Background[edit | edit source]

Hypochlorous acid (HOCl or HClO) is a weak acid that naturally occurs in many living organisms including humans. Hypochlorous acid is non-toxic and has the ability to disable many different pathogens and microorganisms. The advantage of using hypochlorous acid as a disinfectant is that it can be produced in-situ via the electrolysis of an aqueous sodium chloride solution; simply put, it can be made by directing an electric current through a salt water solution. This allows for the production of a powerful disinfectant that doesn’t require the use of expensive or complicated industrial equipment. The goal of this literature review is to collect the background information required to design and build an open-source hypochlorous acid generator that can be used at both the consumer and institutional level.9

Market Survey[edit | edit source]

Company Regular Price Product Specs Additional Information
EWCO Store $299.99 -Electrolysis Cell: Titanium

-Electrolysis Cell Lifespan:

3,000 cycles (8 minutes each)

-Power Supply: 110V/220V, 50/60Hz

-Dimensions: 21 x 15 x 36 cm (8.3 x 5.8 x 14 inches)

-Weight: 0.6 kg (1.3 lbs.)

-Volume: 1L

-Produces concentrations up to 200 ppm of HOCl
Liberty Sprayers $119.00 Premium 2L HOCL Generator

-Electrode is a Mesh Titanium/ Iridium/ Platinum Coated Alloy

-Power Supply: AC adaptor North American Plug

-Weight: 3-4 lbs

Standard 2 Liter HOCl Generator

-Electrode is coil style platinum/ titanium alloy

-Power Supply: AC adaptor North American Plug

-Weight: 3-4 lbs

-Wall plug-in as power source
Force of Nature $69.00 -Shipping Weight: 0.86 kg  (1.9 lbs.)

-Product Dimensions: 33.0 x 21.6 x 8.9 cm (13.0 x 8.5 x 3.5 in)

-Activator Base: 21.6 x 7.6 x 10.8 cm (8.25 x 3.0 x 4.25 in)

-Volume: 0.35L

-Forced to buy their capsules that contain salt and vinegar (acetic acid)

-EPA registered Hospital disinfectant and on EPA’s List N (approved for use against SARS-CoV-2)

Egret Lab Canada $249.00 -Electrolysis Cell: Titanium

-Shipping Weight: 0.90 kg (1.98 lbs.)

-Product Dimensions: 31.8x10.1x24cm

-Volume: 0.800L

-Body Materials: ABS/TPR/PC

-Battery: 1200mAh/13.32Wh

-Rated voltage: DC 11.11v

-Charging V: 5v-2A

Literature[edit | edit source]

Chemistry LibreTexts: Electrolysis[edit | edit source]

K. Song, “Electrolysis,” Chemistry LibreTexts, Oct. 02, 2013. (accessed Jan. 06, 2022).

  • Electrolysis: used to separate a substance into its original components/elements
  • Electrolytic cell: essentially the non-spontaneous reactions' voltaic cell; consist of two electrodes (one that acts as a cathode and one that acts as an anode), and an electrolyte
  • See electrolytic cell diagram here

Electrochemical water disinfection Part I: Hypochlorite production from very dilute chloride solutions[edit | edit source]

A. Kraft et al., “Electrochemical water disinfection Part I: Hypochlorite production from very dilute chloride solutions,” vol. 29, pp. 859–866, Jul. 1999, doi: 10.1023/A:1003650220511.

Abstract: Electrolytic production of hypochlorite in very dilute chloride solutions is investigated using platinum and iridium oxide coated titanium expanded metal electrodes as anodes. The dependence of the hypochlorite production rate on temperature, chloride concentration and current density was determined. It was found that the hypochlorite production rate is consistently higher on iridium oxide coated titanium compared to platinum coated titanium electrodes.

  • Investigates the electrolytic production of hypochlorite for very dilute chloride solutions
  • Relevant results:
    • Hypochlorite production rate is consistently higher on iridium oxide coated titanium compared to platinum coated titanium electrodes
    • The chlorine production rate increases with increasing chloride concentration
    • Current efficiency increases with chloride concentration
      • This result is important when considering which is the limiting resource (reactant versus energy)
    • IrO2 electrode had a remarkable reduction in the active chlorine production rate between 23 and 30 degree C
      • This strong reduction in the active chlorine production rate over a small temperature range was not observed at higher chloride concentrations
    • The active chlorine production rate decreased with increased in temperature
      • This depends strongly on chloride concentration
    • Linear increase in the production rate with increasing current density
    • At lower current densities:
      • Slight increase in the active chlorine production rate
      • Once higher current densities are reached the active chlorine production rate flatlines
    • Chlorine decay rate strongly depends on the temperature and pH
      • Does not appear to depend on concentration of active chlorine
      • An exponential increase in the reaction rate of chlorine decay with increasing temperature
  • Key information:
    • Two steps for electrolytic hypochlorite production:
      1. Primary oxidation of chloride to chlorine at the anode surface:
        • 2 Cl- → Cl2 + 2e-
      2. Solution phase reaction:
        1. Cl2(aq) + H2O → HClO + Cl- + H+
    • In very dilute solutions a relatively large amount of chlorine produced rapidly reacts with oxidizable agents in the water and the apparatus in a process known as chlorine decay/consumption
      • A major problem in accurately determining the exact amount of active chlorine produced
      • All type of water have a chlorine demand which is the quantity of chlorine that will react with the inorganics and organic impurities contained within that water
      • Active chlorine can only forme once the chlorine demand of the water must be satisfied (breakpoint chlorination)
    • The formation of calcareous deposits on the cathode is an issue to be aware of and is a result of OH- being produced
      • 2 H2O + 2 e- → H2 + 2 OH-
      • The OH- causes an increase in pH which leads to the deposits:
        • Ca2+ + HCO3- + OH- → CaCO3 + H2O
        • Mg2+ + 2 OH- → Mg(OH)2
    • Chlorate formation from hypochlorite and hypochlorous acid is another issue to be aware of
      • The formation follows the equation:
        • ClO- + 2 HClO → ClO3- + 2 HCl
      • Chlorate formation increases with solution temperature
  • Important terms:
    • Hypochlorite = hypochlorous acid + hypochlorite anion
    • Active chlorine = chlorine + hypochlorous acid + hypochlorite
      • Cl2, HClO and ClO-
      • In the pH range 6-9, the active chlorine is almost entirely hypochlorous acid (HClO) and hypochlorite (ClO-)
  • Device specifications:
    • Geometrical area of electrodes used:
      1. Platinum:
        • 100 mm X 30 mm
      2. Iridium oxides:
        • 113 mm X 30 mm
      3. Both coatings were applied to 1 mm thick expanded titanium metal
    • Anolyte and catholyte were divided by a Nafion 450 cation exchange membrane to prevent the reduction of the produced hypochlorite at the cathode
    • The electrode distance: 5 mm
    • Reaction compartment volumes:
      • Anolyte compartment: 1250 cm3
      • Catholyte compartment: 280 cm3

