Development Of Cathode Catalysts Research Articles

Electrochemical Reduction of Carbon Dioxide to Value-Added Products: The Electrocatalyst and Microbial Electrosynthesis.

The electrochemical reduction of carbon dioxide (CO2 ) to value-added products obtains great attention and investigation worldwide in recent years. The commercialization of this green process relies on the progress of relating high-performance electrocatalysts and their feasibility with proper reactor design. The microbial electrosynthesis (MES) is an alternative route to reduce CO2 with electroactive bio-film electrode as catalyst. This review presents the research status and development of cathode catalysts, particularly focusing on the active sites and development tendency, for highly efficient electrochemical reduction CO2 from personal viewpoint. Some of our results are also presented to exhibit contributions. MES shows a similar process to the typical electrochemical reduction of CO2 . Their combination is an important trend, and the future research in this field is full of challenges and opportunities.

(Invited) Highly Active and Durable Pt-Based Catalysts for the Oxygen Reduction Reaction in PEFCs

Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is essential for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs), which are applied to fuel cell vehicles (FCVs) and residential co-generation systems. At the present stage, costly Pt or its alloys have been employed as the cathode catalyst, having appreciable ORR activity and durability in strong acidic electrolyte at low operating temperatures <100 °C. The mass activity MA of Pt-based catalysts is defined as MA (A gPt −1) = j S (A m−2) × ECA (m2 gPt −1). To increase the area-specific activity j S (current density per active surface area), Pt-M alloys (Pt-Fe, Pt-Co, Pt-Ni, Pt-Cr, etc.) have been examined. Assuming spherical catalyst particles, the electrochemically active surface area ECA is inversely proportional to the particle diameter. To increase the ECA, it is effective to disperse Pt or Pt-alloy nanoparticles on high-surface-area carbon supports (Pt/C or Pt-M/C). However, for developing such catalysts having both high ORR activity and high durability, there has been long-standing controversy surrounding important issues such as the crystal structures (ordered and disordered) and the chemical composition of Pt-M alloys, as well as their optimum particle size. Furthermore, because the mechanism of enhancement of the ORR activities at Pt-M alloys is still unclear, the strategy for designing new potential catalysts has not yet been established. This presentation focuses on the following topics for the cathode catalysts from bulk single crystals to practical nanoparticle catalysts. 1) Pt particle-size effect on the durability for an accelerated test, which simulates load-cycles for FCVsSo far, much research has suffered from trade-offs in determining the optimal particle-size of the catalysts, due to the different trends of Pt particle-size effects on the MA and durability. However, by the use of our n-Pt/C catalysts with a very narrow size distribution (σd ≤ 10%), the most durable catalyst with the highest MA over the whole test period (65°C, 0.6 ↔ 1.0 V, up to 30,000 cycles) was found to be n-Pt2 nm/C.1 2) Enhanced ORR activities at Pt-M alloys2, 3 2-1) Enormously enhanced ORR activity at Pt-skin layer formed on Pt3Co(111) single crystal electrode Pt-skin/Pt–Co(111) single crystals exhibited extremely high ORR activity, j S. The j S value at 0.9 V reached a maximum at Pt73Co27(111), the value of which is ca. 27 times higher than that on a pure Pt(111) electrode.4 By the use of in situ STM, in situ SXS, ex situ XPS, and DFT calculation, we have succeeded in correlating such a high j S with the specific surface structure. 2-2) ORR activity and durability of ordered- and disordered-Pt3Co/C5 We have recently clarified the effect of the crystal structure of Pt3Co alloy nanoparticles on the ORR activity, H2O2 yield, and durability, for the first time, by the use of the ordered- and disordered-Pt3Co/C catalysts with the nearly identical average particle size, size distribution, and composition.5 3) Enhancement in the ORR activity and durability at stabilized Pt-skin–PtCo alloy catalysts6, 7 We have successfully prepared PtCo alloy nanoparticles, having a stabilized Pt skin (one to two atomic layers: xAL), supported on carbon black or graphitized carbon black (PtxAL–PtCo/C or PtxAL–PtCo/GCB). These new catalysts exhibited high MA for the ORR, together with superlative durability. This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from the NEDO of Japan. References H. Yano, M. Watanabe, A. Iiyama, and H. Uchida, Nano Energy, 8, 13893 (2016).H. Uchida, H. Yano, M. Wakisaka, and M. Watanabe, Electrochemistry, 79, 303 (2011).M. Watanabe, D. A. Tryk, M. Wakisaka, H. Yano, and H. Uchida, Electrochim. Acta, 84, 187 (2012).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, and H. Uchida, J. Phys. Chem. C. 121, 11234 (2017).H. Yano, I. Arima, M. Watanabe, A. Iiyama, and H. Uchida, J. Electrochem. Soc., 164, F966 (2017).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, H. Uchida, Electrochemistry, 84, 133 (2016).

