LiNixMn2−xO4 has been synthesized using sol‐gel and solid‐state methods for 0 < x < 0.5. The electrochemical behavior of the samples was studied in normalLi/LiNixMn2−xO4 coin‐type cells. When x = 0, the capacity of normalLi/LiMn2O4 cells appears at 4.1 V. As x increases, the capacity of the 4.1 V plateau decreases as 1−2x Li per formula unit, and a new plateau at 4.7 V appears. The capacity of the 4.7 V plateau increases as 2x Li per formula unit, so that the total capacity of the samples (both the 4.1 and 4.7 V plateaus) is constant. This is taken as evidence that the oxidation state of Ni in these samples is +2, and therefore they can be written as Li+1Nix+2Mn1−2x+3Mn1+x+4O4−2 . The 4.1 V plateau is related to the oxidation of Mn3+ to Mn4+ and the 4.7 V plateau to the oxidation of Ni2+ to Ni4+ . The effect of synthesis temperature, atmosphere, and cooling rate on the structure and electrochemical properties of LiNi0.5Mn1.5O4 is also studied on samples made by the sol‐gel method. LiNi0.5Mn1.5O4 samples made by heating gels at temperatures below 600°C in air are generally oxygen deficient, leading to Mn oxidation states significantly less than 4. LiNi0.5Mn1.5O4 samples heated above 650°C suffer due to disproportionation into LiNixMn2−xO4 with x < 0.5 and LizNi1−zO with z ≈ 0.2, which occurs above about 650°C. Pure LiNi0.5Mn1.5O4 materials can be made by extended heatings near 600°C or by slowly cooling materials heated at higher temperatures. LiNi0.5Mn1.5O4 made at 600°C has demonstrated good reversible capacity at 4.7 V in excess of 100 mAh/g for tens of cycles.

This Review addresses the technical challenges, scientific basis, recent progress, and outlook with respect to the stability and degradation of catalysts for the oxygen evolution reaction (OER) operating at electrolyzer anodes in acidic environments with an emphasis on ion exchange membrane applications. First, the term "catalyst stability" is clarified, as well as current performance targets, major catalyst degradation mechanisms, and their mitigation strategies. Suitable in situ experimental methods are then evaluated to give insight into catalyst degradation and possible pathways to tune OER catalyst stability. Finally, the importance of identifying universal figures of merit for stability is highlighted, leading to a comprehensive accelerated lifetime test that could yield comparable performance data across different laboratories and catalyst types. The aim of this Review is to help disseminate and stress the important relationships between structure, composition, and stability of OER catalysts under different operating conditions.

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si ͑a-Si͒, and a-Si 0.64 Sn 0.36 as they reversibly react with Li measured by in situ atomic force microscopy ͑AFM͒. Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a-Si and a-Si 0.64 Sn 0.36 show good capacity retention with cycle number.

We have investigated the impact of electrocatalyst loading on rotating ring-disk electrode ͑RRDE͒ experiments for the oxygen reduction reaction on Fe-N-C catalysts ͑ORR͒ in acid medium. In particular, the fraction of H 2 O 2 produced as a function of catalyst loading was studied. A dramatic increase in H 2 O 2 release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst ͑NNMC͒, the fraction of produced H 2 O 2 varied between less than 5% and greater than 95%, depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via H 2 O 2 , and if the catalyst is sparsely loaded, the produced H 2 O 2 cannot be efficiently reduced to H 2 O before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.Polymer electrolyte membrane fuel cells are currently under intensive research because they are a clean energy conversion device. 1 Electrocatalysts which have high activity for oxygen reduction and show stability in acidic environments are highly desired, because a significant fraction ͑70%͒ of voltage loss in a fuel cell originates at the cathode where oxygen reduction occurs. 2,3 Besides being active and stable, the electrocatalysts must meet other requirements, including immunity against poisoning; ease of preparation; affordable cost, and minimum release of H 2 O 2 . The latter is a particularly important attribute because H 2 O 2 is known to decompose into highly reactive intermediates that initiate a chain oxidation, starting with the carboxylic groups that then propagate within the Nafion membrane. This membrane breakdown results in the release of F − ions in the effluent water. There are extensive reports on the role of H 2 O 2 , OH radicals, and the degradation of Nafion membranes in the literature. 4-6 Furthermore, reduction of oxygen to H 2 O 2 is a twoelectron reaction, thus producing smaller electric current per available oxygen molecule. Thus, minimum production of H 2 O 2 and a complete, four-electron reduction of oxygen are highly preferred.Although there are reports of detecting H 2 O 2 produced at the electrocatalyst in real operating fuel cells, 7 the best known method to measure the H 2 O 2 yield is still that of the rotating-ring disk electrode ͑RRDE͒. By fixing the potential ͓1.2-1.5 V vs a reversible hydrogen electrode ͑RHE͔͒ of the ring during cathodic or anodic sweeps of the disk potential, free H 2 O 2 , generated at the disk and passing near the ring by convection, is oxidized and produces a current. By comparing this current to the current produced at the central disk electrode, the percentage of O 2 molecules that are reduced to H 2 O 2 can be calculated. Paulus and Schmidt et al. explain the RRDE method applied to carbon-supported oxygen reduction reaction ͑ORR͒ electrocatalysts in detail. 8,9 Although there is extensive literature on RRDE measurements of ORR activity for Pt-based and non-noble metal catalysts ͑NNMCs͒, 10-13 there have been very few reports of the impact of the cat...

