Presentation of the results. In recent years, due to the vigorous development of electrocatalysts, the efficiency of electrocatalytic reactions has been continuously improved. An effective strategy is to optimize the size of the electrocatalyst to achieve better electrochemical performance while reducing costs. Nanoelectrocatalysts with high specific surface area have been widely used in advanced electrochemical devices such as fuel cells. From an engineering point of view, nano-sized electrocatalysts not only increase the surface area of the electrode, but also improve the performance of the electrode device.
Nanyang Technological University, Singapore, Ming-yong Han, Agency for Science, Technology and Research, SingaporeThey summarized typical examples of the size effect of electrocatalysts and revealed the effect of size changes on the intrinsic properties of electrocatalysts. The size effect of electrocatalysts should be studied from both engineering and basic science perspectives, that is, the observed activity changes are not only the result of surface area changes, but also the relationship between the intrinsic activity and performance of catalysts is quite interesting.
Related work is based on ".size effects of electrocatalysts: more than a variation of surface area" as the title in ".acs nano. The first author is Dr. Wu Tianxue, Nanyang Technological University.
*Introduce. 
Figure 1Understand the size effects of catalysts from engineering and basic science perspectives
Using electrocatalytic ORR, OER, CO2RR, and MOR as examples, the authors discuss the size effects of catalysts from the aspects of engineering and basic science. The authors emphasize that changing the size of the catalyst is not just about changing the usable surface area. Changing the size of the catalyst will lead to changes in the intrinsic properties of the catalyst material and surface, which in turn will affect the intrinsic activity of the catalyst. From an engineering point of view, changing the particle size of the catalyst will have a significant change in the specific surface area of the catalyst. Whereas, the electrochemical reaction takes place on the surface of the electrocatalyst. Thus, by reducing the size of the electrocatalyst, a larger usable surface area is provided for the reaction, thereby improving the performance of the electrochemical device.
Fig.2 Specific surface area of catalysts of different sizes
In order to achieve high geometric activity with limited loading masses, the catalyst can be dimensionally engineered to create more surface for catalyzing electrochemical reactions. For example, nano-Nife alloys oxides, hydroxides, hydroxyhydroxides, have been developed for catalytic OER in alkaline media. The mass activity of Nife-based catalysts is highly dependent on the specific surface area and particle size of the catalyst. However, when the intrinsic activity of the catalyst is investigated, i.e., the activity is normalized by surface area, the dependence of this activity on size cannot be observed. In this case, the size effect is only related to the surface area of the catalyst and has little effect on the intrinsic activity of the catalyst. Therefore, the use of size effects in catalyst development is only applicable to the engineering scope, i.e., reducing particle size to increase the surface area available for electrochemical reactions, thereby improving the performance of electrochemical devices.
Fig.3 Electrochemical remodeling to increase the specific surface area of the catalyst
Increasing the specific surface area of the catalyst is not only possible in the preparation of nanocatalysts. The same results can be achieved with the use of precatalysts. The development of precatalysts relies on thermodynamically unstable materials in electrochemical reactions, which can reconstitute the catalyst material to obtain active species on the surface.
Figure 4Particle size affects the intrinsic activity of the catalyst
From the perspective of basic research, particle size also affects the intrinsic activity of catalysts. Changing the particle size of the catalyst can change the intrinsic properties of the catalyst material or surface, thereby affecting the intrinsic activity of the catalyst. To study this size effect, it is critical to quantify the surface area and surface-active sites. Electrochemical methods such as hydrogen underpotential deposition, CO dissolution, underpotential deposition of heterogeneous metals, redox of surface metals, and illegal Laday electric double layer capacitance have been widely used to quantify surface area and surface active sites.
In addition, non-electrochemical methods such as atomic force microscopy, BET, and electron microscopy can also be used to quantify surface area and surface-active sites. Due to the high cost and scarce reserves of Pt, nano-PT catalysts are essential to improve the mass activity of Pt. It has been widely reported that the mass activity of PT and the size of PT particles show a volcanic pattern. The particle size is 3-5 nm and the mass activity is optimal. After surface area normalization, it can be seen that the decrease in the size of PT particles leads to a decrease in their intrinsic activity. This indicates that as the particle size decreases, the activity of the PT surface decreases.
