摘要          

随着全球经济和人口的快速增长，化石能源的不可再生性和日益增长的消费需求已成为当今社会不可调和的矛盾。作为缓解能源危机的有效策略，探索太阳能、风能、海浪能等可再生能源的高效储能技术已成为当代科学和社会面临的最大挑战。超级电容器和锂离子电池是两种典型的高性能便携式储能器件，前者通过物理吸附电荷和表面快速氧化还原存储能量，后者通过锂离子在电极材料中的化学嵌入反应存储能量。这两种储能器件的电容能力主要取决于电极材料的结构。因此，开发高容量电极材料成为超级电容器和锂离子电池发展的目标之一。

黑腐酸 (BA) 是从腐植酸中分离出的黑色有机大分子混合物。与腐植酸其它级分相比，它具有更高的分子量和碳含量，更丰富的芳香单元，更低的H/C原子比，还含有一定量的羧基和酚羟基，故更接近于氧化石墨烯 (GO) 的结构，适合做复合材料基底，也有更高的石墨烯转变潜质。因此，本研究采用pH调控-超滤组合法对黑腐酸(BA)进行逐级分离，随后对所得级分进行磺甲基化修饰获得改性产物(SBA)。基于配位化学策略，以BA/SBA级分为基底，Co²⁺作为桥联金属中心，三聚氰胺为氮源配体，通过原位自组装合成黑腐酸-钴-三聚氰胺复合前驱体材料（分别标记为BA-Co-Melamine和SBA-Co-Melamine）。通过热解前驱体法最终制备出两类掺氮多级孔碳纳米骨架材料 (GNC)。并对它们用作超级电容器和锂离子电池电极材料时的电容性能进行了研究。同时，对黑腐酸用作电解液添加剂，是否提升超级电容器的电容性能也做了研究。得到如下结论:

pH值为4.16时分离所得Fraction Ⅰ级分在构建多孔GNC材料中起到了决定性作用。GNC-Ⅰ-b材料通过Fraction Ⅰ基前驱体热解构筑，其分级多孔异质结构由高密度多孔多壁碳纳米管与离散分布的介孔石墨烯片层自组装形成。其具有以介孔为主的多级孔结构，一定的石墨化结构和晶格缺陷，以及以吡咯氮和吡啶氮为主的2.27 wt%氮掺杂。组装的GNC-I-b//AC超级电容器在1 A g^-1电流密度下比电容达147 F g^-1，在14.4 kW kg^-1功率密度下能量密度达16.8 Wh kg^-1，在5 A g^-1下循环10000次后电容保持率为81.9%，表现出高的比电容和能量密度，优异倍率性能及循环稳定性。组装的Li//GNC-Ⅰ-b半电池在0.03 A g^-1的电流密度下表现出759 mAh g^-1的高初始放电容量和81.8%的初始库仑效率，同时具有优异的倍率性能 (0.5 A g^-1时为526.2 mAh g^-1) 和循环稳定性 (在0.5 A g^-1下循环180次后容量为527.3 mAh g^-1，库仑效率为99.9%)。

磺甲基化改性黑腐酸级分Fraction-IS-1制备的GNC-IS-1材料含有丰富的多壁孔多裂纹碳纳米管和介孔结构 (比表面积127 m² g^-1，孔径9.53 nm)，4.25 wt%的氮掺杂和1.53 wt%的硫含量。组装的GNC-IS-1//AC超级电容器在1 A g^-1下比电容达到152 F g^-1，并在5 A g^-1时循环10000次后仍保持87.6%初始容量。组装的Li//GNC-IS-1半电池在0.03 A g^-1下初始放电/充电比容量为714.6/606.1 mAh g^-1，首效为84.8%。在0.1 A g^-1循环500次后容量仍达237.6 mAh g^-1，库伦效率稳定于99.99%。表现出高的循环稳定性和首效，但相较于Li//GNC-Ⅰ-b体系，初始放电容量、循环容量和倍率性能略低。

通过向6 mol/L KOH电解液体系引入0.5 g黑腐酸级分(Fraction-I-1)，GNC-IS-1电极材料在0.5 A g^-1电流密度下比容量显著提升至323 F g^-1。基于该材料组装的GNC-IS-1//AC超级电容器在1 A g^-1下展现出161.7 F g^-1的高比容量，在1.2 kW kg^-1功率密度下拥有48.2 Wh kg^-1的高能量密度，该性能提升主要源于电解液添加剂Fraction-I-1中含氧官能团氧化还原带来的法拉第电容贡献。

关 键 词：黑腐酸；分级；磺甲基化；氮掺杂；电解质添加剂；电容性能

研究类型：基础研究

ABSTRACT

With the rapid growth of the global economy and population, the non-renewable nature of fossil fuels and their ever-increasing consumption demand have created an irreconcilable contradiction for modern society. As an effective strategy to mitigate the energy crisis, the exploration of efficient energy storage technologies for renewable energy sources (such as solar, wind, and wave energy) has emerged as one of the greatest challenges facing contemporary science and society. Supercapacitors and lithium-ion batteries represent two typical high-performance portable energy storage devices. The former stores energy through physical charge adsorption and fast surface redox reactions, while the latter relies on chemical intercalation reactions of lithium ions within electrode materials. The capacitive performance of both energy storage devices is primarily determined by the structure of their electrode materials. Consequently, the development of high-capacity electrode materials has become a key objective in advancing supercapacitor and lithium-ion battery technologies.