A multitarget sputtering machine with a water-cooled rotating substrate table has been modified to produce films on 75 mm × 75 mm wafers which map large portions of ternary phase diagrams. The system is unconventional because the stoichiometries of the elements sputtered on the wafer vary linearly with position and in an orthogonal manner. Subsequent screening of film properties is therefore quite intuitive, since the compositional variations are simple. Depositions are made under continuous rotation, so either intimate mixing of the elements (fast rotation) or artificial layered structures (slow rotation) can be produced. Rotating subtables mounted on the main rotating table hold the 75 mm × 75 mm substrates. Stationary mask openings over the targets and mechanical actuators that rotate the subtables in a precise manner accomplish the linear and orthogonal stoichiometry variations. Deposition of a film spanning the range SiSn x Al y (0 < x, y < 1), with Sn content increasing parallel to one edge on the wafer and Al content increasing in a perpendicular direction, is given to illustrate the effectiveness of the method. Since the system was easily and inexpensively built, it has enabled our research program in combinatorial materials synthesis to begin without large scale funding.

No abstract

Si 1Ϫx Sn x samples for 0 Ͻ x Ͻ 0.5 were prepared by magnetron sputtering using a combinatorial materials science approach. The room-temperature resistivity and X-ray diffraction ͑XRD͒ patterns of the samples were used to select materials having both an amorphous structure and good conductivity for further study. The reaction of lithium with amorphous Si 0.66 Sn 0.34 was then studied by electrochemical methods and by in situ XRD. The electrode material apparently remains amorphous throughout all portions of the charge and discharge profile, in the range 0 Ͻ x Ͻ 4.4 in Li x Si 0.66 Sn 0.34. No crystalline phases are formed, unlike the situation when lithium reacts with tin. Using the Debye scattering formalism, we show that the XRD patterns of the a-Si 0.66 Sn 0.34 starting material and a-Li 4.4 Si 0.66 Sn 0.34 can be explained by the same local atomic arrangements as found in crystalline Si and Li 4.4 Si or Li 4.4 Sn, respectively. In fact, the in situ XRD patterns of a-Li x Si 0.66 Sn 0.34 , for any x, can be well approximated by a linear combination of the patterns for x ϭ 0 and x ϭ 4.4. This suggests that predominantly only two local environments for Si and Sn are found at any value of x in a-Li x Si 0.66 Sn 0.44. However, based on differential capacity vs. potential results for Li/a-Si 0.66 Sn 0.34 there is no evidence for two-phase regions during the charge and discharge profile. Thus, the two local environments must appear at random throughout the particles. We speculate that the charge-discharge hysteresis in the voltage-capacity profile of Li/ a-Li x Si 0.66 Sn 0.34 cells is caused by the energy dissipated during the changes in the local atomic environment around the host atoms.

Synthesis and Electrochemistry of LiNixMn2-xO4.-Samples of LiNixMn2-xO4 with 0 ¡ x ¡ 0.5 are prepared by solid state reaction of LiOH, Ni(NO3)2·6 H2O, and MnO2 at 750 • C as well as by a sol-gel method using Mn(OAc)2, Ni(NO3)2, and LiOH as precursors and carbon black as stabilizer (250-800 • C), The electrochemical behavior of the samples is studied in Li/LiNixMn2-xO4 coin-type cells. The potential vs. capacity curves show a plateau at 4. 1 V for the Li/LiMn2O4 cells. With increasing x, the length of this plateau decreases and a new plateau at 4.7 V appears. The shift of the plateau potential is caused by the higher binding energy (by 0.6 eV) of the Ni eg electrons compared to the Mn eg electrons. LiNi0.5Mn1.5O4 prepared by the sol-gel technique at a firing temp. of ≈600 . degree.C exhibits a capacity of ¿ 100 mAh/g at 4.7 V which is maintained over tens of cycles. -(ZHONG, Q.; BONAKDARPOUR, A.; ZHANG, M.; GAO, Y.; DAHN, J. R.; J. Electrochem.