink containing water at a temperature TH that is higher than room temperature to a cold sink containing water at a temperature Tc that is lower than TH; and harnessing the energy created from the flow of water vapor across the membrane.

In one embodiment, TH is from 50°C to 160°C.

DEFINITIONS

The term“substrate” as used herein is to be interpreted broadly to refer to any supporting structure.

The term “layer” when used to describe a first material is to be interpreted broadly to refer to a first depth of the first material that is distinguishable from a second depth of a second material. The first material of the layer may be present as a continuous film, as discontinuous structures or as a mixture of both. The layer may also be of a substantially uniform depth throughout or varying depths. Accordingly, when the layer is formed by individual structures, the dimensions of each of individual structure may be different. The first material and the second material may be same or different and the first depth and second depth may be same or different.

The term“continuous” when used to describe a film or a layer is to be interpreted broadly to refer to a film or a layer that is substantially without gaps or holes or voids across the film or layer. In this regard, a continuous film or a continuous layer is also intended to include a film or a layer that may have trivial gaps or holes or voids that may not appreciably affect the desired properties of the film or the layer. For example, if the layer or film is an oil layer or film, the gaps or holes or voids present (if any) may be at an atomic level such that the desired hydrophobic properties of the layer or film is not compromised.

The term "micro" as used herein is to be interpreted broadly to include dimensions from about 1 micron to about 1000 microns.

The term "nano" as used herein is to be interpreted broadly to include dimensions less than about 1000 nm.

The term“particle” as used herein broadly refers to a discrete entity or a discrete body. The particle described herein can include an organic, an inorganic or a biological particle. The particle used described herein may also be a macro-particle that is formed by an aggregate of a plurality of sub-particles or a fragment of a small object. The particle of the present disclosure may be spherical, substantially spherical, or non-spherical, such as irregularly shaped particles or ellipsoidally shaped particles. The term“size” when used to refer to the particle broadly refers to the largest dimension of the particle. For example, when the particle is substantially spherical, the term “size” can refer to the diameter of the particle; or when the particle is substantially non-spherical, the term“size” can refer to the largest length of the particle. The terms "coupled" or "connected" as used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa.

The term "adjacent" used herein when referring to two elements refers to one element being in close proximity to another element and may be but is not limited to the elements contacting each other or may further include the elements being separated by one or more further elements disposed therebetween.

The term "and/or", e.g., "X and/or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning.

Further, in the description herein, the word“substantially” whenever used is understood to include, but not restricted to, "entirely" or“completely” and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements/components recited after such terms, in addition to other components not explicitly recited. For example, when “comprising” is used, reference to a“one” feature is also intended to be a reference to“at least one” of that feature. Terms such as“consisting”,“consist”, and the like, may in the appropriate context, be considered as a subset of terms such as "comprising", "comprise", and the like. Therefore, in embodiments disclosed herein using the terms such as "comprising", "comprise", and the like, it will be appreciated that these embodiments provide teaching for corresponding embodiments using terms such as“consisting”,“consist”, and the like. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of +/- 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1 % of the disclosed value. Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1% to 5% is intended to have specifically disclosed sub-ranges 1% to 2%, 1 % to 3%, 1% to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. The intention of the above specific disclosure is applicable to any depth/breadth of a range.

Additionally, when describing some embodiments, the disclosure may have disclosed a method and/or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated that the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and/or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

Furthermore, it will be appreciated that while the present disclosure provides embodiments having one or more of the features/characteristics discussed herein, one or more of these features/characteristics may also be disclaimed in other alternative embodiments and the present disclosure provides support for such disclaimers and these associated alternative embodiments.

DESCRIPTION OF EMBODIMENTS

Exemplary, non-limiting embodiments of a membrane, a method of making a membrane, a thermo-osmotic energy conversion system and a thermo-osmotic energy conversion method are disclosed hereinafter.

There is provided a membrane comprising a plurality of pore channels coated with an oil layer. In various embodiments, as the pore channels of the porous membranes are coated/infused with a thin layer of oil, the pore channels of the porous membranes are extremely hydrophobic since oil and water are immiscible.

In various embodiments, the membrane or pore channels is/are suitable for, capable of, configured to or adapted to selectively allow movement of gaseous water vapor across the membrane but substantially prevent movement of liquid water across the membrane under predetermined conditions. Such predetermined conditions may include thermo-osmotic energy conversion (TOEC) conditions and/or temperatures of above about 60°C. Accordingly, in various embodiments, the membrane is one that is suitable for a thermo- osmotic energy conversion process. In various embodiments, the membrane is suitable for or capable of being used to maintain the vapor gap in TOEC process for converting low-grade heat to useful energy. Accordingly, in various embodiments, there is also provided a membrane for a thermo-osmotic energy conversion process, the membrane comprising: a composite having a plurality of pores thereon, the composite being impregnated with oil adsorbing particles; and an oil layer coating the surface of said composite and pores such that under predetermined conditions, the pores selectively allow movement of gaseous water vapor across the membrane but substantially prevent movement of liquid water across the membrane.

In various embodiments, the membrane may be able to maintain a vapor gap between the hot and cold sinks in a TOEC process even at high temperatures for example more than about 60°C, more than about 70°C, more than about 80°C, more than about 90°C, more than about 100°C, more than about 110°C, more than about 120°C, more than about 130°C, more than about 140°C, or more than about 150°C.