Designing a high-efficiency hypochlorite ion generation system by combining cation exchange membrane aided electrolysis with chlorine gas recovery stream[edit | edit source]

S. K. Kim, D.-M. Shin, and J. W. Rhim, “Designing a high-efficiency hypochlorite ion generation system by combining cation exchange membrane aided electrolysis with chlorine gas recovery stream,” Journal of Membrane Science, vol. 630, p. 119318, Jul. 2021, doi: 10.1016/j.memsci.2021.119318.

Abstract: Clean water is an absolute necessity to sustain life on Earth; thus, research and development in water management and disinfection processes are necessary. As an effective method to disinfect water, dilute solutions containing hypochlorous acid and hypochlorite ions are often utilized. Herein, we report a newly designed hypochlorite ion generation process, which involves installation of cation exchange membranes (CEMs) to conventional electrolysis cell to enhance stability and controllability, and addition of Cl2 recovery stream to increase the production rate and efficiency of hypochlorites generation. Electrolysis cells with SPEEK, Nafion 115, and Nafion 324 are evaluated and compared with a cell without CEM, which resulted in up to 17% enhancement of the production rate. In addition, increased energy efficiency and controllability of the electrolysis process are achieved when SPEEK is used as the membrane. The CEM installed electrolysis combined with Cl2 recovery stream, which enables enhancement of stability and performances of the electrolysis-based hypochlorite generation, opens a new possibility for an efficient in-situ water disinfection system.

  • Demonstrates a newly designed hypochlorite ion generation process that improves upon the conventional electrolysis cell by adding cation exchange membranes (CEMs) and a chlorine recovery stream to enhance stability and controllability as well as production rate and efficiency of hypochlorites generation, respectively
  • Relevant results:
    • Chlorine recovery stream harvests the chlorine gas and bubbles through the high pH outflow
      • The Cl2 gas was collected and supplied to the cathode outflow to create more NaOCl and HOCl
    • The two desired reactions that are occurring with the Cl2 recovery model:
      • Electrochemical generation of HOCl and NaOCl (electrolysis of salt water)
      • Spontaneous reaction between NaOH solution and recovered Cl2 gas to create HOCl and NaOCl
    • SPEEK membrane was found to be the most efficient and controllable
    • The CEMs used in this study, including the SPEEK, are not very accessible
      • Nafion series must be bought in bulk from a supplier; cost is quote dependent
        • Creating the SPEEK membrane requires various reactions requiring specific temperatures, timing, and equipment
          • >104 hours to create
          • Vacuum oven required
        • Comment: Due to these factors membrane-based electrolysis using traditional membrane technology does not fit the appropriate technology model very well
    • Higher current densities damage CEMs
    • Hypochlorite generation from recovered Cl2 is more effective when the electric current applied to the electrolysis cell increases, since the following redox reactions become more rapid:
      • Anode:
        • 2 H2O --> 4H+ + O2 + 4 e-
        • 2 Cl- --> Cl2 + 2 e-
      • Cathode:
        • 2 H2O + 2 e- --> H2 + 2OH-
    • Comment: The Cl2 recovery stream appears to be key since the occurrence of unreacted Cl2 raises some issues:
      • Indicates loss of efficiency
      • Safety concern as Cl2 gas is considered toxic
        • The recovery stream therefore increases the efficiency and safety of the system
    • Comment: Although the addition of a CEM to the system increases hypochlorite production it is not significant enough to warrant the additional cost for appropriate technology applications
      • Supported by the fact that the results show that energy consumption for w/o CEM system approximately the same as the w/ CEM systems
  • Key information:
    • Hypochlorous acid (HOCl) and sodium hypochlorite (NaOCl) as an aqueous solution with concentration <40% are chemically stable and relatively safe to store and use (according to the National Fire Protection Association; NFPA 430, 2000)
  • Device specifications:
    • Electrode distance: 4mm
    • CEM-based (sulfonated polyether ether ketone(SPEEK), Nafion 115, or Nafion 324)

A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine[edit | edit source]

S. M. H. Hashemi et al., “A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine,” Energy Environ. Sci., vol. 12, no. 5, pp. 1592–1604, 2019, doi: 10.1039/C9EE00219G.