Superlative Activity and Durability of Stabilized Pt-Skin Pt-M (M=Fe, Co, Ni) Alloy Cathode Catalysts

Development of cathode catalysts with high oxygen reduction reaction (ORR) activity and high durability is one of the most important issues for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs). To obtain high mass activity (MA), Pt alloy nanoparticles dispersed on high-surface-area carbon support have been prepared. However, the durability of conventional PtM alloys (M = Fe, Co, Ni) have been insufficient due to an appreciable dealloying of M component at high temperatures (> 60 °C).1, 2 Recently, to increase both the MA and durability, we have succeeded in preparing new catalyst with two atomic layer of Pt skin (stabilized Pt skin layer) formed on PtCo-core alloys nanoparticles supported on graphitized carbon black (GCB; 150 m2 g-1) 2, 3 and high surface area carbon black (C; 800 m2 g-1).4 The preparation is briefly described below.3, 4 The PtCo alloy (1:1 atomic ratio) nanoparticles with uniform size and composition were highly dispersed on GCB or C was performed by the nanocapsule method.5 The PtCo/GCB or PtCo/C was heat-treated in H2 atmosphere in order to form nearly one atomic layer (Pt1AL) of Pt skin on the surface of the PtCo nanoparticles (denoted as Pt1AL–PtCo) by the segregation of Pt atoms from interior of the alloy core. Then, one more atomic layer of Pt-skin was formed on Pt1AL–PtCo (denoted as Pt2AL–PtCo) via controlled Pt deposition from a Pt-complex-containing aqueous solution with H2 bubbling. Finally, the Pt2AL–PtCo/GCB or Pt2AL–PtCo/C catalysts thus obtained were heat-treated in H2 at 200 oC.The kinetically-controlled mass activity (MA k) and specific activity (j k) for the ORR at Nafion-coated Pt2AL–PtCo/GCB and Pt2AL–PtCo/C electrodes were examined in O2-saturated 0.1 M HClO4 solution at 65 oC by multi-channel flow double electrode cell.1 The accelerated durability test (ADT) was examined by a standard potential step protocol (E = 0.6 V ↔ 1.0 V vs. RHE, holding 3 s at each E) simulated the load-change for the fuel cell vehicle (FCV)6 in N2-purged 0.1 M HClO4 solution at 65 oC.Figure 1 shows the plots of MA k and j k values at Nafion-coated Pt2AL–PtCo/GCB and Pt2AL–PtCo/C electrodes as a function of the potential step cycles (N). The initial values of j k at Pt2AL–PtCo/GCB (d TEM = 2.9 ± 0.2 nm) and Pt2AL–PtCo/C (d TEM = 3.3 ± 0.5 nm) were identical with that of PtCo/GCB, which was 2.5 times larger than that of a commercial standard catalyst c-Pt/C. The value of MA k at c-Pt/C decreased severely to 10% of the initial value after N = 30,000, whereas the j k was nearly unchanged. The MA k and j k values at PtCo/GCB were drastically decreased with increase N and were equal to that for c-Pt/CB after N = 30,000 due to the dealloying of Co. In contrast, the value of j k at the Pt2AL–PtCo/GCB electrode was maintained constant value during the ADT, and the retention of MA k at N = 30,000 was as high as 68%. A similar trend was seen for the Pt2AL–PtCo/C, at least, up to N= 10,000. Thus, the dealloying of Co was almost completely suppressed by the stabilized Pt skin layer on the surface regardless of GCB and C support.We have also succeeded in preparing the Pt2AL–PtM alloy (M = Fe and Ni) dispersed on C support by the same method as described above. The average particle sizes d TEM of the Pt2AL–PtFe/C and Pt2AL–PtNi/C were 2.9 ± 0.4 nm and 3.2 ± 0.4 nm, respectively. The Pt2AL–PtFe/C and Pt2AL–PtNi/C also exhibited high MA k. We will present the effect of alloy components on the ORR activity and durability.This work was supported by the funds for the “SPer-FC” project from the NEDO of Japan.

Effect of Added Graphitized Carbon Black on Both Performance and Durability of Pt/Nb-SnO2 Cathodes for PEFCs

