Electrocatalytic Activities of Chemically Reduced Graphene Are Essentially Dominated by the Adhered Carbonaceous Debris

Due to its simple, scalable, and facile qualities, the chemical reduction of graphene oxide seems to be the most popular approach to prepare graphene. We show that such prepared graphene is strongly adhered with carbonaceous debris that has been produced during the synthesis of graphene oxide by the chemical exfoliation of graphite and still remain on graphene sheets through the chemical reduction steps. Interestingly, the presence of the carbonaceous debris causes a significant impact on the electrochemical behavior of the chemical reduced graphene. Herein, we demonstrate that the electrocatalytical activities of the graphene are greatly boosted by the adhered carbonaceous debris. After the removal of the carbonaceous debris, the electrocatalysis of graphene is not superior to conventional graphite.     

The Effect of KOH Treatment on the Chemical Structure and Electrocatalytic Activity of Reduced Graphene Oxide Materials

Reduced graphene oxide (rG‐O)‐based materials have great potential as metal‐free electrocatalysts for the oxygen reduction reaction (ORR) owing to their electrical and electrochemical properties and large surface area. Long‐term durability and chemical stability of the catalysts in the presence of electrolytes such as aqueous KOH solution are important for their use in practical applications. In this study, three types of rG‐O and rG‐O‐K (rG‐O after reaction with KOH) materials were synthesized. The chemical structures, surface areas, and catalytic ORR performances of the rG‐O materials were compared with those of the corresponding rG‐O‐K materials. The onset potentials of the rG‐O materials for electrocatalytic reduction of oxygen are almost the same as those of the corresponding rG‐O‐K materials; however, the current density and the number of transferred electrons are significantly reduced. These data show that the catalytic ORR performance of rG‐O‐based materials can be altered by KOH.     

Chemical Preparation of Graphene Materials Results in Extensive Unintentional Doping with Heteroatoms and Metals

Chemical synthesis of graphene relies on the usage of various chemical reagents. The initial synthesis step, in which graphite is oxidized to graphite oxide, is achieved by a combination of chemical oxidants and acids. A subsequent chemical reduction step eliminates/reduces most oxygen functionalities to yield graphene. We demonstrate here that these chemical treatments significantly contaminate graphene with heteroatoms/metals, depending on the procedures followed. Contaminations with heteroatoms (N, B, Cl, S) or metals (Mn, Al) were present at relatively high concentrations (up to 3 at %), with their chemical states dependent on the procedures. Such unintentional contaminations (unwanted doping) during chemical synthesis are rarely anticipated and reported, although the heteroatoms/metals may alter the electronic and catalytic properties of graphene. In fact, the levels of unintentionally introduced contaminants on graphene are often higher than typical levels found on intentionally doped graphene. Our findings are important for scientists applying chemical methods to prepare graphene.     

Uniform Functionalization of High-Quality Graphene with Platinum Nanoparticles for Electrocatalytic Water Reduction

Graphene–metal composites have potential as novel catalysts due to their unique electrical properties. Here, we report the synthesis of a composite material comprised of monodispersed platinum nanoparticles on high-quality graphene obtained by using two different exfoliation techniques. The material, prepared via an easy, low-cost and reproducible procedure, was evaluated as an electrocatalyst for the hydrogen evolution reaction. The turnover frequency at zero overpotential (TOF₀ in 0.1 m phosphate buffer, pH 6.8) was determined to be approximately 4600 h⁻¹. This remarkably high value is likely due to the optimal dispersion of the platinum nanoparticles on the graphene substrate, which enables the material to be loaded with only very small amounts of the noble metal (i.e., Pt) despite the very highly active surface. This study provides a new outlook on the design of novel materials for the development of robust and scalable water-splitting devices.     

Effects of Substituents on the Electrocatalytic Activity of Cobalt Phthalocyanines when Conjugated to Graphene Quantum Dots

We report on the π–π interactions between graphene quantum dots (GQDs) and the following cobalt phthalocyanine derivatives: cobalt monocarboxyphenoxy phthalocyanine (complex 1 ), cobalt tetracarboxyphenoxyphthalocyanine (complex 2 ), and cobalt tetraaminophenoxy phthalocyanine (complex 3 ). The conjugates (conj) with GQDs are represented as 1 @GQDs(conj), 2 @GQDs(conj) and 3 @GQDs(conj), respectively. The resulting phthalocyanine/GQDs conjugates were adsorbed on containing a glassy carbon electrode (GCE) using the drop and dry method. We explore the electrochemical properties of phthalocyanines functionalized with both electron withdrawing groups and electron donating groups when non‐covalently linked to the π‐electron rich graphene quantum dots. GCE/ 3, GCE/ 2 @GQDs(conj) and GCE/ 1 @GQDs(conj) had the lowest limits of detection (LOD). Sequentially modified electrodes showed less favourable detection limits compared to the conjugates.     

