Applicability of Waste Engine Oil for the Direct Production of Electricity

New methods for the use of waste products as input for other technologies are a constant subject of research efforts. One such product is waste engine oil. Due to the constantly increasing number of motor vehicles in the world, the recycling or application of engine oils for energy production purposes is currently of considerable importance. This paper contains research regarding the analysis of the electro-oxidation potential of waste engine oil, and thus the possibility of using such oil as a material in fuel cells. The research demonstrates the basic possibility of the electro-oxidation of this oil emulsion on a platinum electrode in an acid electrolyte (aqueous solution of H2SO4). It was shown that in the temperature range of 20–80 °C, the electro-oxidation of the waste engine oil emulsion occurred for all emulsion concentrations (0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume). The maximum current density obtained in the measurements was 21 mA·cm−2 at the temperature of 60 °C (0.030% waste oil and 0.5 M electrolyte). Although this value is small, it encourages further research on the use of used engine oil for the direct generation of electricity.


Introduction
In line with the strategy of reusing waste products as an input base for other technologies, new methods of recycling these products are constantly being developed, both focusing on the recovery of materials as well as energy production [1][2][3][4][5]. Waste engine oil could provide a common example of using a waste product for energy purposes. One of the reasons for this direction of activity is related to the constantly growing number of motor vehicles in the world, which generate huge amounts of waste engine oils [6]. For this reason, the recycling of engine oils forms a challenging study consideration [7][8][9]. The waste oils derived from the automotive industry are mainly previously used engine oils and transmission oils [10]. Such oils are classified as hazardous waste in most countries. Apart from waste oils, in economic practice there is waste contaminated with oils, such as used oil filters and oil packaging. Therefore, the category of waste oil includes oils, water-oil emulsions, and oil-containing sludges [6,7]. These wastes contain contaminants related to the type of oil applied in a specific application (and the amount of improvers), as well as the operation process [10][11][12][13]. An oil's degradation is related to alterations in its physicochemical properties as a result of high temperature and oxygen action in the air, in the presence of catalytically interacting metals and mechanical shear forces [13,14]. During engine operation, the oil is oxidized and the lubricating oil is contaminated with exhaust gases and combustion products of the engine fuel [15,16]. The ongoing changes lead to the formation of waxes, resins and polycyclic aromatic hydrocarbons, and to chemical transformations in enriching additives. The chemical composition of waste oils is complex; such oils are normally toxic, and the reacting elements create compounds that are often hazardous to the environment and humans. The waste oils also contain products of the thermal and mechanical degradation of polymers, as well as metals resulting from the • Restoring oil properties through the application of cleaning processes such as filtration, centrifugation, and evaporation under vacuum in order to use them later, in accordance with their original purpose, or as a lubricant of a lower quality class; • Reprocessing oils, involving the removal of mechanical impurities and water from waste oils to obtain a fuel component of a quality compliant with the substitute fuel specification; • Deep regeneration (re-refining), including appropriate physicochemical processing and obtaining waste petrochemical raw materials from oils that can be used for the production of new lubricating oils or light heating oils; • Recycling by applying waste oil as a resource in a refining process, or in the production of base oil in an installation working in the area of refining; and • Re-using oil directly as a fuel.
One of the most effective ways of reusing waste oils involves their re-refining [21,22]. This process makes it possible to obtain engine oils at lower costs than in the case of the classical method from crude oil [26]. It is also possible to use waste oils as fuel in boilers in order to recover their heat energy. This method of disposal is characterized by low costs, but poses a significant threat to the natural environment [27,28].
There are also many other possible uses of waste oils (e.g., as an additive to asphalt, mixing with water and coal, and decomposition using the microwave pyrolysis process) [29][30][31][32][33][34][35][36], but the majority of such efforts are at the research stage. One of the possibilities of waste oil utilization may also be its direct use to generate electricity in the electro-oxidation process. Therefore, the results of this type of research can be considered in the future as a basis for the analysis of the possibility of using a waste product (such as used engine oil) as an active substance for fuel cells. Fuel cells are characterized by high real efficiency, which can reach up to 80% [37,38]. This high efficiency is due to the fact that the fuel cell converts the chemical fuel into electricity without any intermediate steps [37][38][39][40]. In addition, fuel cells are characterized by a low (or lack of) negative impact on the environment, as well as quiet operation or the absence of moving parts [37,[39][40][41][42]. Although the principle of operation of fuel cells has been known since 1839 [43], it was not until the 1960s that they began to be utilized in the American space program, where cost-related considerations do not hold very important roles [37,38,40,44]. In the case of mobile applications such as FCVs (fuel cell vehicles) or mobile energy sources for laptops and mobile phones, costs form an extremely important, and often decisive, aspect [45,46]. Fuel cells are powered mainly by hydrogen (due to the production of water as the sole by-product), and sometimes also by methanol or hydrazine [37,38,40]. However, problems related to storing hydrogen are responsible for the search for new fuels and biofuels for fuel cells [47][48][49]. On the other hand, the use of waste products for the direct production of electricity in fuel cells would offer the possibility of the simultaneous recycling of this type of substance, combined with due care for the natural environment. Even if the flux of the energy that is generated is inconsiderable, the waste product could become a source of chemical energy for the next process (generating electricity in a fuel cell).
However, in order to talk about the possibility of using waste engine oil as fuel to power fuel cells, it is first necessary to assess whether electro-oxidation of this type of waste product takes place at all. This paper presents research on the possibility of direct generation of electricity from used oils. The research analyzes the possibility of generating electricity through the electrooxidation of used engine oil on a platinum electrode.