Abstract: Renewables challenge the management of energy supply and demand due to their intermittency. A promising solution is the direct conversion of the excess electrical energy into valuable chemicals in electrochemical reactors that are inexpensive, scalable, and compatible with irregular availability of electrical power. Membrane-less electrolyzers, deployed on a microfluidic platform, were recently shown to hold great promise for efficient electrolysis and cost-effective operation. The elimination of the membrane increases the reactor lifetime, reduces fabrication costs, and enables the deployment of liquid electrolytes with ionic conductivities that surpass those allowed by solid membranes. Here, we demonstrate a membrane-less architecture that enables unprecedented throughput by 3D printing a device that combines components such as the flow plates and the fluidic ports in a monolithic part, while at the same time, providing tight tolerances and smooth surfaces for precise flow conditioning. We show that inertial fluidic forces are effective even in millifluidic regimes and, therefore, are utilized to control the two-phase flows inside the device and prevent cross-contamination of the products. Simulations provide insight on governing fluid dynamics of coalescing bubbles and their rapid jumps away from the electrodes and help identify three key mechanisms for their fast and intriguing return towards the electrodes. Experiments and simulations are used to demonstrate the efficiency of the inertial separation mechanism in millichannels and at higher flow rates than in microchannels. We analyze the performance of the present device for two reactions: water splitting and the chlor-alkali process, and find product purities of more than 99% and Faradaic efficiencies of more than 90%. The present membrane-less reactor – containing more efficient catalysts – provides close to 40 times higher throughput than its microfluidic counterpart and paves the way for realization of cost-effective and scalable electrochemical stacks that meet the performance and price targets of the renewable energy sector.

  • Presents a 3D printed membraneless cell design that combines components such as the flow plates and the fluidic ports in a single part and provides tight tolerances and smooth surfaces for precise flow conditioning
  • Relevant results:
    • Using a two-phase millifluidic system can be used to prevent cross-contamination of the anodic and cathodic products without the need for a membrane
    • A membraneless architecture increases the reactor lifetime and reduces fabrication costs
    • Using 3D printing for the fabrication of electrochemical reactors allows for tight tolerances and a reduced number of parts meaning fewer manufacturing steps
    • This reactor showed a 37-fold throughput increased over the first microfabricated prototype presented by Hashemi, Mohammad & Modestino, Miguel & Psaltis, Demetri in their 2015 paper presenting a membrane-less electrolyzer for hydrogen production
    • Inertial separation of the gas bubbles done beyond micro geometries and at higher flow rates
    • Conclusions related to further optimization:
      • Increased electrode size; this study expects optimum dimensions to be in the same order of magnitude
      • Main channel length can be increased so long the largest bubble at the end of the channel does not become bigger than half of the channel's width
        • This can be controlled by the flow rate and current density
      • Parallelization and stacking of several optimized cells
      • Decreasing the ohmic losses at the contacts
      • Introduction of acidified analytes through second dedicated inlet
        • Will enhance the Faradaic efficiency
      • Employing sophisticated 3D printers to reduce the interelectrode gap to few hundreds of microns
  • Key information:
    • 3 main components in the electrochemical cells: an anode, a cathode, and a membrane/separator
      • The role of the membrane:
        • Allows the passage of ions through it’s structure while preventing the mixing of reduction and oxidation products or reactants
          • This makes it a critical component in terms of the cell’s lifetime, price, and manufacturing due to limitations it imposes on the materials of the anode and the cathode
    • Comment: As noted in Boissonneau and Byrne's 2000 study on bubble-induced free convection small electrochemical cells, scaling up channel size can lead to more violent flow patterns that impact the performance of the reactor due to interaction of bubbles and their coalescence dynamics
      • Even more important in this case since almost all inertial microfluidics research is done in microchannels at Reynolds numbers well below 100
        • Flow regimes here were investigated had Re as high as 312 and in fact these higher Re flow regimes were found to be more efficient
          • In these more turbulent flow regimes bubbles experience sudden changes their equilibrium positions after coalescence, followed by their subsequent return towards the electrodes
    • Two hydrodynamic mechanisms lead to the separation of gaseous products in flow-based cells:
      • Inertial migration
      • Bubble coalescence
  • Device specifications:
    • Main body
      • Has fluidic channels, female Luer Lock fluidic connectors, electrode grooves, holes for electrical connections, and assembly screw holes
      • Main channel is 1 x 1 mm, 26 mm long terminating with Y-shaped section connected to a separate outlets
    • Electrodes
      • 4 x 10 mm, 1 mm thick thick titanium substrate coated with 20 um layer containing IrO2, RuO2, and TiO2
      • Inserted into tight, low tolerance grooves in the main body
        • Active area that forms part of channel walls is 1 x 10 mm
        • Other 3 mm is buried in grooves and connected to two copper bars using conductive epoxy
    • Clear top
      • 3 mm thick sheet of PMMA with screw holes
        • 250 mm thick, transparent sealing tape is used to join PMMA and 3DP part and prevent the leakage
        • The two parts are fastened together with M3 and M2 screws and nuts
        • Male Luer Lock connectors connect PTFE tubes to introduce electrolyte in and take products out of device

An experimental investigation of bubble-induced free convection in a small electrochemical cell[edit | edit source]

P. Boissonneau and P. Byrne, “An experimental investigation of bubble-induced free convection in a small electrochemical cell,” Journal of Applied Electrochemistry, vol. 30, p. 9, Jul. 2000, doi: 10.1023/A:1004034807331.

