A new scientific discovery can often generate a chain reaction where additional breakthroughs take place, expanding on existing technology and generating new findings. For example, after single-layer graphene was experimentally isolated in 2004, the two-dimensional material rose to prominence in the scientific community and later earned Andre Geim and Konstantin Novoselov the Nobel Prize in Physics in 2010 (1-3). Graphene has inspired developments in a variety of areas, including in electronics, field emission, gas sensing, energy storage, and energy conversion (3).
Because of the rapid depletion of fossil fuels, a major challenge facing the 21st century scientific community is trying to find a clean, efficient, and renewable energy source (1). In addition to acting as a replacement for presently diminishing sources of energy, renewable energy can be a much more effective and environmentally friendly alternative to nonrenewable sources of energy (4). One of the most promising existing energy sources is the fuel cell, an electrochemical device which derives power from converting chemical power into electrical energy through chemical reactions on the surface of an electrode and an electrolyte (1). Fuel cells have many additional benefits over presently used energy sources, such as the fact that they produce no harmful emissions, thus reducing the output of greenhouse gases and air pollutants in the Earth’s atmosphere. As fuel cells gain more popularity, scientists are always looking for new ways to improve their efficacy by utilizing new technology. Methods they are using range from reducing system size to decreasing cost and increasing durability (4).
Fuel cells use electrocatalysts in order to accelerate oxygen reduction reactions (ORR) within the cell (1, 4). Currently, the catalyst of choice is platinum because it exhibits the highest electrocatalytic activity of available materials (1). However, a major obstacle which prohibits the mass production of fuel cells is their use of platinum catalysts. These catalysts are not ideal due to their high cost, low reliability, and inefficiency (6-8). If platinum catalysts were replaced with a more efficient material, fuel cells could possibly replace current nonrenewable energy sources.
Graphene is one of the materials proposed to replace platinum. It is extremely versatile, having the ability to be shaped into 0D fullerenes, rolled into 1D nanotubes, or stacked into 3D graphite (3). Its versatility and other unique properties such as high charge carrier mobility allow it to be applied in a variety of fields, from biosensing to electronics (2, 9). In addition, it possesses high thermal and electrical conductivity and flexibility (9). It is also much stronger than platinum; in fact, researchers at Columbia University once lauded it as the ‘strongest material in the world’ (10). Being a 2D material, it has a large surface area, which makes it optimal for use not only in electronics, but also as a catalyst support in fuel cells (1, 9).
Although graphene has a multitude of properties which enable its potential usage in many different fields, it still contains innate flaws which need to be improved before it can be used in real world applications. For example, pristine graphene has predetermined energy levels which restrict its ability to be used in many existing electronic devices. (9) The most effective method of mitigating graphene’s defects seems to be doping the graphene sheet. Doping is when carbon atoms within the graphene sheet are substitutionally replaced with other elements (9). Doping can alter the electronic properties and change the local chemistry of graphene, which allows aspects of the graphene sheet to be adjusted to suit different purposes (11). The modification of these properties can also be beneficial in fuel cells because it can increase electrochemical activity.
Carbon-based nanomaterials show high electrocatalytic activity during ORR and are relatively inexpensive and environmentally friendly. Therefore, they provide a welcome alternative to platinum catalysts. Graphene sheets doped with elements such as nitrogen or phosphorus have been displayed even higher electrocatalytic activity than do pure graphene and other carbon-based nanomaterials (7). Originally it was hypothesized that the increased activity of the nitrogen-doped graphene sheet was due to the fact that nitrogen atoms (electronegativity (EN): 3.04) are much more electronegative than the carbon atoms (EN: 2.55) that comprise the graphene sheet. This disparity in charge then created positive charge density in the carbon atoms adjacent to the doped nitrogen atoms, causing increased electrocatalytic activity. However, when atoms such as phosphorus (EN: 2.19) and boron (EN: 2.05) were used as dopants, these doped graphene sheets also exhibited increased activity even though boron and phosphorus atoms possess electronegativities lower than that of carbon. This led to the rationale that positive and negative sites led to increased O2 adsorption in ORR (5). In addition to introducing asymmetrical charge distribution, dopants within the graphene sheet also have the added benefit of providing anchoring points for the adsorption of isolated atoms such as O2 in ORR, further increasing electrocatalytic activity (6).
