CN114300691B - 中自旋铁单原子催化剂的制备和应用 - Google Patents
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- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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Abstract
本发明涉及一种中自旋铁单原子催化剂及其制备方法和应用,S1、在ZIF‑8晶体生长过程中,Fe前驱体与富N配体2‑甲基咪唑配位形成Fe/ZIF‑8;S2、将Fe/ZIF‑8退火得到Fe‑N‑C黑色粉末;S3、Fe‑N‑C丰富的孔隙作为“笼子”原位吸附Pd前驱体;S4、退火形成Fe‑N‑C/PdNC。本发明首次构建了超细Pd纳米簇协同增强型Fe‑N‑C/PdNC,其在酸性介质中获得了优异的ORR性能,半波电位为0.87V,高于商业Pt/C催化剂。同时,Fe‑N‑C/PdNC在H2‑O2燃料电池中能量密度高达839mW/cm2,使其成为质子交换膜燃料电池PEMFCs阴极催化剂的候选材料。
Description
技术领域
本发明属于质子交换膜燃料电池技术领域,具体涉及一种中自旋铁单原子催化剂及其制备方法和应用。
背景技术
专利CN202011093433.8公开了一种吡咯衍生的单原子铁基氮碳氧还原催化剂的制备方法,首先,将铁源化合物与锌源化合物溶于亲水性溶液中,加入吡咯单体,进行一定时间的剧烈搅拌形成混合均一的溶液,结束后产物经去离子水和乙二醇离心洗涤数次,干燥后得到的产物再通过煅烧得到最终产物。该专利采用的合成方法所制备的催化剂,其氧还原性能不足以满足实际应用,且催化剂形貌具有不可控性。
专利CN202010705526.5公开了一种单原子铁基氧还原催化剂的制备方法,该催化剂主要应用于燃料电池阴极催化。其特征在于:将Fe掺杂的ZIF-8前驱体,退火得到高度分散的Fe单原子催化剂,向其中添加二次氮源,然后在惰性气体下高温热处理,即可得到所述的单原子铁基催化剂。本发明在前驱体中增加二次氮源可以增加催化剂中氮的含量,为Fe原子的固定提供更多的配位点,实现Fe单原子含量的提升,从而获得高氧还原催化活性的单原子铁基催化剂。该催化剂中铁原子的含量为1.2at%,是未加二次氮源制备的催化剂的2.7倍。该催化剂在酸性和碱性电解质溶液中都显示出优异的氧还原催化活性,并且在锌-空气燃料电池器件中表现出出色的空气阴极催化性能。但该专利所制备的催化剂的氧还原性能还有待进一步提升以满足实际应用。
发明内容
针对上述技术问题,本专利发明了一种中自旋铁单原子催化剂,解决现有技术所面临的氧还原(ORR)活性差,且在酸性体系中ORR性能低于商业Pt/C催化剂的现状;还解决了现有技术采用Pt基催化剂作为ORR活性组分导致成本高的现状。
具体的技术方案为:
中自旋铁单原子催化剂的制备方法,包括以下步骤:
S1、在ZIF-8晶体生长过程中,Fe前驱体与富N配体2-甲基咪唑配位形成Fe/ZIF-8;
S2、将Fe/ZIF-8退火得到Fe-N-C黑色粉末;
S3、Fe-N-C丰富的孔隙作为“笼子”原位吸附Pd前驱体;
S4、退火形成Fe-N-C/PdNC。
S1具体包括以下过程:将2-甲基咪唑溶于甲醇中,搅拌溶解,随后向其中加入Zn(NO3)2·6H2O和Fe(NO3)3·9H2O的甲醇溶液;溶液在60℃下加热24h后,将沉淀物离心,用乙醇洗涤3遍,去除未反应的2-甲基咪唑,并在60℃真空干燥箱中干燥,得到Fe/ZIF-8粉末。
