In-situ diagnostic tools for hydrogen transfer leak characterization in PEM fuel cell stacks part II: Operational applications Amir M. Niroumand a, * , Hooman Homayouni a , Jake DeVaal b , Farid Golnaraghi a , Erik Kjeang a a School of Mechatronic Systems Engineering, Simon Fraser University,13450-102nd Ave, Surrey, BC, V3T 0A3, Canada b Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, BC, V5J 5J8, Canada highlights  Diagnostic tool for hydrogen leak characterization in operational PEM fuel cells.  Requires only stack voltage and air flow measurements.  Provides rate and distribution of hydrogen transfer leak in fuel cell stacks. article info Article history: Received 25 November 2015 Accepted 6 May 2016 Keywords: Fuel cell Diagnostic Hydrogen transfer leak Pinhole Operation abstract This paper describes a diagnostic tool for in-situ characterization of the rate and distribution of hydrogen transfer leaks in Polymer Electrolyte Membrane (PEM) fuel cell stacks. The method is based on reducing the air flow rate from a high to low value at a fixed current, while maintaining an anode overpressure. At high air flow rates, the reduction in air flow results in lower oxygen concentration in the cathode and therefore reduction in cell voltages. Once the air flow rate in each cell reaches a low value at which the cell oxygen-starves, the voltage of the corresponding cell drops to zero. However, oxygen starvation results from two processes: 1) the electrochemical oxygen reduction reaction which produces current; and 2) the chemical reaction between oxygen and the crossed over hydrogen. In this work, a diagnostic technique has been developed that accounts for the effect of the electrochemical reaction on cell voltage to identify the hydrogen leak rate and number of leaky cells in a fuel cell stack. This technique is suitable for leak characterization during fuel cell operation, as it only requires stack air flow and voltage mea- surements, which are readily available in an operational fuel cell system. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Improvements in fuel cell cost, performance, reliability, and lifetime are key to their successful commercialization. One approach for such improvements is using diagnostic tools at various stages of the fuel cell lifecycle, from Research and Devel- opment (R&D), to operation, and maintenance, in order to detect and correct failures [1,2]. In the first part of this article series [1], we introduced a diagnostic tool for characterizing hydrogen transfer leaks in PEM fuel cells, which is suitable for R&D applications. In the present work, we introduce a diagnostic tool that is suitable for characterizing hydrogen transfer leaks in an operational fuel cell stack. This tool uses available sensory information in an operational fuel cell system, therefore, it is inexpensive and compatible with mass manufacturing practices. In addition, it has the capability for early detection of membrane pinholes, which enables the control system and/or the operator to take compensatory actions. PEM fuel cell membranes develop pinholes as they age, due to a multitude of chemical, mechanical, and thermal processes [3e7]. These pinholes result in reactant gasses to leak across the mem- brane and react with the species of the opposite electrode, which causes several drawbacks. First, the crossover of hydrogen from anode to cathode, results in reduced fuel utilization, which in- creases the operational cost of the fuel cell system. In addition, the reaction of crossover hydrogen with oxygen reduces the cathode oxygen concentration, which negatively affects cell voltage and system performance. Furthermore, as the size and number of the * Corresponding author. E-mail address: amniroum@sfu.ca (A.M. Niroumand). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2016.05.019 0378-7753/© 2016 Elsevier B.V. All rights reserved. Journal of Power Sources 322 (2016) 147e154