鉑鎳正八面體讓燃料電池省鉑90%

高效耐用的催化劑是燃料電池領(lǐng)域取得突破的關(guān)鍵。最近，德國科學(xué)家研發(fā)出一種鉑鎳納米粒子，用其作催化劑，可將燃料電池中鉑的用量減少90%。研究還發(fā)現(xiàn)，新納米粒子的功能由其幾何形狀和原子結(jié)構(gòu)決定。發(fā)表在最新一期《納米!材料學(xué)》雜志上的最新研究將有助于科學(xué)家們提高催化劑的性能。

氫動(dòng)力燃料電池除產(chǎn)生電能外，唯一的副產(chǎn)品是水，因此被看作傳統(tǒng)內(nèi)燃機(jī)的環(huán)保替代品。但這種燃料電池的電極（化學(xué)轉(zhuǎn)化過程在此發(fā)生）需要用到大量“身價(jià)不菲”的鉑。如果沒有鉑作為催化劑，很難獲得必需的轉(zhuǎn)化效率，因此，鉑正成為發(fā)展氫燃料電池的“攔路虎”。

科學(xué)家們解釋道，氫燃料電池的催化過程只發(fā)生在鉑表面，而鉑納米粒子可增加鉑表面積，既節(jié)省材料又能提高電極效率，1公斤納米粒子的表面積相當(dāng)于幾個(gè)足球場(chǎng)。另外，讓鉑和鎳或銅等廉價(jià)金屬相混合，還可以節(jié)省更多鉑。

在這一思路指導(dǎo)下，德國于利希研究中心（FZJ）和柏林工業(yè)大學(xué)的科學(xué)家成功研發(fā)出高效的鉑鎳催化劑粒子，用于將氫氣和氧氣轉(zhuǎn)變成水，其中鉑僅為以前用量的1/10。新催化劑不由以前廣泛使用的圓形納米粒子，而由鉑—鎳合金的正八面體納米粒子組成，正八面體表面的鉑鎳原子擁有獨(dú)特的排列方式，能顯著提高氫氣和氧氣生成水的化學(xué)反應(yīng)速度。

研究人員還使用德國恩斯特魯斯卡電子顯微學(xué)與電子譜學(xué)中心（ER-C）的超高清電子顯微鏡，對(duì)新納米粒子的原子結(jié)構(gòu)進(jìn)行了觀察。ER-C的馬克!海根博士解釋道：“我們發(fā)現(xiàn)，鎳和鉑原子并非均勻地分布在納米正八面體表面，這可以增加其反應(yīng)活性，不過會(huì)減少其壽命?！?/p>

柏林工業(yè)大學(xué)的皮特!斯特拉瑟教授表示：“新研究告訴我們，為獲得最優(yōu)的催化效果，催化劑粒子合適的幾何形狀與其組成及大小一樣重要，我們可借用這一發(fā)現(xiàn)來改進(jìn)用來儲(chǔ)能的功能性材料尤其是催化劑的性能?！?/p>

Platinum-nickel nano-octahedra catalyst for fuel cells save 90% platinum

Efficient,robust and economic catalyst materials hold the key to achieving a breakthrough in fuel cell technology.Scientists from Jülich and Berlin have developed a material for converting hydrogen and oxygen to water using a tenth of the typical amount of platinum that was previously required.With the aid of state-of-the-art electron microscopy,the researchers discovered that the function of the nanometre-scale catalyst particles is decisively determined by their geometric shape and atomic structure.This discovery opens up new paths for further improving catalysts for energy conversion and storage.

The results have been published in the current issue of the respected journal Nature Materials.

Hydrogen-powered fuel cells are regarded as a clean alternative to conventional combustion engines,as,aside from electric energy,the only substance produced during operation is water.At present,the implementation of hydrogen fuel cells is being hindered by the high material costs of platinum.Large quantities of the expensive noble metal are still required for the electrodes in the fuel cells at which the chemical conversion processes take place.Without the catalytic effect of the platinum,it is not currently possible to achieve the necessary conversion rates.

As catalysis takes place at the surface of the platinum only,material can be saved and,simultaneously,the efficiency of the electrodes improved by using platinum nanoparticles,thus increasing the ratio of platinum surface to material required.Although the tiny particles are around ten thousand times smaller than the diameter of a human hair,the surface area of a kilogram of such particles is equivalent to that of several football fields.

Still more platinum can be saved by mixing it with other,less valuable metals,such as nickel or copper.Scientists from Forschungszentrum Jülich and Technische Universit?t Berlin have succeeded in developing efficient metallic catalyst particles for converting hydrogen and oxygen to water using only a tenth of the typical amount of platinum that was previously required.

The new catalyst consists not of the round nanoparticles that were previously in widespread use,but of octrahedral-shaped nanoparticles of a platinum-nickel alloy.The researchers discovered that the unique manner in which the platinum and nickel atoms arrange themselves on the surfaces of these particles serves to optimally accelerate the chemical reaction between hydrogen and oxygen to form water.Round or cubic particles,on the other hand,have different atomic arrangements at the surface and are therefore less effective catalysts for the chemical reaction,something which would have to be compensated by using increased amounts of noble metal.

The way in which the lifecycle of the catalysts depends on and can be optimized by their atomic composition was the subject of the research team's investigation,which made use of ultrahigh-resolution electron microscopy at the Ernst Ruska-Centre (ER-C),a facility of the Jülich Aachen Research Alliance."A decisive factor for understanding the life-cycle of the catalysts was the observation that nickel and platinum atoms prefer not to be evenly distributed at the surface of the nano-octahedra," explains Dr.Marc Heggen from ER-C and the Peter Grünberg Institute at Forschungszentrum Jülich."Although this is advantageous for reactivity,it limits lifetime."

To identify the locationof each element with atomic precision,the researchers used a method in which the electron beam of one of the world's leading ultrahigh-resolution electron microscopes is finely focused,sent through the specimen and,by interactions with the specimen,loses part of its energy.Each element in the specimen can thus be identified like a fingerprint.Conventional electron microscopes are not capable of detecting such chemical signatures with atomic resolution.

"This pioneering experimental work provides direct evidence for the fact that the choice of the correct geometric shape for the catalyst particles is as important for optimizing their function as the choice of their composition and size," says Prof.Peter Strasser from Technische Universit?t Berlin."This provides researchers with new possibilities for further improving functional materials,especially catalysts,for energy storage." The latest experiments from Strasser's research group indicate that substantial increases in efficiency may also be possible for the reaction splitting water to produce oxygen in electrolysers,for which the even more expensive noble metal iridium is used.