S2具体包括以下过程:在管式炉中,氮气条件下以5℃/min的升温速率升温至900℃,将Fe/ZIF-8退火2小时得到Fe-N-C黑色粉末。
S3具体包括以下过程:将Fe-N-C分散在水中,超声10min得到悬浮液;将Na2PdCl4溶液缓慢滴加到上述悬浮液中,连续搅拌24小时后用纯水洗涤3次,在真空干燥箱中以60℃烘干。
S4具体包括以下过程:在体积浓度5%的氢气条件下,以5℃/min的升温速率升至400℃保持2小时退火形成Fe-N-C/PdNC。
本发明获得的中自旋铁单原子催化剂的应用,作为质子交换膜燃料电池的阴极催化剂。
本发明利用Pd纳米晶与Fe单原子之间的协同效应所引发的Fe金属原子的自旋态调控,为提高Fe SACs的ORR性能开辟新的途径。首次构建了超细Pd纳米簇(Pdnanoclusters,PdNC)协同增强型Fe SACs(标记为Fe-N-C/PdNC),其在酸性介质中获得了优异的ORR性能,半波电位(half-wave potential,E1/2)为0.87V,高于商业Pt/C催化剂(E1/2=0.85V),且经过3万次电位循环后活性衰减可忽略不计。同时,Fe-N-C/PdNC在H2-O2燃料电池中能量密度高达839mW/cm2,使其成为质子交换膜燃料电池PEMFCs阴极催化剂的候选材料。实验结果表明,PdNC与Fe单原子之间的强电子相互作用成功地诱导了Fe(II)从低自旋(LS)到中自旋(MS)的自旋态跃迁。
附图说明
图1为本发明的Fe-N-C/PdNC的合成示意图;
图2a为实施例的Fe-N-C/PdNC的SEM图之一;
图2b为实施例的Fe-N-C/PdNC的SEM图之二;
图2c为实施例的TEM图;
图2d为实施例的AC-HAADF STEM图;
图2e为实施例的EDS mapping图之一;
图2f为实施例的EDS mapping图之二;
图2g为实施例的EDS mapping图之三;
图2h为实施例的EDS mapping图之四;
图2i为实施例的EDS mapping图之五;
图2j为实施例的EDS mapping图之六;
图3a为实施例的Fe-N-C/PdNC和Fe-N-C的N2吸附/解吸等温线;
图3b为实施例的孔径分布图;
图4a为实施例的Fe-N-C/PdNC和Fe-N-C的XPS Fe 2p谱图;
图4b为实施例的Fe-N-C/PdNC和Fe-N-C的XPSPd 3d谱图;
图5a为实施例的UPS光谱图;
图5b为实施例的零场冷却磁化率图;
图5c为实施例的57Fe光谱图;
图6a为实施例的催化剂在0.1M HClO4中的ORR极化曲线;
图6b为实施例的Tafel曲线;
图6c为实施例的%H2O2及n曲线;
图6d为实施例的Fe-N-C/PdNC在注入不同浓度CH3OH后的ORR极化曲线;
图6e为实施例的Fe-N-C/PdNC的时间-电流曲线在通入CO前后的变化;
图6f为实施例的Fe-N-C/PdNC经ADTs测试前后的ORR极化曲线;
图7为实施例的催化剂在H2-O2燃料电池中的极化曲线及能量密度曲线;
图8a为实施例的催化剂在0.1M KOH中的ORR极化曲线;
图8b为实施例的催化剂经ADTs测试前后的ORR极化曲线;
图8c为实施例的催化剂在0.1M PBS中的ORR极化曲线;
图8d为实施例的催化剂经ADTs测试前后的ORR极化曲线。
具体实施方式
结合实施例说明本发明的具体技术方案。
Fe-N-C/PdNC是通过ZIF-8的双限制效应合成的,具体步骤为:
