As the population continues to grow around the world, there is an ever-increasing need for clean and efficient sources of energy. As it is also likely that future energy needs will come from multiple power sources that work in concert, there is an implied need to diversify the sources of produced energy [1
]. This diversification will also contribute to the ability to remotely power smaller electronic devices under a variety of situations.
Although it is inconceivable to think that hydrocarbons will be replaced as an energy source any time soon, there is always an ever pressing need to improve the efficiency of the use of all traditional energy resources, including small molecule carbon compounds. One high energy small molecule hydrocarbon that has proved resistant to use as a non-biological energy source is glycerol. The visibility of glycerol has increased in parallel with the development of the bio-diesel industry as the latter has continued to grow over the past 20 years [3
]. The production of bio-diesel utilizes the base-catalyzed transesterification of waste vegetable oil with methanol to make renewable fuel for transportation or other diesel dependent systems. Additionally, as bio-diesel exploitation increases, the large amount of low-grade glycerol byproduct produced has begun to impact the sustainability of the whole bio-diesel production process.
Bio-diesel production results in a volume of approximately 10% (w
) glycerol of the total product [4
]. Although many uses have been touted, the oversupply of crude glycerol byproduct remains a deterrent to the further promotion of bio-diesel production and its use. Entities investing in bio-diesel production also need to begin to consider investing in glycerol storage. As a result, some research has been done to convert glycerol into smaller carbon-containing molecules for alternative energy applications [5
]. This conversion work appears to be too energy intensive and will likely not result in a feasible way to use the glycerol feedstock as an energy source anytime soon.
Glycerol is difficult to use as an energy source in a traditional combustive manner and has been only non-biologically approachable through fuel cell designs [8
]. Biologically, glycerol has been used a variety of ways, including to grow bacteria or livestock, to conserve energy, or to produce an alternative energy source [9
Fuel cells are arguably the most efficient way to harvest energy from a resource as the process avoids the limitations of the Carnot cycle and harnesses an increased amount of the available energy. Research has been invested into the development of fuel cell configurations to generate power directly from glycerol oxidation with different anodic materials and catalysts [10
]. In recent studies, the catalytic oxidation of glycerol was found to be significantly promoted by the presence of gold and platinum catalysts on carbon supports under alkaline conditions [13
]. These are conditions which are similar to fuel cells that have utilized other alcohols or sugar alcohols such as methanol, ethanol, and ethylene glycol [18
For the most part, these fuel cell designs are modeled after the hydrogen fuel cell with expensive precious metal catalysts at the cathode and temperamental separator membranes [10
]. Such cells can require a significantly skilled level of expertise to fabricate them. Of the ten glycerol fuel cells compared in researching these efforts, all but four use a platinum-based catalyst at the cathode. Of the ten, all used some form of a proton exchange membrane. Again, these choices are generally based on modeling these fuel cells after the hydrogen fuel cell. The proton exchange membrane is arguably one of the limiting factors in the commercialization of the hydrogen fuel cell and is really not feasible if a fuel cell for glycerol is going to be further developed. This fact is accentuated by the use of a proton exchange membrane in the example of a bio-fuel cell that uses biological catalysts at the anode and cathode. Even though the catalysts are generally exclusive in terms of their substrate preferences, this cell also uses a proton exchange membrane (Table A1
In this work, extremely simple glycerol fuel cells were made from a purchased MnO2 cathode catalyst material. Various materials, with and without gold-coating, were used as anodic surfaces. A non-conductive material (a piece of plastic with holes) was used as a separator to ensure that the anode and the cathode did not come in contact. The cell electrolyte was aqueous KOH. Glycerol was added to the electrolyte, completing the fuel cell.
These simple cells can collect a significant amount of energy from glycerol under basic conditions without a membrane. The performance of these glycerol fuel cells was also recorded at different temperatures. Additionally, experiments using various concentrations of ethanol, xylitol, and glucose were performed to compare with the glycerol fuel cell. To ensure this cell design was able to generate power from crude glycerol, glycerol from a batch of bio-diesel was produced through the transesterification of oil with methanol and KOH, and the crude glycerol was also used as the fuel for this simple fuel cell.