Abstract: The sodium chlorate production process is run in large electrolysers where electrolyte flows between the electrodes due to the natural convection from hydrogen gas evolution. A brief review is given of electrolytic gas generation at electrode surfaces and of previous studies. A small, enclosed rectangular cell was used to electrolyse both a Na2SO4 and a NaCl/NaClO3 solution, in order to produce hydrogen and oxygen bubbles at one or both of the electrodes. The two-phase flow regimes, bubble sizes, gas fraction and fluid velocities between the electrodes were investigated using microscope enhanced visualisation, laser doppler velocimetry and particle image velocimetry. The practicality of each of the measuring methods is analysed and it is concluded that laser doppler velocimetry is the most robust method for measuring such systems. The experimental results are discussed and conclusions are drawn relating gas evolution to the hydrodynamics of electrolyte flowing through a narrow vertical channel. The major conclusions are that fluid flow in systems with bubble evolution can transform from a laminar to a turbulent behaviour, throughout the length of the cell, and that both turbulence and laminar behaviour can exist across the cell channel at the same horizontal plane.

  • Electrolysis of both a Na2SO4 and a NaCl/NaClO3 solution was performed in order to produce hydrogen and oxygen bubbles at one or both of the electrodes so that the two-phase flow regimes, bubble sizes, gas fraction and fluid velocities between the electrodes were investigated
  • Relevant results:
    • Electrochemical reactions are influenced by electrolyte flow due to an increase in mass transfer to an electrode surface
    • Flow and mass transfer can also be imposed by electrochemically producing bubbles at an electrode surface
      • Relevant to the chlorate process since hydrogen bubbles produced at the cathode surface lead to flow in the channel
    • Electrolysis of a sodium chloride solution (NaCl) produces:
      • Hydrogen gas at the cathode and remains as gas bubbles in the system
      • Chlorine at the anode, but very quickly reacts with other species in the electrolyte and disappears
      • Oxygen gas at the anode and reduces the fluid current of the system by 2-4%
  • Key information:
    • In a scaled-up membrane-less electrolysis reactor, the interaction of bubbles and their coalescence dynamics can lead to more violent flow patterns that impact the performance of the reactor
      • In this context  "scaled-up" refers to a millifluidic application opposed to microfluidic
      • Comment: this is relevant to our purpose which more closely aligns with the design proposed by H. Hashemi et al. 2019

A membrane-less electrolyzer for hydrogen production across the pH scale[edit | edit source]

M. Hashemi, M. Modestino, and D. Psaltis, “A membrane-less electrolyzer for hydrogen production across the pH scale,” Energy Environ. Sci., vol. 8, pp. 2003–2009, Apr. 2015, doi: 10.1039/C5EE00083A.

Abstract: The development of deployable water-splitting devices is hindered by the lack of stable ion conducting membranes that can operate across the pH scale, impose low ionic resistances and avoid product mixing. The membrane-less approach developed in this work breaks this paradigm and demonstrates for the first time an electrolyzer capable of operating with lower ionic resistance than benchmark membrane-based electrolyzers using virtually any electrolyte. Our method separates product gases by controlling the delicate balance between fluid mechanic forces in the device. The devices presented here are able to split water at current densities over 300 mA/cm2, with more than 42% power conversion efficiency, and crossover of hydrogen gas into the oxidation side as low as 0.4%, leading to a non-flammable and continuous hydrogen fuel stream. Furthermore, being able to use buffered electrolytes allows for the incorporation of earth-abundant catalysts that can only operate at moderate to high pH.

  • Demonstrates a membrane-less electrolyzer that is capable of operating with lower ionic resistance than benchmark membrane-based electrolyzers using virtually any electrolyte
  • Relevant results:
    • Increased flow rate resulted in much lower crossover percentages
    • The membrane-less flow-based design has the potential to surpass the performance of equivalent apparatus that rely on ion conductive membranes for separation
    • Scaling this concept to increase throughput can be achieved by employing multi-stack panels or larger area electrodes
  • Key information:
    • Current electrolysis systems employ membrane electrode assemblies (MEAs) utilize the low ionic resistance through Nafion membranes to separate the sites where the hydrogen and oxygen are produced
    • Membrane-based systems allow for production of nearly-pure gas streams and operation at high current densities, but the strongly acidic nature of Nafion requires the incorporation of acid-stable catalysts based on noble metals (i.e. Pt, Ir, Ru)
    • The production of gaseous products from electrochemical reactions has precluded demonstrations of efficient membrane-less electrolyzers in the past since these gaseous products evolve out of the liquid electrolyte to form bubbles which is problematic as it enhances product mixing and slows the reaction
      • To mitigate gas crossover to reduce product mixing in a membrane-less scheme, fluid dynamics can be used to control the position and trajectory of bubbles that evolve from electrodes
    • Membrane-less electrolysis allows for the operation at any pH, reduces the complexity, and decreases the ionic resistance
    • Described method separates product gases using fluid mechanic forces of the device
    • Devices presented in this study are able to split water at current densities over 300 mA cm-2, with more than 42% power conversion efficiency, and crossover of hydrogen gas into the oxidation side as low as 0.4%, leading to a non-flammable and continuous hydrogen fuel stream

3D printed flow plates for the electrolysis of water: an economic and adaptable approach to device manufacture[edit | edit source]

G. Chisholm, P. J. Kitson, N. D. Kirkaldy, L. G. Bloor, and L. Cronin, “3D printed flow plates for the electrolysis of water: an economic and adaptable approach to device manufacture,” Energy Environ. Sci., vol. 7, no. 9, pp. 3026–3032, 2014, doi: 10.1039/C4EE01426J.