S1、在ZIF-8晶体生长过程中,Fe前驱体与富N配体2-甲基咪唑配位形成Fe/ZIF-8:将3.94g 2-甲基咪唑溶于300mL甲醇中,搅拌溶解,随后向其中加入300mL含3.39g Zn(NO3)2·6H2O和200mg Fe(NO3)3·9H2O的甲醇溶液。溶液在60℃下加热24h后,将沉淀物离心,用乙醇洗涤3遍,去除未反应的2-甲基咪唑,并在60℃真空干燥箱中干燥,得到Fe/ZIF-8粉末。
S2、在管式炉中,氮气(N2)条件下以5℃/min的升温速率升温至900℃,将Fe/ZIF-8退火2小时得到Fe-N-C黑色粉末。
S3、Fe-N-C丰富的孔隙作为“笼子”原位吸附Pd前驱体:将40mg Fe-N-C分散在100mL水中,超声10min得到悬浮液。将0.76mL(或0.095mL、0.38mL、0.095mL、1.9mL、3.8mL)Na2PdCl4溶液(19.7mM)缓慢滴加到上述悬浮液中,连续搅拌24小时后用纯水洗涤3次,在真空干燥箱中以60℃烘干。
S4、在体积浓度5%的氢气(H2)条件下,以5℃/min的升温速率升至400℃保持2小时退火形成Fe-N-C/PdNC。
Fe-N-C/PdNC中Pd前驱体的质量分数占总前驱体的4%(或0.5%、2%、10%、20%)。
对获得的物质进行电化学测试:
催化剂墨水的制备方法是:将4mg催化剂分散于1mL含500μL乙醇、490μL水和10μL5%Nafion的混合溶液中,超声波处理1h,形成均匀的催化剂墨水。作为对比,将1mg 20wt%的商业Pt/C催化剂分散在1mL上述溶液中。然后,在抛光的旋转圆盘电极(rotating diskelectrode,RDE,直径0.5cm,表面积0.196cm2)上滴一定量的上述墨水。催化剂的负载量均为0.6mg/cm2,商业Pt/C催化剂的负载量为0.2mg/cm2。
电化学测试方法:测试采用传统的三电极体系,其中负载催化剂的RDE或RRDE作为工作电极,铂片为对电极以及饱和甘汞电极(SCE)为参比电极。ORR性能测试的电解质溶液为0.1M HClO4溶液。电化学测量之前,提前30分钟向电解质通入N2或O2使其饱和。
ORR活性测定方法:ORR极化曲线在10mV/s、工作电极转速为1600rpm下获得。起始电位(Eonset)在电流密度为1mA/cm2时获得。
过氧化氢的产率(%(H2O2))和电子转移数(n)测定方法:通过旋转环盘电极(rotating ring disk electrode,RRDE,直径0.56cm,表面积0.2462cm2)测试得到过氧化氢的产率(%(H2O2))和电子转移数(n)以评价2e-和4e-的选择性。具体计算公式如下:
其中ir和id分别表示环电流和盘电流。N为环电流收集效率(0.37)。
抗甲醇(CH3OH)及一氧化碳(CO)毒化能力测试:向电解质溶液中注入不同浓度(1-20M)CH3OH扫描ORR极化曲线比较前后变化得到抗CH3OH毒化能力;扫描时间-电流(i-t)曲线时向电解质中通入CO比较前后变化得到抗CO毒化能力。
ORR稳定性测定方法:在0.6~1V电位下循环扫描不同圈数(0圈至3万圈),测定扫描前后ORR极化曲线并进行比较测试稳定性,变化越小,稳定性越好。
燃料电池性能测试:按照催化剂:异丙醇:去离子水:5%Nafion=1:12:12:5的比例配制催化剂油墨。将油墨超声处理1小时,然后将其喷到Nafion 212膜的一侧,有效面积约为4cm2,直到负载量达到3mg/cm2(商业Pt/C催化剂(20wt.%,Johnson Matthey)为1.5mg/cm2)。以0.3mgPt/cm2负载量作为阳极电极,将商业Pt/C催化剂喷在碳纸(TGP-H-060,Toray)上。将电极、催化剂包覆的Nafion膜和碳纸(TGP-H-060,Toray)在120℃、2MPa负载下热压1min制备燃料电池膜电极(membrane electrode assemblies,MEAs)。然后在一个单电池和条件控制燃料电池测试站(Scribner 850e)中测试MEAs。H2和O2的背压均保持在20psig,H2和O2的气体流速分别为300和400标准立方厘米每分钟(sccm)。