After the performance of the cell was evaluated and the dependence of the cell’s performance on the concentration of glycerol and KOH was determined, a preliminary survey into the products of this oxidation was begun. Product determination was explored with nuclear magnetic resonance (NMR), an enzymatic peroxidase assay, a pressure assay to detect the production of gasses from the products, and comparative titrations. All of these methods and assays were to help contribute to the future identification of possible products of this oxidation reaction occurring in the fuel cell.
2. Materials and Methods
Glycerol (≥99.5%) was purchased from Fisher Scientific (Logan, UT, USA). Potassium cyanide (KCN) was purchased from Fluka Analytical (München, Germany) Potassium hydroxide (KOH) pellets were purchased from VWR Analytical (New York City, NY, USA). A 20 mM solution of gold chloride (AuCl3) was purchased from Unit-Tech (St. Louis, MO, USA). Methanol, ethanol, xylitol, glucose, equine heart myoglobin, potassium chloride (KCl), Tris base, sodium malonate, potassium oxalate, and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), and manganese (IV) oxide (MnO2) were purchased from Sigma-Aldrich (St. Louis, MO, USA).
2.2. Fuel Cell Materials
Glycerol fuel cells were simply made from the cathodic material utilizing different anodic surfaces. The materials used as anodic surfaces were 99% gold mesh (6.25 cm2), platinum mesh, gold-plated platinum mesh (5.28 cm2), carbon felt, gold-plated carbon felt (5 cm2, 3.18 mm thick), silver-plated nickel mesh, and gold-plated sliver-plated nickel mesh (4.94 cm2). All connecting wires were made of platinum to nullify any corrosive currents from contributing to the produced power.
For the preparation of the gold-plated surfaces, a solution of 10 mM AuCl3
and 1.6 M KCN was prepared, and a 9V battery was used to provide the plating potential. A platinum wire was used as a counter electrode for gold plating. After connecting all surfaces, the surfaces and the counter electrode were dipped into the solution for 10 s. All gold-plated surfaces were prepared this way. The air-breathing cathode (a silver-plated nickel screen electrode with 0.6 mg cm−2
loading using 10% MnO2
on Vulcan XC-72 with micro-porous fluorocarbon backing) was purchased from Electric Fuel (Bet Shemesh, 9905415, Israel). Cells were organized in-house by simply using a heat gun to seal the micro-porous fluorocarbon backing on both sides of the cathode and an adhesive (white Gorilla Glue, Gorilla Glue Company, Sharonville, OH, USA) to strengthen the seal. The resulting cells have a thin V-shaped structure (Figure 1
). The volumes of the cells are approximately 3 mL.
2.3. Experimental and Methods
Fuel cell experiments were performed with different concentrations of glycerol (0.27 molar (M), 0.68 M, 1.4 M, 4.1 M, 8.2 M) and KOH (0.5 M, 1 M, 3 M, 6 M, 8 M). Additionally, experiments with different concentrations of ethanol (0.34 M, 1.7 M, 10 M), xylitol (0.2 M, 0.5 M, 1 M), and glucose (0.2 M, 0.5 M, 1 M) were performed, also in solutions of 8 M KOH. Power was drawn from glycerol cells in which the above listed anodic surfaces were used. The concentration of glycerol and OH−
in the bio-diesel glycerol byproduct was determined by comparing the completion of the transesterification reaction, the solutions pH and was confirmed by using the resulting voltages upon drawing currents and comparing those voltages to a standard curve created from previous experiments (Figure A1
The final product of the oxidation of glycerol in these fuel cells was explored with 13C-NMR, a colorimetric ABTS peroxidase assay, a pressure assay, and a set of comparative titration experiments. The peroxidase enzymatic activity of myoglobin was harnessed to identify the presence of H2O2 as a byproduct of the oxidation of glycerol by monitoring the changing absorbance of the electron donor ABTS in solution with a spectrophotometer. All measurements were performed in 1M tris buffer pH 7.5 with a total volume of 3 mL. The pH of the fuel sample was adjusted to about 7.5, and 0.02 mL of the sample was added along with 0.1 mL of ABTS and 0.2 mL of a 2 mg mL−1 myoglobin solution.
