【作者】 王敏；

【导师】 彭扬；

【作者基本信息】 苏州大学 ， 新能源科学与工程， 2024， 硕士

【摘要】 随着工业化进程的加速,化石燃料燃烧所导致的能源和气候挑战日益突显。电催化二氧化碳还原（Electrocatalytic carbon dioxide reduction reaction,ECO₂RR）技术利用可再生能源生成的电力将CO₂转化成具有高经济价值的燃料和化学品,不仅能有效地缓解能源危机,还能够人为关闭碳循环,解决温室效应引发的环境问题。这对国家能源安全和发展低碳经济具有重要意义。在ECO₂RR过程中,还原产物CO生成步骤简单且选择性高,具有较高的经济效益。研究表明,镍-氮-碳材料中固有缺陷对电还原CO₂制备CO具有超高的催化活性。然而,在碱性体系中,CO₂容易与OH^-反应生成CO₃^2-,导致低的碳单程转化率。随着反应的持续进行,CO₃^2-将以碳酸盐的形式在气体扩散电极表面析出,降低ECO₂RR系统稳定性。另外,碱性体系中低的离子迁移率会降低ECO₂RR系统的能量效率。研究表明,在酸性体系下开展ECO₂RR有望解决上述问题。酸性体系中高浓度的H⁺可以将电极表面生成的碳酸根(CO₃^2-)或碳酸氢根（HCO₃^-）再生成CO₂,提高利用率,同时抑制盐析,而且电解液中质子高的离子迁移率还能够降低电解池的内阻,提高ECO₂RR体系的能量效率。然而,在酸性ECO₂RR过程中存在着析氢副反应严重的挑战。针对上述问题,本论文以镍-氮-碳材料为研究对象,通过设计多孔中空结构的一体化催化剂,限域OH^-,构筑局部高p H的微环境,抑制析氢,并将其应用于膜电极组件,最终在酸性体系中成功地实现高效且稳定地电还原CO₂制备CO。具体研究内容如下:（1）利用中空结构限制OH^-扩散,提高局域碱性并抑制HER,促进酸性电催化CO₂还原为CO针对酸性ECO₂RR中析氢严重的问题,本工作通过硬模板法合成对电还原CO₂制备CO具有高催化活性的中空多孔镍-氮-碳催化剂,利用催化剂自身的中空结构限域OH^-,提高局域p H,抑制析氢,提高ECO₂RR选择性。研究表明,ECO₂RR过程中生成的OH^–会扩散至空腔中,形成空腔内部碱性,同时OH^–扩散出去,外界的酸性电解液迅速中和,碳层内外的浓度梯度逐渐增大,扩散通量增大。当OH^–的扩散和形成速率达到动态平衡时,空腔内部OH^–浓度大致保持不变,而空腔越大,在相同时间内想要达到扩散平衡会更困难,需要的OH^–更多,导致p H值越低。除空腔外,碳壁厚度增大,会降低气体传输和离子的扩散,导致局域p H低。当空腔为280 nm以及壁厚为50 nm时,在p H=2,0.25mol/L K₂SO₄中有接近100%的CO选择性,并在配备传统气体扩散电极的流动池保持40 h的电化学稳定。40 h后电极出现水淹、盐析问题,同时流动池内阻大导致能量效率低。（2）通过静电纺丝构筑分层多孔、高疏水、高导电性一体化电极,在酸性流动池和膜电极器件中实现高选择性和高稳定性的电还原CO₂制备CO针对上述的传统气体扩散电极（Gas diffusion electrode,GDE）存在水淹、盐析以及活性位点利用受限的问题,本研究通过静电纺丝方法制备了一种具有分级多孔结构的一体化GDE（Ni NF）应用于MEA,利用碳纳米管（CNT）和聚四氟乙烯（PTFE）来增强纳米纤维以提高其导电性和疏水性。由于该电极具有一体化、分级多孔的优点,同时高活性的催化位点遍布于纤维上,在酸性流动池中实现了接近于100%的CO选择性。此外,Ni NF GDE在中性MEA中稳定运行超过273小时,总能量效率达到38%,在酸性MEA中单程CO₂转化率达到78%。一系列的反应后表征证实了一体化GDE具有较高的化学和机械稳定性,并将其主要失效机制归因于疏水性的丧失。这项工作通过GDE设计的创新为工业规模的CO₂电解铺平了道路。更多还原