In various embodiments, the pores or pore channels of the membrane are nanosized pores or nanosized pore channels. In various embodiments, the pores or pore channels of the membrane comprise pore sizes of no more than about 10 nm, no more than about 9 nm, no more than about 8 nm, no more than about 7 nm, no more than about 6 nm, no more than about 5 nm, no more than about 4 nm, no more than about 3 nm, no more than about 2 nm or about 1 nm.

In various embodiments, the membrane comprises or is composed of a composite impregnated with oil adsorbing particles. In various embodiments, the composite comprises the plurality of pore channels thereon. The oil adsorbing particles may advantageously allow the oil to be adsorbed thereon. In various embodiments, the walls of the pore channels are at least partially lined or completely lined with the oil adsorbing material (e.g. carbon). The oil adsorbing material may be in the form of particles such as nanoparticles or parts thereof. In various embodiments, a plurality of oil adsorbing particles abut each other such that the pores and/or pore channels of the membrane or composite comprise the spaces/passages formed between the abutting particles. The pores and/or pore channels of the membrane or composite may also comprise pores and/or pore channels within the particles. Consequently, the oil may also be coated on the surface of the composite/membrane and/or onto the pore channels e.g. on the walls of the pore channels such that an oil layer is at least partially or completely formed on composite/membrane surface and/or the walls of the pore channels. Thus, the oil layer may advantageously provide the membrane and/or pore channels with the desired hydrophobicity or non-wetting properties.

In various embodiments, any particles that effectively adsorb oil may be used in embodiments of the composite/membrane disclosed herein. In various embodiments, the oil adsorbing particles are carbonaceous particles, for example, activated carbon particles, powdered activated carbon (PAC) particles, granular activated carbon (GAC) particles, extruded activated carbon (EAC) particles, bead activated carbon (BAC) particles, carbon black particles or mixtures thereof. The oil adsorbing particles may comprise carbon nanoparticles, for example activated carbon nanoparticles. In various embodiments, the activated carbon has a high average surface area of from about 300 m²/g to about 3,000 m²/g. In various embodiments, the activated carbon has a surface area of about 300 m²/g, about 400 m²/g, about 500 m²/g, about 600 m²/g, about 700 m²/g, about 800 m²/g, about 900 m²/g, about 1 ,000 m²/g, about 1 ,250 m²/g, about 1 ,500 m²/g, about 1 ,750 m²/g, about 2,000 m²/g, about 2,250 m²/g, about 2,500 m²/g, about 2,750 m²/g or about 3,000 m²/g. In various embodiments, oil is readily adsorbed and adheres strongly on carbon, and this ensures that the oil will not easily be displaced by water or mechanical shaking. In various embodiments, the carbon particles have large high specific surface area (> 300 m²/g), ensuring more pore channels available for the vapor to pass through.

In various embodiments, the oil has a viscosity in the range of from about 5 to about 15,000 cSt, from about 10 to about 14,500 cSt, from about 15 to about 14,000 cSt, from about 20 to about 13,500 cSt, from about 25 to about 13,000 cSt, from about 30 to about 12,500 cSt, from about 35 to about 12,000 cSt, from about 40 to about 11 ,500 cSt, from about 45 to about 1 1 ,000 cSt, from about 50 to about 10,000 cSt, from about 60 cSt to about 9,500 cSt, from about 65 cSt to about 9,000 cSt, from about 70 cSt to about 8,500 cSt, from about 75 cSt to about 8,000 cSt, from about 80 cSt to about 7,500 cSt, from about 85 cSt to about 7,000 cSt, from about 90 cSt to about 6,500 cSt, from about 95 cSt to about 6,000 cSt, from about 100 cSt to about 5,500 cSt, from about 150 cSt to about 5,000 cSt, from about 200 cSt to about 4,500 cSt, from about 250 cSt to about 4,000 cSt, from about 300 cSt to about 3,500 cSt, from about 350 cSt to about 3,000 cSt, from about 400 cSt to about 2,500 cSt, from about 450 cSt to about 2,000 cSt, from about 500 cSt to about 1 ,500 cSt, from about 550 cSt to about 1 ,000 cSt, from about 600 cSt to about 950 cSt, from about 650 cSt to about 900 cSt, from about 700 cSt to about 850 cSt or from about 750 cSt to about 800 cSt. The oil may be at least one of silicone oil, halogenated oil selected from the group consisting of fluoroether based oil (such as perfluoropolyether (PFPE) oil, perfluoroalkylether (PFAE) and perfluoropolyalkylether (PFPAE) e.g. manufactured under the trademark Krytox™), fluorosilicone oil, polytrifluorochloroethylene oil or mixtures thereof. In various embodiments, due to the fluidity of the oil, the membrane advantageously has self-healing ability. For example, areas of the membrane may be inadvertently damaged resulting in the oil layer in these areas being lost. However, due to the fluidity of the oil, the oil from adjacent areas may flow over the damaged areas at predetermined conditions (e.g. high temperatures), thereby allowing these damaged areas to be re-coated with oil or“repaired” or “healed”.

