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。当电池充放电时，电化学反应发生在电极上，有些是可取的，有些是不可取的。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的电池的关键。科学家们面临的挑战是，要弄清楚电极/电解质界面上发生了什么是非常困难的，因为在一个工作电池中，这个界面很难到达——电极在一边，电解质在另一边。传统上，我们依赖于事后测量。拆卸电池，操作后观察电极表面。这有两个问题，首先，去除电极很可能会改变其组成(例如。通过氧化)。第二个更严重的问题是，这些测量只能告诉你电化学反应之后发生了什么，而不能告诉你反应过程中发生了什么。在反应过程中，表面的状态对于理解它是至关重要的，因此有一个强大的推动力来开发操作测量技术（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来自表面原子的信息。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，这是一种死后技术，需要高真空条件才能操作。开发能够原位研究电化学反应的XPS在技术上是非常具有挑战性的，但潜在的回报非常大——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，这些方法对样品几何形状有严格的限制或需要复杂的样品制造。我在曼彻斯特领导一项研究，开发一种新的电化学XPS方法。我们的方法是独特的通用，可以应用于几乎任何样本。我们的方法包括将一小滴电解质投射到XPS仪器内的样品表面，并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，那里的液体层足够薄，我们可以检测到来自电极/电解质界面的电子。我们最近发表了概念验证结果，展示了该接口的特征。本提案的目的是在这一发展的基础上，扩展电化学XPS技术，使其成为一种可靠和有用的研究工具。然后，我们将应用该工具来深入了解与新兴电池技术相关的电化学问题。氮掺杂石墨烯（将部分碳原子交换为氮的石墨烯）已被证明是氧还原反应（ORR）的优良电催化剂。这种反应是空气电池技术发展的关键瓶颈，空气电池是一种新兴的电池技术，有可能在相同重量的情况下提供10倍的电池容量。然而，由于对n -石墨烯电催化剂的工作原理知之甚少，阻碍了它们的发展。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，从而获得对其工作原理的前所未有的深入了解。

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.