Abstract: The electrolysis of water is considered a promising route to the production of hydrogen from renewable energy sources. Electrolysers based on proton exchange membranes (PEMs) have a number of advantages including high current density, high product gas purity and the ability to operate at high pressure. Despite these advantages the high cost of such devices is an impediment to their widespread deployment. A principal factor in this cost are the materials and machining of flow plates for distribution of the liquid reagents and gaseous products in the electrochemical cell. We demonstrate the production and operation of a PEM electrolyser constructed from silver coated 3D printed components fabricated from polypropylene. This approach allows construction of light weight, low cost electrolysers and the rapid prototyping of flow field design. Furthermore we provide data on the operation of this electrolyser wherein we show that performance is excellent for a first generation device in terms of overall efficiency, internal resistances and current–voltage response. This development opens the door to the fabrication of light weight and cheap electrolysers as well as related electrochemical devices such as flow batteries and fuel cells.

  • Demonstrates the production and operation of a PEM electrolyser that uses silver coated 3D printed components which allows construction of light weight, low cost electrolysers and rapid prototyping
  • Demonstrates excellent performance for a first generation device in terms of overall efficiency, internal resistances and current-voltage response
  • Relevant results:
    • 3D printing and appropriate surface coatings can be used to fabricate electrodes that can be used to create viable, practical electrochemical system
  • Key Information:
    • Flow plates were created using layer by layer deposition with polypropylene
    • Two coats of silver paint were applied to the FPs
      • Curing at 120 degree C after each coat had and dried at ambient conditions
    • The processed used here is that outlined in the paper by Polk et al. 2006 that demonstrates the fabrication and testing of Ag/AgCl microelectrodes
    • The high cost of electrolysers based on proton exchange membranes (PEMs) hinders widespread deployment.
      • Principal cost factor are the materials and machining of flow plates
  • Device specifications:
    • Flow plate thickness: 3mm
    • The final thickness of the coated layer was 300 μm

Ag/AgCl microelectrodes with improved stability for microfluidics[edit | edit source]

B. J. Polk, A. Stelzenmuller, G. Mijares, W. MacCrehan, and M. Gaitan, “Ag/AgCl microelectrodes with improved stability for microfluidics,” Sensors and Actuators B: Chemical, vol. 114, no. 1, pp. 239–247, Mar. 2006, doi: 10.1016/j.snb.2005.03.121.

Abstract: A method for fabricating Ag/AgCl planar microelectrodes for microfluidic applications is presented. Micro-reference electrodes enable accurate potentiometric measurements with miniaturized chemical sensors, but such electrodes often exhibit very limited lifetimes. Our goal is to construct Ag/AgCl microelectrodes reliably with improved potential stability that are compatible with surface mounted microfluidic channels. Electrodes with geometric surface areas greater than or equal to 100μm2 were fabricated individually and in an array format by electroplating silver, greater than 1μm thickness, onto photolithographically patterned thin-film metal electrodes. The surface of the electroplated silver was chemically oxidized to silver chloride to form Ag/AgCl micro-reference electrodes. Characterization results showed that Ag/AgCl microelectrodes produced by this fabrication method exhibit increased stability compared with many devices previously reported. Electrochemical impedance spectroscopy allowed device specific parameters to be extracted from an equivalent circuit model, and these parameters were used to describe the performance of the microelectrodes in a microfluidic channel. Thus, stable Ag/AgCl microelectrodes, fabricated with a combination of photolithographic techniques and electroplating, were demonstrated to have utility for electrochemical analysis within microfluidic systems.

  • Presents a method for fabricating Ag/AgCl planar microelectrodes for microfluidic applications with the goal of reliably constructing Ag/AgCl microelectrodes that are compatible with surface mounted microfluidic channels and have improved potential stability
  • Key information:
    • The electrode surface was electroplated with silver then chemically oxidized to silver chloride to form Ag/AgCl micro-reference electrodes
    • Ag/AgCl microelectrodes produced by this method exhibit increased stability compared with many devices previously reported and were shown to have utility for electrochemical analysis within microfluidic systems

Hypochlorous acid and its pharmacological antagonism: An update picture[edit | edit source]

D. Lapenna and F. Cuccurullo, “Hypochlorous acid and its pharmacological antagonism: An update picture,” Gen. Pharmac., vol. 27, no. 7, pp. 1145–1147, Oct. 1996, doi: 10.1016/S0306-3623(96)00063-8.

Abstract: 1. Some biochemical and pathophysiological aspects of hypochlorous acid (a major oxidant species produced by activated white blood cells) are discussed. 2. Moreover, we have discussed the problem of the pharmacological scavenging of hypochlorous acid, focusing attention on the biochemical tests able to study therapeutically relevant scavenging properties of various drugs against hypochlorous acid itself.