In various embodiments, the composite comprises a curable mixture with the oil adsorbing particles dispersed therein and the mixture is coated onto a porous support/substrate and cured. The porous support/substrate may be a stainless steel wire mesh although other types of porous support/substrate that is capable of acting as a platform for the curable mixture to coat and form a porous structure on it may also be used. Accordingly, in various embodiments, the composite comprises the support/substrate and likewise the membrane comprises the support/substrate. In various embodiments, the porous support/substrate comprises pore sizes of no more than about 10 microns, no more than about 9 microns, no more than about 8 microns, no more than about 7 microns, no more than about 6 microns, no more than about 5 microns, no more than about 4 microns, no more than about 3 microns, or about 2 microns. In various embodiments, the average pore size of the pores/pore channels of the membrane is smaller than the average pore size of the support/structure.

In various embodiments, the curable mixture contains a resin/polymer so that the oil adsorbing particles can be bound together by the polymer/resin after the curable mixture is cured to maintain the membrane structure. In various embodiments, the resin/polymer comprises thermoset polymer/resin. The thermoset polymer/resin may be an epoxy resin (such as bisphenol A epoxy resin), polyester resin, phenolic resin, vinyl ester resin, polyurethane resin or polyamide resin. The curable mixture may also contain a hardener such as an amine hardener. In various embodiments, the curable mixture also contains solvent that is fast evaporating/drying. In various embodiments, the fast evaporating/drying solvent is an organic solvent such as an alcohol. In various embodiments, the alcohol is selected from methanol, ethanol, propanol, isopropyl alcohol or butanol.

The oil adsorbing particles dispersed in the curable mixture may be in an amount from about 10 wt% to about 40 wt%, about 15 wt% to about 35 wt%, about 20 wt% to about 30 wt%, or about 25 wt% of the curable mixture. In various embodiments, the curable mixture comprises epoxy resin-hardener- carbon mixture. In various embodiments, the membrane is capable of achieving a power density of at least about 0.20 W/m² , at least about 0.21 W/m² , at least about 0.22 W/m², at least about 0.23 W/m², at least about 0.24 W/m², at least about

0.25 W/m², at least about 0.26 W/m², at least about 0.27 W/m², at least about

0.28 W/m², at least about 0.29 W/m², at least about 0.30 W/m², at least about 0.31 W/m², at least about 0.32 W/m², at least about 0.33 W/m², at least about

0.34 W/m², or at least about 0.35 W/m² at about 95°C in a thermo-osmotic energy conversion process. In various embodiments, the membrane achieves a power density of 0.28 ± 0.07 W/m² at 95°C, which is otherwise unachievable by PTFE nano-porous membranes.

In various embodiments, the membrane is advantageously capable of maintaining its level of hydrophobicity from about 30°C to about 200°C, about 35°C to about 190°C, 40°C to about 180°C, about 45°C to about 170°C, about 50°C to about 165°C, or about 60°C to about 160°C. In various embodiments, the membrane is advantageously capable of maintaining its level of hydrophobicity when subjected to mechanical stress, for e.g. with rough handling.

In various embodiments, the membrane is mechanically stable, scratch- resistant and robust. In various embodiments, the membrane and/or composite is substantially free from/of fibers or fibrous material. In various embodiments, the membrane and/or composite is substantially free from/of silica particles/nanoparticles. In various embodiments, the membrane and/or composite is substantially free from/of silane groups on its surface from silanization such as fluorosilanization. Accordingly, in various embodiments, the method disclosed herein does not comprise the step of surface functionalization of the membrane by a chemical reaction such as silanization or fluorosilanization.

There is also provided a method of making a membrane, the method comprising providing a curable mixture having oil adsorbing particles dispersed therein; coating a porous support with said mixture; curing the mixture coated on the porous support to form a composite membrane having a plurality of pore channels thereon, the membrane being impregnated with the oil adsorbing particles; and coating the pore channels with an oil layer. In various embodiments, there is also provided a method of making a membrane for a thermo-osmotic energy conversion process, the method comprising: providing a mixture comprising a curable polymer and oil adsorbing particles dispersed therein; coating a porous support with said mixture; curing said coated porous support to form a composite membrane having a plurality of pores thereon, the composite membrane being impregnated with the oil adsorbing particles; and infusing the composite membrane in oil to thereby coat the surface of said composite and pores with an oil layer such that under predetermined conditions, the pores selectively allow movement of gaseous water vapor across the membrane but substantially prevent movement of liquid water across the membrane. The membrane, curable mixture, oil adsorbing particles, porous support, composite membrane may contain one or more features described earlier above.

The step of providing a curable mixture having oil adsorbing particles dispersed therein may comprise mixing a thermosetting resin/polymer, a hardener and oil adsorbing particles together. The thermosetting resin/polymer and the hardener may contain one or more features described earlier above. The thermosetting resin/polymer and hardener may be mixed in a ratio of from about 10:1 to about 10:5 by mass. In various embodiments, the resin and hardener is mixed in a ratio of about 10:1 by mass, in a ratio of about 10:2 by mass, in a ratio of about 10:3 by mass, in a ratio of about 10:4 by mass or in a ratio of about 10:5 by mass. In various embodiments, the method also comprises adding an organic solvent such as an alcohol (e.g. ethanol) to the mixture of thermosetting resin/polymer and the hardener, and stirring (e.g. mechanical stirring) to decrease the viscosity of the resin-hardener mixture. The oil adsorbing particles may then be added to the resin-organic solvent mixture and continuously stirred for uniform distribution. The ratio of organic solvent to oil adsorbing particles by mass may be in the range of from about 1 :2 to about 1 :10. In various embodiments, the amount of organic solvent and oil adsorbing particles used is in a ratio of about 1 :2 by mass, in a ratio of about 1 :3 by mass, in a ratio of about 1 :4 by mass, in a ratio of about 1 :5 by mass in a ratio of about 1 :6 by mass, in a ratio of about 1 :7 by mass, in a ratio of about 1 :8 by mass, in a ratio of about 1 :9 by mass or in a ratio of about 1 :10 by mass. The organic solvent may contain one or more features described earlier above.

