  The theoretical understanding of active matter, which is driven out of
equilibrium by directed motion, is still fragmental and model oriented.
Stochastic thermodynamics, on the other hand, is a comprehensive theoretical
framework for driven systems that allows to define fluctuating work and heat.
We apply these definitions to active matter, assuming that dissipation can be
modelled by effective non-conservative forces. We show that, through the work,
conjugate extensive and intensive observables can be defined even in
non-equilibrium steady states lacking a free energy. As an illustration, we
derive the expressions for the pressure and interfacial tension of active
Brownian particles. The latter becomes negative despite the observed stable
phase separation. We discuss this apparent contradiction, highlighting the role
of fluctuations, and we offer a tentative explanation.
