Patent Description:
<CIT> relates to a printed gas sensor. <CIT> relates to an oxygen sensor with microporous electrolyte layer and partially open cover membrane. <CIT> relates to an electrochemical gas sensor including a plurality of electrodes at least one of which is formed of a catalyst/binder slurry which is halftone printed onto a substrate, the composite printed element and substrate are sintered to form the electrode.

For a more complete understanding of the present invention, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The invention should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims.

The term "comprising" means including but not limited to, and should be interpreted in the manner it is typically used in the patent context;.

The phrases "in one embodiment," "according to one embodiment," and the like generally mean that the particular feature, structure, or characteristic following the phrase may be included in at least one embodiment of the present invention, and may be included in more than one embodiment of the present invention (importantly, such phrases do not necessarily refer to the same embodiment);.

If the specification describes something as "exemplary" or an "example," it should be understood that refers to a non-exclusive example;.

The terms "about" or "approximately" or the like, when used with a number, may mean that specific number, or alternatively, a range in proximity to the specific number, as understood by persons of skill in the art field (for example ±<NUM>%); and.

If the specification states a component or feature "may," <NUM> "can," "could," "should," "would," "preferably," "possibly," "typically," "optionally," "for example," "often," or "might" (or other such language) be included or have a characteristic, that particular component or feature is not required to be included or to have the characteristic. Such component or feature may be optionally included, or it may be excluded.

Embodiments may reduce electrolyte leakage within an electrochemical sensor and/or improve contact between the electrolyte and a plurality of electrodes. The electrodes are formed on a substrate and may contact ambient gas via diffusion holes through the substrate. The electrodes in the sensor may participate in electrochemical reactions via contact with the electrolyte, and may also prevent electrolyte leaking from the diffusion holes. However, in some electrochemical sensors, the electrodes may not be robust enough to prevent electrolyte leakage through the diffusion holes, particularly during high humidity (e.g., more than <NUM>% relative humidity (RH)). Meanwhile, the preparation of the catalyst paste for forming the electrode(s) may easily introduce moisture into the electrochemical sensor, which may have a negative effect on the printing process, and may lead to the electrodes connecting during the printing formation process (and may result in a short out within the sensor).

When traditionally printing (or otherwise applying) the electrolyte on the electrode(s), there may be a high rate of failure because the electrode comprises a hydrophobic material, while the electrolyte is hydrophilic. For effective functioning of the electrochemical sensor, the electrolyte must contact the electrode, and in some cases permeate and/or wet the electrode material.

Embodiments comprise preparing a homogenous catalyst paste for forming an electrode on a substrate which can remove and/or prevent moisture from pervading the electrode and may result in a robust electrode with resistance to leakage through the diffusion holes during high humidity (more than <NUM>% RH). Additionally, embodiments comprise changing the concentration of the mixture of the perfluorinated ion solution and a catalyst (Pt, Au, Ru, Ir, Ag powder, carbon black, graphite, and/or a combination thereof) and solve the problem of leakage from the diffusion holes during high humidity.

Additionally, embodiments comprise printing a transition layer between the electrodes and the electrolyte, where the transition layer may facilitate contact between the electrodes and the electrolyte. The transition layer comprises a thin, hydrophilic material. The transition layer is applied to the electrodes before the electrolyte is applied to the electrodes.

In embodiments, a perfluorinated ion solution (e.g., GEFC, <NUM> wt% in DMF, Golden Energy Fuel Cell Co. ) is used as a binder in a catalyst paste which is printed onto a substrate to form electrodes through annealing. The method comprises mixing the perfluorinated ion solution with a catalyst (e.g., Pt, Au, Ru, Ir, Ag powder, carbon black, graphite, and/or a combination thereof). Then, the mixture is concentrated to make a paste with a suitable viscosity for printing. The paste is printed on to a substrate comprising diffusion holes to form electrodes through annealing. The diffusion holes are formed into the substrate and extend through the substrate thickness.

Typically, the mixing process and/or printing process may be performed in an open system where the mixture may be exposed to an ambient environment, which may allow moisture from the ambient air to penetrate the electrodes and/or other elements of the sensor during printing. The mixing process and optionally the printing process are completed under a vacuum, to prevent moisture and/or other substances from the ambient environment from penetrating the materials of the layers of the electrochemical sensor.