The step of curing the mixture coated on the porous support may be carried out at a temperature range of from about 100°C to about 200°C. In various embodiments, the step of curing the mixture coated on the porous support may be carried out at a temperature above about 100°C, above about 110°C, above about 120°C, above about 130°C, above about 140°C, above about 150°C, above about 160°C, or about 170°C for more than about 1 hours, more than about 2 hours, more than about 3 hours, or at least about 4 hours. In various embodiments, the step of curing the mixture coated on the porous support may be carried out at a temperature above about 170°C for about 4 hours. The step of coating the pore channels with an oil layer comprises infusing the composite membrane in oil at a temperature above about 100°C, above about 110°C, above about 120°C, above about 130°C, above about 140°C, or about 150°C for more than about 24 hours, more than about 26 hours, more than about 28 hours, more than about 30 hours, more than about 32 hours, more than about 34 hours, more than about 36 hours, more than about 38 hours, more than about 40 hours, more than about 42 hours, more than about 44 hours, more than about 46 hours, or more than about 48 hours. In various embodiments, the step of infusing the composite membrane in oil is carried out at a temperature range of from about 100°C to about 200°C for a time period range of from about 20 hours to about 50 hours. In various embodiments, the composite membrane is infused in oil at a temperature of about 150°C for about 2 days.

There is also provided a thermo-osmotic energy conversion system comprising a hot sink for containing water at a temperature TH that is higher than room temperature; a cold sink for containing water at a temperature T_c that is lower than TH; a membrane disclosed herein, the membrane disposed between and separating the hot sink and cold sink. The thermo-osmotic energy conversion system may further comprise an energy conversion unit (e.g. a turbine) for converting energy from water vapor flowing from the hot sink to the cold sink, to electrical energy. For example, without being bound by theory, it is believed that the temperature difference (TH - Tc) between the sinks sets up a vapor pressure difference cross the hydrophonic porous membrane, thereby transporting water vapour across the membrane from the hot sink before condensing on the meniscus at the cold sink. Consequently, volumetric increase in the cold sink causes pressure of the cold sink to increase, which can be used to generate energy. In various embodiments, the hot sink contains water at a temperature TH in the range of from about 50°C to about 160°C or from about 80°C to 150°C and the cold sink contains water at about room temperature. In various embodiments, the thermo-osmotic energy conversion system further comprises a heating tank (for e.g. having a heater) coupled to the hot sink for heating the temperature of the water in the hot sink for e.g. to temperature TH. The thermo-osmotic energy conversion system may further comprise a pump coupled to the heating tank and the hot sink for pumping heated water from the heating tank to the hot sink. There is also provided a thermo-osmotic energy conversion method comprising creating a pressurized water vapor flow across a membrane disclosed herein from a hot sink containing water at a temperature TH that is higher than room temperature to a cold sink containing water at a temperature Tc that is lower than TH; and harnessing the energy created from the flow of water vapor across the membrane. In various embodiments, the temperature T H is in the range of from about 50°C to about 160°C or from about 80°C to 150°C and the temperature T_c is at about room temperature. BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a schematic diagram of a design of an oil-infused hydrophobic membrane (OHM) in accordance with an example embodiment disclosed herein.

FIG. 2 is a schematic diagram for illustrating the application of an oil- infused hydrophobic membrane in a thermo-osmotic energy conversion (TOEC) system in accordance with an example embodiment disclosed herein.

FIG. 3 is a schematic diagram for illustrating the application of an oil- infused hydrophobic membrane in a thermo-osmotic energy conversion (TOEC) system in accordance with another example embodiment disclosed herein. FIGS. 4A-4D are images of a water droplet on the surface of an oil- infused hydrophobic membrane in accordance with various embodiments disclosed herein (FIGS. 4A and 4C), shown alongside with images of a water droplet on the surface of a PTFE membrane (FIGS. 4B and 4D) at varying temperatures. FIGS. 4A and 4B show a cold water droplet on the respective membranes at 25°C. FIGS. 4C and 4D show a hot water droplet on the respective membranes at 95°C.

FIG. 5 is a graph showing changes in the power density of a PTFE membrane when the operating temperature is increased.