The development of cathode catalysts that have both high activity for the oxygen reduction reaction (ORR) and high durability is an important subject for the application of PEFCs in fuel cell vehicles. At present, Pt nanoparticle catalysts supported on carbon black (Pt/CB) are typically being used in PEFC cathodes. While CB supports are essential for the high performance of PEFCs because of their large surface area, high electrical conductivity and well-developed pore structure, they corrode seriously during startup and shutdown of the fuel cell. Graphitized carbon black (GCB), which has a high degree of graphitization, has been used to mitigate such corrosion. According to our previous studies, the high stability of a Pt catalyst supported on GCB (Pt/GCB) was confirmed by use of simulated startup/shutdown evaluation.1 Electrochemical catalysts for PEFCs are required to have both electrically conductive paths and gas diffusion paths in order to form effective catalyst layers (CL). Therefore well-developed pore structures are needed in addition to high conductivity. We have developed Pt catalysts supported on doped SnO2 (Pt/Sb-SnO2, Nb-SnO2, and Ta-SnO2) with fused aggregate structures, which are similar to that of CB, constructed by the fusion of nearest-neighbor particles to form a branched structure.2,3 The Pt/doped SnO2 catalyst provides high electronic conductivity due to the low grain boundary resistance and also has a large micropore volume. Due to the effect of the doped SnO2 with fused aggregate structure, the high performance of these cathodes, which was superior to that of Pt/CB, was confirmed by use of single cell measurements.4,5 In this research,6 we report a detailed investigation of a single cell using Pt/Nb-SnO2 with the fused aggregate structure under actual operating conditions. For instance, we confirmed that the steady-state current-potential (I–E) curves of membrane-electrode assemblies (MEAs) were strongly dependent on the humidity condition of the supplied gases. We also investigated the effects of the addition of GCB into the Pt/Nb-SnO2cathode for the improvement of the cell performance. In Fig.1, the single cell performances using Pt/Nb-SnO2 with/without GCB as function of relative humidity were compared with that of a cell using Pt/GCB at 80◦C and hydrogen/air under 1 atm. The Pt-mass specific power of the Pt/Nb-SnO2 cathode under low humidity conditions was superior to that of the Pt/GCB cathode, because the hydrophilic SnO2 support helped to increase the proton conductivity of the ionomer, which led to high Pt effectiveness. The addition of GCB to the Pt/Nb-SnO2 cathode improved the cell performance under high humidity, and the Pt-mass specific power value reached more than 10 kW gPt −1. The improved performance was attributed to the formation of gas diffusion paths due to the addition of hydrophobic GCB. Fig. 2 shows that the Pt/Nb-SnO2 cathodes, with/without GCB, had greater durability than that of the Pt/GCB cathode after 60,000 cycles of potential sweep cycle evaluation (1.0-1.5 V, 0.5 V s-1). Fig. 3 shows transmission electron microscopic (TEM) images of the Pt/Nb-SnO2 CL and Pt/GCB CL both before and after durability evaluation. The Pt nanoparticle size on the Pt/Nb-SnO2 surface increased to 4.9 ± 0.8 nm (after 60000 cycles) from 3.0 ± 0.6 nm (initial state) in diameter during durability evaluation, becoming spherical in shape (Fig. 3(a) and 3(b)). In contrast, it can be observed that the Pt particles on GCB formed elongated clusters, most likely due to the aggregation of spheres, after 60000 cycles in Fig. 3(d). Moreover, we confirmed that the Pt (111) lattice planes were parallel to those of SnO2 (110), and that the Pt nanoparticles were well oriented on the Nb-SnO2 surface in the initial state, as shown in Fig. 4. We consider that such interaction between the Pt nanoparticles and the Nb-SnO2 support could also have suppressed the migration of the Pt nanoparticles during the durability evaluation. We conclude that the Pt/Nb-SnO2 cathodes, both with and without GCB, exhibited outstanding durability during the startup / shutdown potential sweep evaluation in PEFCs.6 Acknowledgement This research was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells” (HiPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and the JSPS KAKENHI grant Number B24350093. References M. Uchida et al., Physical Chemistry Chemical Physics, 15, 11236 (2012). K. Kakinuma et al., Electrochimica Acta, 56, 2881 (2011). Y. Senoo et al., RSC Advances, 4, 32180 (2014). K. Kakinuma et al., Electrochimica Acta, 110, 316 (2013). Y. Senoo et al., Electrochemistry Communications, 51, 37 (2015). Y. Chino et al., Journal of the Electrochemical Society, 162, F736 (2015). Figure 1

19 石炭を固体高分子形燃料電池の電極触媒に用いた研究(改質・利用技術)

19 石炭を固体高分子形燃料電池の電極触媒に用いた研究(改質・利用技術)

Preparation of cathode catalysts for PEMFC by carbon-alloying techniques

Development of cathode catalysts with lower platinum content or without platinum is an urgent issue for the commercialization of proton-exchange membrane fuel cell (PEMFC). Our idea to deal with the issue is to develop active carbonbased catalysts for the oxygen reduction by employing carbon-alloying techniques. In the first section of the present paper, the idea of carbon-alloy based catalysts will be explained. In the second and the third sections, the oxygen reduction activity, the structure, and the chemical nature of the two types of carbon alloys are given, i.e. metal-alloyed carbons and N-or BN-alloyed carbons. The importance of the surface defects on the shell-like structure and the N atoms located on the edge of graphene are pointed out for providing oxygen reduction activities to carbon materials. In the fourth section, we clarified that the oxygen reduction on the carbon-alloys included direct four-electron mechanism by employing different two electrochemical techniques. Lastly, preliminary results of single cell tests are described with a cobalt phthalocyaninealloyed carbon, which could give a maximum power of 0.2Wcm-2 without platinum.