In the method of the present invention, concentrating of the perfluorinated ion solution and subsequent mixing with a catalyst are combined into a single step. These processes are completed using ultrasonication to generate a slurry made of the perfluorinated ion solution and catalyst powder in a homogenous suspension. Additionally, the preparation is performed by vacuum distillation in a relatively closed system with negative-pressure condition. In particular, the process conditions are approximately <NUM>-<NUM> Pa (<NUM>-<NUM> mbar) and approximately <NUM>-<NUM>. The viscosity of the mixture may be controlled by monitoring the rotation speed of a magnetic stirrer.

The lower temperature (e.g., <NUM>-<NUM>) of the method may not cause undesirable binding of the perfluorinated ion solution. Additionally, the concentrating process may produce more interaction between the perfluorinated ion solution binder and the catalyst particles than traditional simple mixing of concentrated perfluorinated ion solution and catalyst powder, where the interaction contributes to the strength and robustness of the electrodes and may prevent electrolyte leakage in the final sensor.

The method facilitates the printing of a hydrophilic electrolyte onto hydrophobic electrodes. The method comprises printing a transition layer onto the electrodes before applying an electrolyte. The transition layer comprises a water-immiscible solvent and a hydrophilic, inert powder. The added transition layer may not significantly thicken the electrode(s), because the same stencil and/or template may be used during printing of the electrode and printing of the transition layer, and the printed hydrophilic particles of the transition layer may disperse or otherwise fit into holes and/or spaces created on the surface of the electrodes by the catalyst of the electrodes. In some embodiments, there may be substantially no change in the thickness of the electrode due to adding of the transition layer. With the transition layer in place on the surface of the electrode (opposite the substrate and/or direct towards the electrolyte), the surface of the electrodes may become more hydrophilic, and the electrolyte can be more easily printed onto the electrodes. The transition layer may be thin enough to not interfere with interaction between the electrodes and the electrolyte. In some embodiments, the transition layer may comprise conductive properties configured to allow for electric communication between the electrodes and the electrolyte.

Referring to <FIG>, an ultrathin electrochemical (EC) sensor <NUM> is made by printing technology. The sensor <NUM> comprises electrodes <NUM>, <NUM> and <NUM>, an electrolyte <NUM> (wherein the electrolyte typically contacts and provides electrical interaction between all the electrodes), and optionally an encapsulation layer <NUM>, which may be printed layer by layer onto a substrate <NUM>. The substrate <NUM> comprises a plurality of diffusion holes <NUM>, wherein the electrodes <NUM>, <NUM>, and <NUM> are printed over the diffusion holes <NUM> (for example, with each electrode having at least one diffusion hole underlying it). The diffusion holes <NUM> may comprise a size, shape, a first diameter, a second diameter, and/or length that causes a capillary effect within the diffusion holes <NUM>. The electrolyte <NUM> may be printed or otherwise applied over the electrodes <NUM>, <NUM>, and <NUM>. The electrodes may comprise a sensing electrode <NUM>, a reference electrode <NUM>, and a counter electrode <NUM>.

The encapsulation layer <NUM> can be used to seal the electrolyte <NUM> (e.g. creating an envelope applied to the substrates which seals the electrolyte and underlying electrodes). The encapsulation layer <NUM> can comprise any material suitable for bonding to the substrate <NUM> and retaining the electrolyte <NUM> in position on the substrate <NUM>. Any of the materials described herein can be used for the encapsulation layer <NUM>. Additional materials such as silicone rubber or other polymeric materials can also be used as the encapsulation layer <NUM>. The encapsulation layer <NUM> may comprise a silicon or silicone material. Once disposed over the electrolyte <NUM> (e.g., a liquid electrolyte, a gelled electrolyte, and/or a solid electrolyte), the encapsulation layer <NUM> may serve as a water vapor diffusion barrier to seal the electrolyte <NUM> from the external environment. The encapsulation layer <NUM> may be flexible to allow the volume of the electrolyte <NUM> to change over time, for example, in response to a gain or loss of water in a hygroscopic electrolyte.

