Cell voltage versus current density (polarization curves) for several CsHSO4 fuel cells with differing Pt loadings operated under the conditions indicated. Platinum content was varied by changing the thickness of the electrode layer while maintaining the overall composition.

The compound CsH 2 PO 4 has emerged as a viable electrolyte for intermediate temperature (200-300 1C) fuel cells. In order to settle the question of the high temperature behavior of this material, conductivity measurements were performed by two-point AC impedance spectroscopy under humidified conditions (p[H 2 O] = 0.4 atm). A transition to a stable, high conductivity phase was observed at 230 1C, with the conductivity rising to a value of 2.2 Â 10 À2 S cm À1 at 240 1C and the activation energy of proton transport dropping to 0.42 eV. In the absence of active humidification, dehydration of CsH 2 PO 4 does indeed occur, but, in contradiction to some suggestions in the literature, the dehydration process is not responsible for the high conductivity at this temperature. Electrochemical characterization by galvanostatic current interrupt (GCI) methods and three-point AC impedance spectroscopy (under uniform, humidified gases) of CsH 2 PO 4 based fuel cells, in which a composite mixture of the electrolyte, Pt supported on carbon, Pt black and carbon black served as the electrodes, showed that the overpotential for hydrogen electrooxidation was virtually immeasurable. The overpotential for oxygen electroreduction, however, was found to be on the order of 100 mV at 100 mA cm À2. Thus, for fuel cells in which the supported electrolyte membrane was only 25 mm in thickness and in which a peak power density of 415 mW cm À2 was achieved, the majority of the overpotential was found to be due to the slow rate of oxygen electrocatalysis. While the much faster kinetics at the anode over those at the cathode are not surprising, the result indicates that enhancing power output beyond the present levels will require improving cathode properties rather than further lowering the electrolyte thickness. In addition to the characterization of the transport and electrochemical properties of CsH 2 PO 4 , a discussion of the entropy of the superprotonic transition and the implications for proton transport is presented.

No abstract

The ability to store energy on the electric grid would greatly improve its efficiency and reliability while enabling the integration of intermittent renewable energy technologies (such as wind and solar) into baseload supply. Batteries have long been considered strong candidate solutions owing to their small spatial footprint, mechanical simplicity and flexibility in siting. However, the barrier to widespread adoption of batteries is their high cost. Here we describe a lithium-antimony-lead liquid metal battery that potentially meets the performance specifications for stationary energy storage applications. This Li||Sb-Pb battery comprises a liquid lithium negative electrode, a molten salt electrolyte, and a liquid antimony-lead alloy positive electrode, which self-segregate by density into three distinct layers owing to the immiscibility of the contiguous salt and metal phases. The all-liquid construction confers the advantages of higher current density, longer cycle life and simpler manufacturing of large-scale storage systems (because no membranes or separators are involved) relative to those of conventional batteries. At charge-discharge current densities of 275 milliamperes per square centimetre, the cells cycled at 450 degrees Celsius with 98 per cent Coulombic efficiency and 73 per cent round-trip energy efficiency. To provide evidence of their high power capability, the cells were discharged and charged at current densities as high as 1,000 milliamperes per square centimetre. Measured capacity loss after operation for 1,800 hours (more than 450 charge-discharge cycles at 100 per cent depth of discharge) projects retention of over 85 per cent of initial capacity after ten years of daily cycling. Our results demonstrate that alloying a high-melting-point, high-voltage metal (antimony) with a low-melting-point, low-cost metal (lead) advantageously decreases the operating temperature while maintaining a high cell voltage. Apart from the fact that this finding puts us on a desirable cost trajectory, this approach may well be more broadly applicable to other battery chemistries.

CsH 2 PO 4 preparation. Cesium dihydrogen phosphate powder was synthesized by methanol-induced precipitation from aqueous solutions of stoichiometric quantities CsCO 3 and H 3 PO 4 . The resulting precipitate was dried under vacuum at 100 °C.Thermal analysis. Simultaneous gravimetric analysis and differential scanning calorimetry was performed using a Netzsch Jupiter 449c, equipped with a Balzers AMU 200 mass spectrometer for exhaust gas analysis. Data were collected from 30 to 400 °C