Materials and Methods
Waste oils from gasoline and diesel engines were applied in the tests. The oils were obtained from a car service station. The oil used in the research came from diesel, gasoline, and LPG engines. The engine mileage was in the range of 7500 to 30,000 km. As a result, the engine wear was at different levels.
In each car service, used mixed oils from various vehicles are collected and sent for disposal or reprocessing. However, such a mixture contains both engine oils and oils from gearboxes (manual and automatic), differentials, etc. The test plan assumed measurements using only waste engine oils. For this reason, used oils accumulated directly during oil change intervals were applied in the experiment. Table 1 shows the details of the oils used in the measurements. Waste engine oils were mixed in equal proportions (1:1:1:1:1:1:1:1:1:1) to simulate typically used oil obtained from car service stations (or oil change stations).
The oils mixed in equal proportions were initially cleansed of solid impurities resulting from the processes of normal exploitation. First, decantation was performed and then filtered on a viscose (synthetic silk) filter. Due to the addition of oil to the electrolyte during measurements, it was necessary to ensure that the electric current was conducted. The motor oil was not electrically conductive, so to ensure conduction, an agent (detergent) was used to form the emulsion. Syntanol DS-10 was utilized as the detergent [50]. Syntanol DS-10 is a mixture of primary oxygen-ethylene-glycol ethers of fatty alcohol of C 10 -C 18 fraction, and is characterized by high superficial activity, solubilization capabilities, dispersion, and emulsification [51,52]. After electro-oxidation of the emulsion, Syntanol DS-10 can be degraded (e.g., by bacteria) [53,54].
Measurements were carried out in a glass vessel (reactor) using a potentiostat. The emulsion applied in the measurements was obtained by mixing motor oil, detergent, and water with a mechanical agitator working at a speed of 1200 rpm. The proportions of oil, water, and detergent (1:2:1) were selected experimentally. The emulsion stability time was about 20 min. The analysis of the emulsion stability was based on the observation of the substance separation. The observation consisted of the analysis of images of the sample (exposed to white light every minute), that is, the analysis of changes in the uniformity of the color in the entire images of the emulsion sample.
Measurements were made using the method of polarization electro-oxidation curves of the engine oil emulsion on a smooth platinum electrode in an acid electrolyte. An aqueous solution of H 2 SO 4 (0.5 M, 2 M) was used as the electrolyte. Figure 1 contains a diagram of the measurement setup for the testing of waste engine oil electro-oxidation. of the sample (exposed to white light every minute), that is, the analysis of changes in the uniformity of the color in the entire images of the emulsion sample.
Measurements were made using the method of polarization electro-oxidation curves of the engine oil emulsion on a smooth platinum electrode in an acid electrolyte. An aqueous solution of H2SO4 (0.5 M, 2 M) was used as the electrolyte. Figure 1 contains a diagram of the measurement setup for the testing of waste engine oil electro-oxidation. A smooth platinum electrode [55] with an area of 6.28 cm 2 was used as the electrode in all measurements, while a saturated calomel electrode (SCE) was used as the reference electrode [56,57]. The use of a platinum electrode ensures the determination of a catalytic reference for future measurements with the use of other catalysts, mainly those that do not contain noble metals [55,[58][59][60][61].
We measured the electro-oxidation of emulsions based on used engine oil in acid electrolyte (0.5 M, 2 M-aqueous solution of H2SO4), for various concentrations of oil and A smooth platinum electrode [55] with an area of 6.28 cm 2 was used as the electrode in all measurements, while a saturated calomel electrode (SCE) was used as the reference electrode [56,57]. The use of a platinum electrode ensures the determination of a catalytic reference for future measurements with the use of other catalysts, mainly those that do not contain noble metals [55,[58][59][60][61].
We measured the electro-oxidation of emulsions based on used engine oil in acid electrolyte (0.5 M, 2 M-aqueous solution of H 2 SO 4 ), for various concentrations of oil and detergent, at different temperatures (20,40,60, and 80 • C). The concentration of used engine oil emulsion (when measuring electro-oxidation) was 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume.
However, the stationary potential of the working electrode in the electrolyte was measured first. Then, the electro-oxidation of the detergent itself was tested (Syntanol DS-10-concentration 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume) in order to ensure that the electric current was generated from the oil, and not only from the detergent. For measurements of the electro-oxidation of the waste engine oil emulsion, an AMEL System 500 potentiostat (Amel S.l.r., Milano, Italy) with CorrWare software (Scribner Associates Inc., Southern Pines, NC, USA) was used. The emulsion was mixed using a CAT R17 mechanical agitator (Ingenieurbüro CAT M.Zipperer GmbH, Staufen, Germany) and a technoKartell model TK 22 magnetic stirrer with hot plate (Kartell S.p.A.-LABWARE Division, Noviglio, Italy) was used to mix the electrolyte with emulsion in the glass vessel (reactor) (Figure 1). gine oil emulsion (when measuring electro-oxidation) was 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume.