Fabrication of Polyaniline/Graphene/Polyester Textile Electrode Materials for Flexible Supercapacitors with High Capacitance and Cycling Stability

Vertical polyaniline (PANI) nanowire arrays on graphene‐sheet‐coated polyester cloth (RGO/PETC) were fabricated by the in situ chemical polymerization of aniline. The 3D conductive network that was formed by the graphene sheets greatly enhanced the conductivity of PANI/RGO/PETC and improved its mechanical stability. PANI nanowire arrays increased the active surface area of PANI, whilst the hierarchically porous structure of the PANI/RGO/PETC electrode facilitated the diffusion of the electrolyte ions. Electrochemical measurements showed that the composite electrode exhibited a maximum specific capacitance of 1293 F g^?1 at a current density of 1 A g^?1. Capacitance retention was greater than 95 %, even after 3000 cycles, which indicated that the electrode material has excellent cycling stability. Moreover, the electrode structure endowed the PANI/RGO/PETC electrode with a stable electrochemical performance under mechanical bending and stretching.     

ZnO Quantum Dots Coupled with Graphene toward Electrocatalytic N2 Reduction: Experimental and DFT Investigations

Electrochemical reduction of N₂ to NH₃ is a promising method for artificial N₂ fixation, but it requires efficient and robust electrocatalysts to boost the N₂ reduction reaction (NRR). Herein, a combination of experimental measurements and theoretical calculations revealed that a hybrid material in which ZnO quantum dots (QDs) are supported on reduced graphene oxide (ZnO/RGO) is a highly active and stable catalyst for NRR under ambient conditions. Experimentally, ZnO/RGO was confirmed to favor N₂ adsorption due to the largely exposed active sites of ultrafine ZnO QDs. DFT calculations disclosed that the electronic coupling of ZnO with RGO resulted in a considerably reduced activation-energy barrier for stabilization of *N₂H, which is the rate-limiting step of the NRR. Consequently, ZnO/RGO delivered an NH₃ yield of 17.7 μg h⁻¹ mg⁻¹ and a Faradaic efficiency of 6.4 % in 0.1 m Na₂SO₄ at −0.65 V (vs. RHE), which compare favorably to those of most of the reported NRR catalysts and thus demonstrate the feasibility of ZnO/RGO for electrocatalytic N₂ fixation.     

ZIF-67/石墨烯复合物衍生的氮掺杂碳限域Co纳米颗粒用于高效电催化氧还原

燃料电池阴极氧还原(ORR)催化剂目前主要以商业Pt/C为主, 其高成本和稀缺性极大地限制了燃料电池的广泛应用. 为了替代Pt/C催化剂, 廉价高效的非贵金属催化剂目前受到了广泛的研究和关注. 利用氧化石墨烯(GO)为诱导模板, 借助表面丰富的含氧官能团, 实现了Co基金属有机框架材料(MOF) (ZIF-67)在GO表面的原位生长, 构筑了ZIF-67/GO层状复合材料. 热解过程中, 石墨烯的存在有效抑制了Co纳米颗粒的团聚, 并且很好地维持了原始的层状结构. 最终获得的Co@N-C/rGO复合催化剂材料实现了活性位的高度分散, 并且具有丰富的孔结构和优异的导电性能. 在电化学性能测试中Co@N-C/rGO表现出优异的ORR性能, 其起始电位为0.96 V, 半波电位0.83 V, 远优于ZIF-67直接热解得到的Co@N-C材料, 且性能与商业Pt/C催化剂相当. 此外, Co@N-C/rGO复合催化剂还表现出良好的催化稳定性和甲醇耐受性, 显示出该材料作为燃料电池氧还原催化剂的重要潜力.     

Three‐Dimensional Graphene Networks with Abundant Sharp Edge Sites for Efficient Electrocatalytic Hydrogen Evolution

To achieve sustainable production of hydrogen (H₂) through water splitting, establishing efficient and earth‐abundant electrocatalysts is of great necessity. Morphology engineering of graphene is now shown to modulate the electronic structure of carbon skeleton and in turn endow it with excellent ability of proton reduction. Three‐dimensional (3D) graphene networks with a high density of sharp edge sites are synthesized. Electrocatalytic measurements indicate that the obtained 3D graphene networks can electrocatalyze H₂ evolution with an extremely low onset potential of about 18 mV in 0.5 m H₂SO₄ solution, together with good stability. A combination of control experiments and density functional theory (DFT) investigations indicates that the exceptional H₂ evolution performance is attributed to the abundant sharp edge sites of the advanced frameworks, which are responsible for promoting the adsorption and reduction of protons.     

Electrochemical Preparation of N‐Doped Cobalt Oxide Nanoparticles with High Electrocatalytic Activity for the Oxygen‐Reduction Reaction

Nitrogen‐doped CoO (N‐CoO) nanoparticles with high electrocatalytic activity for the oxygen‐reduction reaction (ORR) were fabricated by electrochemical reduction of CoCl₂ in acetonitrile solution at cathodic potentials. The initially generated, highly reactive nitrogen‐doped Co nanoparticles were readily oxidized to N‐CoO nanoparticles in air. In contrast to their N‐free counterparts (CoO or Co₃O₄), N‐CoO nanoparticles with a N content of about 4.6 % exhibit remarkable ORR electrocatalytic activity, stability, and immunity to methanol crossover in an alkaline medium. The Co?N_x active sites in the CoO nanoparticles are held responsible for the high ORR activity. This work opens a new path for the preparation of nitrogen‐doped transition metal oxide nanomaterials, which are promising electrocatalysts for fuel cells.     