  • This study discusses some of the biochemical and pathophysiological aspects of hypochlorous acid
  • Key information:
    • Hypochlorous acid (HOCl) is a major oxidant species produced by activated white blood cells
    • Hypochlorous acid is a major inorganic bactericidal compound of innate immunity and is effective against a broad range of microorganisms

Hypochlorous Acid as a Potential Wound Care Agent[edit | edit source]

L. Wang et al., “Hypochlorous Acid as a Potential Wound Care Agent,” J Burns Wounds, vol. 6, p. e5, Apr. 2007.

Abstract: Objective: Hypochlorous acid (HOCl), a major inorganic bactericidal compound of innate immunity, is effective against a broad range of microorganisms. Owing to its chemical nature, HOCl has never been used as a pharmaceutical drug for treating infection. In this article, we describe the chemical production, stabilization, and biological activity of a pharmaceutically useful formulation of HOCl. Methods: Stabilized HOCl is in the form of a physiologically balanced solution in 0.9% saline at a pH range of 3.5 to 4.0. Chlorine species distribution in solution is a function of pH. In aqueous solution, HOCl is the predominant species at the pH range of 3 to 6. At pH values less than 3.5, the solution exists as a mixture of chlorine in aqueous phase, chlorine gas, trichloride (Cl3−), and HOCl. At pH greater than 5.5, sodium hypochlorite (NaOCl) starts to form and becomes the predominant species in the alkaline pH. To maintain HOCl solution in a stable form, maximize its antimicrobial activities, and minimize undesirable side products, the pH must be maintained at 3.5 to 5. Results: Using this stabilized form of HOCl, the potent antimicrobial activities of HOCl are demonstrated against a wide range of microorganisms. The in vitro cytotoxicity profile in L929 cells and the in vivo safety profile of HOCl in various animal models are described. Conclusion: On the basis of the antimicrobial activity and the lack of animal toxicity, it is predicted that stabilized HOCl has potential pharmaceutical applications in the control of soft tissue infection.

  • Describes the production, stabilization, and biological activity of HOCl for pharmaceutical applications
  • Key Information:
    • This study used stabilized HOCl is in the form of a physiologically balanced solution in 0.9% saline at a pH range of 3.5 to 4.0
    • Chlorine species distribution in solution is a function of pH
    • In aqueous solution, HOCl is the predominant species at the pH range of 3 to 6
    • At pH values less than 3.5, the solution exists as a mixture of chlorine in aqueous phase, chlorine gas, trichloride (Cl3−), and HOCl
    • At pH greater than 5.5, sodium hypochlorite (NaOCl) starts to form and becomes the predominant species in the alkaline pH
    • To maintain HOCl solution in a stable form, maximize its antimicrobial activities, and minimize undesirable side products, the pH must be maintained at 3.5 to 5
  • Relevant results:
    • This stabilized form of HOCl is used to demonstrate the potent antimicrobial activities of HOCl against a wide range of microorganisms
    • HOCl has potential pharmaceutical applications in the control of soft tissue infection
      • Predicted based on antimicrobial activity and the lack of animal toxicity
    • Paper includes multiple tables showing the time to kill and number of pathogens recovered per minute as well as a table showing safety studies done on different animal species

Framework for evaluating the performance limits of membraneless electrolyzers[edit | edit source]

X. Pang, J. T. Davis, A. D. Harvey, and D. V. Esposito, “Framework for evaluating the performance limits of membraneless electrolyzers,” Energy Environ. Sci., vol. 13, no. 10, pp. 3663–3678, Oct. 2020, doi: 10.1039/D0EE02268C.

Abstract: Emerging membraneless electrolyzers offer an attractive approach to lowering the cost of hydrogen (H2) production from water electrolysis thanks to potential advantages in durability and manufacturability that are made possible by elimination of membranes or diaphragms from the device architecture. However, a fair comparison of the performance limits of membraneless electrolyzers to conventional electrolyzers is hindered by the early stage of research and absence of established design rules for the former. This task is made all the more difficult by the need to quantitatively describe multiphase flow between the electrodes in membraneless electrolyzers, which can have a huge impact on gas product purity. Using a parallel plate membraneless electrolyzer (PPME) as a model system, this study takes a combined experimental and modeling approach to explore its performance limits and quantitatively describe the trade-offs between efficiency, current density, electrode size, and product purity. Central to this work is the use of in situ high-speed videography (HSV) to monitor the width of H2 bubble plumes produced downstream of parallel plate electrodes as a function of current density, electrode separation distance, and the Reynolds number (Re) associated with flowing 0.5 M H2SO4 electrolyte. These measurements reveal that the HSV-derived dimensionless bubble plume width serves as an excellent descriptor for correlating the aforementioned operating conditions with H2 crossover rates. These empirical relationships, combined with electrochemical engineering design principles, provide a valuable framework for exploring performance limits and guiding the design of optimized membraneless electrolyzers. Our analysis shows that the efficiencies and current densities of optimized PPMEs constrained to H2 crossover rates of 1% can exceed those of conventional alkaline electrolyzers but are lower than the efficiencies and current densities achieved by zero-gap polymer electrolyte membrane (PEM) electrolyzers.

