在2016年的各大车展中，来自丰田(Toyota)、本田(Honda)、现代(Hyundai)和梅赛德斯-奔驰(Mercedes-Benz)的多款全新燃料电池汽车和SUV，引发了广泛的讨论。但由于这种车的燃料电池组必须采用昂贵的铂催化剂，以加速产生能量的化学反应，因此第一批商用燃料电池车型相当昂贵。

“过去20年中，燃料电池中铂催化剂的用量已经下降了到了原来的10%，”纽瓦克市特拉华大学(Universityof Delaware)化学工程系教授严玉山介绍道，“但我认为在未来很长一段时间内，铂的用量可能很难继续下降。”

当前的燃料电池技术，其成本到底能否降至经济上真正可负担的水平，严教授对此持怀疑态度。大约10年前，严教授和他的同事放弃了现行的质子交换膜型（下简称PEM）燃料电池，转而将精力放在了一种完全不需要铂催化剂的燃料电池研发中。

酸vs碱

1839年，当WilliamGrove发明燃料电池原理时，他选择了硫酸作为电池的电解质。大约过了漫长的100年，人们才终于研制出另一种燃料电池，即碱性燃料电池。具体来说，碱性燃料电池会使用氢氧化钾溶液作为电解质。严教授指出，总体来说，酸性燃料电池和碱性燃料电池内的化学反应非常类似，都是电池正极氧气减少，电源负极的氢气被氧化。

“但当你写下标注电荷移动的化学方程式时，你就会发现用碱性电解质中的OH-替代酸性电解质中的H+还是会产生一些差别。”

上世纪90年代，当汽车行业着力研发PEM氢燃料电池时，大家并未过多考虑酸性电池内极具腐蚀性的酸性反应环境。那时的研究重点是研制允许氢质子(H+)通过的交换膜。后来，杜邦(DuPont)公司推出的Nafion半透性质子交换膜改变了整个燃料电池领域的面貌，即使这种膜看起来与厨房常见的塑料纸没什么差别。

严教授回忆说，虽然当时这种氟化聚合物薄膜的价格非常昂贵，但那时的研究人员“好像就是认定了这种东西，他们似乎对其他技术完全没有兴趣了。”但严教授和他在特拉华大学的团队并未止步于杜邦公司的Nafion膜，他们坚信，氢氧质子交换膜燃料电池的概念一定可以提供与氢质子交换膜同等级别的高性能，而且可以将成本降至前所未有的水平。

严教授解释说，选择碱性环境有一个优势：我们可以用镍、银等相对便宜的金属替代非常昂贵的铂催化剂。“碱性的反应环境更好，”他表示说，“这是因为多数催化金属在碱性环境下都稳定的多，而酸性环境会溶解一切金属，包括铂。”

最近，严教授团队发表的报告描述了团队进行的氢氧质子交换膜燃料电池研究，这种燃料电池原型选择了一种以镍为基础的低成本复合催化剂，并在电池阳极附近的氢氧化反应（见《自然·通讯》，1月14日内容，点击此处直达）。这种复合催化剂特别采用掺氮碳纳米管上的镍纳米颗粒，可在碱性反应环境中，展现与铂族金属类似的催化氧化效果。

目前，碱性燃料电池面对的最大问题在于，与酸性电池相比相对较慢的反应速度。“这是一个问题，碱性环境下的反应速度是酸性环境的1%。”严教授指出，“但我们已经有思路，可以利用催化剂加速反应的发生，不过这可能还需要几年时间才能实现。”

碱性质子膜挑战Nafion膜

几年前，特拉华大学的团队为自己的碱性燃料电池研制了一种与“Nafion”很类似的膜。如同Nafion膜允许氢质子通过一样，特拉华大学的这层薄膜也允许氢氧质子通过。严教授表示，“我们在氢氧质子交换膜上做的相当不错。”

从技术层面而言，这种质子膜的本质是“银-磷高分子离聚物的交汇界面”，效果非常好。使用这种季磷功能化的聚合物可以产出一种材料，这种材料在水中不易膨胀，可以用来打造性能绝佳的氢氧质子交换膜燃料电池。严教授表示，这种材料是一种纳米级拼接物，其疏水区段周围遍布亲水通道，氢氧离子正是通过这些微小通道才能穿过交换膜。

这种新型膜技术的成本更低，主要原因是其采用的碳氢化合物材料比PEM膜采用的氟化聚合物膜更加便宜。

“我们最终的目标是将氢氧质子膜燃料电池的应用推广至汽车领域，打造真正买的起的燃料电池汽车，那时丰田Mirai的价格大概可以下降至23000美元。”严教授推测，“一旦真正让用户买得起的燃料电池汽车成为现实，那氢能源经济所带动的各种基础设施建设与发展也将很快跟上。”

严教授以轻松的语气讲述了他与他的团队是如何在2009年和2010年应对美国能源署部长朱棣文(Steven Chu)，以及其他对燃料电池持怀疑态度人士的质疑。虽然当时进行了非常严格的评估，但最终朱棣文还是批准了严教授团队的经费申请。

如果打动朱棣文的不是严教授的研究成果，那可能是严教授努力研制新型燃料电池时的大胆无畏打动了这位诺贝尔物理学奖得主，因为朱棣文很少为氢燃料电池技术拨付经费。
 

Rethinking the route to lower-cost fuel cells

New fuel cell-powered cars and SUVs from Toyota, Honda, Hyundai and Mercedes-Benz are capturing plenty of publicity on this year's auto show circuit. But the first commercial models are expensive, in part because their fuel cell stacks use costly platinum catalysts to speed the key power-producing chemical reactions.

