The carbons simultaneously exhibit electrical conductivity more than 3x more than activated carbons; very high electrochemical activity at high mass loading; and high stability, as demonstrated by supercapacitors and lithium–sulfur batteries with excellent performance.

The synthesis can be readily tuned to make a broad range of graphitic carbons with desired structures and compositions for many applications.

High surface area porous carbon materials are of great technological importance due to their diverse functionalities and excellent physical/chemical robustness. Their high electronic conductivity, large surface area, and good chemical and electrochemical stability are of particular interest for electrochemical energy storage devices, such as electrochemical capacitors (or supercapacitors) and batteries.

… Traditional porous carbon materials, such as activated carbons (ACs), have high surface area (up to 3000 m²/g), but their large pore tortuosity and poor pore connectivity severely limit electrolyte ion transport to the surface. Furthermore, they are generally synthesized from coal or biomass (e.g., coconut shell, rice husk) containing a large amount of impurities. As a result, extensive purification is needed to achieve high-quality supercapacitor-grade AC, which substantially increases the cost. Soft or hard templates can be used to prepare mesoporous carbons to achieve better pore size control and tunable pore connection; however, complicated and costly synthesis is required, prohibiting their practical applications.

Porous graphitic carbons, such as three-dimensional (3D) porous graphene network, are attracting increasing interest owing to their high intrinsic electronic conductivity and large surface area. However, bulk graphene powder made from random stacking of individual sheets often suffers from severe aggregation, which dramatically decreases its surface area, pore connectivity, and electronic conductivity, leading to moderate charge storage performance. While some specially designed 3D porous graphene networks show good pore connectivity and conductivity, large-scale and low-cost fabrication of such graphene networks remains a challenge. The general strategy toward the above-mentioned graphene networks is to use graphene oxides (GOs) as building blocks. However, making conductive graphene from GO building blocks (normally by Hummer’s method ) requires strong oxidative and subsequently reductive chemicals, which is unfavorable for large-scale production. In this context, efficient synthesis of 3D interconnected graphitic carbon networks remains highly desired.

—To et al.

The team used a 3D cross-linked precursor from a conjugated polymeric molecular framework without using any sacrificial templates for the low-cost and low-temperature synthesis of the carbon networks.

According to Zhenan Bao, the senior author of the study and a professor of chemical engineering at Stanford, the new designer carbon represents a dramatic improvement over conventional activated carbon.

The process begins with conducting hydrogel, a water-based polymer with a spongy texture similar to soft contact lenses. Hydrogel polymers form an interconnected, three-dimensional framework that’s ideal for conducting electricity, Bao said. The framework also contains organic molecules and functional atoms, such as nitrogen, which allow the tuning of the electronic properties of the carbon.

For the study, the Stanford team used a mild carbonization and activation process to convert the polymer organic frameworks into nanometer-thick sheets of carbon.

We call it designer carbon because we can control its chemical composition, pore size and surface area simply by changing the type of polymers and organic linkers we use, or by adjusting the amount of heat we apply during the fabrication process.

—John To, a co-lead author

For example, raising the processing temperature from 400 ˚C to 1,650 ˚C resulted in a 10-fold increase in pore volume.

To see how the new material performed in real-world conditions, the Stanford team fabricated carbon-coated electrodes and installed them in lithium-sulfur batteries and supercapacitors.

In the supercapacitors, electrical conductivity improved threefold compared to supercapacitor electrodes made of conventional activated carbon.

Electrochemical performance of 3D HPG carbon for Li−S batteries. (a) Charge/discharge voltage profiles at a C/5 current rate for HPG carbon/polysulfide and KB/polysulfide electrode after equilibrium, respectively. (b) Long-term cycling stability of HPG carbon/polysulfide (3.2 mg cm−2), AC- 1/polysulfide (2.56 mg cm−2), and KB/polysulfide (1.28 mg cm−2) electrodes, respectively. After initial activation, high coulombic efficiency (CE, ∼99.8%) was maintained for HPG carbon electrode during all the cycles. (c) Comparison of areal capacity and cycling life between HPG carbon/ sulfur electrodes and recently reported high-performance sulfur electrodes. Previously reported sulfur electrodes often had areal capacity of below 3 mAh g−1 and cycling lifetime of less than 200 cycles. Credit: ACS, To et al. Click to enlarge.

Tests were also conducted on lithium-sulfur batteries. The Stanford team discovered that electrodes made with designer carbon can trap the problematic polysulfides in the Li-S battery and improve the battery’s performance.

We can easily design electrodes with very small pores that allow lithium ions to diffuse through the carbon but prevent the polysulfides from leaching out. Our designer carbon is simple to make, relatively cheap and meets all of the critical requirements for high-performance electrodes.

—Zhenan Bao

Other Stanford co-authors of the study are graduate student Jiajun He; postdoctoral scholars Hongbin Yao, Kwanpyo Kim and Ho-Hsiu Chou; visiting scholar Lijia Pan, and professors Jennifer Wilcox and Yi Cui.

Resources

• John W. F. To, Zheng Chen, Hongbin Yao, Jiajun He, Kwanpyo Kim, Ho-Hsiu Chou, Lijia Pan, Jennifer Wilcox, Yi Cui, and Zhenan Bao (2015) “Ultrahigh Surface Area Three-Dimensional Porous Graphitic Carbon from Conjugated Polymeric Molecular Framework” ACS Central Science 1 (2), 68-76 doi: 10.1021/acscentsci.5b00149