Proton exchange membrane fuel cells (PEMFCs) suffer severe performance loss in the high current density (HCD) region as Pt‐loading decreases. A smaller electrocatalyst size inducing a higher electrochemically active surface area (ECSA) is critical for solving this issue. However, the poor electrocatalytic activity and stability of sub‐2 nm nanoclusters limit the potential to reduce their size. In this study, 1.69 nm Co‐doped Pt nanoclusters with a large ECSA (116.19 m2 gPt–1) are synthesized. The mass activity (MA) (0.579 A mgPt–1) and stability (9% MA loss after 30k potential cycling) refresh the record of sub‐2 nm nanoclusters. The structural characterization and theoretical calculations reveal that doping reduces the total energy required to stabilize the nanoclusters. Dopant tailoring of the d‐band center and vacancy formation energy account for the activity and stability enhancement, respectively. Due to the larger ECSA and MA induced by doping, HCD voltage loss due to lower Pt‐loading is significantly reduced compared with commercial Pt/C. The peak power density of low‐Pt‐loading PEMFCs (0.075 mgPt cmMEA–2) with a doped nanocluster cathode is 0.811 W cm–2 (H2–air condition), which far exceeds commercial Pt/C (0.5 W cm–2) and that of most reported electrocatalysts.

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Long‐term stable and high active catalysts for oxygen reduction reaction (ORR) are required for the commercialization of proton exchange membrane fuel cells (PEMFCs). Platinum (Pt) catalyst is the preferred choose for ORR, but the stability and activity of existing Pt catalyst are unsatisfactory for the commercialization of PEMFCs. Here the ternary Pt–Au–Ni/C and binary Pt–Ni/C are synthesized by a rapid microwave‐assisted polyol reduction. The Pt–Au–Ni/C exhibits superior ORR activity to both of Pt–Ni/C and commercial Pt/C. Moreover, the Pt–Au–Ni/C exhibited the long‐term stability in both of half‐cell and single‐cell accelerated degradation tests. The above results indicate that the addition of Au into binary Pt–Ni catalyst can not only enhance the stability but also improve the electrocatalytic activity.

We demonstrate a portable intrinsic scalar magnetic gradiometer composed of miniaturized cesium vapor cells and verticalcavity surface-emitting lasers (VCSELs). Two cells, with an inner dimension of 5 mm x 5 mm x 5 mm and separated by 5 cm, are driven by one VCSEL and the resulting Larmor precessions are probed by a second VCSEL through optical rotation. The off-resonant linearly polarized probe light interrogates two cells at the same time and directly reads out the amplitude difference between magnetic fields at two cell locations. The intrinsic gradiometer scheme has the advantage of avoiding added noise from combining two scalar magnetometers. We achieve better than 18 fT/cm/√Hz sensitivity in the gradient measurement. Ultra-sensitive short-baseline magnetic gradiometers can potentially play an important role in many practical applications, such as nondestructive evaluation and unexplode ordnance (UXO) detection. Another application of the gradiometer is for magnetocardiography (MCG) in an unshielded environment. Real-time MCG signals can be extracted from the raw gradiometer readings. The intrinsic gradiometer greatly simplifies the MCG setup and may lead to ubiquitous MCG measurement in the future.Over the last two decades, many breakthroughs have been achieved in atomic magnetometer research. For example, the discovery of the spin-exchange relaxation-free (SERF) phenomenon at high atomic density and low magnetic field leads to a great improvement in the magnetometer noise performance [1]. Sensitivities comparable with [2] or even outperforming [3] those of superconducting quantum interference devices (SQUIDs) have been reported with SERF magnetometers. Another example is the successful fabrication of atomic magnetometers using the technique of Micro-Electro-Mechanical Systems (MEMS) [4,5,6,7,8]. MEMS techniques enable chip-scale devices, significantly reducing size and power-consumption of atomic magnetometers. Chip-scale magnetometers can have sizes approaching 10 mm 3 and dissipate less than 200 mW. Despite all these advances, applications of atomic magnetometers are still limited. Highly sensitive SERF magnetometers require a magnetically shielded environment while chip-scale total-field magnetometers have subpar noise performances [5], although a scalar magnetometer with a sensitivity of 100 fT/√Hz has been demonstrated using a MEMS-based cesium vapor cell [9]. With bigger cells, scalar magnetometers can reach sensitivities of sub-10 fT/√Hz [10] or even sub-fT/√Hz [11]. In practical applications in an unshielded environment, the output noise of scalar magnetometers is often dominated by the background field fluctuation, instead of their fundamental sensitivities. To overcome this problem, a common solution is to set up a gradiometer system using two or more magnetometers [12,13,14]. By taking the reading difference between adjacent magnetometers, this conventional gradiometer configuration suppresses the common field fluctuations at the cost of worsening the fundamental sensitivity by at least a factor...

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