Results
However, the stationary potential of the working electrode in the electrolyte was measured first. Then, the electro-oxidation of the detergent itself was tested (Syntanol DS-10-concentration 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume) in order to ensure that the electric current was generated from the oil, and not only from the detergent.
Measurements of the electro-oxidation of Syntanol DS-10 itself (without waste engine oil) in an acidic electrolyte (aqueous solution of H2SO4) were performed for various concentrations of detergent at the temperatures of 20, 40, 60, and 80 °C. Only in the next step were measurements of the electro-oxidation of used engine oil emulsion in aqueous solution of H2SO4 electrolytes performed for various concentrations of emulsions at 20, 40, 60, and 80 °C. The comparison of these two processes (electro-oxidation of Syntanol DS-10 and waste engine oil emulsion) offered the possibility of determining whether the current is generated during the electro-oxidation of used engine oil or only from detergent (in this case it was based on Syntanol DS-10).
For measurements of the electro-oxidation of the waste engine oil emulsion, an AMEL System 500 potentiostat (Amel S.l.r., Milano, Italy) with CorrWare software (Scribner Associates Inc., Southern Pines, NC, USA) was used. The emulsion was mixed using a CAT R17 mechanical agitator (Ingenieurbüro CAT M.Zipperer GmbH, Staufen, Germany) and a technoKartell model TK 22 magnetic stirrer with hot plate (Kartell S.p.A.-LABWARE Division, Noviglio, Italy) was used to mix the electrolyte with emulsion in the glass vessel (reactor) (Figure 1).    A saturated calomel electrode was used in the research to make the measurements easier. However, after conversion, all research results (Figures 2-9) included in the study refer to an RHE.          ous concentrations of detergent and emulsion (0.005%, 0.010%, 0.030%, and 0.060% of reactor volume). Figures 6-9 present details of the electro-oxidation curves of Syntanol DS-10 (orange lines) detergent and waste engine oil emulsion (black lines) in 2 M H2SO4 solution for temperatures in the range of 20-80 °C. The concentrations of Syntanol DS-10 detergent (orange lines) and waste engine oil emulsion (black lines) were equal to 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume.   emulsion (black lines) at the temperature of 80 °C in a 0.5 M aqueous solution of H2SO4, for various concentrations of detergent and emulsion (0.005%, 0.010%, 0.030%, and 0.060% of reactor volume). Figures 6-9 present details of the electro-oxidation curves of Syntanol DS-10 (orange lines) detergent and waste engine oil emulsion (black lines) in 2 M H2SO4 solution for temperatures in the range of 20-80 °C. The concentrations of Syntanol DS-10 detergent (orange lines) and waste engine oil emulsion (black lines) were equal to 0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume.     A saturated calomel electrode was used in the research to make the measurements easier. However, after conversion, all research results (Figures 2-9) included in the study refer to an RHE.