A solution of perfluorinated ion electrolyte solution (GEFC-IES the copolymer of perfluorosulfonic acid and PTFE) commercially available from Golden Energy Fuel Cell Co. or Nafion® (copolymer of tetrafluoroethylene (Teflon®) and perfluoro-<NUM>,<NUM>-dioxa-<NUM>-methyl-<NUM>-octene-sulfonic acid) commercially available from DupontTM, is used as a binder. The catalyst (as described above) may be Pt, Au, Ru, Ir, Ag powder, carbon black, graphite, and/or a combination thereof. Glycol or other similar chemicals can be used as a diluent to form a catalyst slurry, recipe or catalyst system, which can be printed on a PTFE membrane by a printer. The printed element is sintered at an elevated temperature to form an electrode which can be used in an electrochemical sensor.

The sensing electrode <NUM>, the reference electrode <NUM>, and the counter electrode <NUM> can be arranged in a co-planar, non-overlapping arrangement on the surface of the substrate <NUM>. While shown in <FIG> as having three electrodes, the sensor <NUM> can also be used with only two electrodes, for example including the sensing electrode <NUM> and the counter electrode <NUM>. Four or more electrodes may also be present. For example, two or more sensing electrodes can be present and each sensing electrode may operate at a different potential to enable the detection of more than one target gas. Alternatively, four or more electrodes may be present to enable diagnostic tests to be conducted during operation of the sensor <NUM>, continuously, periodically, or aperiodically. In some contexts, the sensing electrode <NUM> may also be referred to as a working electrode.

The composition, size, and configuration of the electrodes <NUM>, <NUM>, <NUM> can depend on the specific species of target gas or gasses being detected by the sensor <NUM>.

One or more (or all) of the electrodes <NUM>, <NUM>, <NUM> can comprise a porous, gas permeable membrane. The electrode (e.g., the sensing electrode <NUM>), may be placed over the aperture or capillary (which may also be called a diffusion hole <NUM>). The gas diffusing through the capillary <NUM> may then contact and diffuse through the permeable membrane to react with the electrolyte <NUM> at the opposite surface of the electrode. Such an electrode can be formed of any of the materials described herein. In addition to any of the materials for forming the electrode, various hydrophobic components such as PTFE can be combined with the electrode material and/or used as a backing layer ( e.g., as a tape or support) for the electrode on the substrate <NUM>. For example, sensing electrode <NUM> can comprise a catalyst such as platinum or carbon, supported on a PTFE membrane. Such as toxic gas sensors, the counter electrode <NUM> may comprise a catalyst mounted on a PTFE backing tape, in the same manner as the gas sensing electrode <NUM>.

The electrodes comprise hydrophobic materials. Various coatings such as PTFE coatings can be used to provide a hydrophobic surface while maintaining a degree of porosity for gas diffusion of the target gas. The electrode material can be formed to exhibit hydrophobicity or super-hydrophobicity. The electrode material can be formed using a template material to form a patterned surface for the electrode, where the pattern may impart hydrophobic properties to the electrode. The patterning material can include any suitable material that can be removed once the electrode is formed. Nanosized polymer spheres (e.g., nanosized latex spheres-which are commercially available) can be arranged on a suitable sacrificial substrate (e.g., a metal such as copper). The electrode metal can then be electroplated around the assembled spheres to produce a suitable hydrophobic surface. The resulting electrode surface may also have porosity for gas diffusibility. Plating bath additives may be added as appropriate. Alternatively, other templating techniques such as self-assembled surfactant molecules can be used. The templating material can then be subsequently removed, for example by dissolution, heat, or the like. The resulting electrode material can then be used for one or more of the electrodes <NUM>, <NUM>, <NUM> while exhibiting hydrophobic properties. Electrodes may be formed using one or more of the methods/processes described herein.

The electrodes <NUM>, <NUM>, <NUM> may be at least partially covered by or in contact with the electrolyte <NUM> (and typically might all be entirely covered by the electrolyte. Electrical contact can be made with an external contact lead through one or more electrical conductors such as wires <NUM>. The wires <NUM> can comprise foils, wires, or deposited materials on the substrate <NUM>. The electrical conductors may comprise noble metals ( e.g., platinum), such as by being formed from noble metals or coated with noble metals if the conductors are in contact with the electrolyte <NUM>. The electrical conductors may not be formed from noble metals if the electrical conductors are not in contact with the electrolyte <NUM>.

