Power Density of a Magnesium Air Fuel Cell with Titanium Dioxide Added to Activated Carbon Derived from Marine Plastic Waste
Authors: Osae Ryota , Tashima Daisuke 
Published: December 2024

Plastic waste is widely recognized as one of the major sources of marine pollution. Not only does it have a severe effect on marine ecology, but it also releases cancerogenic chemicals into the waterways. The amount of global plastic waste has doubled from 234 million tonnes in 2000 to 460 million tonnes in 2019, of which only 9% was recycled. In 2010, an estimated 275 million tonnes of plastic waste were collected from 192 countries located in coastal areas, with approximately 4.8–12 million tonnes ultimately finding their way into the seas and oceans. Global health, animal wildlife, and the environment are bearing a hefty price [1].

In addition to increasing demand for portable energy storage in electronics and electric vehicles, batteries better than the current Li-ion batteries (LIBs) are a necessity. Magnesium (Mg) ion batteries have emerged as an attractive alternative because of the unique advantages of Mg metal. These include as large specific capacity, easy handling, high abundance, and theoretically smooth electrodeposition on Mg metal [2]. In addition, the reasons why magnesium is attracting attention as a battery include the following. A magnesium battery has been proposed as a promising multivalent-ion battery candidates due to its large specific capacity, high energy density, high security, low cost, low equivalent weight and low toxicity. Moreover, it shows many advantages in comparison with the widely used lithium ion batteries. Firstly, Mg ions are present in the form of +2 valence cations, implying that its energy storage density is twice of that of Li ion battery. Secondly, an Mg battery has a higher volume capacity than graphite and, even lithium ion batteries. Furthermore, an Mg element is an environmental element and abundant in the earth’s crust [3]. In this research, we developed electrode materials for magnesium air fuel cells using marine plastic waste, mainly Poly Ethylene Terephthalate, and aimed to apply them to energy storage device materials and reduce waste.

The first step in the procedure was to produce activated carbon. Activated carbon is a carbon with micropores made from carbon materials such as coal and coconut shells by reacting them with gases and chemicals at high temperatures. Compared to charcoal, activated carbon has smaller micropores, larger specific surface area, and higher adsorption performance, and is used as a food additive and medicinal charcoal. Activated carbon was prepared by the two-step activation of organic materials (i.e., Poly Ethylene Terephthalate). The resulting activated carbon, conductivity aid, and binding material were added and mixed. Titanium dioxide was added as an additive. Currently, the annual production of titanium dioxide exceeds 4 million tonnes, and this molecule is used as an excipient in the pharmaceutical industry, in the production of sunscreen creams in the cosmetics industry, as a colorant in white plastics, and as a relatively inexpensive non-toxic food pigment approved by the relevant EU authorities regarding food additive safety, it is used in numerous household products. Its high dielectric constant gives the material many capabilities, such as its use as an antireflective surface and as a dielectric gate material in metal-oxide-semiconductor field-effect transistors [4]. The mixed activated carbon and titanium dioxide were prepared in three different ratios (1:1, 1:5, and 1:10) and mixed further. The mixture was then used to prepare two types of cathode materials. Magnesium-air fuel cells with sizes of 10 mm × 10 mm were also fabricated. Sodium polyacrylate was used as the solid electrolyte. The electrolyte was then prepared with a 1:25 ratio of sodium polyacrylate to a 3.0% NaCl concentration solution. The current and voltage were measured when connected to a solid electrolyte 1:25, 40kΩ~1Ω resistance, and the current density per reaction area, weight, and volume of the magnesium air fuel cell were calculated to obtain the power density. The maximum power density was 7.18 mW/cm2 at 1:25 without titanium dioxide added, 2.02 mW/cm2 at 1:1, 4.23 mW/cm2 at 1:5, and 4.06 mW/cm2 at 1:10 with titanium dioxide added.

The results show that the power density was highest when titanium dioxide was not blended. Initially, it was considered that the power density would increase due to the high ionic conductivity of titanium dioxide, but this was not the case. The reason for this is thought to be that the redox reaction did not work due to the oxide film of titanium dioxide. In the future, titanium dioxide will be treated with hydrochloric acid to eliminate the oxide film so that the redox reaction can take place.

Figure 1