FIG. 6 is a schematic diagram for illustrating an experimental setup designed for studying the performance of oil-infused hydrophobic membranes in accordance with various embodiments disclosed herein. FIG. 7 is a graph showing the performance (i.e. power density in W/m²) of an oil-infused hydrophobic membrane in accordance with various embodiments disclosed herein, in comparison with control samples. The control samples are membranes prepared in the absence of oil adsorbing particles (i.e. a membrane prepared with resin only, a membrane prepared with resin and infused oiland a PTFE membrane). The oil-infused hydrophobic membrane in accordance with various embodiments disclosed herein is prepared by coating a carbon-resin-hardener mixture comprising 25 wt% activated carbon particles on a 2 pm stainless steel wire mesh, curing and then infusing with 50 cSt silicone oil.

FIG. 8A-8D are scanning electron microscopy (SEM) micrographs of an oil-infused hydrophobic membrane prepared in accordance with various embodiments disclosed herein, with the scale bar representing 10 pm. FIG. 8A is a SEM micrograph showing a pristine stainless steel mesh with 2 pm pore size. FIG. 8B is a SEM micrograph showing a top view of the membrane prepared by coating the stainless steel mesh with a carbon-resin-hardener mixture comprising activated carbon nanoparticles, prior to oil infusion. FIG. 8C is a SEM micrograph showing a cross-section view of the membrane prepared by coating the stainless steel mesh with a carbon-resin-hardener mixture comprising activated carbon nanoparticles, prior to oil infusion. FIG. 8D is a SEM micrograph showing the membrane prepared by coating the stainless steel mesh with a carbon-resin-hardener mixture comprising activated carbon nanoparticles and after oil infusion.

FIGS. 9A-9B are image of an oil infused-hydrophobic membrane prepared by coating a carbon-resin-hardener mixture comprising 25 wt% activated carbon particles on a 2 pm stainless steel wire mesh, curing and then infusing with 50 cSt silicone oil in accordance with various embodiments disclosed herein (FIG. 9A), shown alongaside with image of a 2 pm mesh soaked in 50 cst silicone oil; control (FIG. 9B).Both samples were left to stand in water at 150°C after shaking.

FIG. 10 is a graph showing the performance (i.e. power density in W/m²) of an oil-infused hydrophobic membrane prepared in accordance with various embodiments disclosed herein (i.e. with Krytox oil and with 10,000 cSt silicone oil respectively), in comparison with a membrane that is prepared in the absence of oil (i.e. resin without oil). EXAMPLES

Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following examples, tables and if applicable, in conjunction with the figures. It should be appreciated that other modifications related to structural, and chemical changes may be made without deviating from the scope of the invention. Example embodiments are not necessarily mutually exclusive as some may be combined with one or more embodiments to form new example embodiments. The example embodiments should not be construed as limiting the scope of the disclosure.

Example 1 : Design and Application of an oil-infused hydrophobic membrane

FIG. 1 is a schematic diagram for illustrating a design of an oil-infused hydrophobic membrane (OHM) in accordance with an example embodiment disclosed herein. The oil-infused hydrophobic membrane 100 comprises a plurality of pore channels 102a, 102b, 102c, 102d which are used as channels for water vapor particles to pass through the membrane. The insides 104 of these pore channels (and the surface of the membrane 100) are coated/infused/lubricated with a thin layer of oil 106, making the pore channels of the porous membrane (and the surface of the membrane) extremely hydrophobic, i.e. giving the membrane its liquid repellency, since oil and water are immiscible. The structure of the membrane 100 is maintained by binding oil adsorbing particles in the form of carbon nanoparticles 108 together with a resin for e.g. a thermoset resin 110.

In FIG. 1 , the membrane 100 is used as a hydrophobic porous membrane placed between a hot sink and a cold sink. As a result of the temperature difference between the hot and cold sinks, a vapour pressure difference is set up across the membrane 100, which drives water vapour particles across the membrane through the plurality of pore channels. Referring to FIG. 1 , the water evaporates from the meniscus 112 at the hot sink, passes across the membrane through the pore channels and condenses on the opposite meniscus 114 of the cold sink.

While the oil-infused hydrophobic membrane 100 is shown to be used as a membrane between a hot and cold sink for a TOCE process, it may be appreciated that the membrane 100 may also be used in many other applications which require a hydrophobic porous membrane at high temperatures ranging from about 60°C to about 150°C. As shown in FIG. 1 , the oil-infused hydrophobic membrane may be used in a thermo-osmotic energy conversion (TOEC) system for generating energy.

FIG. 2 is a schematic diagram for illustrating the application of an oil- infused hydrophobic membrane in a thermo-osmotic energy conversion (TOEC) system in accordance with an example embodiment disclosed herein. A hydrophobic porous membrane 202 is placed in between a hot sink

204 containing water at a temperature TH and a cold sink 206 containing water at a temperature Tc, with a thin gas barrier trapped due to the membrane. The water in the hot sink 204 is kept at an elevated temperature ranging from about 80°C to about 150°C while the water in the cold sink 206 is kept at room temperature. The temperature difference (i.e. DT = TH - T_c) between the sinks sets up a vapour pressure difference across the hydrophobic porous membrane 202, thereby transporting water vapour across the membrane through the plurality of pore channels from the hot sink 204 (in the direction of the arrows shown in FIG. 2) before condensing on the meniscus at the cold sink 206. The volumetric increase in the cold sink 206 causes the pressure of the cold sink 206 to increase, which can be used to generate energy. As mass transfer continues, the cold sink 206 becomes increasingly pressurized and sets up pressure in a valve leading to an energy conversion unit 208. This resultant increase in pressure is then directed to the energy conversion unit 208 (for e.g. a turbine) for converting energy (from the water vapour flowing from the hot sink to cold sink) to electrical energy.