同构MOFs复合氧化石墨烯电极材料构筑高性能超级电容器

在水热条件下一步自组装合成系列同构X-MOF （X₆O （TATB）₄（H⁺）₂·（H₂O）₈·（DMF）₂,X=Zn、Co、Ni; H₃TATB=4,4'',4″-s-triazine-2,4,6-triyl-tribenzoic acid; DMF=N,N-二甲基甲酰胺）和氧化石墨烯（GO）的复合材料（X-MOF@GO）,并探究其作为超级电容器电极材料的电化学性能。通过X射线粉末衍射、X射线光电子能谱和扫描电子显微镜测试证明GO和MOFs复合成功。其中,性能最优的Ni-MOFs@1.5GO （GO的添加量为1.5 mL）的比电容高达694.8 F·g^-1（0.5 A·g^-1）,约是Ni-MOF的2倍。电化学测试结果表明：复合材料X-MOF@1.0GO较其原MOF表现出更大的比电容和更好的倍率性能。在3.5 A·g^-1的电流密度下,1 000次循环充放电后,Ni-MOFs@1.0GO仍保持初始比电容量的81.2%。与活性炭（AC）组装的非对称超级电容器Ni-MOF@1.5GO//AC的性能最优,其功率密度为754.3 W·kg^-1时,能量密度为15.4 Wh·kg^-1,且循环3 000次后比电容保持率约为70.0%,显示出较长的循环寿命。     

Synergistic Enhancement of Electrocatalytic Activity toward Oxygen Reduction Reaction in Alkaline Electrolytes with Pentabasic (Fe,B, N,S, P)‐Doped Reduced Graphene Oxide

As alternatives to Pt‐based electrocatalysts, the development of nonprecious metal catalysts with high performance in the cathodic oxygen reduction reaction (ORR) is highly desirable for widespread use in fuel cells. Here we report a simple approach for preparing pentabasic (Fe, B, N, S, P)‐doped reduced graphene oxide (rGO) via a two‐step doping method of adding boric acid and ferric chloride to ternary (N, S, P)‐doped rGO (NSPG). Electrochemical investigation of the composites for the ORR revealed that simultaneously doping appropriate amounts of Fe and B into the NSPG produced a synergistic effect that endowed the prepared catalyst with both a positively shifted ORR half‐wave potential and high selectivity for the 4e^? reduction of O₂. The optimized Fe₂B‐NSPG catalyst approached a 4e^? process for the ORR with a half‐wave potential (E_1/2=0.90 V vs. RHE) even 30 mV higher than that of the commercial Pt/C catalyst in alkaline solution. Furthermore, relative to the Pt/C catalyst, the Fe₂B‐NSPG demonstrated superior stability and excellent tolerance of the methanol cross‐over effect. This simple method afforded pentabasic (Fe, B, N, S, P)‐doped rGO as a promising nonprecious metal catalyst used for alkaline fuel cells.     

An Electrochemical Route to Quantitative Oxidation of Graphene Frameworks with Controllable C/O Ratios and Added Pseudocapacitances

We report an electrochemical oxidation route to tunable C/O ratios in the graphene framework, creating enhanced pseudocapacitance with increasing oxygen content. Controlled surface functionalities on graphene enable a high specific capacitance and negligible electric conductivity loss. A specific capacitance of up to 279 F g^?1 was achieved for the functionalized graphene at a discharge current of 1 A g^?1 in 1 M H₂SO₄ electrolyte; this capacitance remained as high as 152 F g^?1 at 100 A g^?1. These values are much higher than those of non‐oxidized graphene. These excellent performances of the functionalized graphene signify the importance of precise control of the surface chemistry of graphene‐based materials.     

Application of Different Carbon Materials for Carbon Paste Electrodes to Simultaneous Electrochemical Detection of four DNA Bases with High Simpleness

Three different carbon materials, graphite, graphene and multiwalled carbon nanotubes (MWCNTs), were applied to fabricate carbon paste electrodes and used directly as working electrodes without any further modification in a simple electrochemical system for simultaneous detection of four DNA bases, guanine, adenine, thymine and cytosine. EIS and SEM were used to characterize the formed carbon paste electrodes made from different carbon nanomaterials and silicon oil, respectively. Conditions for bases detection were studied, such as ratio of carbon nanomaterials to silicon oil, types of buffer saline and pH. An unexpected result was discovered that compared with graphite and graphene, MWCNTs in carbon paste electrodes were not able to obtain admirable electrochemical behavior, the possible reason of which was preliminary discussed. Individual and simultaneous detection of four bases were successfully carried out, with acceptable linear ranges and low detection limits. Furthermore, this facile method had admirable reproducibility, stability and acceptable recovery in real urine sample (97.62 % ∼103.36 %), indicating certain practical potential.