  • Explores performance limits of parallel plate membraneless electrolyzers (PPME) and quantitatively describes the trade-offs between efficiency, current density, electrode size, and product purity
    • Achieved experimentally using in situ high-speed videography (HSV) to monitor the width of H2 bubble plumes produced as a function of many parameters
    • Quantitatively describes the trade-offs and optimization by employing different mathematical relationships that relate the sides of a theoretical model called the unfortunate tetrahedron
  • Relevant results:
    • The efficiencies and current densities of optimized PPMEs constrained to H2 crossover rates of 1% can exceed those of conventional alkaline electrolyzers
    • PPMEs have lower efficiencies and current densities than those achieved by zero-gap polymer electrolyte membrane (PEM) electrolyzers
    • Analyzes the trade-offs between current density, efficiency, and product purity
    • Analyzes the trade-offs between electrode size, efficiency, and product purity
    • Expression for calculating the electrolyzer efficiency is given
    • The dimensionless bubble plume width was determined to be a good descriptor that can be used to predict H2 crossover rates across a wide range of operating conditions for the PPME
      • An empirical relationship was described using Buckingham Pi Theorem to relate plume width to channel width & electrode length and operating conditions (i, velocity (U)) that are expected to influence plume width
  • Key information:
    • Performance metrics can be graphically illustrated using a model dubbed the ‘‘unfortunate tetrahedron’’
      • The four corners are:
        • Opex (related to efficiency)
        • Capex (related to electrode size and scalability)
        • Current density
        • Product purity (a proxy for safety)
    • Gas chromatography used to quantify gas production and crossover rate
  • Device specifications:
    • Cell body made from PLA
    • Rectangular fluidic channel
      • 145 mm long
      • 2 or 4 mm wide
      • 5 mm high
      • 29 mm long by 0.4 mm thick divider placed 12 mm downstream from end of the electrodes
    • Electrodes 2 nm of titanium (Ti) and 50 nm platinum (Pt) onto Ti foil substrates
    • CAD files for the two cells are freely available for download at echem.io

Development of a low-cost Arduino-based potentiostat[edit | edit source]

J. R. Crespo, S. R. Elliott, T. Hutter, and H. Águas, “Development of a low-cost Arduino-based potentiostat,” p. 21.

Abstract: A simple Arduino-based potentiostat has been developed. This potentiostat design is cost effective for low budget applications and educational purposes. The resolution and linear response of the developed potentiosat was evaluated. A cyclic voltammetry of potassium ferricyanide K3[Fe(CN)6] was performed to compare the potentiostat electrochemical performance with a commercial one. To show the portable potentiostat capabilities, cyclic voltammograms were also performed at different scan rates. Finally, the diffusion coefficient of potassium ferricyanide was calculated in a solution containing a known concentration of the salt using the Randles-Sevcik equation.

  • Design for a simple, cost-effective Arduino-based potentiostat
    • A potentiostat is an electronic circuit used to study the electrochemical events taking place at one specific electrode and allow for specific tuning of the applied conditions
  • Abbreviations
    • RE – Reference Electrode
    • CE – Counter Electrode
    • WE – Working Electrode
    • TIA - Transimpedance Amplifier
  • Relevant results:
    • Demonstrates the circuit being simulated using the LTspice simulation software and it’s resulting transient response graphs
    • Provides and explains the Arduino software
    • Data acquisition through the serial USB interface can be displayed in real time using the open-source processing software environment as a serial terminal visual interface, which can also save the data in text file format for analysis
    • Circuit functions by comparing the measured Cell voltage with the desired voltage
      • Using this it drives current into the cell to force the voltages to be the same in an inverting configuration to provide the negative feedback
    • See original paper for circuit diagram
      • U1 is a Differential Amplifier:
        • Functions to drive the input signal from the Arduino PWM output and add offset voltage value, from the voltage divider composed by the 100K resistors and the 100k potentiometer in the non-inverting input, to shift the applied voltage to the cell in the desired range
        • The potentiometer is used so the user can tune manually the applied voltage range
      • U2 is the Control Amplifier
        • Measured Cell voltage compared with the desired voltage
        • Drives current into the cell to force the voltages to be the same in an inverting configuration to provide the negative feedback
        • Equations describing this provided in paper
      • U3 and U4 are voltage Followers
        • Isolates the input from the output to prevent loading of the input signal
        • Voltage output from U3 is connected to the CE
      • U5 and U7 are voltage shifters
        • Sums 3.3 V from the Arduino to the output signals of the potentiostat
          • i.e. the voltage and the current measured
        • Used because Arduino only can read voltage values from 0 to 5000 mV
          • Thus, the maximum applied voltage ranges that can be read by the potentiostat can be determined which is from -3300 mV to +1700 mV
      • U6 is a transimpedance amplifier
        • Converts the measured current in the WE into a voltage through Rf
  • Key information:
    • To evaluate the fabricated potentiostat and to verify proper function use 10 and 20 kΩ resistors to simulate the electrolyte
      • The resistors were connected between the working electrode and reference electrode (simulating the cell resistance) and a small resistor (100 Ω) was connected between the reference and the counter electrode to have the control of the potential applied
    • The 10-100 kΩ gain potentiometer was used to read currents in the ranges from µA to mA
      • Range where most electrochemical reactions occur
    • The Potentiostat Circuit presented was inspired by the potentiostat circuit proposed in “A Simplified Microcontroller Based Potentiostat for Low-Resource Applications” by Aremo et al

Highly-customizable 3D-printed peristaltic pump kit[edit | edit source]

T. Ching et al., “Highly-customizable 3D-printed peristaltic pump kit,” HardwareX, vol. 10, Oct. 2021, doi: 10.1016/j.ohx.2021.e00202.

Abstract: Commercially available peristaltic pumps for microfluidics are usually bulky, expensive, and not customizable. Herein, we developed a cost-effective kit to build a micro-peristaltic pump (~ 50 USD) consisting of 3D-printed and off-the-shelf components. We demonstrated fabricating two variants of pumps with different sizes and operating flowrates using the developed kit. The assembled pumps offered a flowrate of 0.02 ~ 727.3 μL/min, and the smallest pump assembled with this kit was 20 × 50 × 28 mm. This kit was designed with modular components (i.e., each component followed a standardized unit) to achieve (1) customizability (users can easily reconfigure various components to comply with their experiments), (2) forward compatibility (new parts with the standardized unit can be designed and easily interfaced to the current kit), and (3) easy replacement of the parts experiencing wear and tear. To demonstrate the forward compatibility, we developed a flowrate calibration tool that was readily interfaced with the developed pump system. The pumps exhibited good repeatability in flowrates and functioned inside a cell incubator (at 37 °C and 95 % humidity) for seven days without noticeable issues in the performance. This cost-effective, highly customizable pump kit should find use in lab-on-a-chip, organs-on-a-chip, and point-of-care microfluidic applications.

