CN101019255A - 气体扩散电极、薄膜电极组件及其制造方法 - Google Patents

气体扩散电极、薄膜电极组件及其制造方法 Download PDF

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CN101019255A
CN101019255A CNA2005800200045A CN200580020004A CN101019255A CN 101019255 A CN101019255 A CN 101019255A CN A2005800200045 A CNA2005800200045 A CN A2005800200045A CN 200580020004 A CN200580020004 A CN 200580020004A CN 101019255 A CN101019255 A CN 101019255A
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electrode
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noble metal
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安德里·F·古洛
罗伯特·J·艾伦
艾莫里·德卡斯特罗
恩里科·拉蒙尼
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BASF Fuel Cell GmbH
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Abstract

本发明涉及气体扩散电极,用于燃料电池和其他电化学应用中,所述气体扩散电极通过具有低的铂载荷的气体扩散电极的直接金属化而获得,并且本发明涉及包含有该气体扩散电极的薄膜电极组件。

Description

气体扩散电极、薄膜电极组件及其制造方法
技术领域
本发明涉及燃料电池以及其他电化学应用中使用的气体扩散电极,并且涉及相关制造方法。
背景技术
质子交换薄膜燃料电池(PEMFC)被认为是未来清洁电能最有希望的来源之一。PEMFC是电化学发电机,其从气体燃料(典型的,纯的或混合的氢气)和气体氧化剂(通常包括氧气或空气)直接产生直流电。该电池的核心部件是薄膜电极组件,该组件包括离子交换薄膜,其是支持整个过程的固体电解质和阳极与阴极电池分隔室的物体分隔器,被粘到或以其他方式接合到气体扩散电极。气体扩散电极,一个阴极和一个阳极接触离子交换薄膜的各端,通常包括气体扩散介质和催化层。对于这些部件,现有技术中已知数种技术方案:在某些情况中,将催化层在与薄膜接合前应用到气体扩散介质,和/或在其上应用未催化的气体扩散介质之前将它们直接涂覆在薄膜表面。气体扩散介质通常包括:导电网和一个或多个气体扩散层;该导电网可以是金属的或基于碳的,并且可以由金属网格、泡沫或布构成,由编织的或非编织的碳布构成,由碳纸或任何其他优选多孔或穿孔的介质构成。设置气体扩散层来提供用于电极结构内的气体反应物向催化部位扩散的适当路径,在所述催化部位处发生燃料氧化(阳极侧)和氧化剂还原(阴极侧)的电化学反应:它们通常是基于导电惰性填充物和适当的,优选为疏水结合剂(例如,PTFE或其他氟化结合剂)的混合物。应当仔细设计气体扩散层,以提供可渗透且平滑的结构,以确保气体反应物的正确分配,而不引发严重的大量传送后果,并且来提供与薄膜良好的接触。例如,在US6103077中公开了改进的燃料电池气体扩散结构。例如,如US6017650中所述,然后可以将催化层应用到气体扩散层;现有技术的催化层包括:贵金属催化剂,如铂,可选将其支撑在碳或石墨微粒上;适当的结合剂,其可以是与气体扩散层中已经存在的相同的疏水结合剂;以及离聚成分,通常是离聚过氟化碳物质。离聚成分可以添加到催化剂-结合剂混合物和/或它可以随后施加作为润湿先前施加的催化剂和结合剂微粒的外部层。这种类型的气体扩散电极,其接合到现有技术中已知的质子交换薄膜,例如基于氟碳酸如Nafion(美国公司DuPont的商标),它导致产生以极好的性能为特征的薄膜电极组件;然而,在这种结构中将贵金属成分使用到很低的程度,以至于需要非常高的特定载荷(通常在商用产品中对于阳极和阴极侧铂的范围在0.3到1mg/cm2内)。在燃料电池中为获得合适的性能需要大量的贵金属,这是妨碍PEMFC(以及其他类型的燃料电池,如DMFC,直接甲醇燃料电池)获得商业成功的最重要的一个因素。