FIG. 3 is a schematic diagram for illustrating the application of an oil- infused hydrophobic membrane in a thermo-osmotic energy conversion (TOEC) system in accordance with another example embodiment disclosed herein. The thermo-osmotic energy conversion system 300 contains a hot sink 304, a cold sink 306 and a hydrophobic porous membrane 302 in a configuration that is similar to that in FIG 2. However, the thermo-osmotic energy conversion system 300 further comprises a pump 308 coupled to a tank 310 containing a heater 312 and the hot sink 304 for pumping heated water from the tank 310 to the hot sink 304. The thermo-osmotic energy conversion system 300 also additionally comprises a tank 314 that is coupled to the cold sink 306.

Similar to that shown in FIG 2, the hydrophobic porous membrane 302 is placed in between a hot sink 304 containing water that is heated to a temperature TH in the range of from about 80°C to about 150°C and a cold sink 306 containing water at a temperature Tc, with a gas barrier formed due to the membrane 302. In use, the heater 312 heats up water contained in the tank 310 to reach a temperature TH. The heated water is then transported to the hot sink 304 by the pump 308. At the same time, water in the cold sink 306 is kept at about room temperature. The temperature difference (i.e. DT = TH - T_c) across the membrane 302 between the sinks sets up a vapour pressure difference across the membrane, thereby driving transport of vapour (in the direction of the arrows shown in FIG. 3), creating a pressurised flow. As mass transfer continues, the cold sink 306 becomes increasingly pressurized, in other words, the volumetric (V) increase in the tank 314 coupled to the cold sink 306 sets up pressure in the valve. The pressurized flow can then be driven through an energy conversion unit (for e.g. a turbine) for generating electrical energy. As will be shown below, the oil-infused hydrophobic membrane prepared according to the method disclosed herein exhibit non-wetting property to water at high temperatures (see FIG. 4), self-healing ability and can maintain its properties over a long duration.

FIG. 4A shows a water droplet on the surface of an oil-infused hydrophobic membrane in accordance with various embodiments disclosed herein at 25°C and FIG. 4C shows a water droplet on the surface of an oil- infused hydrophobic membrane in accordance with various embodiments disclosed herein, both water droplet and membrane heated at 95°C. When the temperature is increased from 25°C to 95°C, the contact angle of the water droplet remains fairly constant (c.f. FIGS. 4A and 4C), therefore clearly showing that the oil-infused hydrophobic membrane prepared according to the method disclosed herein is able to maintain its hydrophobicity even at high temperatures such as at 95°C.

In contrast, it has been shown that a PTFE membrane which is typically used in thermo-osmotic systems fails to maintain its hydrophobicity at high temperatures such as at 95°C. FIG. 4B shows a water droplet on the surface of a PTFE membrane at 25°C and FIG. 4D shows a water droplet on the surface of a PTFE membrane, both water droplet and membrane heated at 95°C. When the temperature is increased from 25°C to 95°C, a significant change in the contact angle of the water is observed (c.f. FIGS. 4B and 4D). Advantageously, the oil-infused hydrophobic membrane prepared according to the method disclosed herein can maintain its hydrophobicity even at temperatures as high as 150°C. On the other hand, a PTFE membrane which is typically used in thermo-osmotic systems is unable to maintain its hydrophobicity at an elevated temperature such as at 95°C. In fact, it is shown in FIG. 5 that the performance of a PTFE membrane peaks at 60°C and decreases significantly when the operating temperature was further increased. Therefore, it may be appreciated that the performance of TOEC systems which typically use a PTFE membrane would decrease as temperature increases above 60°C as there is a loss of hydrophobicity of the PTFE membrane, which resulted in a reduced net forward flux. Example 2: General steps for making an oil-infused hydrophobic membrane

The general steps for making an oil-infused hydrophobic membrane (OHM) in accordance with various embodiments disclosed herein are provided as follows:

(1) Preparation of carbon-resin-hardener mixture

Thermoset resin and hardener are mixed in a 10 to 3 ratio by mass. Ethanol is then added to the resin-hardener mixture to decrease the viscosity of the mixture under mechanical stirring. Carbon is then added to the resin-ethanol mixture with continuous stirring to ensure uniform distribution. The amount of ethanol and carbon used is in a 1 to 5 ratio by mass.

(2) Coating of mixture onto a support

The carbon-resin-hardener mixture is then coated onto a support (in the examples, a wire mesh was used) by any coating technique. In the examples, brush coating technique is used. The support used may be any material/structure that gives the membrane its shape.

(3) Curing of membrane on support

The coated membranes are then cured at 170°C to harden the resin for 4 hours.