材料表征:
测试结果:
Fe-N-C/PdNC的形貌:如图2a所示,Fe-N-C/PdNC很好地继承了ZIF-8的十二面体形状。图2b显示Fe-N-C/PdNC中具有高密度的PdNC,其平均尺寸约为~1.8nm。此外,像差校正的高角度环形暗场扫描透射电子显微镜(AC-HAADF STEM)图像显示,Fe-N-C/PdNC由密集的Fe单原子和PdNC组成,如图2c所示。对PdNC的进一步研究显示出Pd(111)晶格条纹,如图2d所示。能量色散X射线能谱(EDS)元素分析证实了在PdNC中存在均匀分布的Pd物种和Fe物种,同时通过元素线扫图证实,如图2e-图2j所示。N2吸附/解吸等温线显示Fe-N-C/PdNC具有丰富的比表面积和孔隙,有利于电子传输和活性位点的接近,如图3a和图3b。
Fe-N-C/PdNC的电子结构分析:X射线光电子能谱(XPS)研究表明,在引入PdNC后,Fe2p峰向结合能较高的方向发生了明显的偏移,而其Pd 3d谱图则与N-C/PdNC相反向低价态偏移,如图4a-图4b所示,证实了PdNC与Fe单原子之间存在较强的相互作用。
Fe-N-C/PdNC的自旋态分析:利用紫外光电子能谱(UPS)了解了能带结构信息。如图5a,Fe-N-C/PdNC和Fe-N-C的截止能量(cutoff)分别为17.24eV和17.05eV。根据公式eΦ=21.22eV-cutoff计算出它们的功函数(eΦ)分别为3.98eV和4.17eV,表明Fe-N-C/PdNC更有可能从催化剂表面向O2/中间体提供电子。同时,Fe-N-C/PdNC和Fe-N-C的最高占据轨道(HOMO)的能量分别为2.70eV和2.44eV,表明PdNC整合后HOMO向更高的能量转移。换句话说,引入PdNC诱导Fe的3d电子离域,Fe中心电子密度降低。采用零场冷却磁化率(χm)测试分析了Fe的电子自旋结构(图5b)。从1/χm图中可以看出,PdNC的引入降低了Fe物种的顺磁状态,进一步计算得到Fe-N-C/PdNC中Fe的单电子个数为2,高于Fe-N-C(0)。采用光谱法对不同种类的Fe进行了鉴别。如图5c所示,根据同分异构体位移(IS)和四极分裂(QS)的值,将57Fe光谱拟合为D1、D2和D3三个双峰,分别属于低自旋(LS)、中自旋(MS)和高自旋(HS)Fe(II)。与Fe-N-C相比,Fe-N-C/PdNC的D2含量从19.64%增加到40.32%,而D1比例呈下降趋势(从73.05%减少到50.94%)。由此可见,Fe单原子与PdNC之间的强相互作用有效地重塑了Fe的电子结构,实现了Fe从LS到MS的3d电子自旋构型转变。
Fe-N-C/PdNC在酸性条件下的ORR活性:采用RDE在饱和O2的0.1M HClO4电解质中评价了Fe-N-C/PdNC的电催化活性。如图6a所示,Fe-N-C/PdNC的起始电位(onset potential,Eonset)和E1/2分别为0.97V和0.87V,相对于Fe-N-C得到了明显的提升(Eonset和E1/2分别为0.92V和0.82V),证实了PdNC的引入可以有效提高Fe单原子的ORR活性。更重要的是,Fe-N-C/PdNC在酸性介质中的E1/2超过了Pt/C催化剂(E1/2=0.85V),进一步表明Fe-N-C/PdNC巨大的实际应用潜力。此外,通过Tafel曲线验证了快速的ORR动力学过程,如图6b。RRDE结果证实了低H2O2产率,遵循4e-转移过程,如图6c。图6d及图6e表明Fe-N-C/PdNC具有优异的抗甲醇(CH3OH)及一氧化碳(CO)的能力,为其在甲醇燃料电池等中的应用奠定了基础。