Discussion
On the basis of the measurements of the electro-oxidation of Syntanol DS-10 (orange lines in Figures 2-9) and the measurements of the electro-oxidation of the waste engine oil emulsion (black lines in Figures 2-9), we could determine the range of temperatures at which the oxidization of the emulsion occurred faster. Therefore, we determined the boundary temperature for the use of waste engine oil as fuel in the analyzed electrolyte (aqueous solution of H2SO4). It was demonstrated that until the temperature of 60 °C, the value of the current density increased continuously (Figures 2-4 and 6-8). However, when a comparison was made with the results of the current density, with a further increase in temperature to 80 °C, the electro-oxidation of Syntanol DS-10 (orange lines in Figures 5  and 8) took place first, and only then did the electro-oxidation of waste engine oil emulsion occur (black lines in Figures 5 and 8).

Discussion
On the basis of the measurements of the electro-oxidation of Syntanol DS-10 (orange lines in Figures 2-9) and the measurements of the electro-oxidation of the waste engine oil emulsion (black lines in Figures 2-9), we could determine the range of temperatures at which the oxidization of the emulsion occurred faster. Therefore, we determined the boundary temperature for the use of waste engine oil as fuel in the analyzed electrolyte (aqueous solution of H 2 SO 4 ). It was demonstrated that until the temperature of 60 • C, the value of the current density increased continuously (Figures 2-4 and Figures 6-8). However, when a comparison was made with the results of the current density, with a further increase in temperature to 80 • C, the electro-oxidation of Syntanol DS-10 (orange lines in Figures 5 and 8) took place first, and only then did the electro-oxidation of waste engine oil emulsion occur (black lines in Figures 5 and 8).
Within the temperature range of 20-80 • C, electro-oxidation of the waste engine oil emulsion occurred for all emulsion concentrations (0.005%, 0.010%, 0.030%, and 0.060% of the reactor working volume) and both electrolyte concentrations (0.5 M and 2 M) (Figures 2-8). The current density for all measurements of the electro-oxidation of used engine oil emulsion was in the range of 4-21 mA·cm −2 . The current density value increased until the temperature reached 60 • C (Figures 2-4 and Figures 6-8), where it reached its maximum values, and then decreased (Figures 5 and 8). The highest value of current density (21 mA·cm −2 ) was recorded for the 0.030% concentration of the reactor volume, at a temperature of 60 • C in 0.5 M electrolyte (aqueous solution of H 2 SO 4 ) (Figure 4). At the temperature of 40 • C, a considerable increase in the current density was recorded (maximum up to 18 mA·cm −2 ; Figures 3 and 7) in relation to the temperature 20 • C (maximum of 6 mA·cm −2 ; Figures 2 and 6). The successive increase in the current density (maximum to 21 mA·cm −2 ; Figures 4 and 8) when the temperature was increased to 60 • C was not equally abrupt. However, for the temperature of 80 • C, the current density fell (maximum to 14 mA·cm −2 ; Figures 5 and 8) to a level below that recorded for the measurements carried out at 40 • C.

Conclusions
This research investigating the possibility of using waste engine oil as fuel for fuel cells showed the basic possibility of the electro-oxidation of this oil emulsion on a platinum electrode in an acid electrolyte (aqueous solution of H 2 SO 4 ). In the temperature range of 20-80 • C, the electro-oxidation of the waste engine oil emulsion took place for all emulsion concentrations (0.005%, 0.010%, 0.030%, and 0.060% of the reactor volume) and both concentrations of acid electrolyte (0.5 M and 2 M aqueous solutions of H 2 SO 4 ). The maximum current density obtained in the measurements was 21 mA·cm −2 at the temperature of 60 • C (0.030% waste oil and a 0.5 M electrolyte). Although this value is inconsiderable, it indicates the need to carry out research with regard to the application of waste engine oils for direct electricity production. It seems, however, that further research should also include an analysis of the selection of a suitable reaction catalyst-preferably one that does not contain noble metals [58][59][60][61][62][63][64][65].
Author Contributions: Data curation, P.P.W. and B.W.; investigation, P.P.W. and B.W.; methodology, P.P.W.; writing-original draft, P.P.W. and B.W.; writing-review and editing, P.P.W. and B.W.; supervision, P.P.W. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.