(4) Infusing oil to make OHM

The membranes are then infused with oil of different viscosity (ranging from 10 cSt - 10000 cSt) at 150°C for 2 days. Example 3: Method for making a silicone oil infused-hydrophobic membrane and experiment setup designed for studying performance of oil-infused hydrophobic membranes The method for preparing a silicone oil infused-hydrophobic membrane is described as follows. Firstly, thermosetting epoxy resin was prepared by mixing 2 g of resin (Huntsmen Araldite LY 1564) and 0.6 g of formulated amine hardener (Huntsmen Aradur 3487). This mixture was then dissolved in 6.5 ml_ of tech-grade ethanol. 1.3 g of activated carbon (SS Nano, super activated carbon nanoparticles (Product Code 0530HT)) was then added to the resin- ethanol mixture under continuous stirring. The activated carbon has a surface area of about 1 ,000 m²/g. The coating solution was then coated on stainless steel wire meshes with 2 pm nominal pores. The membranes were then cured at 170°C to harden the resin for 4 hours. The membranes were then infused with silicone oil with viscosity of 50 cSt at 150°C to increase its hydrophobicity for 2 days.

Experiments were conducted to study the performance of the respective oil infused-hydrophobic membranes prepared according to embodiments of the method disclosed herein. A schematic diagram showing a membrane cell set-up 600 designed for the experiments is provided in FIG. 6. Referring to FIG. 6, a temperature-controlled water bath 608 containing a heater 610 (Thermo Scientific Haake SC 100) and a pump 612 (Longerpump) were used in the setup. The oil-infused membrane 602 was placed in between a hot side cell 604 and a cold side cell 606 in the designed membrane cell setup 600 to measure its performance. A pump 612 was used to drive hot water from the hot water bath 608 containing the heater 610 into the hot side cell 604. A syringe 614 was attached to the cold side cell 606 to measure the volumetric change in the cold side cell/sink 606. The readings of the volumetric change were taken in two- minute intervals for 20 min. By using the rate of increase in the volume of water in the cold sink 606 and flow rate, the power density was calculated using: f x V

p =—

wherein P is power density (W/m²), f is flow rate (m³/s), V is vapour pressure (Pa) and A is the surface area of the membrane (m²).

The experiment was then repeated by varying the temperature set in the heater. To ensure reliability, 3 replicates and 3 repeats were performed for membranes of different specification.

Example 4: Performance of oil-infused hydrophobic membranes comprising activated carbon nanoparticles vs. membranes prepared in absence of activated carbon (in terms of power density)

The experiments were repeated according to the method and experimental setup described in Example 3, except that (i) 25 wt% of activated carbon nanoparticles was used instead of 1.3 g of activated carbon; (ii) only thermosetting epoxy resin was used (i.e. coating thermosetting epoxy resin- hardener mixture on stainless steel mesh and cured, with no subsequent oil infusion step); and (iii) thermosetting epoxy resin was used (i.e. coating thermosetting epoxy resin-hardener mixture on stainless steel mesh and cured), followed by infusing with silicone oil having a viscosity of 50 cSt. Membranes (ii) and (iii) are control samples that are prepared in the absence of activated carbon.

The result obtained on the performance of membrane (i) was compared with those of (ii), (iii) and (iv) a PTFE membrane as shown in FIG. 7. The best performing oil-infused hydrophobic membrane is membrane (i) prepared by coating a carbon-resin-hardener mixture comprising 25 wt% activated carbon particles on a 2 pm stainless steel wire mesh, curing and infusing with silicone oil having a viscosity of 50 cSt, which achieved a power density of as high as 0.28 ± 0.07 W/m² at 95°C. As shown in FIG. 7, said best performing oil-infused hydrophobic membrane outperforms a PTFE membrane at least by more than 10 times when tested at 95°C. Said best performing oil-infused hydrophobic membrane (i) also outperforms the membranes that are prepared with the resin only or with the resin and infused oil, i.e. in the absence of oil adsorbing particles.

Example 5: Scanning electron microscopy and dye experiments on oil-infused hydrophobic membranes

The following experimental results demonstrate that the method of preparing oil-infused hydrophobic membranes disclosed herein is effective and the oil is successfully infused and adhered to the oil adsorbing particles.

Firstly, scanning electron microscopy (SEM) is performed on a pristine stainless steel mesh/support (see FIG. 8A), which shows visible pores having a pore size of approximately 2 pm. After coating the stainless steel mesh with a carbon-resin-hardener mixture comprising activated carbon particles, the SEM images show that the intended textured surface was incorporated successfully onto the stainless steel mesh surface (see FIGS. 8B and 8C). After oil infusion, the SEM image shows a smooth membrane surface, indicating that the oil had completely covered the surface of the membrane and had successfully“wicked” into the membrane (see FIG. 8D).

Next, to demonstrate that the oil is indeed infused and adhered to the carbon, an experiment was conducted by immersing an oil infused-hydrophobic membrane prepared in accordance with membrane (i) of Example 4 (i.e. by coating a carbon-resin-hardener mixture comprising 25 wt% activated carbon particles on a 2 pm stainless steel wire mesh, curing and infusing with silicone oil having a viscosity of 50 cSt (FIG. 9A)) in water at 150°C. As a control, a 2 pm mesh soaked in 50 cst silicone oil (FIG. 9B) is also immersed in water at 150°C. The silicone oil was dyed with methylene red to provide visual aid.

When the oil infused-hydrophobic membrane was left to stand in water after shaking, no red layer of oil is observed to form on the water surface (FIG. 9A). This demonstrates that the infused oil adheres to the carbon of the oil infused-hydrophobic membrane. On the other hand, a layer of red-colour oil was observed on the water surface for the set-up containing the 2 pm mesh soaked with silicone oil (FIG. 9B; circled in dotted line). Accordingly, this test gives reassurance that the silicone oil would not desorb from the oil infused- hydrophobic membrane when faced with mechanical stress in applications such as in a TOEC cell.