A pressure assay was also performed which can relate an increase in pressure to the release of a gas from solution. This assay can be used to detect the presence of either H2O2 or the carbonate ion. These two types of experiments were performed by either adding MnO2 to detect H2O2, or HCl to detect the presence of the carbonate ion. Solutions of (a) 8 M KOH and 100 µL H2O2, (b) 8 M KOH and an unreacted sample of 1.4 M glycerol, and (c) a reacted sample which initially contained 8 M KOH and 1.4 M glycerol were compared to each other. To show MnO2 can decompose H2O2 under neutral and basic conditions, two controls were set up. Control 1 was suspension of MnO2 with 100 µL H2O2; control 2 was a suspension of MnO2 with 100 µL H2O2 and 8 M KOH. The pressure changes were monitored through a Micro LAB interface. Additionally, to determine if the gas produced by the reacted solutions was something other than oxygen, an oxygen sensor was used to detect its relative concentration before and after the pressure change in the reaction vessel. The change in pressure can also indicate the amount of gas and thus the amount of either reactant in the initial sample.
A titration was also performed on the reacted fuel cell sample as well. The reacted sample (8 M KOH and 1.4 M glycerol after oxidation) was titrated with 0.1 M HCl. Three compounds were used to attempt to simulate the titration curve of the sample. These compounds were carbonate, malonate and oxalate in a comparable solution of KOH. The titration curve was also monitored by a Micro LAB interface in which the pH was monitored vs. the volume of added HCl.
All electrochemical measurements were obtained using a Bio-Logic 3-channel VSP galvanostat/potentiostat (Bio-Logic Science Instruments, Seyssinet-Pariset, France). Electrochemical cells were monitored at different current levels to determine sustainable voltages. The anodic surfaces were coated with a thin gold surface through applying voltage to the anodes’ surfaces in the mixture of potassium cyanide, potassium hydroxide, and gold chloride with a platinum wire as the counter electrode. The ABTS assay was monitored by the kinetics application of the Vernier SpectroVis Plus spectrophotometer software (Vernier Software & Technology, Beaverton, OR, USA). Additionally, the pressure assay and titrations were performed on a FS-522 MicroLab system (MicroLab, Moscow, Russia) with qualitative oxygen measurements taken with a DO200 Oxygen sensor from YSI Environmental (Yellow Springs, OH, USA). An UltraShield 300 NMR instrument (Bruker, Billerica, MA, USA) was used to help in the identification of the final products after glycerol oxidation.
The results here clearly indicate that glycerol can be used as a fuel in these simple, membrane-less fuel cell configurations. As the concentration of glycerol increases, there is an increase in power production. The lack of linearity of power production above 2 M glycerol is interesting and worth exploring in the future. There is a possibility that as the concentration of glycerol increases, the viscosity of the solution begins to alter the activity of the glycerol at the anode surface. It is also a possibility that the higher concentration of glycerol results in the production of more carbonate product which would lower the concentration of the available activity of the electrolyte in solution. However, with 8 M KOH in solution, it is also possible that any carbonates are not significantly interfering with the required available electrolyte. The influence of the possible production of carbonates would be more pronounced over time as the cell were to run toward completion. The experiments performed where this was done to examine the products of the reaction did not show any significant decrease that would seem to be attributed to the presence of carbonates, but this is a study that should be performed in the future to identify the impact of the presence of carbonates on the cell’s performance.
There is also a clear dependence on the concentration of the hydroxide ion. This concentration of hydroxide adjusts the redox potential for the glycerol and allows the reaction to occur more favorably. The fuel cell also shows a degree of activity on other small carbon alcohols including glucose, ethanol and xylitol. Besides glycerol as a fuel source, the power generated from the oxidation of glucose in this type of fuel cell setup is also relatively interesting.