已经提出了利用催化剂层将离子交换薄膜直接金属化,来作为实现更好的催化剂-薄膜界面的手段,允许更好地利用催化剂并因此使用较低的贵金属载荷。然而,到目前为止,还没有任何用于薄膜直接金属化的手段被证明是有效和实用的。溅射或超高真空淀积(UHV)所要求的高温注定会对精细的离子交换薄膜产生相应的影响,并且甚至普通的物理和化学气相淀积技术(PVD或CVD)已经证明难以控制且难以比例放大。在US6077621中公开了薄膜金属化的一种实质性的改进,其中提出了双IBAD以用于此目的。双IBAD是离子束辅助淀积(IBAD)技术的演进技术,其具有低温处理、易于比例放大的优点。初始清洗薄膜,并通过具有不高于500eV的第一低能离子束,例如Ar+束流,对其进行纹理处理;然后将第二束流聚焦在薄膜上,其包含高能离子(如,O2 +、N2 +)和要淀积的金属的离子,优选的通过电子束来蒸发。双IBAD比起常规IBAD(其中使用单束流)具有许多优点,其允许形成具有需要的密度和多孔性的更好的受控膜,同时在薄膜结构中引入最小的应变。由于在连续的金属化过程中处理大尺寸的离子交换薄膜不是很容易,因此在US6673127中公开了对此技术的进一步改进,在此情况下,在气体扩散结构上形成非常薄的离子交换薄膜层,然后使其经双IBAD处理。尽管这一技术允许在燃料电池中获得高功率密度,并具有降低的铂载荷,它仍然表现出某些缺点,而这正是本发明所针对的。首先,尽管这些电极的性能较高,但是由于该技术的可靠性受离聚物膜的特性的影响,而离聚物膜的特性根据制备条件而变化,因此这些性能在某种意义上不可预期。现有技术目前的液态离聚物膜是氟碳化合物性质的,因为这是仅知的允许高功率密度操作的离聚材料,并且其必须从氟碳酸(比如DuPont的商业化的产品“Liquid Nafion”)的醇或水醇悬浮液重塑。这些悬浮液的特性并不总是一致的,因为平均分子量、悬浮微粒的形态特征参数、流变参数和其他因素在一个批次和另一批次间显著变化。而且,也是在最佳的情况下,具有液态离聚物嵌入的微粒的催化剂的利用率从不一致。用于气体扩散电极的液态离聚物最先描述在US4876115中,作为用于扩展三维的催化层的填隙空间内质子传导路径的手段,从而提高催化剂的利用率(这是对作为期望的反应的部位的催化剂自身的可用性和可接入性的一种量度)。这一手段在一定程度上是有效的,但只是模拟其中催化剂以非常薄且平滑的准两维层形式存在并且与薄膜表面直接接触的理想情况。除了解决降低燃料电池电极中铂的载荷(或者更通常的,贵金属载荷)的问题外,需要解决的另一问题是,在一定的处理条件下薄膜电极组件中的基于氟碳化合物的离聚成分的低稳定性。在某些应用中(如汽车的应用),燃料电池根据瞬时功率需求工作在非连续的方式中;由于PEMFC因其非常快的启动及其满足急剧变化的功率需求的要求的能力而著名,因此它们是在这一领域中操作的最有希望的备选者。然而,在零或接近零的功率需求的条件下,即,当产生很小或不产生电流(开路电压条件)时,很可能在阳极产生过氧化物。过氟化碳材料常常不能在这些条件下稳定,尤其是长时间使用。也由于此原因,开发出用于燃料电池应用的替换薄膜(例如,基于聚苯并咪唑、聚醚酮或聚砜)。不管怎样,根据US4876115的教导,这些材料都没有被证明适合用作电极界面的质子传导材料,并且总是使用过氟化碳材料,比如前述的“Liquid Nafion”。因此出于多种理由,不仅出于成本和可靠性,而且还出于在特定处理条件下总的化学稳定性考虑,消除这一成分将是有利的。
出于所有上述理由,在过去以数种不同技术尝试了气体扩散介质的直接金属化,但是没有获得重大的成功。尽管例如US6159533声称利用在气体扩散介质上铂的PVD淀积可以获得优异的性能,但是一些实例表明,在具备非常薄的薄膜(20微米)、在相对高的压力(大约2bar)下馈以非常高的气体流速(3.5化学计量比的空气、2化学计量比的纯氢气)的燃料电池中,实际记录的性能不超过谨慎估计的在0.358V下732mA/cm2
发明内容
本发明的目的是提供一种克服了现有技术的限制的气体扩散电极。
在另一方面,本发明的目的是提供一种通过具有低的铂载荷和高的性能的气体扩散介质的直接金属化而获得的气体扩散电极,优选的没有离聚的氟碳化合物成分;以及包含有该气体扩散电极的薄膜电极组件。
在又一方面,本发明的目的是提供一种通过直接金属化而在气体扩散介质上形成贵金属覆层的方法。