Example 6: Performance of oil-infused hydrophobic membranes vs. non oil- infused membranes (in terms of power density)

The experiments were repeated according to the method and experimental setup described in Example 3, except that (i) 10,000 cSt silicone oil was used for oil infusion instead of 50 cSt silicone oil and (ii) Krytox oil was used for oil infusion instead of 50 cSt silicone oil.

The results obtained were compared with that obtained for a non oil- infused hydrophobic membrane (see FIG. 10). FIG. 10 shows the improved performance of the oil-infused hydrophobic membrane comprising 10,000 cSt silicone oil as the infused oil. FIG. 10 also shows the improved performance of the oil-infused hydrophobic membrane comprising Krytox oil as the infused oil. The results also show that the membranes (i) that are infused with 10,000 cSt silicone oil performs better than membranes (ii) that are infused with Krytox oil as well as membranes that are prepared with resin only but without infused oil.

It will be appreciated by a person skilled in the art that other variations and/or modifications may be made to the embodiments disclosed herein without departing from the spirit or scope of the disclosure as broadly described. For example, in the description herein, features of different exemplary embodiments may be mixed, combined, interchanged, incorporated, adopted, modified, included etc. or the like across different exemplary embodiments. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A membrane comprising:

a plurality of pore channels coated with an oil layer for selectively allowing movement of gaseous water vapor across the membrane but substantially preventing movement of liquid water across the membrane, wherein the membrane is composed of a composite impregnated with oil adsorbing particles.

2. The membrane of claim 1 , wherein the oil adsorbing particles comprises carbon nanoparticles.

3. The membrane of claim 2, wherein the walls of the pore channels are at least partially lined with carbon.

4. The membrane of any one of the preceding claims, wherein the oil has a viscosity in the range of 5 to 15,000 cSt.

5. The membrane of any one of the preceding claims, wherein the oil is selected from the group consisting of silicone oil, Krytox oil, halogenated oil, fluoroether based oil, perfluoropolyether (PFPE) oil, perfluoroalkylether (PFAE), perfluoropolyalkylether (PFPAE), fluorosilicone oil, polytrifluorochloroethylene oil and mixtures thereof.

6. The membrane of any one of the preceding claims, wherein the composite comprises a curable mixture with oil adsorbing particles dispersed therein coated onto a porous support and cured.

7. The membrane of claim 6, wherein the oil adsorbing particles dispersed in the curable mixture are in an amount from 10 wt% to 40 wt% of the curable mixture.

8. The membrane of claim 6, wherein the porous support comprises pores having pore sizes of no more than about 10 microns.

9. The membrane of any one of the preceding claims, wherein the membrane is capable of achieving a power density of at least about 0.20 W/m² at 95°C in a thermo-osmotic energy conversion process.

10. A method of making a membrane, the method comprising:

providing a curable mixture having oil adsorbing particles dispersed therein;

coating a porous support with said mixture;

curing the mixture coated on the porous support to form a composite membrane having a plurality of pore channels thereon, the membrane being impregnated with the oil adsorbing particles; and

coating the pore channels with an oil layer.

11. The method of claim 10, wherein the step of providing a curable mixture having oil adsorbing particles dispersed therein comprises mixing a thermosetting resin, a hardener and oil adsorbing particles together.

12. The method of claim 11 , wherein the thermosetting resin comprises an epoxy resin.

13. The method of claim 11 or 12, wherein the hardener comprises an amine group containing hardener.

14. The method of any one of claims 10 to 13, wherein the oil adsorbing particles comprises carbon nanoparticles.

15. The method of any one of claims 10 to 14, wherein the step of coating the pore channels with an oil layer comprises infusing the composite membrane in oil at a temperature above about 100°C for more than about 24 hours.

16. The method of any one of claims 10 to 15, wherein the oil has a viscosity in the range of 5 to 15,000 cSt.

17. The method of any one of claims 10 to 16, wherein the oil is selected from the group consisting of silicone oil, Krytox oil , halogenated oil, fluoroether based oil, perfluoropolyether (PFPE) oil, perfluoroalkylether (PFAE), perfluoropolyalkylether (PFPAE), fluorosilicone oil, polytrifluorochloroethylene oil and mixtures thereof.

18. A thermo-osmotic energy conversion system comprising:

a hot sink for containing water at a temperature TH that is higher than room temperature;

a cold sink for containing water at a temperature T_c that is lower than TH;

a membrane of any one of claims 1 to 8, the membrane disposed between and separating the hot sink and cold sink.

19. The thermo-osmotic energy conversion system of claim 18 further comprising:

an energy conversion unit for converting energy from water vapor flowing from the hot sink to the cold sink, to electrical energy.

20. A thermo-osmotic energy conversion method comprising:

creating a pressurized water vapor flow across a membrane of any one of claims 1 to 8 from a hot sink containing water at a temperature TH that is higher than room temperature to a cold sink containing water at a temperature T_c that is lower than TH; and

harnessing the energy created from the flow of water vapor across the membrane.

21. The method of claim 20, wherein TH is from 50°C to 160°C.