Another notable advantage of doped graphene is that, depending on the dopant atoms, doped graphene can be a metal-free catalyst. An advantage of metal-free catalysts is that they are not susceptible to gas poisoning and also show tolerance to methanol crossover effect, which can be beneficial in methanol fuel cells (12, 13). However, there are still problems with using doped graphene as catalysts. Methods of producing doped graphene often yield low concentrations of the dopant element within the graphene sheet. For example, when researchers used a thermolysis method to synthesize phosphorus-doped graphene, the graphene sheet had a phosphorus concentration of less than 0.3% (13). Since the concentration of dopant atoms in the graphene sheet correlates with the amount of charge introduced into the sheet and the number of anchoring points for oxygen atoms, the creation of a graphene sheet with a low dopant concentration is not ideal (4). However, synthesizing phosphorus-doped graphene sheets is much more difficult than with other dopant atoms because phosphorus atoms are much larger than the carbon atoms. Recent methods such as using graphite oxide and triphenylphosphine to synthesize phosphorus-doped graphene have proven successful. The resulting phosphorus-doped graphene sheet acted as an efficient catalyst in ORR (13).
Since single-atom doped graphene has shown higher electrocatalytic activity than pure graphene, some researchers have experimented with graphene doped with two or more elements. Graphene doped with more elements is harder to synthesize, but has been shown to have significantly higher electrochemical activity than single-atom doped graphene.
Researchers at the Korea Advanced Institute of Science and Technology attempted to dope graphene with a combination of boron, nitrogen, and phosphorus elements. They obtained results which state that boron nitrogen-codoped graphene showed 1.2 times higher ORR activity than nitrogen-doped graphene. Phosphorus nitrogen-codoped graphene exhibited even higher ORR activity, having results which were 2.1 times higher than that of nitrogen-doped graphene. However, in conjunction with the previous theory that higher electrocatalytic activity was due to the asymmetric charge density and the anchoring points introduced by dopants, the ternary-codoped graphene sheet had the highest activity, 2.3 times higher than that of nitrogen-doped graphene (14).
These researchers also provided additional reasons to explain why doped graphene yielded higher electrochemical activity than pure graphene. They asserted that phosphorus doping increased the difference in charge between the carbon atoms and the number of edge sites within the graphene sheet. Therefore, since introducing phosphorus into the graphene sheet provided the largest difference in electrochemical activity, they concluded that the number of edge sites and charge delocalization in the graphene sheet was the major factor in determining ORR activity (14).
The feasibility of graphene replacing platinum catalysts in fuel cells is high, as in October 2014 researchers at Rice University developed boron and nitrogen-doped graphene quantum dots which outperformed commercial platinum catalysts in fuel cells by about 15 millivolts in the ORR. These quantum dots also showed about 70% higher charge density than the platinum catalysts. These dots were synthesized from graphene quantum dots (GQD) and graphene oxide, which makes them much more inexpensive than commercial platinum catalysts. Using the GQD-graphene oxide hybrid, these quantum dots combine the overall advantages of graphene (15). The GQD dots add to the number of edges sites, which Choi et al. found was beneficial in increasing electrochemical activity, while the final hybrid quantum dots retain the large surface area and high electrical conductivity of graphene (14, 15).
Replacing the platinum catalysts in fuel cells with graphene catalysts is the best option for an alternative to non-renewable energy. Graphene solves the two major problems platinum incurs—its high cost and inefficiency. Carbon-based nanomaterials are much more inexpensive than metal catalysts such as platinum, and both theoretical and experimental studies have proven that pure graphene exhibits higher electrocatalytic activity than their platinum counterparts. Doped graphene, both single-doped and codoped, exhibits even higher electrocatalytic activity than pristine graphene. The possibility of graphene catalysts is no stretch of the imagination. Researchers such as those at Rice University are already developing graphene catalysts which can be commercially sold. The substitution of platinum with graphene may finally make fuel cells widely accessible, providing an environmentally friendly energy source and eliminating the need for fossil fuels.
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