The electrolyte <NUM> may comprise any material capable of providing an electrically conductive pathway between the electrodes <NUM>, <NUM>, <NUM>. The electrolyte <NUM> may be non-reactive with the substrate <NUM> material. If the electrolyte <NUM> and the substrate <NUM> can react, an insulting, non-reactive layer may be placed over the substrate prior to disposition of the electrodes <NUM>, <NUM>, <NUM> and the electrolyte <NUM>. The electrolyte <NUM> can comprise a liquid electrolyte, a gelled electrolyte, a solid electrolyte, or the like. The electrolyte <NUM> can be contained in or retained by a porous or absorbent material.

In an embodiment, the electrolyte <NUM> comprises an aqueous electrolyte in particular a solution of acid. The electrolyte can comprise a hygroscopic acid such as sulfuric acid for use in an oxygen sensor. For example, the electrolyte can comprise sulfuric acid having a molar concentration between about <NUM> to about <NUM>. Since sulfuric acid is hygroscopic, the concentration can vary from about <NUM> to about <NUM> wt% (<NUM> to <NUM> molar) over a relative humidity (RH) range of the environment of about <NUM> to about <NUM>%.

In addition to aqueous based electrolytes, ionic liquid electrolytes can also be used to detect certain gases. The ionic liquids may have a greater viscosity than a corresponding aqueous electrolyte. In any of the electrolytes, a viscosifier may be added to provide an increased viscosity, which may aid in retaining the electrolyte in contact with the electrolytes. The electrolyte can be present in the form of a gel or a semi-solid.

The electrolyte <NUM> can comprise a solid electrolyte. Solid electrolytes can include electrolytes adsorbed or absorbed into a solid structure such as a solid porous material and/or materials that allow protonic and or electronic conduction as formed. The solid electrolyte can be a protonic conductive electrolyte membrane. The solid electrolyte can be a perfluorinated ionexchange polymer such as Nafion or a protonic conductive polymer such as poly(ethylene glycol), poly(ethylene oxide), poly(propylene carbonate). Nafion is a hydrated copolymer of polytretafluoroethylene and polysulfonyl fluoride vinyl ether containing pendant sulfuric acid groups. When used, a Nafion membrane can optionally be treated with an acid such as H3PQ4, sulfuric acid, or the like, which improves the moisture retention characteristics of Nafion and the conductivity of hydrogen ions through the Nafion membrane. The sensing, counter and reference electrodes can be hot-pressed onto the Nafion membrane to provide a high conductivity between the electrodes and the solid electrolyte. The electrolyte <NUM> can be disposed on the substrate <NUM> as a drop or in a solid form so that the electrolyte is in electrical contact with the electrodes <NUM>, <NUM>, <NUM>.

As shown in <FIG>, the electrochemical sensor <NUM> also comprises a transition layer <NUM> positioned between the hydrophobic electrode(s) <NUM> (which may be similar to those described in <FIG>) and the electrolyte <NUM>. Electrode <NUM>, <NUM> and/or <NUM> formed using a method as described herein may comprise increased hydrophobic characteristics when compared to traditional electrodes. In this case, the possibility of issues when applying the electrolyte to the electrode(s) may be increased as well. Therefore, a transition layer <NUM> is applied between the electrode(s) and electrolyte to allow contact between the two.

In <FIG>, the sensing electrode <NUM> is shown, but the transition layer <NUM> may be applied to any or all of the electrodes <NUM>, <NUM>, and <NUM> (e.g. of <FIG>). The transition layer <NUM> facilitates contact between the electrode(s) <NUM> and the electrolyte <NUM>. The transition layer <NUM> comprises a thin, hydrophilic material. The transition layer <NUM> is applied to the electrode <NUM> before the electrolyte <NUM> is applied to the electrode <NUM>.