Fe-N-C/PdNC在酸性条件下的ORR稳定性:Fe-N-C/PdNC的ORR稳定性通过在0.6-1.0V电位窗口进行加速稳定性测试(accelerated stability test,AST)进行评估。从图6f可以看到,经过3万圈循环伏安测试后,Fe-N-C/PdNC比Fe-N-C和商业Pt/C催化剂都保留了更多的初始ORR性能,即表现出更为优异的稳定性。
Fe-N-C/PdNC的实际应用潜力:将Fe-N-C/PdNC组装到燃料电池膜电极(membraneelectrode assemblies,MEAs)对其H2-O2燃料电池性能进行了测试。如图7所示,Fe-N-C/PdNC的开路电压接近0.95V,表明其在实际工作条件下具有较高的本征ORR活性。以Fe-N-C/PdNC为阴极催化剂的MEAs在2.79A/cm2时的最大功率密度(maximum power density,Pmax)为920mW/cm2,远高于商业Pt/C催化剂(2.62A/cm2时的Pmax为839mW/cm2)和Fe-N-C(1.31A/cm2时的Pmax为419mW/cm2)。以上结果表明,Fe-N-C/PdNC在实际应用条件下也具有突出的ORR活性,并说明Fe-N-C/PdNC中的PdNC对改善Fe单原子的ORR性能具有显著作用。
Fe-N-C/PdNC在碱性和中性条件下的ORR性能:Fe-N-C/PdNC除了在酸性介质中表现出良好的ORR活性外,其在碱性和中性介质中也表现出优越的活性。与商业Pt/C催化剂和Fe-N-C相比,其具有0.94V(0.1M KOH)和0.83V(0.1M PBS)的半波电位E1/2,并表现出优异的稳定性,进一步说明了PdNC与Fe单原子之间的协同效应,如图8a到图8d。
Claims (3)
1.中自旋铁单原子催化剂的制备方法,其特征在于,包括以下步骤:
S1、在ZIF-8晶体生长过程中,Fe前驱体与富N配体2-甲基咪唑配位形成Fe/ZIF-8;
S1具体包括以下过程:将2-甲基咪唑溶于甲醇中,搅拌溶解,随后向其中加入Zn(NO3)2·6H2O和Fe(NO3)3·9H2O的甲醇溶液;溶液在60 ℃下加热搅拌24 h后,将沉淀物离心,用乙醇洗涤3遍,去除未反应的2-甲基咪唑,并在60 ℃真空干燥箱中干燥,得到Fe/ZIF-8粉末;
S2、将Fe/ZIF-8退火得到Fe-N-C黑色粉末;
S2具体包括以下过程:在管式炉中,氮气条件下以5°C/min的升温速率升温至900 ℃,将Fe/ZIF-8退火2小时得到Fe-N-C黑色粉末;
S3、Fe-N-C丰富的孔隙作为“笼子”原位吸附Pd前驱体;
S3具体包括以下过程:将Fe-N-C分散在水中,超声10 min得到悬浮液;将Na2PdCl4溶液缓慢滴加到上述悬浮液中,连续搅拌24小时后用纯水洗涤3次,在真空干燥箱中以60 ℃烘干;
S4、退火形成Fe-N-C/PdNC;
S4具体包括以下过程:在体积浓度5%的氢气条件下,以5°C/min的升温速率升至400 ℃保持2小时退火形成Fe-N-C/PdNC。
2.中自旋铁单原子催化剂,其特征在于,根据权利要求1所述的制备方法所得。
3.根据权利要求2所述的中自旋铁单原子催化剂的应用,其特征在于,作为质子交换膜燃料电池的阴极催化剂。
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