The official teaching of the Catholic Church is that Catholics should not receive communion in an Episcopalian church except in situations of grave necessity where access to a Catholic priest is impossible.
The Episcopalian Church, while sharing many similarities with Catholicism, does not have the same understanding of the Eucharist and valid ordination. The Catholic Church believes the Eucharist is the true body and blood of Christ, consecrated by a validly ordained priest. The Catholic Church does not recognize Episcopalian priests as validly ordained due to historical breaks in apostolic succession, therefore they do not believe the Eucharist consecrated in an Episcopalian church is the true body and blood of Christ.
The Code of Canon Law outlines the rules for communicatio in sacris (sharing in sacred things) with non-Catholic churches. Generally, it restricts Catholics from receiving sacraments outside the Catholic Church.
However, there are exceptions, usually involving danger of death or grave necessity, judged according to the norms issued by the diocesan bishop or conference of bishops and whenever impossibility of approaching a Catholic minister or priest.
Therefore, while exceptions are possible in very limited circumstances, generally a Catholic should not receive communion in an Episcopalian church.