Fig.5 Relationship between particle size, activity and structure of PT nanoparticles
N Rskov et al. conducted a theoretical study of the particle model of octahedral PT (Fig. 5A), and their work explained the binding of OHADS to PT surface sites at different PT particle sizes. The results showed that the oxygen species bound to the undercoordination PT of the step site or edge site were much stronger than those bound to the platform site. Therefore, during the ORR, the PT of the step site or edge site is more likely to be poisoned by the binding of oxygen, thus limiting the ORR. Therefore, it can be considered that the PT of the platform site is the true ORR active site. The results showed that there was a good correlation between the ORR specific activity and the proportion of plateau sites in the Pt nanoparticles.
Fig.6 Particle size affects the intrinsic activities of Cu and Pt catalysts
Non-*** catalysts such as Cu also exhibit a similar size-to-activity dependence on CO2RR. Figure 6a normalizes the Faraday current density of Cu NPS and plots it against the diameter of CU NPS. The current density increases with the decrease of the size of Cu nanoparticles, especially when the size of nanoparticles is less than 5 nm. From the selectivity of H2, Co, CH4 and C2H4 in Figure 6B, the bulk copper foil favors the generation of CH4 and C2H4, while the generation of H2 and CO becomes dominant when the Cu size is less than about 15 nm.
The surface atomic coordination of spherical Cu nanoparticles was revealed through DFT simulations of Cu particles, as shown in Figure 6C. The results show that when the particle diameter is less than 5 nm, the proportion of low-coordination atoms (coordination number <8) increases significantly. The results show that the CO2RR performance of Cu NPS is related to size and the formation of low coordination sites in Cu, resulting in high selectivity for H2 and Co. A similar size effect has been reported in MOOR studies using PT NPS as a catalyst. As the particle size decreases, the binding strength of the surface PT site to oxygen (*OH or *CO) increases. *CO is a MOR intermediate, and it is easy to poison the PT surface due to the strong binding of PT-CO. On the other hand, PT-OH on the surface can promote the oxidation of *CO to CO2, while too much PT-OH can hinder the dehydrogenation of methanol on the surface. Therefore, the intrinsic activity of MOR decreased due to the decrease of PT particle size.
Fig.7 Particle size affects the surface electronic structure of the catalyst
In addition to surface chemistry, changes in particle size also affect the surface electronic structure of the catalyst. For example, in LACOO3, CO3+ is in a mixture of high and low spin. By decreasing the particle size of LACOO3, the proportion of CO3+ with high spin on the surface increases. This spin state transition also regulates the EG filling on the CO 3D orbital. The results showed that the LaCoO3 nanoparticles at 80 nm exhibited the highest OER specific activity. In addition, changing the size of the catalyst leads to a change in the atomic structure of the catalyst, resulting in the formation of a different catalytic structure. This can be demonstrated by studying OER's size-selective PD cluster catalysts. It was found that the OER activity of PD4 clusters was much lower than that of PD6 clusters and PD17 clusters.
Fig. 8 Methods for studying size effects and understanding the underlying principles of high-performance catalysts
The size effect in electrocatalysis is not just about considering surface area. It should be noted that, in a scientific context, size effects may be related to the surface chemistry or electronic structure of the electrocatalyst. In general, the highest mass activity can be obtained at the optimal particle size under the competition or cooperation of the size effect of the catalyst specific surface area and the specific activity. In order to reveal a more scientific understanding of the size effect in electrocatalysis, the general steps of studying the size effect are summarized: first, the intrinsic activity analysis is the basis of the size effect study. As a first step, quantifying surface area is crucial. Second, a more scientific understanding of size effects can include the electronic properties of catalysts. The size effect has been shown to affect the band structure, quantum properties, and magnetic properties of materials.
Bibliographic information. size effects of electrocatalysts: more than a variation of surface area，acs nano，2022.
10.1021/acsnano.2c04603