Understanding N-doped graphene electrocatalysts through in-situ characterisation

在电池技术中，很好地理解固体电极和液体电解质之间的界面是至关重要的。当电池充电和放电时，在电极处发生电化学反应，一些是期望的，一些是不期望的。了解这些反应以及如何促进理想的反应和消除不理想的反应是制造更好电池的关键。科学家面临的挑战是，要弄清楚电极/电解质界面发生了什么是非常困难的，因为在工作的电池中，这个界面很难到达--一边被电极埋着，另一边被电解质埋着。传统上，我们依赖于死后测量-即。拆卸电池，并在操作后观察电极表面。这有两个问题，首先，移除电极很可能会改变其成分（例如，氧化）。第二个也是更严重的问题是，这些测量只能告诉你电化学反应之后发生了什么，而不是过程中发生了什么。反应过程中的表面状态对于理解它至关重要，因此大力推动开发操作测量技术（可以在电化学过程中进行测量的技术）。X射线光电子能谱（XPS）是一种提供样品表面化学信息的分析技术。它的工作原理是向样品发射X射线，并检测相应发射的电子。这些电子携带着它们所来自的表面原子的信息。它是最通用和最强大的表面化学探针，已在电池研究中使用多年。然而，这是一种事后技术，需要高真空条件才能操作。开发XPS使其能够原位研究电化学反应在技术上非常具有挑战性，但可能非常有益-原位研究电化学界面的能力可能是革命性的。在这一领域有激烈的活动和几种竞争的方法，对样品的几何形状有严格的限制或需要复杂的样品制造。我在曼彻斯特领导研究，开发一种新的电化学XPS方法。我们的方法具有独特的通用性，几乎可以应用于任何样品。我们的方法包括将电解质的小液滴投射到XPS仪器内的样品表面上，并使用该液滴创建电化学电池。然后，我们可以使用XPS研究液滴的边缘，其中液体层足够薄，我们可以检测到来自电极/电解质界面的电子。我们最近发表的概念验证结果显示此接口的特性。本提案的目的是建立在这一发展和扩展电化学XPS技术，使之成为一个可靠和有用的研究工具。然后，我们将应用此工具来深入了解与应急电池技术相关的电化学问题。氮掺杂的石墨烯（其中一些碳原子交换为氮的石墨烯）已被证明是用于氧还原反应（ORR）的优异的电催化剂。这种反应是空气电池技术发展的关键瓶颈，空气电池技术是一种有前途的新兴电池技术，有可能在相同重量下提供10倍容量的电池。然而，N-石墨烯电催化剂的开发受到对其工作原理的非常差的理解的阻碍。电化学XPS将使我们能够在这些催化剂运行时跟踪其表面化学性质，从而获得前所未有的对其工作原理的深入了解。

In battery technology, a good understanding of what goes on at the interface between the solid electrodes and the liquid electrolyte is critical. As a battery charges and discharges, electrochemical reactions occur at the electrodes, some desirable, some undesirable. Understanding these reactions and how to promote the desirable ones and eliminate the undesirable ones holds the key to making better batteries.The challenge to scientists is that working out what is going on at the electrode/electrolyte interface is very difficult as in a working battery, this interface is hard to get to - buried by the electrode on one side and the electrolyte on the other. Traditionally we relied on post mortem measurements - ie. dismantling the battery, and looking at the electrode surfaces after operation. There are two problems with this, firstly that the removing the electrode will most likely change its composition (eg. by oxidation). The second, and more serious, problem is that these measurements only tell you what is happening after an electrochemical reaction, not during. The state of the surface during reaction is critical to understanding it, so there is a strong push to develop operando measurement techniques (ones that can take measurements during electrochemical processes).X-Ray Photoelectron Spectroscopy (XPS) is an analytical technique which provides chemical information about the surface of a sample. It works by firing X-rays at a sample and detecting the electrons emitted in response. These electrons carry with them information about the surface atoms they have come from. It's the most versatile and powerful probe of surface chemistry and has been in use in battery research for many years. It is, however, a post-mortem technique, requiring high vacuum conditions to operate. Developing XPS such that it can study electrochemical reactions in-situ is very technically challenging but potentially very rewarding - the ability to study electrochemical interfaces in-situ could be revolutionary. There is intense activity in this area and several competing approaches which place stringent restrictions on sample geometry or require complex sample fabrication.I am leading research in Manchester to develop a new approach to electrochemical XPS. Our approach is uniquely versatile and can be applied to practically any sample. Our approach involves projecting a small droplet of electrolyte onto the sample surface inside our XPS instrument and creates an electrochemical cell with that droplet. We can then study the edges of the droplet using XPS, where the liquid layer is thin enough that we can detect electrons from the electrode/electrolyte interface. We have recently published proof-of-concept results showing characterisation of this interface.The purpose of this proposal is to build on this development and to extend the electrochemical XPS technique so that is a reliable and useful research tool. We will then apply this tool to gain insight to an electrochemical problem relevant to emergent battery technology. Nitrogen - doped graphene (Graphene with some of the carbon atoms swapped for nitrogen) has been shown to be an excellent electrocatalyst for the oxygen reduction reaction (ORR). This reaction is a key bottleneck in the development of air battery technology, a promising emergent battery technology which has the potential to deliver batteries with 10 times the capacity for the same weight. However, development of N-graphene electrocatalysts is hampered by a very poor understanding of how they work. Electrochemical XPS will allow us to follow the surface chemistry of these catalysts whilst they are operating and therefore gain unprecendented insight into how they work.