The transition layer <NUM> comprises a water-immiscible solvent and a hydrophilic, inert powder. As an example, the transition layer <NUM> may comprise a perfluorinated ion solution and an inert silicon dioxide (SiO<NUM>) powder. As shown in <FIG>, the hydrophilic particles of the transition layer <NUM> may dispense into spaces fabricated by the catalyst of the electrode(s) <NUM>, wherein the electrode(s) <NUM> may comprise approximately spherical material in the example shown in <FIG>. With the transition layer <NUM>, a top surface of the electrode(s) <NUM> may become more hydrophilic, and the electrolyte <NUM> can be more easily printed onto (e.g. atop) the electrode(s) <NUM> (e.g. on the surface opposite that contacting the substrate). The transition layer <NUM> may be thin enough to not interfere with interaction between the electrode(s) <NUM> and the electrolyte <NUM>.

In an embodiment the method may comprise mixing approximately <NUM> grams (g) of perfluorinated ion solution (e.g., <NUM> wt% GEFC) and approximately <NUM> of catalyst (e.g. one of the catalyst materials described above) to obtain a slurry. The slurry is treated with ultrasonication, e.g., for approximately <NUM>-<NUM> minutes, where the slurry is mixed under vacuum distillation. During the mixing, the temperature may be controlled to be approximately <NUM>, and the pressure is approximately <NUM>-<NUM> Pa (<NUM>-<NUM> mbar). At the beginning of the mixing, the rotation speed of a magnetic stirrer may be set to approximately <NUM> rpm. As the mixture thickness, the rotation speed may decrease due to the increased thickness. When the rotation speed of the magnetic stirrer has decreased to approximately <NUM> rpm, mixing and distillation may be stopped, producing a paste. The paste is printed onto a substrate to form the electrode(s). A stencil may be placed onto and/or above the substrate, and the paste may be applied over the stencil to form the electrode(s). The paste may be applied under a vacuum (as described above). The paste is applied over diffusion holes that extends through the substrate.

Forming the transition layer may comprise mixing approximately <NUM> of perfluorinated ion solution (<NUM> wt% GEFC) and approximately <NUM> of inert SiO2 powder (e.g., AEROSIL ® <NUM>). In other words, the mixture may comprise <NUM> part perfluorinated ion solution to <NUM> part inert powder. After mixing, the mixture may be rested until bubbles disappear. Then, the mixture may be printed to form the transition layer with the same stencil as that used in printing the electrode onto the substrate. The transition layer may be heated at approximately <NUM>-<NUM> for approximately <NUM> to <NUM> minutes. The heating temperature and time may be determined to maintain the hydrophilicity of the transition layer. Then, the electrolyte may be applied (e.g., printed) onto the electrode(s) comprising the transition layer. This method of forming a transition layer may also be used on electrodes formed using the above method of forming electrodes.

Claim 1:
A method of manufacturing an electrochemical sensor (<NUM>), the method comprising:
providing a plurality of electrodes (<NUM>, <NUM>, <NUM>) over a substrate (<NUM>) comprising diffusion holes (<NUM>), the plurality of electrodes (<NUM>, <NUM>, <NUM>) having a hydrophobic coating and covering the diffusion holes (<NUM>) extending cylindrically through the thickness of the substrate (<NUM>);
printing a transition layer (<NUM>) over the plurality of electrodes (<NUM>, <NUM>, <NUM>), the transition layer (<NUM>) comprising at least a first mixture of a water immiscible liquid and a hydrophilic inert substance over the plurality of electrodes (<NUM>, <NUM>, <NUM>); and
applying an electrolyte layer (<NUM>) comprising at least a second mixture of an acid solution and a solid polymer over the transition layer (<NUM>), such that the transition layer is disposed between the plurality of electrodes (<NUM>, <NUM>, <NUM>) and the electrolyte layer (<NUM>),
wherein providing the plurality of electrodes (<NUM>, <NUM>, <NUM>) comprises:
mixing perfluorinated ion solution and catalyst powder to obtain a slurry,
concentrating the slurry using ultrasonication for a predetermined time period, during which the slurry is mixed under vacuum distillation at a temperature of approximately <NUM>-<NUM> and a pressure of approximately <NUM>-<NUM> Pa (<NUM>-<NUM> mbar), to produce a paste, and
printing the paste over the substrate (<NUM>) to form the plurality of electrodes (<NUM>, <NUM>, <NUM>) through annealing.