According to the previous research [27
], as the temperature increases, it is possible that the reaction at the cathode is limited. Secondly, based on the proposed mechanism of oxidation of glycerol [16
], it requires oxygen to cleave the bond between Au and glycerol. The air-breathing cathode material is capable of capturing oxygen from the air. However, as the temperature increases, the solubility of oxygen decreases. Therefore, the separation between Au and glycerol oxidation product is limited. As a result, more glycerol would remain on the Au surface and would possibly interrupt the oxidation process of other glycerol molecules. Although higher temperatures might result in a faster reaction, the fact that glycerol cannot separate from the Au surface might be the main contribution to the decrease in power production.
A sample of crude glycerol from a transesterification process also produced power in this fuel cell configuration, albeit a significantly lower amount. This may be due to the concentration of glycerol being lower than what was calculated. It may be that the actual concentration of the glycerol was not attainable with the standard curve used because additional components in the solution such as methanol or soaps could interfere with the standardization of the sample. The presence of residual methanol would also compete for active sites and be oxidized itself as well. This activity would alter the current and the potential of the cell.
To produce any significant amount of power, the anodic surfaces need to have a gold-plated catalyst. Additionally, the surface under the gold plating plays a significant role. Although it is known that gold catalyzes the oxidation of glycerol [12
], it was relatively surprising to see how poor of a catalyst gold mesh was. Based on these results, the gold surface had a lower performance compared to other gold-plated surfaces. The gold-plated platinum mesh clearly showed a much greater power generation than the gold surface. This increase in activity of the platinum anode coated in gold is likely due to the contribution that the different surfaces bring to this reaction. The platinum is possibly responsible for binding the oxygen atoms on either the glycerol molecules or possibly the needed molecular oxygen for the reaction, while the gold may kinetically favor the oxidation of the glycerol. It was also hypothesized that the different arrangement of the gold-plated atoms compared to that of the pure gold surface was responsible for the different degree of oxidation. To test this, a surface of gold mesh was gold-plated the same way the other electrodes were. The result was no improvement in the performance of the cell. This suggests that the plating method itself was not the influential part of contributing to the improved oxidation of glycerol. This is a clear indication that the best catalyst for glycerol oxidation is likely going to require at least two different elements.
By performing various assays, it is clear that there was no detectable H2O2 produced from the oxidation that occurs in this cell. It is possible that the cathode material which contains MnO2 promotes the decomposition of H2O2. Thus it could be conceived that H2O2 produced from the oxidation of glycerol would be immediately decomposed into O2 and H2O at the cathode. This is unlikely to occur so quickly and it would still seem possible to identify some residual peroxide if it was being produced at the anode and needing to migrate to the cathode to be consumed. This process would also seem to be associated with a decrease in voltage of the individual runs as this theoretical concentration of H2O2 is accumulated. Such a shift in voltage is not seen within the individual runs.
It was interesting to see that after acidifying the spent fuel sample, a distinct pressure increase was detected. Identifying the gas produced upon acidifying the spent fuel sample will help clarify the products from this glycerol oxidation, but with the concentration of oxygen decreasing upon the production of this gas, it is clearly not oxygen. The pressure results are consistent with the presence of the carboxylate ion. Therefore, it is a possibility that the oxidation of glycerol in this fuel cell results in a decarboxylation which releases CO2. Assuming that the gas generated was CO2, the CO2 generated from decarboxylation should have a ratio of 1:1 with glycerol as it is less likely that the resulting two carbon species would be able to further decompose to another unit of carboxylate. The resulting calculations of the volume of gas produced is very close this ratio.
Additionally, this cell configuration was able to endure over 150 runs without any indication of power decrease due to the possible accumulation from the product on the cathodic or anodic surfaces. According to the mechanism proposed [16
], molecules leave the surfaces after being reacted which would be consistent with the longevity of these cells. As a result, it is likely that the accumulations from the products of this oxidation process are minimal which result in no indication of power decrease.
To further investigate the product of the glycerol oxidation, experiments which were run so that the fuel in the fuel cell was completely oxidized were used to count the amount of electrons that transferred from the glycerol. According to the resulting calculations, these experiments are very close to the electrons transferable from glycerol, assuming an oxidation to CO2. This is further evidence to suggest that the glycerol is significantly oxidized.