  • 3D-printed peristaltic with the following specifications (variant A):
    • L × W × H [mm]: 30 × 85 × 17
    • Number of channels: 4
    • Flow rate range [μL/min]: 0.05 ~ 727.3
    • Cost of hardware [USD]: ~$30
    • Cost per channel [USD]: ~$8
  • Comment: this pump will not be used in the final functional design due to the relatively low flow rate; it will be used for testing and characterization of the flow cell architecture but the final design will use a simpler, less precise peristaltic pump with a flow rate since a lower level of precision is required for production versus testing/characterization
  • Designed to allow for variations in size, flow rate, and the number of channels
  • Peristaltic pumps deliver fluids by alternating compression and relaxation of a flexible tube
  • The pump consists of three rollers
    • Three-roller assembly is used to generate rotational torque from a single motor to induce fluid propulsion
    • The rotation of the rollers compress and relax the flexible tubing to create negative and positive pressures to draw the fluid through the tube
  • Variant A was a four-channel peristaltic pump with a footprint of 30 x 85 x 17 mm with flow rates of 0.05 ~ 727.30 mL/min
  • A standard 2-mL Eppendorf tube was modified to create a fluid reservoir which was secured with 3D printed a tube holder
  • Modular design: deconstructable into small subunits called modules
    • Can be independently printed, modified, replaced, or exchanged with other modules in the same system or between different systems

References[edit | edit source]

[1]G. Chisholm, P. J. Kitson, N. D. Kirkaldy, L. G. Bloor, and L. Cronin, “3D printed flow plates for the electrolysis of water: an economic and adaptable approach to device manufacture,” Energy Environ. Sci., vol. 7, no. 9, pp. 3026–3032, 2014, doi: 10.1039/C4EE01426J.

[2]M. Hashemi, M. Modestino, and D. Psaltis, “A membrane-less electrolyzer for hydrogen production across the pH scale,” Energy Environ. Sci., vol. 8, pp. 2003–2009, Apr. 2015, doi: 10.1039/C5EE00083A.

[3]S. M. H. Hashemi et al., “A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine,” Energy Environ. Sci., vol. 12, no. 5, pp. 1592–1604, 2019, doi: 10.1039/C9EE00219G.

[4]B. J. Polk, A. Stelzenmuller, G. Mijares, W. MacCrehan, and M. Gaitan, “Ag/AgCl microelectrodes with improved stability for microfluidics,” Sensors and Actuators B: Chemical, vol. 114, no. 1, pp. 239–247, Mar. 2006, doi: 10.1016/j.snb.2005.03.121.

[5]P. Boissonneau and P. Byrne, “An experimental investigation of bubble-induced free convection in a small electrochemical cell,” Journal of Applied Electrochemistry, vol. 30, p. 9, Jul. 2000, doi: 10.1023/A:1004034807331.

[6]S. K. Kim, D.-M. Shin, and J. W. Rhim, “Designing a high-efficiency hypochlorite ion generation system by combining cation exchange membrane aided electrolysis with chlorine gas recovery stream,” Journal of Membrane Science, vol. 630, p. 119318, Jul. 2021, doi: 10.1016/j.memsci.2021.119318.

[7]A. Kraft et al., “Electrochemical water disinfection Part I: Hypochlorite production from very dilute chloride solutions,” vol. 29, pp. 859–866, Jul. 1999, doi: 10.1023/A:1003650220511.

[8]K. Song, “Electrolysis,” Chemistry LibreTexts, Oct. 02, 2013. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Electrochemistry/Electrolytic_Cells/Electrolysis (accessed Jan. 06, 2022).

[9]“Hypochlorous acid,” Wikipedia. Dec. 31, 2021. Accessed: Jan. 08, 2022. [Online]. Available: https://en.wikipedia.org/w/index.php?title=Hypochlorous_acid&oldid=1063015952

[10]D. Lapenna and F. Cuccurullo, “Hypochlorous acid and its pharmacological antagonism: An update picture,” Gen. Pharmac., vol. 27, no. 7, pp. 1145–1147, Oct. 1996, doi: 10.1016/S0306-3623(96)00063-8.

[11]L. Wang et al., “Hypochlorous Acid as a Potential Wound Care Agent,” J Burns Wounds, vol. 6, p. e5, Apr. 2007.

[12]X. Pang, J. T. Davis, A. D. Harvey, and D. V. Esposito, “Framework for evaluating the performance limits of membraneless electrolyzers,” Energy Environ. Sci., vol. 13, no. 10, pp. 3663–3678, Oct. 2020, doi: 10.1039/D0EE02268C.

[13]J. R. Crespo, S. R. Elliott, T. Hutter, and H. Águas, “Development of a low-cost Arduino-based potentiostat,” p. 21.

[14]T. Ching et al., “Highly-customizable 3D-printed peristaltic pump kit,” HardwareX, vol. 10, Oct. 2021, doi: 10.1016/j.ohx.2021.e00202.

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Type Literature review
Authors Cameron Brooks
Published 2021
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