在第一方面,本发明的气体扩散电极由通过双IBAD淀积而设有贵金属覆层而无离聚成分的气体扩散介质构成。发明人惊讶地发现,与其他的直接金属化技术比如溅射或PVD不同,双IBAD能够形成薄且平滑的贵金属覆层,特别是铂覆层,而对下面的衬底没有损伤且具有优异的电化学特性。甚至更令人惊讶的,催化剂利用率(这是对淀积的贵金属覆层的催化效率的一种量度)不仅比现有技术的电极高得多,而且这是不借助根据US4876115的教导的质子传导材料而实现的。更令人惊讶的,发明人发现,根据US4876115的教导的质子传导材料的悬浮液的可选添加在大多数情况下对性能有害:根据本发明涂覆有铂的本发明的电极的伏安特性表明,Liquid Nafion的0.5mg/cm2层的添加将Pt表面有效度降低到一个相当显著的程度。应用了双IBAD的贵金属覆层的平滑度和密度对于获得高性能的电极是极其重要的:特别是在铂的情况下,采用100-500eV的第一低能束流来清洗该气体扩散介质的表面并使其形成纹理,以及采用蒸发的金属离子的第二高能束流,优选500-2000eV,以获得具有优选包括在5和500nm之间的厚度以及优选包括在0.01和0.1mg/cm2之间的载荷的覆层,可以得到最佳结果。在本发明中,引用铂来作为用于本发明的气体扩散电极的示例性的催化剂材料,但是也可以使用所有其他的贵金属或不同金属,贵金属和非贵金属的组合。
该气体扩散介质的特性对于以非常低的贵金属载荷获得要求的电化学性能是极其重要的;在优选的实施例中,在导电网,例如金属或碳布或碳纸上获得应用了本发明的贵金属覆层的气体扩散介质,该导电网预先涂覆有包括导电填充物的气体扩散层,所述导电填充物可选地由碳微粒或纤维以及结合剂,优选的疏水的,可选的氟化结合剂构成。使用非常平滑的气体扩散介质,优选的具有比1000加里(Gurley)秒更高,并且更优选的比5000加里秒更高的平滑度的气体扩散介质,能够获得最佳结果。这样高等级的平滑度例如可以通过乙炔碳黑微粒和PTFE或其他等效氟化结合剂的油墨的凹版涂覆或其他类型的机械涂覆来获得,优选采用如在相同申请人的待决临时专利申请中所公开的合适的平滑添加剂。
本发明的气体扩散电极特别适合于整合在薄膜电极组件结构中,优选通过粘结到离子交换薄膜,可选通过本领域中已知的热压来进行。在优选实施例中,该离子交换薄膜是非氟化类型,例如基于聚苯并咪唑、聚醚酮或聚砜,使得在所获得的薄膜电极组件中没有氟碳化合物成分。
根据本发明而获得的薄膜电极组件在用于即使具有非常低的铂载荷的PEMFC中时也具有极好的性能,并且对于其他类型的燃料电池,如DMFC(直接甲烷燃料电池),或对于其他电化学应用,如薄膜电解过程,也是有用的。
附图说明
图1示出本发明的电极在应用Liquid Nafion悬浮液之前和之后的伏安曲线。
具体实施方式
选择气体扩散介质,其由涂覆有Shawinigan Acetylene Black碳微粒和PTFE的三维编织碳布构成,总厚度410微米,基本重量210g/m2,密度0.54g/cm3,电阻率525mΩcm,空气渗透率0.74加里,25微米的多孔率,平均孔大小6微米。该气体扩散层已改善了张力特性和表面粗糙度,良好地适合随后的表面金属淀积;具体的,标准的平滑度测试给出5000加里秒的值。如此获得的气体扩散介质经铂金属的双IBAD淀积:样品首先经200至500eV的第一低能束流,以清洗和使表面部分形成纹理,然后经受气相的铂离子,其被从等离子中提取并被加速以1000-2000eV的能量进入在该气体扩散层表面上的生长的铂覆层。在IBAD处理中,离子轰击是控制膜特性的关键因素,引入相当的能量到该覆层和覆层/衬底界面。这实现衬底加热(这通常提供紧密的更均匀的膜)而没有显著加热下面的气体扩散材料,而所述加热可能使其体特性恶化。离子还与覆层原子相互作用,驱使它们进入到衬底,并产生分级材料界面,这增强了粘接。获得了0.04mg/cm2的总的淀积,这对应于25nm的总厚度。
从样品中切出两个小片,其中之一涂覆有0.5mg/cm2的现有技术中已知的Liquid Nafion。对被涂覆和未被涂覆的样品两者执行循环伏安法,如图1所示,其中(1)表示对应于后者的曲线,而(2)是与前者对应的曲线。尽管与现有技术的教导不一致,但它清楚地表明了有效表面如何由于Nafion覆层而降低。
其余的未被涂覆的电极,用来经热压过程(例如在120℃和25bara下10分钟)以“三明治”的方式将其结合到Nafion112,制备薄膜电极组件。与现有技术中已知的常规MEA组件相反,在制造的MEA中不存在额外的液态离聚物。在随后的燃料电池表征中,能够在1.5bara下以化学计量比2和80℃的电池温度下进料纯氢气和空气,在大约0.8V产生0.3A/cm2,和在大约0.7V产生0.7A/cm2
上述说明不应当认为是限制本发明,本发明可以根据不同实施例进行实践,而不脱离本发明的范围,并且其范围仅由所附权利要求限定。在本申请的说明书和权利要求书中,词“包括”及其变形,如“包含”不意图排除另外成分的其他要素的存在。

