When fluidized by the upward flowing liquid electrolyte, the resulting fluidized particle bed electrodes can reduce reaction polarization, thereby reducing voltage and consequent power requirements. In some cases this effect can also avoid undesirable side reactions.
Particle bed materials attractive for these electrodes include copper, boron carbide, carbon and copper-coated polymer or glass. Particle sizes and shapes are related to fluidization characteristics and to pressure loss for liquids flowing through the beds.
Electrolytes currently under investigation include sodium and potassium hydroxide, sulfuric acid and sodium chloride. Study of some weak electrolytes is also considered.
In conjunction with the electrolysis investigation, some testing has been started on the use of scrap aluminum and sodium hydroxide reactants to generate hydrogen. The scrap aluminum is available from General Pneumatics manufacturing machining operations. This material has historically been sold as scrap.
A test loop has been set up in a separate room in the General Pneumatics main building consisting of a DC power supply, an electrolyte head tank, replaceable test cell units, a peristaltic pump, a flow meter and a separator vessel. All fluid components are interconnected with tygon tubing. All hydrogen-containing components are housed in a specially fabricated laboratory hood. A very small experimental photovoltaic unit has been procured and is to be modified for testing in our test apparatus.
Four test cell design configurations are being evaluated. These are referred to as circular, rectangular, hexagonal and annular. These designations are based on the geometric cross-section of the cathode half-cells. Initial tests indicate that the annular cell is most convenient for this phase of the investigation. It consists of an outer transparent cylindrical tube, a current feeder wrapped around its inner wall, a cathode comprised of an annular bed of copper particles (or copper coated polymer beads), a carbon anode array located in the center tube of the test cell and a membrane separator supported on the center tube. A bed of glass beads is located at the bottom of the cell to provide even flow distribution when the electrolyte is flowing and to support the particulate bed when flow is stopped. Sparger pipes designs are planned for larger size units.
The large cathode surface area of the particle bed electrode is designed to yield increased hydrogen production rates compared to solid plate electrodes. The agitation achieved under fluidization is expected to result in improved disengagement of the evolved hydrogen gas bubbles and decreased cathode polarization for the fluidized bed electrode.
The US Department of Energy has set a targeted untaxed price of $2.00 – $3.00 per gasoline gallon equivalent on an energy basis for hydrogen production and delivery (Ref. 2). The department has noted that one of the barriers to cost reduction is the lack of manufacturing capacity for producing the significant quantities of small-scale systems that will be needed for distributed production of hydrogen systems.
Technical Discussion
The required emf for the reversible generation of hydrogen at the cathode of an electrolytic cell as a function of concentration is given by the relation:
E=EoRT/2F ΣVjlnC j
Where,
E = emf
E0 = standard electrode potential
R = universal gas constant
T = absolute temperature
Cj = equilibrium concentration of species j
F = Faraday = 96,500 coulombs
Vj = stoichiometric factor of species j
This is used to estimate the effects of electrolyte concentrations for the cell testing. For the estimating of emf requirements as a function of hydrogen pressure we note that:
E=Eo + RT/2F ln(P/Patm)
Where,
P = pressure in atmospheres
F = Faraday = 96,500 coulombs
R = Gas Constant
Ep, Eo = Electromotive force at pressure P and at standard conditions, respectively
A plot of voltage versus pressure for the cell- H2: HCl (0.1N), HgCl; Hg shown in Ref. 3. demonstrates the dramatic effect on developed hydrogen pressure that can be obtained from relatively small increases in impressed cell voltage. In this case, a cell voltage increase from 0.4 volts to 0.46 volts increases the pressure from 1 (100) to over 100 (102) atmospheres. This characteristic could be useful for fuel cell hydrogen feed units.
Flow Rate - Minimum fluidization velocity will be tested for various cell configurations and for a number of particle materials as guided by calculated velocities.
Phase I of the program includes technical analysis, design and physical configuration studies, manufacturing and production analysis, and small scale proof of concept study/demonstration to evaluate feasibility of the proposed technology. Phase II involves one or more prototype systems to be fabricated for operational evaluations of the fluidization parameters, safety validations and sustained stability/capacity. This is to be followed by Phase III that covers marketing and production for commercial and governmental applications.
Electrolysis is one of the approaches under investigation in the current government programs. In the National Academy of Science report, NAS recommendation ES-7 states: “DOE should increase emphasis on electrolyzer development” (Ref. 1). Small, distributed hydrogen generating units have the advantages of lower developmental costs, savings in distribution expenses, more tractable safety issues.
Electrolysis has the added advantage of accommodating renewable energy source devices such as small wind turbines and photovoltaic units. The low voltage requirements of the electrolysis units make it possible to use less expensive electricity generators for evaluation testing.
In addition to hydrogen production, fluidized bed electrodes can be useful for other electrochemical processes. The information obtained from this investigation is expected to be applicable to other technical areas such as environmental cleanup. An advantage in these applications is that the pollutant concentrations are frequently low which tends to result in efficient energy use.
Safety Analyses
Evaluations of the potential hazards and safety measures associated with the substances, processes and operation of the hydrogen generation units will be performed periodically at various stages of the project. This discussion summarizes the initial safety analysis for the pre-testing phase.
The initial safety analysis addresses the hazards of fire, explosion and toxic releases. Fire and explosion hazards from hydrogen and oxygen are evaluated using the Dow fire and explosion index given in Ref 4. In the case of hydrogen the lower flammability limit in air is 4% and the upper limit (ufl) is 75% (Ref 5). It is planned to continue to collect hydrogen at concentrations exceeding 95%, which will maintain the hydrogen well above the upper limit.
Quantities of hydrogen allowed to collect have been limited so as to limit the size of any potential explosion. This limit has tentatively been set at 350 standard cubic centimeters.
Potential leakage of hydrogen into the surrounding air must be considered and steps taken to assure that such leakage does not occur. Reviews of potential leak paths will be performed for each configuration of the test apparatus. Local enclosed venting in the vicinity of hydrogen-containing sections of the test apparatus is being provided with the use of a ducted, hooded enclosure.
Sparking in this low voltage apparatus has a low probability. Precautions to avoid cell voltage reversal will be incorporated into design and the operating procedures. Nevertheless, humidification of the test area will be maintained as a precaution against static electricity. Possible effects from potential detonations will be limited based on the hydrogen quantity limits discussed above. In addition, a shield to protect against any potential projectiles has been installed at the front of the apparatus protective enclosure.
References Cited:
1. NATIONAL RESEARCH COUNCIL AND NATIONAL ACADEMY OF ENGINEERING, “THE HYDROGEN ECONOMY: Opportunities, Costs, Barriers, and R&D Needs,” THE NATIONAL ACADEMIES PRESS Washington, D.C, (2004), page 6. 2. U.S. Department of Energy, “Small Business Innovation Research Program and Small Business Technology Transfer Program, FY 2006 Funding Opportunity Notice, OFFICE OF BASIC ENERGY SCIENCES, PROGRAM AREA OVERVIEW, Topic 20, page 1. 3. MacInnes, Duncan A., “The Principles of Electrochemistry, Dover Publications, (1961) pg.117. 4. Crowl, Daniel A., and Louvar, Joseph F., “Chemical Process Safety”, Section 10-2, “Hazard Surveys”, (1990), page 311. 5. Air Products and Chemicals, Inc, Hydrogen Gas Material Safety Data Sheet No. 300000000074, (2004), page 3.