“The level of platinum use in fuel cells has come down ten-fold in the last 20 years,” observed Yushan Yan, chemical engineering professor at the University of Delaware in Newark, “but I have a feeling that the platinum level will stay where it is for some time to come.”

Yan is skeptical that current fuel cell technology can be become truly affordable. About a decade ago he and his colleagues turned away from current proton exchange membrane (PEM) fuel cells in favor of a type that needs no platinum at all.

Acid versus base

When William Grove invented the principle of the fuel cell in 1839, he used sulfuric acid as the electrolyte. But it took another 100 years for the alternative—the alkaline, or basic, fuel cell—to be developed. The alkaline fuel cell used KOH, or potassium hydroxide, as its electrolyte. Yan noted that the reactions of both types are similar at a high level—oxygen reduces on the positive electrode, and the hydrogen oxidizes on the negative electrode.

"But when you write down the chemical reaction with the charge-carrying ions, it’s different because you use an OH- instead of an H+,” he said.

In the 1990s when the auto industry focused development on PEM hydrogen fuel cells, there wasn't much concern about their extremely corrosive, acidic operating environment. The main issue was the other key component, the membrane that passes protons (H+) between the two electrodes.The ready availability of DuPont’s Nafion semi-permeable polymer film was the game-changer, despite it looking like ordinary plastic kitchen wrap.

Though the fluorinated polymer membrane was itself premium-priced, researchers “felt like that was it; they never wanted to work with other technologies,” Yan recalled. Rather than settle on Nafion, he and his group at Delaware bet that their hydroxide (OH-) exchange membrane fuel cell concept can offer high performance at an unprecedented low cost.

Opting for the high end of the pH range has an advantage: it enables replacement of platinum catalysts with cheaper metals like nickel or silver, Yan explained. “A basic operating environment is better," he said, "because many catalytic metals are much more stable, while everything dissolves in acid, including platinum.”

Yan’s team recently published an account of their work on a hydroxide exchange membrane fuel cell that uses a prototype low-cost nickel-based catalyst for the hydrogen oxidation reaction at the anode. (See www.nature.com, January 14.) The composite catalyst, which features nickel nanoparticles that are supported on nitrogen-doped carbon nanotubes, exhibits levels of hydrogen oxidation activity similar to those of platinum-group metals in an alkaline electrolyte.

The key remaining issue to address in the catalyst is the comparative slowness of the alkaline reaction compared to its acidic platinum counterpart. “It’s a problem; the reaction occurs 100 times slower in basic conditions," Yan noted, "but we have our ideas about how we can get the catalyst to do what we want. Still, it’s probably a couple of years away.”

Basic membrane challenges Nafion

Several years ago the Delaware group developed a “Nafion-equivalent” membrane for its alkaline fuel cell, a thin polymer membrane that does for hydroxide ions what Nafion does for protons. “We have a good handle on the hydroxide exchange membrane,” Yan asserted.

Technically, the prototype membrane is classified as “an efficient silver-phosphonium ionomer interface.” Using a quaternary phosphonium-functionalized polymer yields  a material that is less susceptible to swelling with water while providing excellent hydroxide exchange membrane fuel cell performance. According to Yan, the material is a nanoscale patchwork of hydrophobic domains abutting hydrophilic water channels; it is via these tiny passages that hydroxide ions come streaming through.

The new membrane technology would also be cheaper because it would replace the PEM’s high-priced fluorinated polymer membrane with a cheaper hydrocarbon material, another boost to economic viability.

“Our real hope is that we can put hydroxide exchange membrane fuel cells into cars and make them truly affordable—maybe $23,000 for a Toyota Mirai,” Yan speculated. “Once the cars themselves are more affordable, that will drive development of the infrastructure to support the hydrogen economy.”

Yan recounted with amusement how he and his team’s contrary R&D path somehow passed measure with Steven Chu, the U.S. Department of EnergySecretary and notorious fuel-cell skeptic, in 2009 and 2010. Despite a hard-eyed evaluation, Chu green-lighted Yan's group for funding.

If it wasn't the result of sheer spite on the part of the Nobel Prize-winning physicist, perhaps it was the sheer audacity of building a new kind of fuel cell that impressed the Secretary, because it was one of only a few grants that Chu ever provided for hydrogen fuel cell technology.

Author: Steven Ashley
Source: SAE Automotive Engineering Magazine