Black acid (BA) is a black organic macromolecular mixture isolated from humic acid. Compared to other humic acid fractions, it possesses higher molecular weight and carbon content, richer aromatic units, lower H/C atomic ratio, and contains a certain amount of carboxyl and phenolic hydroxyl groups. These characteristics make its structure more similar to graphene oxide (GO), rendering it suitable as a composite matrix substrate and exhibiting higher potential for graphene transformation. Therefore, this study employed an integrated pH-controlled ultrafiltration approach for stepwise fractionation of black acid (BA), followed by sulfomethylation modification of the obtained fractions to prepare sulfonated derivatives (SBA). Utilizing a coordination chemistry strategy, the BA/SBA fractions served as carbonaceous matrices, with Co²⁺ ions acting as bridging metal centers and melamine as nitrogenous ligands, enabling in situ self-assembly synthesis of black acid-cobalt-melamine composite precursors (denoted as BA-Co-Melamine and SBA-Co-Melamine, respectively). Through pyrolysis of these precursors, we ultimately prepared two types of nitrogen-doped hierarchical porous carbon nano-skeleton materials (GNC). We systematically investigated their capacitive performance when used as electrode materials for both supercapacitors and lithium-ion batteries. Concurrently, we examined whether black acid, when used as an electrolyte additive, could enhance the capacitive performance of supercapacitors. The main conclusions are as follows:

The Fraction I isolated at pH 4.16 played a pivotal role in constructing porous GNC materials. The GNC-I-b material, synthesized via pyrolysis of the Fraction I precursor, features a hierarchical porous heterostructure formed by the self-assembly of high-density porous multiwalled carbon nanotubes and discretely distributed mesoporous graphene lamellae. This architecture exhibits a mesopore-dominated hierarchical porosity, partial graphitic ordering with lattice defects, and 2.27 wt% nitrogen doping predominantly comprising pyrrolic-N and pyridinic-N configurations. When configured into a GNC-I-b//AC asymmetric supercapacitor, the device delivered a specific capacitance of 147 F g^-1 at 1 A g^-1, with an energy density of 16.8 Wh kg^-1 at a power density of 14.4 kW kg^-1. Remarkably, it retained 81.9% capacitance after 10,000 cycles at 5 A g^-1, demonstrating superior specific capacitance, energy density, rate capability, and cycling stability. In Li//GNC-I-b half-cell configurations, the system exhibited a high initial discharge capacity of 759 mAh g^-1 with 81.8% initial Coulombic efficiency at 0.03 A g^-1, coupled with exceptional rate performance (526.2 mAh g^-1 at 0.5 A g^-1) and cycling durability (527.3 mAh g^-1 capacity retention with 99.9% Coulombic efficiency after 180 cycles at 0.5 A g^-1).

2) The GNC-IS-1 material, prepared from sulfomethylated-modified humic acid fraction (Fraction-IS-1), features abundant multi-walled porous and cracked carbon nanotubes with a mesoporous structure (specific surface area: 127 m² g^-1, pore size: 9.53 nm), along with 4.25 wt% nitrogen doping and 1.53 wt% sulfur content.The assembled GNC-IS-1//AC supercapacitor delivered a specific capacitance of 152 F g^-1 at 1 A g^-1 and retained 87.6% of its initial capacity after 10,000 cycles at 5 A g^-1. When configured as a Li//GNC-IS-1 half-cell, it exhibited an initial discharge/charge specific capacity of 714.6/606.1 mAh g^-1 at 0.03 A g^-1, corresponding to a first-cycle Coulombic efficiency (CE) of 84.8%. After 500 cycles at 0.1 A g^-1, the capacity stabilized at 237.6 mAh g^-1 with a near-ideal CE of 99.99%, demonstrating high cycling stability and initial efficiency. Compared with the Li//GNC-I-b configuration, the system exhibited a moderate reduction in initial discharge capacity, cycling capacity retention, and rate capability.

3) Upon introducing 0.5 g of humic acid-derived Fraction-I-1 into the 6 mol/L KOH electrolyte system, the GNC-IS-1 electrode material exhibited a significant enhancement in specific capacitance to 323 F g^-1 at a current density of 0.5 A g^-1. The assembled GNC-IS-1//AC supercapacitor based on this material demonstrated a high specific capacitance of 161.7 F g^-1 at 1 A g^-1 and delivered a remarkable energy density of 48.2 Wh kg^-1 at a power density of 1.2 kW kg^-1. This performance improvement is primarily attributed to the Faradaic capacitance contribution arising from the redox reactions of the oxygen-containing functional groups in the Fraction-I-1 electrolyte additive.

Key words: Black humic acid; Fractions; Sulfonylation; Nitrogen doping; Electrolyte additives; Capacitance performances

Thesis: Basic research