【Abstract】 As the industrialization process accelerates,the energy and climate challenges posed by the combustion of fossil fuels are becoming increasingly prominent.The electrocatalytic carbon dioxide reduction reaction（ECO₂RR）technology,which uses electricity generated from renewable energy sources to convert CO₂ into fuels and chemicals of high economic value,can effectively alleviate the energy crisis and artificially close the carbon cycle,addressing the environmental problems caused by the greenhouse effect.This is of significant importance for national energy security and the development of a low-carbon economy.Within the ECO₂RR paradigm,the production of CO as a reduction product is noted for its simplicity and heightened selectivity,conferring considerable economic advantages.Empirical studies have elucidated that Ni-N-C materials,characterized by intrinsic defects,demonstrate exceptional catalytic efficacy in the electrocatalytic CO₂to CO.Nevertheless,in alkaline milieus,a propensity for CO₂ to react with OH^–,forming CO₃^2–,has been observed,which consequently leads to diminished carbon conversion efficiency.As the reaction progresses,the precipitation of carbonates on the gas diffusion electrode surfaces manifests,adversely impacting the ECO₂RR system’s stability.Moreover,the diminished ion migration rate prevalent in such alkaline systems detracts from the overall energy efficiency of the ECO₂RR process.Implementing ECO₂RR in acidic conditions could potentially ameliorate these limitations.In such environments,the abundance of H⁺ions can reconvert carbonates or bicarbonates back to CO₂,thereby augmenting utilization rates and impeding salt precipitation.Furthermore,the elevated ion migration rate of protons within the electrolyte significantly reduces the electrolyzer’s internal resistance,enhancing the ECO₂RR system’s energy efficiency.However,a considerable challenge in acidic ECO₂RR processes is the pronounced hydrogen evolution reaction.To tackle these issues,this dissertation concentrates on Ni-N-C catalysts,employing an innovative approach through the synthesis of porous hollow structured catalysts.This design aims to confine OH^–,engendering a localized high p H microenvironment that suppresses hydrogen evolution.Applied within membrane electrode assemblies,this strategy has successfully facilitated efficient and stable CO₂electroreduction to CO in an acidic electrolyte.The specific research content is delineated as follows:1.Utilizing hollow structures to restrict OH^–diffusion,enhancing local alkalinity and suppressing hydrogen evolution reaction,thereby facilitating acidic electrocatalytic CO₂ reduction to COIn addressing the pronounced issue of HER within acidic ECO₂RR,this study pioneers the synthesis of hollow porous Ni-N-C catalysts,exhibiting significant catalytic prowess for CO₂electroreduction to CO,via a hard templating method.These catalysts,through their intrinsic hollow structure,effectively confine OH^–,elevate local p H levels,and consequently mitigate HER,thereby enhancing ECO₂RR selectivity.It has been demonstrated that OH^–permeates into the cavity,engendering an alkaline microenvironment,while its diffusion outwards swiftly neutralizes with the surrounding acidic electrolyte,progressively intensifying the concentration gradient across the carbon layer and amplifying diffusion flux.Upon reaching a dynamic equilibrium between OH^–diffusion and formation,the concentration of OH^–within the cavity stabilizes.Larger cavities,however,present challenges in achieving equilibrium in equivalent timeframes,necessitating greater OH^–concentration,which correlates with a lower p H.Additionally,an increase in carbon wall thickness impairs gas transport and ion diffusion,culminating in reduced local p H levels.Optimal conditions,represented by a cavity size of 280 nm and a wall thickness of 50 nm,yielded nearly100%CO selectivity in 0.25 M K₂SO₄（p H=2）,sustaining electrochemical stability in a conventional gas diffusion electrode-equipped flow cell for 40 hours.After 40 hours,electrode flooding and salt precipitation occur,and the high internal resistance of the flow cell leads to low energy efficiency.2.Constructing a porous,highly hydrophobic,and highly conductive integrated electrode via electrospinning to achieve high selectivity and stability for electrochemical reduction of CO₂ to CO in acidic flow cell and membrane electrode assembliesBuilding on the identified shortcomings of traditional gas diffusion electrodes（GDEs）,including flooding,salt precipitation,and restricted active site utilization,a novel integrated GDE（Ni NF）with a tiered porous structure was engineered using electrospinning.This was further enhanced with carbon nanotubes and polytetrafluoroethylene to augment its conductivity and hydrophobic properties,respectively.Owing to its integrated,stratified porous architecture and the ubiquitous presence of highly active catalytic sites on the fibers,the Ni NF GDE achieved near-perfect CO selectivity in acidic flow cells.Moreover,it demonstrated stable operation exceeding 273 hours in neutral MEA,attaining a total energy efficiency of 38%,and achieving a 78%single-pass CO₂ conversion rate in acidic MEA.Subsequent characterizations post-reaction affirmed the high chemical and mechanical stability of the integrated GDE,with its principal degradation mechanism attributed to a diminution in hydrophobic properties.This innovative design in GDE development paves the path for large-scale industrial CO₂ electrolysis.更多还原