Claims (16)

1.一种在基本没有离聚成分的气体扩散介质上形成贵金属覆层的方法,包括:使导电网经受具有不高于500eV的能量的第一离子束,和经受具有至少500eV的能量的第二束,其包含至少一种贵金属的离子。
2.如权利要求1所述的方法,其中,所得到的贵金属覆层具有被包括在5和500nm之间的厚度以及0.01至0.1mg/cm2的载荷。
3.如权利要求1所述的方法,其中,所述第一离子束具有被包括在100和500eV之间的能量,且所述第二离子束具有被包括在500和2000eV之间的能量。
4.如前述任一权利要求所述的方法,其中,所述至少一个贵金属是铂。
5.如权利要求1所述的方法,其中,所述导电网预先设置有无贵金属的气体扩散层,该气体扩散层包括至少一个导电填充物和至少一种结合剂。
6.如权利要求5所述的方法,其中,所述导电填充物包括碳微粒,可选的,乙炔碳黑微粒。
7.如权利要求5或6所述的方法,其中,所述结合剂是疏水结合剂,可选的,是氟化的。
8.如权利要求5至7中任一所述的方法,其中,所述气体扩散层具有至少1000加里秒的平滑度。
9.一种气体扩散电极,包括:导电网;非催化的气体扩散层,其包括至少一个导电填充物和至少一种结合剂;以及贵金属覆层,其通过权利要求1至5中任一个的方法而获得。
10.如权利要求9所述的气体扩散电极,其中,该导电网是金属或碳布或者碳纸。
11.如权利要求9所述的气体扩散电极,其中,所述至少一个导电填充物包括碳微粒,可选的,乙炔碳黑微粒。
12.如权利要求9所述的气体扩散电极,其中,所述至少一种结合剂是疏水的,可选的,是氟化的。
13.如权利要求9所述的气体扩散电极,其中,所述气体扩散层具有至少1000加里秒的平滑度。
14.一种薄膜电极组件,包括:至少一个权利要求9至13中任一个的气体扩散电极,以及离子交换薄膜。
15.如权利要求14所述的薄膜电极组件,其中,所述至少一个气体扩散电极和所述离子交换薄膜通过热压而相互结合。
16.如权利要求14或15所述的薄膜电极组件,其中,所述离子交换薄膜是非氟化的,并且其中不存在离聚氟化成分。
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