In 2008, professor Jeremy P. Meyers, in the Electrochemical Society journal "Interface" wrote, "While fuel cells are efficient relative to combustion engines, they are not as efficient as batteries, due primarily to the inefficiency of the oxygen reduction reaction. ... make the most sense for operation disconnected from the grid, or when fuel can be provided continuously. For applications that require frequent and relatively rapid start-ups ... where zero emissions are a requirement, as in enclosed spaces such as warehouses, and where hydrogen is considered an acceptable reactant, a [PEM fuel cell is becoming an increasingly attractive choice exchanging batteries is inconvenient". The practical cost of fuel cells for cars will remain high, however, until production volumes incorporate economies of scale and a well-developed supply chain. Until then, costs are roughly one order of magnitude higher than DOE targets.